Air Pressure & Wind

Air Pressure & Temperature Air pressure is linked to temperature. Colder air is denser than warm air. Air pressure exerted by colder air > that by warmer air. Lines on a map that connect points of equal temperature are called isotherms. Air Pressure & Temperature http://www.middleschoolscience.com/isotherms.htm

Air Pressure Measurement Air pressure is measured with a barometer. Originally measured in inches of mercury using a mercury barometer. Average pressure at sea level is 29.92 inches of mercury. Air Pressure Measurement

Homework: Make your own barometer Materials: 1 large can, open at one end 1 plastic straw 1 large latex balloon 1 sheet of cardboard tape cut the balloon to create a large sheet of latex. on a day of average barometric pressure, stretch the latex tightly over the open end of the can and seal with tape. attach the straw and cardboard sheet as shown. record barometric readings (high, low, average) twice daily for 7 days.

Measruring Air Pressure Currently measured with an aneroid barometer. Meteorologists use units called millibars, average at sea level is 1013 mb.

Wind is the result of unequal heating of Earth’s surface Wind is the result of unequal heating of Earth’s surface. Unequal heating = unequal densities. Air moves from areas of high pressure to areas of low pressure. Greater pressure differences = higher wind speeds.

Pressure differences can exist due to temperature differences causing differences in density.

Factors That Affect Wind Pressure differences Coriolis effect Friction

Pressure changes across a given distance occur over a continuous spectrum of values, like a number line. Steep pressure gradient is like a steep hill – causing greater acceleration of wind speed. Pressure Gradient

Isobars - lines connecting points of equal pressure. Closely-spaced isobars = steep pressure gradient = strong winds. Widely-spaced isobars = light winds.

Coriolis Effect 4 Coriolis Effect 5 Coriolis effect causes moving objects to be deflected to right in Northern hemisphere, left in Southern Coriolis Effect 1 Coriolis Effect 2 Coriolis Effect 3 Coriolis Effect 4 Coriolis Effect 5 http://www.youtube.com/watch?v=49JwbrXcPjc

Friction Friction is the drag on moving air molecules by a solid or liquid surface. Friction slows air movement and can change its direction. Only acts close to Earth’s surface (friction layer).

Friction & Coriolis Effect Above friction layer, Coriolis Effect & Pressure Gradient balance each other, causing air to move parallel to isobars – jet stream. Within friction layer, slower wind speeds decrease Coriolis Effect. Wind As Art Global Wind Friction & Coriolis Effect

Activities - Isotherm Activity Isobar Activity Isotherm Activity – draw your own isotherms every 10 degrees (40, 50, 60, etc. After you draw each one, select the isotherm you drew under “Reveal Answer” to see how you did. Erase and redraw as needed. Isobar Activity - Weather maps often abbreviate air pressure readings to save space by using only the last two whole digits plus one decimal place. Ex: 1020.4 mb would read 204. Click on the box that says ‘Decode Pressures to see the entire numbers. Then draw your own isobars, checking and redrawing each one as you did with isotherms.

Pressure Centers and Winds Air pushes into warm, low pressure areas, spiraling in counterclockwise cyclone (a - stormy weather) From cold air mass, denser air pushes downward and spirals out clockwise, causing anticyclone (b - dry, fair weather)

High Pressure Systems Cool air is more dense, causes high pressure areas. Highs are associated with cool (but warming), drying, sinking air—clear skies, fair weather.

Low Pressure Systems Warm air is less dense, causes low pressure areas. Lows are associated with warm, moist, rising air & cloud formation—rain and storms.

Air Pressure & Weather Forecasting Air spirals into low pressure systems to fill void left as column of less dense air rises, increasing air pressure – self-limiting. Rising air causes cloud formation & precipitation.

Air Pressure & Weather Forecasting Surface conditions affect/reflect what is going on in the atmosphere above.

Air Pressure & Weather Forecasting Weather systems generally move west-to-east across North America. Observing changes in temperature, air pressure, wind speed and direction, and cloud cover help forecasters tell what is coming.

Global Winds Unequal heating from equator to poles. Non-rotating Earth Model: cold air would sink at the poles and flow all the way to the equator, where it would rise. Global Winds

Wind Measurements Speed – anemometer – knots or mph Direction – named for direction from which they blow – compass direction or degrees – indicated by wind vane. Prevailing wind – more often blows from one direction than any others.

Global Winds Global Circulation Rotating Earth Model: results in 6 global wind cells, 5 bands of high/low pressure, and global wind patterns due to Coriolis Effect. Wettest Places on Earth Global Winds Global Circulation

Continental Influence Land masses interrupt global pressure belts. Develop seasonal highs in winter, seasonal lows in summer. Result in seasonal wind changes – monsoons.

Regional Winds Valley Breeze - Sunny day, air of mt. slopes heats more than air at same elevation over valley, less dense, blows upslope. Mountain Breeze - At night, mt. slopes cool, air’s more dense, flows downslope.

Land & Sea Breezes Low pressure system develops over land by day, over sea by night. Air flows from high to low.

No Anemometer? Beaufort Scale - Developed in 1805 by Sir Francis Beaufort, U.K. Royal Navy

El Nino Periodic abnormal warming of ocean water, pushes into normally cold water. Disrupts jet streams and normal weather patterns – North American winters may be warmer in north, wetter in south. El Nino

La Nina Surface temps. in eastern Pacific colder than average Sends colder air over Pacific Northwest & Great Plains, warms rest of U.S. El Nino-La Nina El Nino-La Nina Explainer