Copyright © 2013 Pearson Education, Inc. The Atmosphere: An Introduction to Meteorology, 12 th Lutgens Tarbuck Lectures by: Heather Gallacher, Cleveland State University Chapter 6: Air Pressure and Winds Copyright © 2013 Pearson Education, Inc.

Wind and Air Pressure  Wind is the result of horizontal differences in atmospheric pressure.  Atmospheric (air) pressure is the force exerted against a surface by continuous collision of gas molecules.  The air pressure at sea level is about 1 kg/cm 2.

Copyright © 2013 Pearson Education, Inc. Measuring Air Pressure  Air pressure is measured in millibars.  1 millibar (mb) = 100 newtons/m 2  mb = sea level pressure  The mercury barometer is the standard pressure- measuring instrument.  Air pressure pushes on a pool of mercury, which forces the mercury up a tube.  High pressure: The mercury goes further up the tube.  Low pressure: The mercury stays lower.

Copyright © 2013 Pearson Education, Inc. Measuring Air Pressure

Copyright © 2013 Pearson Education, Inc. Pressure Changes with Altitude  Air pressure is defined as the force exerted against a surface by continuous collision of gas molecules.  Air pressure decreases with altitude.  Air pressure increases with depth.

Copyright © 2013 Pearson Education, Inc. Why Does Air Pressure Vary?  Temperature influences air pressure.

Copyright © 2013 Pearson Education, Inc. Why Does Air Pressure Vary?  Water vapor affects air pressure.  The amount of water vapor reduces the density of air.  Cold dry air has higher pressure.  Warm, dry air has higher pressure than equally warm, moist air.

Copyright © 2013 Pearson Education, Inc. Why Does Air Pressure Vary?  The movement of air can also cause variations in air pressure.  Convergence occurs when a net airflow into a region causing pressure to rise.  Divergence results when there is a net outflow of air from a region and surface pressure drops.

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  Pressure gradient force  Coriolis force  Friction

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  Pressure gradient force:  Isobars are lines connecting places of equal pressure.  If isobars are close together a steep gradient and high winds result.  If isobars are far apart, the result is a low gradient and lower wind speeds.

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  How temperature differences generate wind

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  Isobars on a surface chart:  Low-pressure systems (L) that occur in the middle latitudes are called midlatitude cyclones.  High-pressure systems (H) are called anticyclones, which tend to be associated with clearing conditions.

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  The Coriolis force cannot generate wind; it modifies airflow.  In the Northern Hemisphere, a deviation to the right is due to the Earth’s rotation.  In the Southern Hemisphere, the deviation is to the left.

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  The Coriolis force is a deflecting force.  It is always directed at right angles to the direction of airflow.  It controls only wind direction, not speed.  It is affected by wind speed (the stronger the wind, the greater the force).  The Coriolis force is strongest at the poles and is nonexistent at the equator.

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  Coriolis force

Copyright © 2013 Pearson Education, Inc. Factors Affecting Wind  Friction significantly influences airflow near Earth’s surface, but its effect is negligible at higher altitudes.

Copyright © 2013 Pearson Education, Inc. Winds Aloft  Geostrophic flow occurs when the pressure gradient force and the Coriolis force are balanced.  The wind flows parallel to the isobars.  They flow in relatively straight paths.  Buys Ballot’s Law states that if you stand with the wind at your back, low pressure will be found on your left, high pressure on the right.

Copyright © 2013 Pearson Education, Inc. Winds Aloft  Curved airflow and the gradient wind

Copyright © 2013 Pearson Education, Inc. Winds Aloft  Curved airflow:  Winds around cells of high pressure or low pressure follow curved paths.  Gradient winds blow at a constant speed.  They are parallel to the curved isobars.  Centers of low pressure are called cyclonic; winds flow counter-clockwise in the Northern Hemisphere.  A trough is the result of isobars curving to form elongated regions of low pressure.  Centers of high pressure are anticyclonic; winds flow clockwise in the Southern Hemisphere.  A ridge is the result of isobars curving to form elongated regions of high pressure.

Copyright © 2013 Pearson Education, Inc. Winds Aloft  Curved airflow and the gradient wind

Copyright © 2013 Pearson Education, Inc. Surface Winds  Surface winds travel at an angle across isobars, toward low pressure.

Copyright © 2013 Pearson Education, Inc. How Winds Generate Vertical Air Motion  Factors that promote vertical airflow:  Friction  Increased friction causes a drop in wind speed resulting in a pileup of air upstream from the ocean to land.  Convergence can result in cloudy weather.  Decreased friction causes and increase in wind speed from the land to the ocean.  Subsidence and divergence results in clearing weather.

Copyright © 2013 Pearson Education, Inc. How Winds Generate Vertical Air Motion  Vertical airflow is associated with cyclones and anticyclones.

Copyright © 2013 Pearson Education, Inc. Wind Measurement  Wind direction is always measured from the direction it is coming.  A North wind blows north to south.  A prevailing wind consistently blows more often than not from one direction.

Copyright © 2013 Pearson Education, Inc. Wind Measurement

Copyright © 2013 Pearson Education, Inc. Wind Measurement  A wind vane shows wind direction.  Wind speed is often measured with a cup anemometer, which has a dial that shows wind speed.  An aerovane is a combination of wind vane and anemometer.

Copyright © 2013 Pearson Education, Inc. End Chapter 6