 Air pressure is a measure of how hard air is pushing on the ground  Barameter – measures this pressure barometer.gif

 Air pressure decreases with altitude

 Air pressure changes daily  Caused by changes in air density  related to temperature  Warm air is less dense than cooler air  Larger pressure changes occur as weather changes  Used to forecast weather

The FALL of the barometer (decreasing pressure)  In very hot weather, the fall of the barometer denotes thunder. Otherwise, the sudden falling of the barometer denotes high wind.  In frosty weather, the fall of the barometer denotes thaw.  If wet weather happens soon after the fall of the barometer, expect but little of it.  In wet weather if the barometer falls expect much wet.  In fair weather, if the barometer falls much and remains low, expect much wet in a few days, and probably wind.  The barometer sinks lowest of all for wind and rain together; next to that wind, (except it be an east or north-east wind). The RISE of the barometer (increasing pressure)  In winter, the rise of the barometer presages frost.  In frosty weather, the rise of the barometer presages snow.  If fair weather happens soon after the rise of the barometer, expect but little of it.  In wet weather, if the mercury rises high and remains so, expect continued fine weather in a day or two.  In wet weather, if the mercury rises suddenly very high, fine weather will not last long.  The barometer rises highest of all for north and east winds; for all other winds it sinks. The barometer UNSETTLED (unsteady pressure)  If the motion of the mercury be unsettled, expect unsettled weather.  If it stands at "MUCH RAIN" and rises to "CHANGEABLE" expect fair weather of short continuance.  If it stands at "FAIR" and falls to "CHANGEABLE", expect foul weather.  Its motion upwards, indicates the approach of fine weather; its motion downwards, indicates the approach of foul weather.

 Falling air pushes hard  high pressure  Compresses and warms  Rising air doesn’t push as much  low pressure  Expands and cools

 Low pressure = rising air = expanding air = cooling air = water condenses = clouds and precipitation  High pressure = falling air = compressing air = heating up = air holds more water = fair weather (hard for clouds to form)

 “Dry” barometer  Construct in class – monitor every day for a week  “Wet” barometer

 Air moves because the unequal heating and cooling of Earth (and the atmosphere)  Air moves from areas of high pressure (cool, sinking air) to areas of low pressure (warm, rising air)  creates wind

 Unequal heating of air over land and water result in breezes near shorelines  Land warm during day  rises; cool sea breeze replaces rising air  Land cool during night  air sinks; cool land breeze blows out to sea  See Animation See Animation

 If Earth did not rotate and was all land or water – we’d have two big convection currents, one in the northern hemisphere, one in the southern  Air would heat up at equator, rise, cool down over poles and fall

 However, Earth DOES rotate…  Results in Coriolis effect:  Moving particles deflect to the right in the northern hemisphere and the left in southern hemisphere  See Animation See Animation

 Coriolis effect combines with heat imbalance to create global wind systems that transport colder air to warmer areas and vice versa  balances Earth’s heat energy  Animate it! Animate it! L H L

 Narrow bands of fast, high-altitude, westerly winds form between wind zones  Polar and Subtropical jet streams  Disturbances form along jet streams and cause large- scale weather systems  Weather systems follow jet stream  Position of jet streams varies tream3.jpg

Video Explanation