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Clouds and Rain Unit (Topic 8A-2) – page 1
Name:
Clouds and Rain Unit
Section:
As air rises, it cools due to the reduction in atmospheric pressure
Air mainly consists of oxygen molecules and nitrogen molecules. Remember that warm
molecules move faster than cold molecules. This allows warm air molecules to push aside nearby
molecules and spread out, which lowers their density and causes them to rise. Atmospheric
pressure is caused by the weight of the air above. Thus, up in the mountains, air pressure is
lower, because there is less atmosphere above you (less air pressing down on top of you).
Therefore, as warm air rises higher into the atmosphere, it experiences lower pressure. Since the
group of warm, rising air molecules are no longer being squeezed together as strongly by the air
above, the group of warm, rising air molecules can now push outward (i.e., the warm air expands
as it rises). However, in pushing outward against the neighboring cooler air molecules, they give
their energy to the neighboring air, causing the warm, rising air to cool down.
Experiment: Blow into your hand. First, keep your mouth opening small, then open wide as if
yawning. In which case does the air feel warm? In which case does it feel cool? When the
opening is small, the air is forced together, and quickly expands once outside your mouth.
If the water molecules in the air cool
down enough, they will begin to bond
with one another. (The water molecules
are no longer moving fast enough to fly
apart when they get to close to one
another and strong hydrogen bonds
form between them.) Thus, rising air
produces cloudy and rainy skies. As the
rising air cools down more and more, it
loses its water as rain. By the time the
air reaches the location where air sinks,
it is completely dry; dry air cannot
produce clouds or rain.
water has
fallen out
as rain
lower
pressure
low
pressure
mountain
high
pressure
Note: In most situations water vapor needs additional help from aerosols – tiny solid particles
like dust or drops of liquid in the air – to condense: it is easier for water molecule to bond with
big, slower-moving objects. The kind and size of aerosols available can have a big impact on
whether clouds form and rain occurs – and how much rain occurs.
1. As air rises, does it become warmer or cooler?
2. Does water vapor cool and condense into clouds and rain when air rises
or when air sinks?
Clouds and Rain Unit (Topic 8A-2) – page 2
Surface Air Pressure, Temperature, and Cloudy Skies and Clear Skies
In the previous section, we saw that as air rises, it cools, causing the water vapor in the air to
condense into clouds and rain (if enough is present). If we apply this idea to a convection cell,
clouds and rain will be more common above the warm spot. The sinking air at the cold spot will
not have any water vapor (it was lost at the warm spot) and in any case air warms as it sinks, so
clear skies will be more common above the cold spot.
Clear
Skies
High
Pressure
High Altitude Winds
Surface Winds
Low
Pressure
Warm
Cold
Ocean
Beneath regions of warm, low-density, rising air, the pressure at the surface of the Earth is lower
(fewer air molecules above), and beneath regions of cold, high-density, sinking air, the pressure
at the surface of the Earth is higher (more air molecules above). Another way to think about this:
if the air is rising – going up – it is not pressing down very hard, and if the air is sinking – going
down – it is pressing down harder. Thus, lower air pressure at the surface is associated with
cloudy and rainy skies, and high pressure is associated with clear skies. (Just listen to weather
forecasters on the news!)
3. Where are the cloudy and rainy skies, at the warm spot or at the cold spot?
4. Where are the clear skies, at the warm spot or at the cold spot?
5. Where is the surface air pressure higher, at the warm spot or at the cold spot?
6. When do we typically get more clouds and rain, when air pressure is higher or lower?
Clouds and Rain Unit (Topic 8A-2) – page 3
The Global Rainfall Pattern
Memorize the global rainfall pattern shown below, as well as the global wind pattern. If you
know where surface winds come together and the air rises, then you know where it rains.
Similarly, where surface winds move apart, air sinks, and the skies are clear. (Remember that the
dotted arrows show air rising and sinking – air going towards or away from the surface of the
Earth.)
(90o N)
North
Pole
Clear
60o N
Rainy
30o N
Clear
Equator (0o )
Rainy
30o S
Clear
60o S
Rainy
South
Pole
(90o S)
Clear
7. At what latitudes does air rise?
8. At what latitudes does air sink?
9. At which latitudes are cloudy and rainy skies more common?
10. At which latitudes are clear skies more common?
Clouds and Rain Unit (Topic 8A-2) – page 4
Weather, Climate, & Fronts
Up till now, we have been discussing climate, not weather. Climate is the long-term average of
weather conditions (what the weather is usually like). For example, Southern California has a
warm, dry climate. This does not mean that it is always warm (we have our cooler days) or that it
does not rain in Southern California; it means that our weather is warm most of the time and that
rain is less common here than elsewhere. Another way to think about it: weather is what
conditions are like a particular day, climate is what conditions are like over a season or a year.
Your own experience of actual storms and rain may contradict something that I said before:
warm, rising air leads to clouds and rain. Many of you will say: wait a minute, the weather is
cold when it rains!
Before:
After:
Warm Air
Storms often form along what meteorologists
Cold Air
Warm Air
Cold Air
call fronts, a place where 2 air masses meet. An
air mass is a collection of air with similar
properties (e.g., temperature, moisture), often
determined by where it comes from. For
example, warm, moist air moves up into the United States from the Gulf of Mexico, while cool,
dry air comes down from Canada. We also use the word front to describe the location where two
opposing armies meet and are shooting at one another. As in the military, the frontlines typically
are where the action is (clouds, rain, hail, snow, etc.) in the atmosphere. At the locations where
air masses meet (the front), the cooler air pushes the warmer air up, sliding in underneath to
replace it, or the warmer air can move up and over the cooler air.
As the warmer air rises, it becomes cooler, and if the change in temperature is strong enough and
the rising air contains enough moisture, the water vapor in the rising air will condense into rain.
If the warmer, rising air does not contain water, there cannot be rain along the front.
Thus, the weather is cooler when it rains, because cooler air is coming in and lifting up the
“warmer” air. (Remember, the “warmer” air might not be very warm, it is just “warmer” than the
cooler air on the other side of the front.)
11. When warm air pushes into cold air, which one rises up on top, the warm air or the cold
air?
12. What happens to the temperature of the air as it rises? Does it get warmer or cooler?
Clouds and Rain Unit (Topic 8A-2) – page 5
Distribution of Heat from the Sun
In this section, you will learn why temperature changes with the seasons, why some parts of the
world are warmer than others, and how the motion of the ocean and atmosphere keep the warm
places from getting too hot and the cool places from getting too cold.
The Equator is warmer than the Poles, because it receives more heat from the Sun. Sunlight
shines directly down upon the Equator, but approaches the Poles at an angle. As a result, sunlight
is spread out over a wider area at
the Poles (It is less
Spread Out
“concentrated,” so these places
are colder.) In addition, sunlight
that comes in at an angle is more North
Pole
likely to get reflected back into
space (the white snow and ice at
Sun
Equator
the Poles help a lot too) rather
than absorbed, and passes
Concentrated
through more of the atmosphere (which absorbs a little bit more
light than normal).
An experiment that you might try: Get a flashlight. Hold your hand flat with your fingers
pointing towards the ceiling. Hold the flashlight horizontal and shine it on your hand. Now, tilt
you palm upwards towards the ceiling. What happens to the circle of light on your hand? You
can see how sunlight is spread out at the Poles because it strikes the surface at an angle.
13. Which receives more light from the Sun, the Equator or the Poles?
14. Why does the Equator receive more heat than the Poles?
Clouds and Rain Unit (Topic 8A-2) – page 6
Heat Distribution and the Seasons
These factors also help explain why some parts of the year are warmer than other parts of the
year (in other words, why we have seasons). Notice how the Earth is “tilted” relative to the Sun;
the Earth’s North Pole always points towards a star we call Polaris (creative, huh?), also known
as the “North Star.” So, as the Earth orbits (travels around) the Sun, its tilt never changes. (The
Earth’s tilt is called its declination.)
During our summer, the northern hemisphere is tilted towards the Sun, so we get more sunlight
and become warmer. (The warmest spot is north of the Equator.) On the other hand, the southern
hemisphere is tilted away from the Sun, so it gets less sunlight and becomes cooler. It takes the
Earth 1 year to travel all the way around the Sun, so in 6 months, the Earth will be on the other
side of the Sun. The tilt does not change (it always points towards the north star, Polaris), so now
the northern hemisphere is tilted away from the Sun. (The warmest spot is south of the Equator.)
We get less sunlight during this part of the year, so it is our winter.
Earth
N. Pole
Equ
ato
r
N. Pole
Equ
ato
r
Sun
S. Pole
S. Pole
Northern
Hemisphere
Winter
Northern
Hemisphere
Summer
Earth
Sun
Earth
Actually, the Earth’s
tilt wobbles very
slowly in a small
circle over thousands
of years due to the
gravitational pulls of
Jupiter and other
planets on the Earth.
15. How long does it take the Earth orbit the Sun one time?
16. Does the Earth’s “tilt” change as it orbits the Sun over year?
17. Why are there seasons? For example, why does summer become winter?
Clouds and Rain Unit (Topic 8A-2) – page 7
Temperature Distribution and Heat Emission
The temperature of a place is not merely a matter of how much heat it receives, because if an
object only gains heat, then it continues to get hotter and hotter. Objects also lose heat by
conducting it to the neighboring environment (for example, your hand if you touch a cold
surface) or radiating it away as infrared “light” (invisible to us because our eyes cannot capture
it, but we can feel its heat when we get close to a hot object). Irrespective of how the heat is lost,
the basic rule of heat loss is: The hotter an object is, the more heat it gives off. As an object
gives away heat, it cools down, and therefore it gives away less and less heat over time. Even
frozen objects give off heat, and therefore get even colder! (They just get colder slower and
slower.)
Every moment of the day and night, the Earth gives away heat to the atmosphere (via
conduction) and radiates the rest towards outer space as infrared light. Over 99% of the
atmosphere is made of nitrogen and oxygen. Infrared light goes right through these gases. The
remaining less than 1% of the atmosphere includes greenhouse gases like carbon dioxide and
water vapor which absorb infrared light, trapping its heat in the atmosphere. The heat in the
atmosphere is eventually radiated into space too, helped by the fact that warm air rises upward
(transporting the heat through the greenhouse gases). The Earth does not run out of heat, because
it gains more heat each day by absorbing visible light from the Sun.
The Poles are colder than the Equator, so they give off less heat than the Equator, but they still
radiate heat into space. Interestingly, observations from satellites show that the Poles give off
more heat each day than they receive from the Sun. Similarly, the Equator radiates less heat into
space then it receives from the Sun. If the Poles are sending away more heat than they receive,
they should get colder, and if the Equator sends away less heat than it receives, it should get
warmer. But, of course, they are not getting warmer or colder; their temperatures are stable
(global warming issues aside). An object’s temperature is stable (does not increase or decrease)
if the amount of heat it receives is exactly equal to the amount it gives away (just like how your
bank account won’t go up or down if the deposits are exactly equal to the withdrawals).
18. Which emits (“gives away”) more heat, a hot object or a cold object?
19. True or false? “Cold objects emit heat, but less heat than hot objects.”
20. If an object gives away (emits) and much heat as it receives, what happens to its
temperature? In other words, does its temperature increase, decrease, or stay the same?
Clouds and Rain Unit (Topic 8A-2) – page 8
21. What 2 gases is the atmosphere primarily made of?
22. Give 2 examples of greenhouse gases.
23. Do greenhouse gases warm or cool the atmosphere (and thus the Earth)?
Temperature Distribution and the Motions of the Atmosphere and Ocean
The temperatures of the Poles and Equator are not increasing or decreasing, because the ocean
and atmosphere are moving (“transporting”) heat from the Equator towards the Poles (so the
Equator has less to “spend” and the Poles more to “spend”). In convection cells, the cool air
moves away from the “cold spot” and towards the “warm spot.” The air then warms up at the
“warm spot,” and rises (absorbs heat from the “warm spot,” cooling it down). Similarly, the
“cold spot” cools the air above it. In other words, heat goes from the air to “cold spot,” warming
the cold spot. Thus, the air moving in the convection cell is cooling down the “warm spot” (the
Equator) and warming up the “cool spot” (the Poles). As we will see in the next lecture, the
ocean does the same thing by moving warm water from the Equator towards the Poles and cool
water from the Poles towards the Equator.
The movement of water between the ocean and atmosphere also plays an important role in
transporting heat from low latitudes (e.g., the Equator) towards high latitudes (e.g., the Poles).
Warm ocean water evaporates under the clear skies of 30oN/S (e.g., southern California), moving
heat from the ocean into the atmosphere. (Remember: the “hot,” fastest-moving water molecules
tend to be the ones that evaporate.) Some of the air moves towards the Poles in the winds called
the “westerlies” (the convection cell between 30oN/S and 60oN/S). The air gives up its heat to the
cooler ground beneath (e.g., Seattle), causing the water to condense into clouds and rain.
Thus, the motion of the atmosphere keeps the Poles from becoming too cold and the Equator
from becoming too hot. As the air moves, it carries the heat away from the hot places and moves
cold air away from the cold places. The motion of the ocean – the ocean currents – performs a
similar job, making the Earth a much more pleasant place to live.
24. Does the motion of the atmosphere warm or cool the Equator?
25. Does the motion of the atmosphere warm or cool the Poles?