Name _______________________
ATMOSPHERIC CIRCULATION
(adapted from Dr. S. Postawko, U. of Ok.)
INTRODUCTION
Why does the wind blow? Why do weather systems in the mid-latitudes typically move
from west to east? Now that we've explored the various parameters used to describe
the state of Earth's atmosphere (temperature, pressure, and moisture), we want to put
them all together and see how they determine our weather and climate.
Part 1: Global Temperature and Pressure Variations
Our atmosphere is an open system - when a region of air is heated, the molecules
move faster and move farther apart from one another (that is, we say that the air
expands). When air expands, it means the number of molecules in a given volume
decreases, that is, the density of the air decreases (density = mass/volume). Therefore
the air in that region weighs less - and so it exerts less pressure than an equivalent
volume of cold air.
In addition, when air in a given region is heated and becomes less dense than air
surrounding it, this heated volume of air will rise (think "hot air balloon").
As we saw earlier, sunlight doesn't hit all parts of the Earth equally. This is one of the
reasons that there are temperature variations across the planet. Now we also know that
when regions of air are at different
temperatures, they also will have
different pressures.
(Open containers work differently than
sealed containers!) If we had two open
cartons of air that are the same size
and at the temperatures given, which
would exert the greatest pressure?
Why?
Assume you have a sealed box, as shown at right.
A divider in the box separates part of the box that
has air in it from part of the box that has no air in it
(i.e., a vacuum). If the divider is removed, will the
air move? If so, which direction? Why?
In the atmosphere, when pressure is higher in one region than in an adjoining region, air
will flow from the area of higher pressure to the area of lower pressure.
On the map of California at left, the atmospheric
pressure at many different weather stations and
buoys have been recorded in millibars.
Step 1: Connect the dots of those stations that
reported the same pressure (these lines of equal
pressure are called "isobars").
Step 2: Using the fact that air will flow from areas of
high pressure to areas of low pressure, indicate the
wind direction on your map with one or more arrows.
Step 3: Winds are named by where they blow from.
The winds you just labelled would be called
_____________________ winds.
We can get a general idea of the large-scale
circulation of Earth's atmosphere in a similar fashion.
For simplicity we will assume that the Earth is not
spinning nor tilted on its axis, and that the surface of
the planet is flat and of uniform composition. The diagram below represents this
simplified Earth. General regions of high and low pressure at the surface are indicated.
Based on this information, indicate the direction the winds would blow in the latitude
"belts" between each of the high and low pressure regions indicated.
Because Earth is a sphere and spins on its axis, there is an "apparent" force called the
Coriolis force that deflects the flow of air as it tries to travel in any direction across the
planetʼs surface. In the Northern Hemisphere the Coriolis force causes winds to turn to
the right of their initial direction of motion. For example, air moving in the latitude belt
between the Equator and 30°N doesn't flow directly from North to South. Because of the
Coriolis force, the air actually moves from Northeast to Southwest, as shown on the
diagram below.
In the Southern hemisphere, the Coriolis force moves air to the left of its initial direction
of motion, as shown in the latitude belt between the Equator and 30°S latitude.
Based on this information, draw the wind patterns in the remaining 4 latitude
belts, taking the Coriolis force into consideration.
The wind patterns in each of the latitude belts are given names, depending on the
direction the wind is coming from. Add these names onto your diagram above:
> Equator-30°N: Northeast Trade Winds
> Equator - 30°S: Southeast Trade Winds
> 30°N-60°N: Westerlies
> 30°S-60°S: Westerlies
> 60°N - North Pole: Polar Easterlies
> 60°S - South Pole: Polar Easterlies
Explain the statement, “The atmosphere socializes humanity, making the entire world a
spatially linked society.” Illustrate your answer with some examples.
So far we have only discussed horizontal motions of air. However, the atmosphere is 3dimensional, and we know that air not only moves horizontally, but vertically (up and
down) as well. Low pressure typically indicates air that is rising, expanding, and cooling.
If saturated air at Earth's surface rises and cools, the water vapor in the air condenses
to form clouds. On the diagram on the previous page, label those areas where you
would expect persistent cloud cover.
High pressure typically indicates air that is sinking (moving downward). As air sinks it
gets compressed and heats. The warmer air evaporates clouds. On the previous page,
label areas that you would expect to be clear most of the time. Does your diagram
predict generally clear or cloudy skies for Southern California?
Part 2: Influences on Local Wind Patterns
Earth's surface certainly isn't uniform in composition - over 70% of Earth's surface is
covered by water. Differences in how areas of land and water heat and cool can result
in temperature and pressure variations that influence local wind patterns.
When you go to the beach on a sunny summer's day, how does the temperature of the
sand compare to the temperature of the water?
It takes a long time for water to heat up or cool down when compared to land. That is,
we say that water has a high specific heat (definition of specific heat, the amount of
energy required to raise the temperature of 1 gram of a substance by 1°C).
The differences in the temperature of the land and water affect the temperature and
pressure of the atmosphere above. And as we now know, differences in temperature
and pressure are what cause air to flow.
On the diagram below, indicate what you think the relative air pressure would be over
land and over water, given the temperature of the land and the water. You can use a
two-step approach to determine this:
Step 1: First ask yourself if air temperature would be higher over the land or over
the water
Step 2: Based on your understanding of the relationship between temperature
and pressure, draw an L or H to indicate that pressure would be higher over land
or over water.
Step 3: Now that you have determined the relative air pressures over land and
over water, use your knowledge of the general relationship between air pressure
and vertical motion of air to indicate with arrows on your diagram if air is moving
upward or downward over land and over the water.
Step 4: Based on where air is rising and descending, draw in arrows to indicate
horizontal flow of air (wind).
The circulation that you have just drawn is called a "sea breeze". During the day, as the
land area heats up more than the water, air at the surface flows from over the cooler
water onto the land. This type of local circulation will dominate over the general
circulation of the atmosphere in a given area.