Chapter 19 The Atmosphere in Motion
Section 1 • Air pressure is the weight of the air as it pushes down on Earth • Ave:14.7 lbs/square inch, and this is from all directions
As you move up in atmosphere there is less air above to press down- so air pressure lessens
Why your ears pop when air pressure changes • When you move up in altitude the air pressure outside your body is less- so your ear drum pushes out. • “popping” your ear equalizes the pressure • When you descend in altitude there is greater pressure outside the ear.
Measuring air pressure • Two types of barometers • Mercury barometer (page 414) • Aneroid barometer (page 415)
Why does air pressure change? 1. elevation 2. temperature 3. Humidity metric unit for pressure called a millibar.
elevation • the higher the elevation the thinner the air (less molecules) so less pressure
temperature • Air pressure decreases as temperature increases. • When air is warm it moves apart • When air is cool it moves closer together
humidity • Water vapor is lighter than nitrogen and oxygen. • So air has less pressure when then is more water vapor
Using pressure to predict the weather Decrease in pressure = warmer, humid air, may bring rain/snow Low pressure=lousy weather Increase in pressure =cool, dry air, air weather High pressure= happy weather
Break down the words • Isotherm • Isobar
Isobar • On weather maps-used to show areas of equal pressure • Isobar lines may never cross or touch. • Low pressure is shown in red • High pressure is shown in blue
A high pressure center is where the pressure has been measured to be the highest relative to its surroundings. • That means, moving in any direction away from the "High" will result in a decrease in pressure
A low pressure center is where the pressure has been measured to be the lowest relative to its surroundings. • That means, moving in any horizontal direction away from the "Low" will result in an increase in pressure
winds flow counterclockwise around a low in the northern hemisphere.
On the weather map strong winds are represented by isobars drawn closely together
What makes the wind blow? • Air moves from areas of high to low pressure • The bigger the difference btw. The high and the low- the stronger the wind
Land Breeze and Sea Breeze • A land breeze -blows from the land to the ocean. • A land breeze occurs at night when the land cools faster than the sea. The air above the warmer surface water rises, pulling in air from the cooler land surface.
Animation of sea breeze and land breeze • http://www.classzone.com/books/earth_science/terc/content/visualizations/es1903/es1903page01.cfm
Winds are named for their origins -winds that blow from west to east are called west winds (westerlies)