1. Air Pressure Although air molecules are invisible, they still have weight and take up space. Air pressure is the force applied by air to any surface in contact with it. Air pressure can change because air particles can move farther apart or closer together. Information about air pressure is important for predicting weather, because changes in air pressure mean the weather is about to change.

2. High to Low Air molecules always want to move from areas of high pressure to areas of low pressure. Why? The surroundings are more spacious in areas of low pressure than in areas of high pressure.

3. High Pressure High pressure occurs when the air pressure is greater than surrounding areas. As air cools, it becomes more dense and sinks; cool, sinking air forms high pressure. High pressure usually means dry weather and cooler temperatures. On a weather map, a high pressure weather system is represented by a big, blue ‘H’.

4. Low Pressure Low pressure occurs when the air pressure is lower than surrounding areas. As air warms, it expands (becomes less dense) and rises; warm, rising air forms low pressure. Low pressure usually means wet weather and warmer temperatures. On a weather map, a low pressure weather system is represented with a big, red ‘L’.

5. Air Pressure & Altitude The lower we travel down in the atmosphere, the more weight there is above, so the tighter the air is squished or compacted. Therefore, air pressure decreases as you increase altitude and increases as you decrease altitude.

6. Links On this webpage, on the right hand side, there’s a link to click that says ‘Current Air Pressure’; this will allow you to see varying air pressure readings across the country for the day: http://kids.earth.nasa.gov/archive/air_pressure/weather.html Watch this video to see the effects air pressure has on objects: https://www.youtube.com/watch?v=28TIyWdfxxc&src_vid=bN7 E6FCuMbY&feature=iv&annotation_idchannel%3A52a3f25d-0- 2d5b-8975-20cf301cc0cc

7. Exit Ticket 1.What is air pressure? 2.What type of weather can be expected from a high pressure weather system? From a low pressure weather system?