The atmosphere and the Weather

Air in Motion

Surface Charts

In order to understand how pressure influences winds, we will first look at charts that show variations in pressure.

The simple task of reading the height of the mercury column to obtain the air pressure is actually not that simple. First, mercury is sensitive to changes temperature: it will expand when heated and contract when cooled. Because the earth is not a perfect sphere, small variations in gravity exist from place to place, and these should be taken into account. Finally, each barometer has a "build-in" error, called the instrument error.

Therefore, the station pressure is defined as the barometer reading at a given meteorological station after being corrected for temperature, gravity, and instrument error.

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. 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.

Isobars are lines connecting points of equal pressure. The analysis of the sea level pressure data allows for the pressure pattern to be visualized. These "maps" are called sea level pressure charts.

Current weather examples of these charts can be found at WW2010 at the University of Illinois and at the UofA Atmospheric Sciences Weather Page.

Additional resources related to weather charts are very nicely presented at WW2010 at the University of Illinois

Why the Wind Blows

Newton's Laws of Motion

Issac Newton (1642-1727) formulated several fundamental laws of motion:

First Law: An object at rest will remain at rest and an object in motion will remain in motion (and travel at a constant velocity along a straight line) as long as no force is exerted on the object.

Second Law: The force exerted on an object equals its mass times the acceleration produced.

A Force is in its simplest form a push or a pull. Acceleration is the speeding up, the slowing down, or the changing in direction of an object. (Acceleration is the change of velocity over a period of time).

Newton's second law always refers to the net, or total, force that results. An object will always accelerate in the direction of the total force acting on it. Therefore, to determine in which direction the wind will blow, we must determine all the forces that affect the horizontal movement of the air. These forces include:

1. pressure gradient force
2. Coriolis force
3. centripetal force
4. friction

We will first focus on the forces that influence the winds away from the surface. Then we will see what forces affect the wind near the ground.

Forces that Influence the Winds

Pressure Gradient Force.

Horizontal differences in pressure cause the air to move, hence, the wind to blow. The pressure gradient force (PGF) is directed from higher to lower pressure. The magnitude of the force is directly proportional to the pressure gradient.

If the PGF were the only force acting upon air, we would always have wind blowing directly from higher to lower pressure.

However, the moment the air starts to move, it is deflected in its path by the Coriolis force.

Winds flow clockwise around a high pressure center in the Northern Hemisphere, while in the Southern Hemisphere, winds flow counterclockwise around a high.

A low pressure center is indicated on a weather map by a red "L" and winds flow counterclockwise around a low in the northern hemisphere. The opposite is true in the southern hemisphere, where winds flow clockwise around an area of low pressure.

The Coriolis Force describes an apparent force that is due to the rotation of the Earth.

The Coriolis force causes the wind to deflect to the right of its path in the Northern Hemisphere and to the left in the Southern Hemisphere.

This process is further demonstrated by the movie on the left.

Real Life Example: This movie of a ball rolling across the surface of a rotating merry-go-round is a helpful demonstration of the Coriolis force.

While the ball moves in a straight line path, the merry-go-round rotates beneath it.

By the time the ball reaches the opposite side, the catcher has moved.

To anyone on the merry-go-round it seems as if there is some force causing the ball to move to the right.

An air parcel initially at rest will move from high pressure to low pressure because of the pressure gradient force (PGF). However, as that air parcel begins to move, it is deflected by the Coriolis force to the right in the northern hemisphere (to the left on the southern hemisphere). As the wind gains speed, the deflection increases until the Coriolis force equals the pressure gradient force. At this point, the wind will be blowing parallel to the isobars.

When the isobars are curved, there is a third force -- the centripetal force. This apparent force, pushes objects away from the center of a circle. The centrifugal force alters the original two-force balance.

In this case, the centrifugal force acts in the same direction as the Coriolis force. As the parcel moves north, it moves slightly away from the center -- decreases the centrifugal force. The pressure gradient force becomes slightly more dominant and the parcel moves back to the original radius. This allows the gradient wind to blow parallel to the isobars.

The surface of the Earth exerts a frictional drag on the air blowing just above it. This friction can act to change the wind's direction and slow it down -- keeping it from blowing as fast as the wind aloft. Actually, the difference in terrain conditions directly affects how much friction is exerted. For example, a calm ocean surface is pretty smooth, so the wind blowing over it does not move up, down, and around any features. By contrast, hills and forests force the wind to slow down and/or change direction much more.