+ 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. Atmosphere is without walls but the force of gravity prevents its escape.
+ Measuring Air Pressure Air pressure is measured in millibars. 1 millibar (mb) = 100 newtons/m2 1013.25 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.
+ 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. Because of the decrease of pressure above you Air pressure increases with depth. Because of the increase of pressure above you
+ Why Does Air Pressure Vary? Recall that variations in air pressure causes the wind to blow, which in turn causes changes in temperature and humidity 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.
+ 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.
+ Why Does Air Pressure Vary? Temperature influences air pressure.
+ Factors Affecting Wind Pressure gradient force Coriolis force Friction
+ Factors Affecting Wind- Pressure Gradient Force The force that generates winds results from horizontal pressure differences. Greater differences the greater the wind blows Pressure gradient force Spacing of the isobars indicate the amount of pressure change occurring over a given distance.
+ Factors Affecting Wind- Pressure Gradient Force Isobars on a surface chart: Low-pressure systems (L) that occur in the middle latitudes are called midlatitude cyclones. Produce stormy weather High-pressure systems (H) are called anticyclones, which tend to be associated with clearing conditions.
+ Factors Affecting Wind- Coriolis Force TheCoriolis force cannot generate wind; it modifies airflow. The Coriolis force causes all free flowing objects including wind: 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.
+ Factors Affecting Wind- Coriolis Force 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.
+ Factors Affecting Wind- Coriolis Force Coriolis force
+ Factors Affecting Wind- FrictionRemember pressure gradient force is the primary driving force for winds. Air moves from high to low concentrations but wind speeds don’t increase indefinitely.Friction significantly influences airflow near Earth’s surface, but its effect is negligible at higher altitudes.
+ Winds Aloft Geostrophic flow occurs when the pressure gradient force and the Coriolis force are balanced. Thewind flows parallel to the isobars. They flow in relatively straight paths. Wind Direction is directly related to prevailing pressure patterns. So if we know direction we can establish a rough approximation of the pressure distribution.
+ Winds AloftCurved airflow and the gradient windCurved airflow: Winds around cells of high pressure or low pressure follow curved paths.
+ Winds Aloft 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. Opposite is true in the Southern Hemisphere
+ Winds Aloft Curved airflow and the gradient wind
+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.
+How Winds Generate Vertical Air MotionVertical airflow is associated with cyclones and anticyclones.
+ Wind Measurement Wind direction is always measured from the direction it is coming. A North wind blows north to south. A East wind blows east to west. A prevailing wind consistently blows more often than not from one direction.
+ 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.