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In describing weather, wind is generally taken to mean the horizontal movement of air. By convention, the wind direction is the direction from which the wind is blowing (e.g., a north wind means the air is moving from north toward south) and the windspeed is the speed at which the air is moving relative to the ground.
Winds result from horizontal differences in pressure. Basically, air is forced or pushed from high pressure toward lower pressure. This explains why air rushes in when you open a vacuum sealed container (like a jar of food); it explains why air rushes out when you open a carbonated beverage sealed under high pressure (like soda or beer); and how it is that we breath (expanding our lungs initally lowers the air pressure inside, causing higher pressure air outside to rush in). In all cases, air is forced to move from high toward low pressure and the greater the difference in pressure, the faster the air moves.
In the atmosphere, winds result from horizontal differences in air pressure. Recall from the previous lecture that the average sea level pressure is 1013 mb, but the actual sea level pressure at any location and time varies. The pattern of sea level pressure is what causes surface winds to blow. We will now look at how the sea level pressure pattern is plotted on a weather map called a surface chart.
Station pressure is defined as the barometer reading at a given meteorological station. A barometer is an instrument used to measure air pressure. But cities separated by just a few hundred kilometers might have very different station pressures. The differences between the station pressures are due primarily to the cities being at different altitudes above sea level. Thus, to properly monitor horizontal changes in pressure, barometer reading must be corrected for altitude.
Altitude adjustments are made so that a barometer reading taken at one elevation can be compared with a barometer reading taken at another. Station pressures are normally adjusted to a level of mean sea level, and thus called Sea level pressure. The size of the correction depends primarily on how high the station is above sea level.
The diagram drawn during the lecture should help you to understand the difference between station pressure and sea level pressure.
Sea level pressure (in millibars) is what is plotted on surface weather charts. Isobars are lines connecting points of equal pressure. The analysis of the sea level pressure data allows for the pressure pattern to be visualized. Again, the reason we plot out the pressure pattern is that winds are forced by the pressure pattern. These "maps" are called sea level pressure charts.
The current sea level weather chart for the United States can be found at WW2010 at the University of Illinois. (Click on the image labeled "isobars")
Horizonal winds blowing at different altitudes above sea level are also very important in determining what is going on with the weather. Upper air weather charts are drawn to visualize pressure patterns at different altitudes. We have also used the 500 mb upper air chart to get a picture of the large-scale weather pattern around the world.
While surface weather charts depict the pressure pattern at a fixed altitude (sea level), upper air charts depict a pattern showing how the altitude of a fixed pressure surface changes. There are maps showing the pattern at 850 mb, 700 mb, 500 mb, 300 mb, and so on. It is very important to realize that the height patterns shown on upper air maps gives you the same information about horizontal winds that surface maps do, just at different altitudes. Thus, the pattern of height contours indicate how air pressure varies along horizontal surfaces. Air is forced or pushed from higher heights toward lower heights.
Because the driving force for all wind is the horizontal change in pressure, the greater the horizontal change in pressure (or more precisely the pressure gradient), the greater the windspeed. The pressure gradient is the horizontal change in pressure divided by the horizontal change in distance. On a weather chart, the magnitude of the pressure gradient can be seen by examining the spacing between the contour lines of the map (isobars on the surface map or height contours on the upper air map). Where the lines are closest together, the horizontal change in pressure is stronger, and the winds are stronger. In other words, higher windspeeds are found where the contour lines are closest together.
Over short distance scales, air moves in the direction forced by the horizontal pressure changes, i.e., directly from high toward low pressure. This is the case for the examples of opening a jar of food or a can of soda mentioned above. However, for large-scale air motions (like the ones depicted on weather maps), the actual wind direction is turned away from this direction because the Earth is rotating. This phenonemon is called the Coriolis effect or Coriolis force. The details of the Coriolis effect are difficult to understand, so we will not go into them. Basically, it comes about because we are observing the wind from a rotating frame of reference (we are attached to the surface of the Earth and are rotating with it), while the air above is not attached and thus does not have to rotate with it. I would like to point out that the Coriolis effect is only important for motions that traverse long distances or last long enough for the Earth to move significantly in its rotation. Thus, the Coriolis effect is not significant when shooting a basketball and does NOT affect the direction that water swirls down a drain. The Coriolis effect is significant for determining the direction of large scale winds from weather charts, the direction of ocean currents, or the paths of long-range missles and airplanes.
The Coriolis effect is demonstrated in these Annimations for the Coriolis Effect
As the annimations show, the Coriolis force turns the wind to the right in the northern hemisphere and to the left in the southern hemisphere. We will only worry about the northern hemisphere. At all altitude levels above the ground surface (includes all upper air charts, but not surface charts), the wind direction is 90 degrees to the right of the pressure force. Recall the pressure force is directed from high heights (or pressures) toward low heights (or pressures). Thus, on upper air charts the wind moves parallel to the height contours, with lower heights to the left of the wind direction. I showed you how to estimate wind speed and direction on 500 mb charts in previous lectures. Now you should understand more clearly why the wind blows as it does at 500 mb.
Because air moving along the ground surface is slowed by friction with the ground, the wind direction is only turned about 60 degrees to the right of the pressure force. Thus, on surface weather charts the wind direction, rather than being parallel to the isobars, points about 30 degrees toward lower pressure. In class, I will show you how to estimate wind direction and speed on a surface weather chart. An important consequence of the fact that winds do not blow exactly parallel to the isobars on a surface weather map, but slightly toward low pressure, is that surface winds cause air to converge toward surface low pressure (forcing rising motion) and to diverge away from surface high pressure (forcing sinking motion). Where rising motion is forced, clouds and possibly precipitation may be found, while where sinking motion is forced, fair weather is most likely.