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. elevation2. temperature3. Humiditymetric 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 weatherHigh 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

• Winds blow clockwise out of a high pressure center

• 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.

Sea breeze

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)