Ω, the Greek letter omega, represents the angular rotation of the earth — Page 159, Lesson 134

I want to walk you through the Coriolis force and the geostrophic wind now. Let's start with the Coriolis force itself. The Coriolis force is not a true force in the physical sense — it's actually an explanation of the effect that the rotation of the earth has on a free moving body that is not in contact with the earth. So if you imagine an air parcel moving freely through the atmosphere, not touching the ground, the earth is rotating underneath it, and that creates an apparent deflection. That deflection is what we call the Coriolis force. The Coriolis force is the combination of four factors, and it's expressed by the formula: CF = 2 Ω ρ V sin θ Let me break that down for you. Ω, the Greek letter omega, represents the angular rotation of the earth. ρ, the Greek letter rho, is the density of the air. V is the wind speed. And θ, theta, is the latitude. Now, here's an important point: the Coriolis force is directly proportional to both wind speed and latitude. That means if you increase the wind speed, the Coriolis force increases. If you move to a higher latitude, the Coriolis force also increases. So it's strongest at the poles and weakest at the equator, and stronger in faster winds. Let's move on to the geostrophic wind. The geostrophic wind blows parallel to straight isobars. Isobars are lines of constant pressure on a weather chart. So the geostrophic wind can only blow in a straight line. If the wind were to follow a curved path, it cannot be considered a geostrophic wind, because there will be additional forces involved — namely the centrifugal or centripetal forces. The gradient wind, which we'll discuss later, uses the pressure gradient force, the Coriolis force, and the centrifugal force. That's the model for wind that follows a curved path. So how can we know the direction of the geostrophic wind along the isobar? This is where Buys Ballot's Law comes in. You may remember this from earlier lessons. In the Northern Hemisphere, with your back to the wind, the low pressure is to your left. In the Southern Hemisphere, with your back to the wind, the low pressure is on your right. If we look at the diagram here, using Buys Ballot's Law, we can see a geostrophic wind direction of 180°, which is a southerly wind — blowing from south to north. So to summarise: the Coriolis force is an apparent force due to the earth's rotation, given by CF = 2 Ω ρ V sin θ, and it's proportional to wind speed and latitude. The geostrophic wind is a straight-line wind parallel to straight isobars, and its direction is determined by Buys Ballot's Law.