The dominant forces of pressure gradient force and Coriolis force combine to form geostrophic wind flows. In this lesson, we’ll examine why many of the winds in the atmosphere actually blow parallel to the prevailing isobars.
Introduction to Geostrophic Wind
There are a number of forces that can either change the force or direction of wind. Two of the biggest forces of wind vectors are the pressure gradient force and the Coriolis force.
When these two major forces are combined and are equal to each other we get a type of wind called a geostrophic wind. In this lesson, we’ll study how pressure gradient force and Coriolis force are created and the formation of geostrophic winds.
Pressure Gradient Force
One of the largest such drivers is the pressure gradient force.
The pressure gradient force is the force that drives air from high to low pressure. Systems in nature are always trying to stay at the lowest energy state possible. High pressure systems are high energy states. If there is a nearby lower pressure area, air will freely move from the high pressure to low pressure area in an attempt to equalize this energy gradient.You may have been introduced to the idea of isobars, which are imaginary lines of equal pressure. Because the air is wanting to move as quickly and efficiently from high to low pressure it will move the shortest distance between these areas, which is always perpendicular to the isobars.
This is described in the following equation for the horizontal portion of the pressure gradient force:P= inverse -1/p * pThe pressure gradient force, here shown as P, is equal to the inverse of the density times the pressure gradient. The inverse sign is an upside down triangle. The negative (-) at the beginning of the equation designates that we move from high to low across the pressure gradient.Now, if this was the only force acting on the atmosphere, we could easily predict wind direction. Wind should move from high to low pressure perpendicular to the isobars; however, we observe that higher in the atmosphere, wind is actually moving parallel to the isobars. How can that happen?
We must look at some other forces that act on wind.
Another dominant force on wind direction is the Coriolis effect. The Coriolis effect is the diversion of the path of air due to the rotation of the Earth. The Earth is spinning from west to east. The Earth is a sphere, and therefore all points are traveling with the same angular velocity. However, because a point at the equator must travel a much further distance than a point near the pole for full rotation, areas near the pole are actually traveling at a much higher linear velocity.
When an object moves either closer or further from the equator its original momentum is preserved, giving the path a diversion off its original course. Paths in the Northern hemisphere are drug to the right, and paths in the Southern hemisphere are drug to the left. So, if we have wind that is originally blowing according to the pressure gradient force, this wind will be deflected by the Coriolis force. Now, the Coriolis force is not present at the equator, but it increases in intensity the further you approach the poles.
The increase in force is due to the greater divergence in linear speed observed at the equator.Coriolis force is described by this equation:ƒc = 2 * omega * sin * PhiIn this equation the Coriolis for is ƒc, omega is the rotational rate in radians per second, and phi is the latitude. Thus, the higher the latitude, the higher the Coriolis force. Also, the Coriolis force only acts on air that is already set into motion. The Coriolis force will not set wind into motion, but will only deflect the direction of wind that is already moving.
Thus it follows that the faster that air is moving, the stronger it is affected by the Coriolis force.
Combining these two major forces in the atmosphere gives us the geostrophic winds that are the topic of this lesson. Geostrophic winds are winds that are moving parallel to the isobars under the effect of the pressure gradient force and the Coriolis effect. As winds begin to move with the pressure gradient force from areas of high pressure to low, the Coriolis force will cause the winds to deflect – the higher the wind speed, the greater the deflection. Eventually these forces will balance each other out, which produces winds that are parallel to the isobars.This is only possible in areas of low friction – over a kilometer above the Earth. Frictional forces are a drag force resulting from interaction with the Earth’s topography or ocean surface.
When a geostrophic wind encounters a frictional force, this will cause it to move from its original direction of parallel to the isobars to oblique to the isobars.
In review, the pressure gradient force causes air to move from high pressure to low pressure areas across the shortest distance possible, which is perpendicular to the isobars. Isobars are lines of equal pressure in the atmosphere. The other major driver of wind vectors is the Coriolis effect, which causes moving wind to deflect from the original path due to the rotation of the Earth. This force deflects wind to the right in the Northern hemisphere and to the left in the Southern hemisphere, and areas high in the atmosphere above the surface of the Earth. These two forces will balance each other to produce geostrophic wind flows. Geostrophic winds are parallel to the isobars because they are winds resulting from the Coriolis effect equaling the pressure gradient force.