Chapter 28 Atmospheric Pressure and Winds BFRB Pages

Air Pressure the weight of the air in a certain areathe weight of the air in a certain area Air pressure is measured using an instrument called a barometer (aneroid or mercury)Air pressure is measured using an instrument called a barometer (aneroid or mercury) It is measured in inches (height of the mercury column in the barometer), or with the metric units called millibarsIt is measured in inches (height of the mercury column in the barometer), or with the metric units called millibars Use your reference tables for “converting” inches to millibarsUse your reference tables for “converting” inches to millibars (ESRT’s Page 13 Right Top)(ESRT’s Page 13 Right Top)

Right here tells you which side is millibars and which side is inchesRight here tells you which side is millibars and which side is inches FYI – the increments are not the same on both sides. Figure out each sides’ increments and write them in right now…FYI – the increments are not the same on both sides. Figure out each sides’ increments and write them in right now… Inches: Increment = __________ Millibars:

Air pressure decreases with increasing altitude (less air above you means less pressure pushing down on you)Air pressure decreases with increasing altitude (less air above you means less pressure pushing down on you) This is an inverse (indirect) relationshipThis is an inverse (indirect) relationship Weather maps show sea level air pressure. It ranges from 960mb – 1050mbWeather maps show sea level air pressure. It ranges from 960mb – 1050mb Isobars are lines on the weather maps which connect areas of equal air pressureIsobars are lines on the weather maps which connect areas of equal air pressure VIF!!!! - The same rules apply for isobars as isotherms and contour lines. If the isobars are spaced close together, the pressure gradient is steep and winds are fast.VIF!!!! - The same rules apply for isobars as isotherms and contour lines. If the isobars are spaced close together, the pressure gradient is steep and winds are fast.

Changes in Air Pressure How does temperature affect air pressure?How does temperature affect air pressure? Warm air’s molecules are further apart than cold air’s molecules!Warm air’s molecules are further apart than cold air’s molecules! It is less dense and not as many molecules are on top of an area to add pressure.It is less dense and not as many molecules are on top of an area to add pressure. Therefore, as temperature ________________, air pressure _________________.Therefore, as temperature ________________, air pressure _________________.

Changes in Air Pressure How does humidity (the amount of water vapor held in a parcel of air) affect air pressure?How does humidity (the amount of water vapor held in a parcel of air) affect air pressure? Humid (moist) air contains more water vapor molecules (H 2 O)Humid (moist) air contains more water vapor molecules (H 2 O) These H 2 O molecules have less atomic mass than N 2 and O 2 molecules that they take the place of!!!These H 2 O molecules have less atomic mass than N 2 and O 2 molecules that they take the place of!!! Let’s PROVE it!!!!!Let’s PROVE it!!!!!

Winds and Air Pressure Winds are caused by the unequal heating of the atmosphere (remember that temperature causes changes in air pressure)Winds are caused by the unequal heating of the atmosphere (remember that temperature causes changes in air pressure) Warm air is lighter (lower pressure) and colder air is heavier (higher pressure)Warm air is lighter (lower pressure) and colder air is heavier (higher pressure) –Ex: Sea breezes and land breezes

Sea Breeze – the surface wind is blowing in FROM the sea.

Land Breeze – the surface wind is blowing out FROM the land.

Winds blow out of high pressure and into low pressureWinds blow out of high pressure and into low pressure The greater the difference between the high pressure and low pressure, the faster the wind speed (this is called the pressure gradient)The greater the difference between the high pressure and low pressure, the faster the wind speed (this is called the pressure gradient) This is shown on a weather map by closely spaced isobars (think of closely space contour lines on a topographic map – steeper slope)This is shown on a weather map by closely spaced isobars (think of closely space contour lines on a topographic map – steeper slope) Air Pressures Effect on Winds on a Larger Scale

What parts of the US and Canada have the fastest winds? the slowest?

Coriolis Revisited! The Coriolis Force (caused by the spin of the Earth) causes winds to bend to the right in the northern hemisphereThe Coriolis Force (caused by the spin of the Earth) causes winds to bend to the right in the northern hemisphere Winds blow out of the High and in to the Low and bend to the right, causing a High Pressure System to spin clockwiseWinds blow out of the High and in to the Low and bend to the right, causing a High Pressure System to spin clockwise Winds blow into the Low from the High and bend to the right, causing the Low Pressure System to spin counter-clockwiseWinds blow into the Low from the High and bend to the right, causing the Low Pressure System to spin counter-clockwise FYI - Hurricanes are areas of extreme low pressure and always flow counter- clockwise in the Northern HemisphereFYI - Hurricanes are areas of extreme low pressure and always flow counter- clockwise in the Northern Hemisphere

In a High, the air wants to move directly out from High to Low Pressure but is deflected to its right by the Coriolis Force, creating a Clockwise flow out from the High. VIF!!!!!! H

In a Low, the air wants to move directly in from High to Low Pressure but is deflected to its right by the Coriolis Force creating a Counterclockwise flow into the Low. VIF!!!!!! L

MEASURING WIND Wind direction is determined by using a wind vane or a wind sock (like you see at the airport)Wind direction is determined by using a wind vane or a wind sock (like you see at the airport) Wind speed is measured using an anemometer (units are knots; 1 knot = 1.15 mph)Wind speed is measured using an anemometer (units are knots; 1 knot = 1.15 mph)

Pressure Belts and Winds The jet stream is a narrow zone of high speed air found in the upper troposphere (about 30,000 feet)The jet stream is a narrow zone of high speed air found in the upper troposphere (about 30,000 feet) –Wind speeds here may be over 300 knots! Large convection currents of air occur on the Earth’s surface due to the unequal heating of the atmosphereLarge convection currents of air occur on the Earth’s surface due to the unequal heating of the atmosphere ESRT’s Page 14 Bottom Link to global_circ_anim.swf

3-D Version of the ESRT’s Please note the deflection of the wind to ITS right in the N.H. due to CF

This is what the air circulation would look like if the Earth DID NOT ROTATE – NO CF!!!!!!