Atmospheric Forces Nick Bassill April 8 th 2009

Why Are Forces Important? When we speak of “forces,” we’re really describing why the air in the atmosphere moves the way it does This describes both how the air moves horizontally (which direction it moves), and why the air moves vertically For example, precipitation occurs because something forces air upward until it condenses

Horizontal Movement When a studying the horizontal movement of air, three primary forces are often discussed: -The Pressure-gradient force -The Coriolis force -The Frictional force

The Pressure-Gradient Force (PGF) Most simply, the word “gradient” just means a “change” in the quantity being observed (in this case, pressure) When the gradient is large, that means that the quantity changes a lot over a small distance Therefore, if we say that there is a “strong pressure gradient” we are saying that pressure changes quickly over a small distance

Strong Pressure Gradient Weak Pressure Gradient

The PGF Continued Remember that pressure effectively measures how much air (or the force of that air) is above you So a pressure difference in the horizontal means that the force of the air above the two locations is different This is not a balanced state Therefore, the atmosphere will attempt to correct for this state by allowing air to flow from high pressure to low pressure A larger pressure gradient will result in a stronger wind

From:

The Coriolis Force The Coriolis force is a “fake” force which is a result of the fact that the Earth rotates The Coriolis force causes moving objects to turn to their right in the northern hemisphere and to the left in the southern hemisphere However, if an object is at rest, the Coriolis force does not apply (The Coriolis force is due to the fact that even though the angular velocity of the Earth is the same at all points, the tangential velocity is not. The Coriolis force can be thought as preserving an objects tangential velocity)

- When the PGF and Coriolis force are balanced, the atmosphere is said to be in “geostrophic balance” - The resultant wind is called the “geostrophic wind”

The Geostrophic Wind The geostrophic wind always blows parallel to the isobars (lines of constant pressure) A stronger PGF (when the isobars are closer) results in a stronger geostrophic wind

What direction would you expect the geostrophic wind to blow in?

Almost … but why the difference?

The Friction Force Close to the surface, geostrophic balance is not a very good approximation This is because friction is quite strong near the Earth’s surface The Earth’s surface is very rough (buildings, trees, mountains, etc.), which induces friction Therefore, as you get farther away from the Earth’s surface, the friction force decreases This means that geostrophic balance becomes more realistic as you move away from the Earth’s surface Friction always acts to oppose the wind (with a strength proportional to the strength of the wind)

Friction Continued However, if friction acts to slow the wind, then the Coriolis Force will weaken At the same time, the PGF remains the same strength Therefore, geostrophic balance is no longer in effect This causes the wind to blow slightly across isobars, towards low pressure

The New Force Balance From: rgetics/Spiral_Winds/Spiral_Winds.html

Constant Pressure vs. Constant Height Maps So far we’ve looked at Sea Level Pressure maps (so pressure varies while the height is constant everywhere - 0 meters) However, meteorologists often look at constant pressure maps (so the height changes, rather than the pressure) As we’ll learn more about later, you can think of “high” heights as being analogous to high pressures, and “low” heights as being analogous to low pressures Similarly, the geostrophic wind will blow parallel to lines of constant height, with lower heights to the left of the direction of the wind

Heights and winds at 200 mb Notice how much closer the winds are to geostrophic balance at this level, compared with the surface