Snow! We skiers think about snow a lot this time of year: Where it
comes from, how likely it is, but who really understands the weather?
Let’s see if we can shift our imagination into high gear and give this
a shot.
There are two huge engines that drive the weather. One is the sun
heating up the air, evaporating moisture into it, and causing it to
circulate in convection currents that take masses of air up from the
surface of the earth and back down again as they are heated and cooled.
The other driving force is the rotation of the earth. This rotation,
usually not apparent to us, is the basis of the Coriolis force, which,
as we shall see, can have quite spectacular effects.
Imagine a sphere in empty space. Let’s say it’s about 25,000 miles
around (the same size as our earth, in fact). And, we’ll cover it with
a smooth linoleum floor (no oceans or land at this point). But we will
give it a spin, so that it rotates once every twenty-four hours. And,
to better see this rotation, we’ll put a big light in nearby space.
Say, 93,000,000 miles away.
Now, because our sphere rotates, it has an axis with two poles.
Arbitrarily, we’ll call one of these the North Pole, and the other the
South Pole. Imagine standing exactly at the North Pole with your arms
outstretched. Standing on this immense linoleum plain, you are slowly
rotating counter clockwise. Once every twenty-four hours your right
arm moves slowly forward, your left arm moves backward, and you make a
complete turn.
An ice skater goes into a spin by pulling her arms and legs in after
she starts a turn. You could do the same. If there were a pivot point
under your feet with no friction and you pulled in your arms you would
actually begin a slow spin. Instead of rotating once in 24 hours, you
might now “spin” around once every two or three hours. This sort of a
spin is simply the conservation of angular momentum you already
possess.
Imagine a bunch of huge merry-go-rounds each a mile in diameter and
scattered all around this linoleum landscape at our North Pole.
Imagine looking down on all these merry-go-rounds. Each one appears
perfectly still, but it nevertheless rotates once every 24 hours along
with the rest of our great sphere. Now imagine thousands of people
getting onto the merry-go-rounds and rushing into their centers. What
happens? The merry-go-rounds (and the people), already spinning once
each 24 hours now begin to spin faster (because of the angular momentum
of the people). They spin counter clockwise because that’s the way our
sphere is already spinning. The more the angular momentum already
possessed by these systems (of merry-go-rounds and people) is
concentrated toward the axis of the system, the more quickly the whole
system spins.
Now, let’s go from this perfect sphere back to the familiar but
chaotic world around us. When water in a bathtub moves toward the
drain, it tends to spin in a counter clockwise direction. This is in
the Northern Hemisphere where things are already spinning that way (in
the Southern Hemisphere, it’s the opposite). The same thing happens to
a mass of air. If air is warmed it expands; if cooled it contracts.
Imagine a huge mass of air being cooled. It contracts. It moves
inward. It was already rotating along with the rest of the earth once
every 24 hours. Now this rotation is amplified. This is why northern
air masses always rotate counter clockwise around a region of low
pressure. Likewise, if air rises, it is replaced by air flowing in
from the surrounding area near the ground. The spin picked up by air
rising in a column can be most dramatic, because the column is often
very thin, concentrating the spin very tightly. We call such a column
a tornado.
We know that the sun heats the air more at the equator than it does
at the poles. All other things being equal, this causes the warm air
to rise at the equator and flow north and south at high altitude
towards the poles. It then cools, settles, and returns to the equator
by being pushed along the ground. Both the air and the ground at the
equator are moving over a thousand miles an hour (the equator is 25,000
miles around and it rotates once every 24 hours; 25,000 divided by 24
is a little over 1000). What happens to this speed as the air moves
toward the North Pole? It becomes the jet stream. The air, already
moving with the earth at 1000 miles an hour begins to move over regions
of the earth that are moving slower and slower, until the north pole is
reached and the ground isn’t moving at all (it is simply rotating in
place). Of course, a lot of this speed is lost to friction, but the
effect is that the jet stream in the upper atmosphere is seen as a
current of air moving from west to east often exceeding two hundred
miles an hour. This can cause quite a head or tail wind for aircraft
flying west or east at high altitudes.
Now, what happens when the air cools in the polar regions, sinks to
the ground, and is pushed back to the equator? The reverse occurs. As
the air moves away from the pole it is going slower than the ground
under it. The ground is moving to the east faster than the air, so the
air appears to be moving to the west. This is what causes the trade
winds that move (mostly across the oceans) from east to west. The
regions closer to the poles, before these winds have a chance to build
up, are known to sailors as the doldrums. As the winds finally begin
to match speed with the rotating earth beneath them, between about
thirty degrees north and south latitude, we have what sailors called
the horse latitudes. Between these two were the trade routes for
sailing ships driven by the trade winds.
The Chinook and Santa Ana winds are different forms of the jet
stream and trade winds, respectively. The Chinook happens when the jet
stream touches down over the Rocky Mountains. The Santa Ana forms
across the deserts of the southwest. The Chinook usually comes from a
westerly direction, the Santa Ana from the east.
Other factors also contribute to the weather. Land warms in the
sunlight more quickly than water. When dawn breaks, warm air begins to
rise on shore and this pulls in the cooler air from the sea causing a
sea breeze in the morning. When the sun goes down, the air cools more
quickly over the land, and the air blows back out to sea later in the
day and during the night. When water picks up the energy from the sun
it evaporates into the air. This energy is released when the water
condenses as rain or snow.
Why does it rain or snow? Because when the warm air rises, the
pressure on it, higher in the atmosphere, goes down. The temperature
also drops. The lower temperature and pressure cause the moisture to
condense, form tiny droplets that are first seen as clouds, and then
aggregate into larger drops to fall as rain, sleet, or snow, depending
on the temperature and the amount of water vapor in the air. To
complicate the process further, when the clouds form, they shade the
sun and cause temperatures to fall quickly and unevenly. Between
clouds, of course, the sun shines through. It hits and warms the
ground, and causes the air in contact with the ground to rise. Great
convective cells can form along with great differences of temperature
and pressure. Intense low-pressure regions can give rise to the
swirling air of tornadoes on the local scale, and to hurricanes on a
much larger scale. Great differences of temperature in convective
cells can cause drops of rain to cycle high into the atmosphere until
they freeze and become large enough to fall as hailstones.
The transport mechanisms of heat and global rotation move the
atmosphere all over the world. In November, a lot of the moisture that
evaporates into the air from the Pacific rises, moves north, and finds
itself turning into the jet stream and moving east. As it moves first
north, then east, and rises higher, snow falls on the Sierras, the
Cascades, the LaSals, and the Rockies. This causes skiers to appear on
the mountain slopes, sales of Chapstick to increase, and thoughts of
snow and winter wonderlands to occupy people’s minds.
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