But what does temperature really mean? In the largest sense temperature is measure of how much energy is present. Let's think about this. We can recall that when a substance is hot, the molecules inside move faster than if the substance is cold. This is consistent with the thought that the hotter something is, the more energy it has. What do we know about the difference between the temperature in the shade and out in the the full sunlight. Where is there more energy? Where is the temperature higher? Out in the full sunlight there is more energy because there is radiant energy from the Sun, and we know that it can be much hotter in the Sun than in the shade. So in general, when the temperature of the air is high, there's more energy present than when the air temperature is low.

The air temperature can affect the weather by creating movement of the air. As the air near the ground warms, it expands and becomes less dense and rises. As it rises, it cools, becomes denser and sinks again. This cycle creates what are commonly called convection currents.

If you've ever been in the Mid-West or East during the Summer, you know what humidity is! The air is thick and sticky. It's hard to breathe and your clothes are soaking wet. An air mass can be cold or hot and contain more or less water vapor. The amount of water vapor in the air is called the "humidity." Warm air can hold more water vapor than cold air. Typically, the humidity is measured in percent of totally saturated air.

For example, when a weatherman reports that the relative humidity is 50%, it means that at the current air temperature, the air is holding the one-half of the water vapor that would totally saturate the air. The higher the air temperature, the more water vapor that can be contained in the air.

Humidity is measured by taking two temperature readings. One of the readings is a normal air temperature reading. The second is done with the "bulb" for the thermometer wet. In this second "wet bulb" reading, the water evaporates from around the bulb and lowers the temperature. If the humidity is low (the air is dry), the "wet bulb" temperature could be much lower than the temperature measured from a "dry bulb." If the humidity is high (the air has a lot of moisture), the difference between the wet and dry bulb readings will be small.

You will need:

• a thermometer
• cup of room temperature water
• a patch of a disposable dishcloth
• a rubber band
  

So why is the air pressure an important part of the weather? Well, pressure differences between air masses causes the air masses to MOVE! So air masses of high and low pressure are constaltly moving over the surface of the Earth. This is one way the weather changes, because the characteristics (for example,the temperature, pressure, and humidity) of the moving air masses moves with them. What if the temperature of these air masses are different? What would happen if these air masses move over you? You would feel the movement of the air (that's the wind), and you would feel the change on the air temperature. So you see, the air pressure plays a big role in what we experience as the weather. There are many high and low pressure areas over the surface of the Earth, so there is plenty of energy available that is capable of moving air masses and changing our weather.

One other thing about air pressure is that it changes with the altitude. For example, the air pressure is higher at sea level than on top of a high mountain. This pressure difference also helps air to move up and down. (Watt, Fiona and Francis Wilson, Weather & Climate, Usborne Publishing Ltd.,1992.)

Air also circulates around high and low pressure areas in very specific ways. In the northern hemisphere, air circulates clockwise around a high pressure area. The opposite is true for a low pressure area; that is, air circulates counterclockwise around a low pressure area.

Interestingly, the air circulation is exactly opposite in the southern hemisphere. The direction of circulation is due to the Earth's rotation, specifically to something called the Coriolis effect.

This circulation can help explain why the wind direction isn't always what you think it should be. For example, a strong low pressure air mass is moving toward you from the west. You live in the northern hemisphere. You might think that the winds would be blowing from the west, but they come from the south. Can you explain this?

The average air pressure is 14.7 pounds per square in. That means at on average, you have a force of 14.7 pounds on every square inch or your body! Usually the air pressure is reported in inches of mercury. This is related to the how high the air pressure can push a column of mercury on a curve tube. To convert pressure reported in inches of mercury, multiply that number by 0.4912 to obtain the number in pounds/sq. in (psi).

1. Can you feel pressure?
   Try this. Fill a large bucket with water about 3/4 full. Put your hand in a plastic bag up to about your elbow. Now slowly put it into the bucket. Do you feel the pressure? Does the pressure increase as your hand goes deeper?

2. Lifting a Book with Pressure
   

   You will need: a large zip-lock bag straw modeling clay a book

In this experiment, we will demonstrate how pressure can be used to move heavy objects. Place the straw in the bag as shown above and seal the zipper. Use the clay to help maintain a good seal.

Place the book on the bag and slowly blow into the straw. You will notice that you can easily lift the book.

This experiment shows how a little pressure acting over a large area can generate a large force. Remember this when a mild wind is blowing and you're trying to carry a poster board to school.

Additional Links
Humidity
Precipitation
Cloud Cover
Wind Direction