Download Chapter 23B

Chapter 23B
Air Pressure and Wind
A. Atmospheric Pressure
- Air gets thinner as you go up in the atmosphere.
- 50% of the air is in the lowest 5km (15,000 ft) of the atmosphere
- 90% of the air is in the troposphere, other 10% is in 3 layers above
- Air pressure is a measure of the weight of the atmosphere.
- One measure of this weight is about 1kg/cm2 or 14.7 lbs/in2.
- This means that a column of air one centimeter on a side pounds would weigh a kilogram
(2.2lbs) - Or a column 1 inch on a side weighs 14.7
- The way we sense air pressure is by our ears “popping” as we go up and down
1. Mercury Barometers
- A mercury barometer measures how far up a column of mercury will be pushed up to
balance the weight of air.
- Mercury barometers are accurate, expensive, hazardous, and difficult to read
- Average air pressure will push up the mercury 76 cm or 29.92 inches
- Metric measure of air pressure is millibar (mb) – average sea level pressure is 1013.2mb
- A water barometer would push water up 32 feet
- High pressure (heavier air) will push the mercury up above 30 inches
- Lower pressure (lighter air) pushes the mercury up only about 29.8 inches
2. Aneroid Barometers
- Aneroid means “without liquid” so they do not contain mercury
- The most important part is a metal container with a partial vacuum inside
- This container changes shape as the air pressure pushing on it changes
Normal pressure
High pressure
Low pressure
- A series of springs, chains, and pulleys transfer the shape change to movement of a
needle on a dial.
- These barometers are cheap, safe, but less accurate
3. Altimeters
- Because as you go up the pressure drops a barometer can be used to tell altitude
- These special barometers are called altimeters
- Since all barometers will act like altimeters, a new barometer must be corrected to sea
level before it can be used.
23.3 Winds
A. Why Does the Wind Blow?
- All winds are caused by pressure differences.
- Winds blow from areas of high pressure to areas of low pressure.
- Pressure differences are caused by temperature and humidity differences
1. Temperature differences
- Convection currents explain how temperature differences create pressure differences.
- Warm rising air creates low pressure
- Cold sinking air creates high pressure
2. Humidity differences
- Humid air is lighter so it creates low pressure
- This is because water vapor molecules weigh less than nitrogen or oxygen molecule
- Dry air is heavier so it creates high pressure
3. Summary:
- LOW PRESSURE – WARM HUMID AIR
- HIGH PRESSURE – COLD DRY AIR
- ALL WINDS BLOW FROM HIGH TO LOW.
B. Global Winds
- The cold dry air at the poles creates a permanent high pressure area
- The warm humid air at the equator creates a permanent low pressure area
- Based on what we know, all winds in the Northern Hemisphere should blow from pole to
equator creating a north wind.
- Unfortunately the atmosphere is too big and there is the Coriolis Effect which creates 3
wind belts.
- The Coriolis Effect is due to the Earth’s rotation and bends all winds to the right in the
Northern Hemisphere (to the left in the Southern Hemisphere)
- There are 3 major wind belts (horizontal movement of air): trade winds, westerlies, and
polar easterlies.
- Between the 3 wind belts are 3 pressure belts (vertical movement of air)
1. Doldrums – Hot humid air in the tropics near the equator
2. Horse Latitudes – Hot dry air at 30°N or S creating deserts
3. Polar Front – Stormy area at 60°N or S: cold polar and warm tropical air meet
- Wind and pressure belts move 23 1/2° north in the summer and 23 1/2° south in winter
- All the winds in the upper atmosphere are westerlies.
- The jet streams are bands of very strong westerlies (500 km/hr, 300mph) at the top of the
troposphere.
- They are important for jet travel and they determine the track of storms.
C. Local Winds
- All winds are named by the direction from which they blow.
- A north wind blows from the north and a southwest wind blows from the southwest.
- Monsoons are seasonal winds which blow on and off the continents.
- Land and sea breezes are day to night winds caused by uneven heating of land and water
during the day.
- Mountain and valley breezes are similar to land/sea breezes but the uneven heating is
due to the fact that thin mountain air heats fast and cools fast
- Wind speed is measured with an anemometer that spins.
- Wind direction is measured with a wind vane that points into the wind
- A wind sock measures both wind speed and direction
- Wind speed on a weather map is shown with an arrow for direction then bars and flags
for wind speed.
- Start with a station circle and draw an arrow showing the direction the wind is coming
from, then add a short bar for 5 mph, long bars for 10 mph, and a flag for 50 mph