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All living creatures on the earth
and all material objects on it

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are subject to the pull of one great force:
The force of gravity.

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Were that to be suspended, even for a moment,
the most extraordinary things would happen.

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I, for example, would suddenly float into the air
because I at the moment...

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...am flying in an aircraft on a very special cours
which in effect cancels out the effect of gravity.

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So I float easily through the air.

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Our plane is climbing and diving
as though it were on a giant roller coaster,

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and as it goes over the crest of its climb, it rea
lifts you out of your seat and keeps you there.

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If there were no gravity on earth,
seas would rise from their beds

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just as this water lifts out of its cup
and disintegrates into droplets.

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Nothing would remain where it was placed.
There would be no up and no down.

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There would no longer be the sense
of earthly order that we take so much for granted.

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Some creatures have overcome the force
of gravity sufficiently to enable them to fly,

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but the only ones that match
this total freedom in the air that I have now

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are those that are so small
that they are, in effect, weightless.

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And there are more of them...
both plant and animal...

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...than you might think.

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The force of gravity holds the clouds
around the earth and the air in which they float.

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You can't touch air,
it's invisible and all-pervasive,

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so it's easy to forget that it has real substance.

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But it's only by exploiting the presence of air
that seeds, insects, birds and man

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are able to overcome gravity
and float above the earth's surface.

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Dandelion seeds rise
because a puff of air carries them up

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and they fall slowly
because their parachutes catch the air beneath.

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A tuft of fluff will serve the same purpose.

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Milkweed and cotton grass,
willowherb and thistles,

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all provide their seeds with downy floats.

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These delay the fall of the seeds for so long
that currents in the air, winds,

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can carry them for hundreds of miles
from their parents.

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Seeds like these have crossed the widest oceans
and landed on the loneliest islands.

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Pollen grains are so small,
they don't even need fluff to keep in the air.

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The microscopic roughness
of their surface is enough.

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Spores, shot out from a puffball

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and shed in tens of millions
from the gills of fungi, are smaller still.

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The merest breath of air
sweeps them away like smoke.

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The gossamer,
that sometimes carpets the meadows,

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is the animal equivalent of downy seeds.

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It's produced
by thousand upon thousand of tiny spiders.

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The young of many species of spider,
soon after they hatch,

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climb to the top of grass stems
or onto the tiny pinnacles of stones

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and lift their abdomens upwards.

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Then, from the spinnerets at the tip,
they produce a thread of finest silk.

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As it lengthens and the wind catches it,
the spiderling turns,

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grabs the thread with its forelegs
and away it goes.

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Only the tiniest and the lightest of animals
and plants can defy gravity in this way.

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Many seeds are far too heavy to be lifted
by the breeze, no matter how downy they are.

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But if they are produced at the top of a tall tree
they can exploit the pull of gravity.

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These, hanging in the jungle of Venezuela,
grow wings.

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The wing is so shaped and weighted,
with the seed at one end,

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that as it falls through the air, it spins.

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This protracted fall gives the breeze a chance
to deflect the seeds sideways

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so that they will land some distance away
from the parent tree.

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The seed is functioning
like the blade of a helicopter.

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Its wing is so shaped that as it sweeps round,

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it puts pressure on the air below
and reduces pressure just above

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so that the seed hangs in the air
much longer than it would otherwise do.

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Sycamore seeds spin and glide in the same way.

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And animals glide too.

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The flying frog of Central America
has a parachute on each foot,

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formed by the web of skin between its toes.

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So one jump from a high branch
is enough to carry it from one tree to another.

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In South-East Asia lives a gecko
that not only has a parachute on each foot,

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but flanges on its body and tail.

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Another lizard glides through the forests

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by extending even bigger wings of skin
from its flanks supported by elongated ribs.

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And the best glider of all: A flying squirrel.

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Its huge cloak of floppy skin
sometimes serves as a simple parachute.

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But in horizontal flight
it does more than just trap air beneath it.

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As air passes over the front edge,
it's deflected slightly upwards,

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creating a slight reduction
in the air pressure on the upper surface,

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like on an aircraft wing
or the spinning blade of a sycamore seed,

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so the squirrel creates a little lift
and floats through the air.

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All those creatures are gliders.

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Some can control to some extent
the direction in which they glide,

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but none of them can climb in the air
except with the help of rising air currents,

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like the breezes
which sweep up these downs in southern England,

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carrying with them whole populations
of seeds and spores and spiders.

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But there are no such breezes
down below the grass stems.

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Down there, if creatures want to climb into the ai
they have to have true powered flight.

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The most demanding moment is at take-off.

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The insect has to haul itself into the air
by sheer unaided muscle power.

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The downward sweep of the wings produces
greater pressure in the air beneath than above,

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so, in a slightly different way
from the cloak of the squirrel,

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beating wings also create lift,
and the insect is sucked upwards.

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Bigger insects, like grasshoppers,
boost their take-off with a powerful spring.

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Birds are even bigger and heavier.

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For them, too, getting into the air is
the most energetic and demanding part of flying.

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They also use their well-muscled legs
to assist their labouring wings.

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They jump
even before their wings begin their downbeat.

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But really big birds, to get airborne,
have to generate the extra lift

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by increasing the speed of air
streaming over their wings,

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so they get up a lot of speed
on the ground or over water,

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just as an aircraft does, before they can take off

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Once in the air,
a whole new environment is open to them,

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and flying animals of all kinds
exploit it to the full.

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Damsel flies catch their food in the air,
mate in the air and even fight in the air.

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As males squabble over territory, they flutter
their patterned wings in an aggressive display.

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This hawkmoth lays its eggs on flowers
while it's still flying,

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for it's too heavy to land on them.

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It feeds by hovering in front of a blossom
and sucking out the nectar

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with a tube-like proboscis as thin as thread.

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One of the smallest of all birds, the bee
hummingbird, even smaller than a hawkmoth,

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is equally skilled,
beating its wings 80 times a second

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to keep itself stationary in the air
as it drinks from the flowers.

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Bird wings are more versatile
than those of insects,

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for their feathers fit so closely alongside
one another and slide so easily past each other

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that the bird can change
the shape and size of its wing

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while maintaining its air-deflecting surface,

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so the wing can be spread wide
on the downstroke,

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and then, on the upstroke, be made small
to offer less resistance to the air.

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This kestrel is maintaining a steady position
in the sky, relative to the ground,

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by facing into the wind and flying with such
accuracy that it exactly matches the wind speed.

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The reduction of air pressure,
creating lift on the surface of the wings,

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can be seen quite clearly,
for it sucks up the smaller feathers.

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The albatross also habitually gets lift
by gliding into the wind,

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and the reduction in pressure produced as
the air blows over the wings ruffles its feathers.

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When it wants to travel against the wind,
it drops down close to the surface of the water,

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where the roughness of the waves
slows down the wind blowing over them.

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Albatrosses spend most of their lives in the air.

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Occasionally, for a minute or so,
they alight on the water to collect food.

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Once every year or so they come to
their nesting grounds to meet their mates again,

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greeting one another
with a charming courtship dance.

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It's difficult to appreciate how big these birds a
when you see them gliding over the ocean.

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It's only when you come to one of their nesting
sites that you really see how big they are.

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When they open these wings,
they are 11 feet across,

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the biggest wingspan of any bird.

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Long, narrow wings are the most efficient shape
for uninterrupted gliding,

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and no bird glides better than the albatross,

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but such wings are hard to flap
fast enough to give take-off,

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so many species of albatross nest on the edge
of cliffs, where they can just fall into the air.

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Cliffs are much favoured by gliders,

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for the wind from the sea striking the cliff face
is deflected upwards,

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and an albatross can hang on it.

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If it wants to fly slower
and prevent itself from being swept away

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or carried too high by a sudden gust,

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it uses its tail and webbed feet as air breaks,

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and reduces its lift by pulling in its wings,
so making their surface smaller.

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With such techniques,
an albatross will glide all day above a line of cl

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travelling effortlessly
along this highway in the sky.

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Land birds also exploit the air currents
above cliffs in the same way.

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This is the coast of Paracas in Peru.

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As the day wears on, the sun heats up
these desert sands, causing rising air,

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and that in turn sucks in cold air from the sea,
often bringing mists with it.

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As this cold air hits the cliffs,
so it's deflected upwards,

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providing just the sort of conditions
that soaring birds need.

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The condor, one of the heaviest of all flying bird

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Yet its skill in soaring is so consummate

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that it can remain in the air for hours
with scarcely a wingbeat,

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sustained entirely by those air currents
swept upwards by the cliffs.

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And something else
produces columns of rising air: Heat.

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When we turn on these burners,
they will create a current of rising air so powerf

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that it'll lift this balloon,
this basket and us up into the sky.

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We're in Africa, floating
over the great game plains of the Serengeti.

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I'm now about 100 feet up
and kept up entirely by hot air.

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But gas burners aren't the only things
which produce rising currents of hot air.

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The sun, as it rises, heats up the landscape,

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but all parts of the landscape
don't react in the same way.

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Some parts absorb the heat. Other parts,
bare slopes of grass or patches of rock,

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reflect the heat, and that causes
those uprising currents of air, the thermals.

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That's a moment those big birds down there
are waiting for.

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They are vultures,
and at the moment they're grounded.

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They're big birds with large wings, so large
that beating them is a very laborious business,

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and the vultures don't do so unnecessarily.

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At this time in the morning, they don't try
to battle against gravity and climb high,

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but flap from one low tree to another.

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They're waiting for the land to heat up
and the thermals to form.

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But we have our own thermal,
created by our burner, and up we go.

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This bird begins to follow us.

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An outcrop of rock is already warming

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and providing it with the thermal it needs
for effortless flight.

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And now the vultures are beginning
to come up here to join me.

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They will use the thermals to provide them
with an observation post in the sky

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from which they can scan the plains below,

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and I'm getting the same kind of view as they are,
and it's a very, very exciting one.

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Below me must be
the biggest concentration of meat on the hoof

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to be found anywhere in the world: Wildebeest.

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Last night or in the early dawn, somewhere,

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lions or hyenas or hunting dogs will have killed.

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The vultures, several thousand feet up in the sky,
quickly spot a kill or deduce its presence

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from the behaviour of birds
in a neighbouring thermal,

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and when they do, they swiftly glide down to it.

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Once one bird finds a carcass,
dozens arrive within a few minutes.

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These are tearing apart
the body of a wildebeest calf.

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Most of these are medium-sized vultures:
Ruppell's griffon and white-back.

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But among them is the biggest and most powerful
of African vultures: The lappet-faced.

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With a heavy load of meat,
the vultures won't fly far, to a nearby tree,

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to perch and digest and wait for tomorrow's
thermals to carry them effortlessly aloft again.

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But all the sustenance
has not yet been extracted from the carcass.

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In the African mountains,
as well as in Asia and Europe,

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lives a species of vulture with
a very specialised diet indeed: The lammergeier.

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It feeds, though it sounds extraordinary,
not only on marrow but on the bones themselves,

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and to do so,
it has developed a special technique.

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First it brings bones from a carcass to a special
workshop which several birds may share.

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A patch of bare rock near the top edge of a cliff.

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It chooses a cliff top
so that when it takes off again with a heavy bone,

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it has the least difficulty in getting into the ai

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Now it has to gain height.

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And this is why it chooses a patch of bare rock
for its operations.

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So that the bone will land so heavily that it crac

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One drop, however, may not be enough.

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White-collared ravens
often hang about the scene of operations.

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The ravens are starting to learn the technique
but haven't mastered it.

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They tend to drop their bones on grass,
where they don't break.

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The lammergeier eats the splinters of bone,
impossibly spiky though they appear to be.

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Some birds exploit the force of gravity by droppin
not their food but themselves from the sky.

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The pied kingfisher hovers
as it searches the water beneath.

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Terns dive with such speed, they can strike fish
several feet beneath the surface,

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pulling back their wings at the last moment
so as to get a clean entry into the water.

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Gannets do the same thing.

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During the nesting season,
concentrated in their colonies,

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huge flocks set out on fishing trips, and when
they find a shoal of fish near the surface,

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they subject it to an aerial bombardment
of devastating intensity.

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00:29:06,778 --> 00:29:11,511
But the ace of dive-bombers, which can reach
at least 80 miles an hour in a dive,

214
00:29:11,783 --> 00:29:13,444
is the peregrine falcon.

215
00:29:17,422 --> 00:29:21,188
It patrols the skies,
high above the flight path of other birds.

216
00:29:21,426 --> 00:29:24,418
When it has selected its victim, it folds its wing

217
00:29:24,596 --> 00:29:29,192
steering almost entirely with its tail,
and hurtles downwards.

218
00:30:26,724 --> 00:30:29,625
The talons are brought forward for the strike

219
00:30:29,794 --> 00:30:33,958
and to make last-second adjustments
to the accuracy of its final run.

220
00:30:46,911 --> 00:30:48,572
A hunter of the night.

221
00:30:48,947 --> 00:30:55,716
Owls, this is a barn owl, don't rely on speed
like the peregrine, but on a slow, silent approach

222
00:30:58,189 --> 00:31:02,489
Their flight feathers have special
soft edges to them which serve as silencers.

223
00:31:02,994 --> 00:31:06,828
Their wings are large
and support the bird so easily

224
00:31:06,998 --> 00:31:09,728
that there's no need for any noisy flapping,

225
00:31:09,901 --> 00:31:13,962
and the owl can waft its way in silence
through the trees.

226
00:31:18,443 --> 00:31:23,471
Although owls hunt after dark, they find their way
with their large, sensitive eyes,

227
00:31:23,648 --> 00:31:31,145
and, because their flight is virtually soundless,
they can listen for the squeak of voles and mice.

228
00:31:33,391 --> 00:31:38,852
But on the darkest nights, even an owl can't see,
and it seldom ventures into the air.

229
00:31:39,097 --> 00:31:41,691
Such nights belong to bats.

230
00:31:43,935 --> 00:31:47,336
They are able to navigate
without the aid of vision.

231
00:31:47,538 --> 00:31:53,966
Instead they use sonar, squeaking ultrasonically
and guiding themselves by the reflected echoes.

232
00:32:10,128 --> 00:32:15,430
They do this so skilfully
that they can pluck a flying moth from the air.

233
00:32:48,366 --> 00:32:52,427
It's been known for a long time
that bats use sounds in this way,

234
00:32:52,637 --> 00:32:59,406
but it's less well known that one or two birds
have, independently, evolved the same technique.

235
00:33:01,179 --> 00:33:04,637
This cave in Venezuela
is the home of one of them.

236
00:33:15,893 --> 00:33:19,761
These, flying all around me, are oilbirds.

237
00:33:20,231 --> 00:33:24,930
Most of the noise that they're making
is nothing to do with navigation.

238
00:33:25,103 --> 00:33:29,369
It's their alarm calls.
They're alarmed by the brightness of my light.

239
00:33:29,774 --> 00:33:32,902
So what I'm going to do
is to put on a deep-red filter.

240
00:33:33,177 --> 00:33:37,443
That will disturb them less,
but it will enable us to watch them

241
00:33:37,615 --> 00:33:42,109
with a special electronic device
called an image intensifier.

242
00:33:47,458 --> 00:33:51,986
They're big, relations of the nightjars,
and about the size of pigeons.

243
00:33:52,263 --> 00:33:57,462
Their nests are compiled from their droppings
and bits of regurgitated food.

244
00:33:59,137 --> 00:34:03,369
When their alarm calls subside,
you can hear the clicks by which they navigate.

245
00:34:04,275 --> 00:34:10,771
These calls are lower in frequency than
the signals of bats, and they're less accurate,

246
00:34:10,948 --> 00:34:14,475
so the oilbirds can't detect objects
much smaller than a foot across.

247
00:34:15,019 --> 00:34:19,251
That's quite good enough to prevent the birds
crashing into the cave walls or one another.

248
00:34:39,043 --> 00:34:41,568
Their favourite food is the fruit of a jungle tree

249
00:34:41,813 --> 00:34:45,305
and the cave floor is covered
by a soggy carpet of seeds.

250
00:34:45,616 --> 00:34:49,848
Many germinate, though in the dark
they can't develop chlorophyll,

251
00:34:50,054 --> 00:34:53,785
and they remain pallid, leggy seedlings
which soon die.

252
00:34:54,225 --> 00:34:58,594
The fruits are too small
for the oilbirds to locate with their clicks,

253
00:34:58,863 --> 00:35:02,458
but out in the moonlit forest, where the trees gro

254
00:35:02,667 --> 00:35:05,158
there's enough light
for the birds to find them by eye.

255
00:35:09,040 --> 00:35:12,498
The mastery of the air
and the strength to remain in flight for days

256
00:35:12,677 --> 00:35:17,410
has enabled birds to become
the greatest of all animal travellers.

257
00:35:18,883 --> 00:35:21,909
In the skies above Panama
every October and November,

258
00:35:22,120 --> 00:35:24,680
there is a great aerial traffic jam.

259
00:35:24,922 --> 00:35:29,518
Hawks and turkey vultures,
fleeing from the winter in North America,

260
00:35:29,760 --> 00:35:32,854
are on their way
to spend a few months in the south.

261
00:35:34,065 --> 00:35:38,365
As the day warms up, they find the thermals
in which they can spiral upwards,

262
00:35:38,569 --> 00:35:43,006
to give them the altitude they need
to make the day's flight with the least effort.

263
00:35:48,746 --> 00:35:51,306
These long journeys require a lot of fuel.

264
00:35:51,616 --> 00:35:55,211
Big birds, like hawks,
can draw it from their body tissues.

265
00:35:56,954 --> 00:36:02,415
But north-east of Panama, across the Caribbean,
on the Atlantic coast of the United States,

266
00:36:02,627 --> 00:36:08,065
smaller wading birds, sandpipers and phalaropes,
are preparing for theirjourney.

267
00:36:08,599 --> 00:36:11,295
They must put on fat before they start off,

268
00:36:11,536 --> 00:36:17,133
and they find food in the quantities they need
in the rich waters of the Bay of Fundy.

269
00:36:40,998 --> 00:36:46,994
In a few days of intensive feeding, each tiny bird
will increase its weight by half as much again,

270
00:36:47,371 --> 00:36:51,603
and they need all that fat,
for they are about to travel across the ocean,

271
00:36:51,776 --> 00:36:54,267
and then they can't feed at all.

272
00:37:15,099 --> 00:37:20,594
On the other side of the Atlantic, migration route
also run predominantly north and south,

273
00:37:20,805 --> 00:37:24,571
as birds move back and forth
to get the best of the changing seasons.

274
00:37:25,977 --> 00:37:30,107
In Scandinavia, every autumn
great numbers make their way south.

275
00:37:30,781 --> 00:37:34,808
Most land birds
prefer to keep their flights over water short,

276
00:37:35,052 --> 00:37:40,319
and huge flocks assemble on the shores of
the narrow straits between Sweden and Denmark

277
00:37:40,491 --> 00:37:42,618
to make the crossing into southern Europe.

278
00:37:45,463 --> 00:37:48,557
Small birds often fly in parties, close to the wat

279
00:37:58,109 --> 00:38:01,044
Buzzards, experts at soaring and gliding,

280
00:38:01,245 --> 00:38:05,181
use the thermals to climb so high
that they cover the distance

281
00:38:05,349 --> 00:38:08,341
in what amounts to one long, shallow glide.

282
00:38:10,921 --> 00:38:16,917
Red-breasted geese spend their summer
much farther east in the tundra of western Siberia

283
00:38:17,194 --> 00:38:19,253
They too move south in the autumn.

284
00:38:33,644 --> 00:38:39,514
Theirjourney is almost entirely over land,
so they're able to stop each night to refuel.

285
00:38:57,601 --> 00:39:02,436
After several weeks, they reach
their wintering grounds south of the Caspian Sea,

286
00:39:02,606 --> 00:39:05,769
many of them on the marshes
of the Danube delta.

287
00:39:11,749 --> 00:39:16,686
Birds are not the only creatures
to make these immense transcontinental flights.

288
00:39:16,887 --> 00:39:21,950
Almost unbelievably, a few small,
seemingly frail creatures do so as well.

289
00:39:22,693 --> 00:39:29,758
Insects, flying with just as steadfast a purpose,
achieve journeys as long as many migrating birds.

290
00:39:30,067 --> 00:39:33,366
In South America, in a high valley in Mexico,

291
00:39:33,537 --> 00:39:38,770
hundreds of thousands of monarch butterflies
roost in just a few special trees.

292
00:39:45,883 --> 00:39:52,015
They hatched in the autumn woods of North
America and have flown 2,000 miles to hibernate.

293
00:39:52,289 --> 00:39:57,625
They won't feed here, but they're spared
the lethal frosts and snows farther north.

294
00:39:57,928 --> 00:40:01,625
In spring they will set off back,
travelling ten miles a day,

295
00:40:01,799 --> 00:40:04,791
feeding, courting and laying eggs as they go.

296
00:40:05,169 --> 00:40:10,072
But only a few will live long enough to reach
the northern woods where they were hatched.

297
00:40:13,077 --> 00:40:16,638
The world is criss-crossed
by the flight paths of animal migrants.

298
00:40:16,881 --> 00:40:22,444
In the Americas, nearly all pass through Panama.
A few hardy travellers cross the Caribbean.

299
00:40:23,320 --> 00:40:28,690
On the other side of the world there's more land,
and birds and insects have more routes,

300
00:40:28,859 --> 00:40:33,319
travelling north and south
but also east and west between Asia and Africa.

301
00:40:35,132 --> 00:40:38,761
Although the journeys
may be thousands of miles long,

302
00:40:39,036 --> 00:40:43,769
the earth's wrapping of air
is less than six miles deep.

303
00:40:44,308 --> 00:40:47,675
On rare occasions the gases
from which it's formed become visible.

304
00:40:47,845 --> 00:40:53,306
Subatomic particles from space, attracted
to the poles by the earth's magnetic field,

305
00:40:53,484 --> 00:40:58,786
energise the gases of the atmosphere
so that they glow and form shifting veils of light

306
00:40:58,956 --> 00:41:00,981
the aurora borealis.

307
00:41:04,428 --> 00:41:06,862
The atmosphere is not composed entirely of gas

308
00:41:07,031 --> 00:41:10,831
and at certain times
you can see evidence of other things.

309
00:41:11,635 --> 00:41:18,063
Dust particles are scattered through its lower
layers, and when the sun shines across the earth,

310
00:41:18,242 --> 00:41:20,870
they scatter its white light, turning it red.

311
00:41:21,812 --> 00:41:27,773
Minute droplets of water, being translucent,
act like tiny prisms and produce a rainbow,

312
00:41:28,052 --> 00:41:32,318
and at high altitudes
tiny ice crystals create a similar effect.

313
00:41:34,458 --> 00:41:40,693
Up away from the earth, the gases become
thinner and the temperature becomes colder.

314
00:41:53,110 --> 00:41:57,877
The balloon taking us to these heights
must be bigger than that we used in Africa

315
00:41:58,048 --> 00:42:03,680
for, as we climb, we will require a greater volume
of the rarefied air to give us the necessary lift.

316
00:42:04,822 --> 00:42:10,783
A rubber bladder, sealed with a cork, gives us a
rough idea of the drop in pressure as we ascend.

317
00:42:19,236 --> 00:42:27,200
We are now at 8,000 feet, and you might think
that no living creature would come as high as this

318
00:42:27,444 --> 00:42:29,878
except perhaps some rather foolhardy men.

319
00:42:30,114 --> 00:42:35,017
But no. Some small creatures
are swept up as high as this

320
00:42:35,185 --> 00:42:38,518
by the convection currents
rising from the surface of the ground,

321
00:42:38,756 --> 00:42:45,423
and we're going to try and catch some
using this rather curious machine.

322
00:42:46,230 --> 00:42:53,796
Inside there's a fan which will suck in air
through this end when I turn it on here,

323
00:42:54,104 --> 00:42:56,800
and I'll lower it over the side
to see what we catch.

324
00:43:05,149 --> 00:43:11,054
And now we're going to go higher still
and it's going to get very, very cold,

325
00:43:11,255 --> 00:43:14,247
so I shall need all this warm clothing I've got,

326
00:43:14,425 --> 00:43:19,692
but, perhaps even more seriously,
the oxygen is going to get thinner and thinner,

327
00:43:19,897 --> 00:43:27,326
and so I shall have to put on this mask in order
to breathe oxygen as we go higher and higher.

328
00:43:52,563 --> 00:43:58,126
And now an indication of our height
can come from this balloon.

329
00:43:58,335 --> 00:44:02,795
Before it had those corners to it
and now it's swollen quite considerably,

330
00:44:02,973 --> 00:44:08,411
so the pressure here is really considerably lower
than it was when we were on the ground.

331
00:44:13,951 --> 00:44:18,854
We are now getting on for four miles
above the surface of the earth.

332
00:44:19,390 --> 00:44:25,954
It certainly looks very far away.
And it's shrouded beneath a pall of clouds.

333
00:44:26,397 --> 00:44:33,098
And we're getting very close
to the outermost frontier of life on earth.

334
00:44:33,871 --> 00:44:40,276
It's very cold and I certainly wouldn't be able
to talk at all if I hadn't got this oxygen,

335
00:44:40,511 --> 00:44:47,110
so conditions here are really
very much more severe than you might imagine

336
00:44:47,284 --> 00:44:52,984
when you sit in your aircraft
flying comfortably from one continent to another.

337
00:44:53,390 --> 00:44:57,258
But let's see what we've caught...

338
00:44:58,295 --> 00:45:01,128
in our apparatus.

339
00:45:05,069 --> 00:45:06,127
Turn it off.

340
00:45:08,238 --> 00:45:09,398
And...

341
00:45:12,276 --> 00:45:14,005
...take off the end.

342
00:45:21,919 --> 00:45:23,011
Well...

343
00:45:25,422 --> 00:45:30,655
We certainly haven't caught anything large.

344
00:45:32,763 --> 00:45:37,894
But if we examine this mesh,
when we get down to earth, with a microscope,

345
00:45:38,168 --> 00:45:45,836
it's very likely that, at the very least, we shall
have some pollen grains and spores of fungus.

346
00:45:47,077 --> 00:45:50,513
But bigger creatures are found at these heights

347
00:45:50,981 --> 00:45:55,714
and I've some of them here, in this phial,
that were caught here.

348
00:45:57,488 --> 00:46:03,324
I'll pour them out on a dish
to get a better look at them.

349
00:46:09,466 --> 00:46:15,564
There are tiny spiders that must have sailed up
hanging from their threads of gossamer.

350
00:46:16,340 --> 00:46:23,371
And winged aphids. At these altitudes
they can be carried halfway around the world

351
00:46:23,580 --> 00:46:26,048
and, amazingly, be frozen solid,

352
00:46:26,216 --> 00:46:30,209
and yet revive when they fall to lower altitudes.

353
00:46:31,488 --> 00:46:37,950
But now we are very close
to the top of our environment,

354
00:46:39,229 --> 00:46:45,327
for all the weather goes on
within these five brief miles,

355
00:46:45,502 --> 00:46:50,269
the envelope of atmosphere
that wraps round the world.

356
00:46:50,507 --> 00:46:53,943
It's here that the weather is manufactured.

357
00:46:55,512 --> 00:47:00,381
Molecules of water, evaporating in the heat
of the sun from the surface of the sea and lakes,

358
00:47:00,484 --> 00:47:02,679
or breathed out by plants as vapour,

359
00:47:02,853 --> 00:47:08,018
rise up from the land
and cool and condense into clouds of droplets.

360
00:47:08,792 --> 00:47:13,923
Driven by the winds, the clouds evaporate
and condense, form and re-form.

361
00:47:34,117 --> 00:47:38,816
The summit of Mount Everest
is less than six miles above the sea,

362
00:47:38,989 --> 00:47:41,355
yet few clouds ever sail much above it.

363
00:47:43,060 --> 00:47:47,793
The earth, as it spins,
creates vast eddies within the atmosphere.

364
00:47:48,232 --> 00:47:51,690
If they become intense,
they will develop into hurricanes.

365
00:47:51,969 --> 00:47:55,871
From a satellite
22,500 miles away from the earth,

366
00:47:56,139 --> 00:48:00,439
the build-up and dissipation
of these huge storms over 15 days

367
00:48:00,611 --> 00:48:04,570
can be seen with pictures taken every hour
and run continuously.

368
00:48:07,718 --> 00:48:12,018
Away to the east of Brazil in the Atlantic,
a hurricane is forming.

369
00:48:13,757 --> 00:48:17,352
As it spins, it moves west across the Caribbean.

370
00:48:22,266 --> 00:48:27,704
Northwards it goes towards Florida, while
up in the north, air sweeping over North America

371
00:48:27,871 --> 00:48:33,332
moves across the Atlantic towards Europe
in another immense, swirling storm.

372
00:48:41,184 --> 00:48:46,451
Other disturbances in the atmosphere are caused
when the sun builds up gigantic thermals

373
00:48:46,623 --> 00:48:49,148
in a sky already loaded with moisture.

374
00:48:49,526 --> 00:48:55,021
As the air is driven upwards, the tops of
the towering clouds burgeon with fearsome speed.

375
00:48:55,933 --> 00:49:00,495
The water molecules within the clouds
condense to form bigger and bigger droplets,

376
00:49:00,837 --> 00:49:05,934
but the speed of the rising air is now so great
that it keeps them suspended within the cloud.

377
00:49:08,211 --> 00:49:12,204
Eventually, the droplets become so big
that they cannot be supported,

378
00:49:12,382 --> 00:49:14,441
and they fall as torrential rain.

379
00:49:14,851 --> 00:49:19,754
The molecules of gas surging upwards
create a build-up of electricity

380
00:49:19,923 --> 00:49:23,620
that eventually becomes so great,
it discharges down to earth.

381
00:49:27,464 --> 00:49:31,491
The water droplets
may have been carried so high that they freeze

382
00:49:31,668 --> 00:49:34,728
and eventually tumble out of the cloud as hail.

383
00:49:53,123 --> 00:49:57,651
If the storm is really intense,
they may rise and fall several times.

384
00:49:57,894 --> 00:50:03,526
In the lower parts of the cloud, the ice
forms relatively slowly and is clear and black.

385
00:50:03,734 --> 00:50:07,670
But when they get to the top again,
the ice forms quickly,

386
00:50:07,838 --> 00:50:10,602
trapping air bubbles,
which makes the ice look white.

387
00:50:10,841 --> 00:50:17,178
So big hailstones may be banded, like an onion,
with alternate rings of black and white ice.

388
00:50:34,097 --> 00:50:40,297
Really big hailstones are often a sign that a trul
devastating storm is about to strike the earth.

389
00:50:42,472 --> 00:50:46,636
A strong, high-altitude wind,
linked with a severe storm such as this,

390
00:50:46,810 --> 00:50:51,372
may vacuum up lower-level air,
increasing the updraught dramatically,

391
00:50:51,615 --> 00:50:54,982
and beginning a spiral motion
in part of the storm.

392
00:50:55,452 --> 00:51:00,480
If these converging winds are powerful enough,
the vortex at the centre of this great whirl

393
00:51:00,657 --> 00:51:06,220
reaches down to the surface of the earth
as a suction funnel, a tornado.

394
00:51:37,961 --> 00:51:42,796
Winds up to 300 miles an hour
devastate the land, tearing things apart,

395
00:51:42,966 --> 00:51:48,666
ripping the roofs from buildings, sweeping
animals and trees and sometimes even people

396
00:51:48,839 --> 00:51:51,740
high into the sky and throwing them down.

397
00:51:52,676 --> 00:51:56,544
When it strikes the land,
it's seldom more than 500 yards across,

398
00:51:56,713 --> 00:52:04,085
but it lashes the earth with the most powerful
and destructive of all atmospheric forces.

399
00:52:35,719 --> 00:52:39,485
Storms like that may bring death and destruction,

400
00:52:39,689 --> 00:52:43,250
but they also bring life,
because the rain that comes from them,

401
00:52:43,426 --> 00:52:49,626
distilled by the sun from the surface of the ocean
is fresh water, salt-free,

402
00:52:49,833 --> 00:52:54,930
and that is something
that all life on land must have.

403
00:52:55,305 --> 00:52:59,935
And when that rain, that sweet fresh water,
accumulates in rivers and lakes,

404
00:53:00,110 --> 00:53:04,444
then it supports
a community of plants and animals all of its own,

405
00:53:04,648 --> 00:53:08,914
and it's those communities that we're going to be
looking at in the next programme.