All large storms in the Northern Hemisphere rotate the same direction. The same is true in the Southern Hemisphere. Which way is it? And why?

To explain, consider the “force diagram” below.

To the left hand side of the figure is a low pressure system; to the right is a high pressure system. Because of the unequal pressure, the wind starts to blow from the high to the low. The green arrow represents the pressure gradient force (PGF). If we were in an inertial reference frame, that’s all that would happen, and it would be pretty boring. But luckily we aren’t. We rotate once about earth’s axis every 24 hours. Because we are in a rotating accelerating system, we need to introduce the Coriolis Force or Coriolis effect (CF). That’s the yellow-orange arrow above. In the Northern Hemisphere, it always points to the right of the direction of travel. The magnitude of CF depends on latitude, but we’re not really concerned about that. Finally, we add a frictional force due to the terrain of the earth, which acts to turn the wind towards the low pressure system. So you can see that in the Northern Hemisphere, the wind blows counter-clockwise around a low and clockwise around a high. (The above isn’t really a force diagram, it shows the wind direction based on the force diagram.)

But I haven’t answered the question. To fully answer the question, we need to know if large storms are associated with high pressure or low pressure systems. Anyone that has watched a weather forecast should know the answer is low pressure. Why? Look again at the wind diagram above. See that wind due to friction? The winds spiral in towards the center of the low. The opposite is true with a high pressure system, the winds spiral outwards from the high. That’s called convergence (in) and divergence (out). As air converges around the low pressure system, it is forced to rise.

The rising motion cools the air, due to adiabatic cooling. If the rising air does not have liquid water (a cloud), then it follows the dry adiabatic lapse rate (~10 C/km), otherwise it follow the moist lapse rate (~6.5 C/km).

A cooler air parcel has a lower saturation vapor pressure. The relative humidity is defined as the vapor pressure (the actual pressure of water in the air) divided by the saturation vapor pressure (the maximum water there can be in the vapor phase at a given temperature). As the temperature goes down, assuming no loss of water content, the relative humidity goes up. The exact relation between temperature and saturation vapor pressure is given by the Clausius-Clapeyron Relation.

Water in the atmosphere is continuously condensing to a liquid and evaporating into a gas. If the relative humidity is below 100%, the rate of evaporation is greater than the rate of condensation. This means that there will be no net condensation, and cloud droplets won’t form. Once the temperature cools enough that the relative humidity equals 100%, the rate of condensation is greater then the rate of evaporation, so that there is a formation of cloud droplets. These cloud droplets collide and coalesce into larger rain droplets. Once these grow large enough to overcome the updrafts, they fall as rain.

Low pressure systems are associated with rain and storms. Therefore, large storms in the Northern Hemisphere rotate counter-clockwise.

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