Atmosphere and Weather



Air Pressure

Air pressure can be thought of as simply the weight of the air above you. As you sit and read this screen, there is 14.7 pounds of air siting on your shoulders! That's like draping the family dog over your back! Why don't you feel it? Our bodies push back against this force with an equal force, balancing it out. If our bodies did not push back, then we would feel like a dog was sitting on our shoulders at all times.

Air pressure is greatest at sea level, and decreases with altitude. That's because the effect of gravity is closer at the surface of the Earth so there are more particles of air drawn nearer to the Earth's surface. Air pressure is measured with a barometer. The units of measurement are inches of mercury and/or millibars.


Air temperature and air pressure-

When the air temperature is cold, it causes more air molecules to become tightly packed together. The air pressure is high when the air is cold. At the equator since the temperature is warmer, the air molecules are more spread out. The air pressure is low when the air is hot.

Two Factors that determine the amount of solar energy received in an area and thus the air temperature and pressure are:

1. The color of the Earth's Surface- Black and other dark colors absorb heat, light colors reflect heat. Snow is light and reflects heat, and urban pavement absorbs heat.

2. Specific Heat Capacity- The ability of an object absorb heat without heating up.

Liquid water takes 1 calorie of heat to raise the temperature1 degree C.

Lead takes 1 calorie of heat to raise the temperature 33 degree C.


Changes in State

When a substance changes its phase, as in ice turning into water, we call that a change in state. The act of melting ice or boiling water requires the addition of energy. Condensing water vapor to liquid water or freezing water requires that energy be taken away. Strangely when matter is changing state, it does NOT change in temperature. Why?

The heat is busy making or breaking bonds. This phenomenon is called Latent Heat or hidden heat. Check out this video on latent heat involved in changing ice to water vapor.

Heat Flow

Heat always moves from Hot to Cold

3 types: Conduction, Convection and Radiation



The horizontal movement of air from high to low pressure is called winds. Air in the atmosphere moves due to differences in density. Air expands when it warms and contracts with it cools. Wind is the air's attempt at making up the differences in air density.

Winds are named by the direction they are coming from. For example, the feared Nor'easter storms are violently bitter winds that come from the cold northeastern Atlantic Ocean waters to New England.

What causes wind?

Bill Nye The Science Guy

Appreviated Video

Full Video




Global Winds

Wind Bands

In general, the earth has different zones of wind. The equatorial air is hottest, so it slowly rises, expanding as it gains altitude. It cools at the higher altitudes, and then sinks at 30 degrees latitude. On the poles, the cold air over the poles sinks, and rises as it heats at the 60 degree latitude line. This sets up wind convection cells or Hadley Cells.




As this is going on, the earth is spinning, giving the moving air a twist. This is due to the Coriolis effect. The Coriolis effect is the effect of the rotation of the earth on the path of an object traveling over the surface of the Earth. Although we cannot feel the earth spinning, the earth is actually rotating at an incredible speed. This spinning causes deflections in the paths of things like airplanes, cannon balls, and cyclones.

The coriolis in the Northern Hemisphere deflects objects to the right

While in the Southern Hemisphere, objects bend to the left


Another effect of the rotation of the Earth are the Jet Streams- These 300 mph winds scream through the upper troposphere like a powerful river. Interestingly the path of the jet streams wanders up and down in lazy S-curves. Their location has a major impact on the local weather. Check out this picture of an unusual dip in the Jet Stream in the winter of 2002.


Local Winds-

Sea Breezes: During the day, the air over the land becomes hot, and therefore the pressure gets lower. The cool air over the sea has higher pressure and rushes toward the land to make up the difference in pressure.

Land breezes: At night, however, the land releases the heat that it absorbed during the day. The water is now warmer than the land and has a lower pressure. The breeze blows from the land to the sea. (Dangerous if you are doing night wind surfing!)







Right before it rains, it feels like you have a wet blanket wrapped around you. Why do you feel so uncomfortable? The air is humid and holding lots of water vapor. The humidity in the air is measured in several ways.

Absolute humidity- the actual amount of water in the atmosphere measured in grams.

Relative humidity- how close the air is to being filled with water vapor.
Its the amount of moisture in the air divided by the amount of water vapor it takes to fill the air X 100%

Which do you think is a better indication of rain? The relative humidity in a desert is often 1% but the rain forest reaches a daily relative humidity of 100%. (That's why its called a RAIN forest!!) The higher the relative humidity, the closer you are to forming clouds and rain.

Now things can get a little sticky. How can the humidity level of the air be changed? Obviously, if you just add more water vapor, relative humidity will rise. But what if the temperature of the air changes? Consider the Golden Rule of Meteorology: The colder the air, the less water vapor it can hold. So why does it rain in a rain forest if it is so hot?

Hot air rises in the jungle, and the higher you go, the colder it gets. The cold air stores less water vapor so cleverly it changes its water vapor to liquid water- the stuff of clouds and rain. Something called Adiabatic cooling also helps- the higher the air rises, the lower the pressure so the molecules spread out further decreasing the temperature of the air. Dew Point Temperature is the name for the temperature to which the air must be cooled to become saturated.

The real reason why we feel uncomfortable right before it rains is that the higher the humidity of the air, the less water the air can absorb. That leads to less absorption of sweat, which makes us feel uncomfortable. Sheets of perspiration cover our body with hope of evaporating. The only relief is to find a place that is already saturated with water- a pool!

Recipe For a Cloud-

1. Add Water

2. A pinch of Condensation Nuclei (salt or dust)

3. Cool the air- orographic lifting, frontal lifting or convection.

Weather Map


Air Masses-

Martitime air masses (m) form over large bodies of water such as oceans or seas. They are wet!
Continental air masses (c) form over the land and are therefore dry.
Polar air masses are cold and Tropical air masses are warm. These are capitalized and written as P and T.

Therefore a tropical maritime air mass would be written as mT.


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