1. Weather Part 2: Air Pressure

2. Air Pressure… is a measure of the force of the air pressing down on Earth’s surface. at any point on the Earth is equal to the weight of the air directly above that point. at any point on Earth depends on the density of the air. Video: Air Pressure ExplainedAir Pressure Explained

3. Factors that Affect Air Pressure Temperature  Temp. =  Air Pressure  Temp. =  Air Pressure Elevation (Altitude)  Altitude =  Density =  Air Pressure Water Vapor Because the mass of H 2 O is less than the mass of O 2 and N 2, increasing water vapor causes a decrease in density and therefore a decrease in air pressure  Water Vapor=  Air Pressure

4. Measuring Air Pressure Barometer: an instrument used to measure air pressure Two types: Mercury and Aneroid Standard air pressure is measured at 0°C at sea level. (FYI: 1 atmosphere = 29.92 in Hg = 14.7 psi = 101,325 Pa = 1,013.25mb = 1.013 bar) 1Pascal (Pa)= 1 N/m 2

5. Mercury Barometers Invented 1643 by Evangelista Torricelli Glass tube closed at one end and filled with mercury Open end put in container of mercury The column of Hg changes height with changes in air pressure

6. Aneroid Barometer Airtight metal box with air removed Needle moves across dial as air pressure changes

7. Air Pressure and Weather High pressure generally means fair weather High pressure occurs when large air masses come together in the upper atmosphere. The air presses down on layers below and blocks warm, moist air from rising, so clouds do not form. Low pressure generally leads to cloudy, rainy weather Low pressure occurs when large air masses move apart in the upper atmosphere. This allows warm, moist air to rise and form clouds.