Air Pressure and motion Air pressure is the weight of air at any particular point. It is also called barometric pressure. – The units are given in the ESRT in millibars and inches of mercury and as atmospheres. – A barometer is a device used to measure pressure, – Air pressure depends on how tightly packed the molecules are and how heavy they are.

Units of air pressure: page 13 ESRT For normal, sea level pressures, the air pressure will be found on this chart.

What’s a ‘falling barometer’? When air is ‘heavy’ it pushes down (and around) on objects. The mercury will ‘rise’ in the vacuum tube. So a rising barometer means ________ weather. When air is ‘light’, it does not push down as much and the mercury in the barometer will ‘fall’ in the vacuum tube. So a falling barometer means ____________ weather.

Remember…… Heavy Air: (high pressure) – cold, because the molecules are closer together. – Dry, because nitrogen and oxygen are heavier than water vapor. Light Air: (low pressure) – warm; the molecules have more energy and are farther apart. – Humid, because water vapor is a very light molecule.

The reason some air is light and some air is heavy is due to the unequal heating of the atmosphere due to our round tilted planet. ffden-2.phys.uaf.edu

Why wind? AIR PRESSURE AND WIND…… Air flows from areas of high pressure to areas of low pressure. Air flows from regions of descending, heavy air to regions of ascending, light air. Descending air is clear weather. Ascending air happens at fronts and in very warm, wet air (afternoon thunderstorms) jason.org

Practice wind direction and gradient

Weather maps show pressure systems, using isobars. The closer the isobars are on the map, the greater the difference in pressure and the faster the wind is blowing, There is a rapid change in air pressure.(popping a balloon)

education.psu.edu/public/meteo/meteo101demo/Examples/Images/Section2/contour_tool_p0207.swf education.psu.edu/public/meteo/meteo101demo/Examples/Images/Section2/contour_tool_p0207.swf allows students to practice making isobars. It will convert from station model pressure to millibars for them and draw the lines for them to check

From high to low High pressure systems air is heavy, is probably falling creating convection. High pressure air blows outward and clockwise across the surface. Low pressure systems indicate the air is light and therefore it is probably rising, creating convecction. Air blows inward and counterclockwise and up.

– Forecasters know that bad weather tends to move from west to east in the continental U.S. because of the prevailing winds. The jet stream :discovered by WWII pilots flying cross country. The jet stream is caused by interactions in upper levels of the troposphere. Most dramatic during our winter season; cold, dry Canadian air meets warmer, moist air of the Gulf of Mexico. the jet helps determine the weather patterns for the country.

free-online-private-pilot-ground-school.com Local winds caused by daily heating and cooling

Self-check: What are 2 characteristics of heavy air? Of Light air? What air is rising: heavy or light? High pressure or low pressure? Why does wind blow? Does it blow from high to low or low to high? Can you draw wind around a high? Around a low? Which way do storms move across the US? Review book pages and ogin.aspx ogin.aspx