What factors determine how hurricanes form? Meteorologists have long known that two factors play crucial roles. First, the temperature of the sea determines the updraft of air that leads to a storm. Second, the latitude governs the strength of the Coriolis Force which triggers the initial vorticity of the storm (which is why hurricanes do not form at the equator and rotate in opposite senses in each hemisphere).

Today, Robert Ehrlich, a physicist at George Mason University in Washington DC, shows how these two variables alone can account for the probability density of a hurricane or tropical storm forming. No other factors need be taken into account.

Ehrlich’s approach is a to create an elegant mathematical model of the system that relies on only two variables: the temperature of the sea above a threshold of 25.5 degrees C and the latitude of the ocean at that point.

He then fits the function to the data from real hurricanes ie sea surface temperatures and latitude data from satellite images from 1960 until 2007. This determines that the power law has an exponent of 3.5 for most parts of the globe.

Fitting the data to a curve by no means proves that a model is correct but Ehrlich is able to make some interesting observations using it. One problem that climatologists have puzzled over in recent years is that the number of hurricanes have increased in the north Atlantic but not in the Pacific, despite similar temperature increases. Many say that this is proof that other factors must influence hurricane formation.

However, there’s an important difference between these regions: in the Atlantic, the water tends to be cooler to start with and the hurricanes tend to form at a slightly higher latitude.

When you take this into account, the difference in the number of hurricanes is exactly what Ehrlich’s model predicts. He says the specific form of his mathematical model “yields larger percentage increases when a fixed increase in sea surface temperature occurs at higher latitudes and lower temperatures”.

That could help to solve an important climate change puzzle but before greater reliance can be placed on Ehrlich’s, it needs to show its colours by accurately forecasting the numbers of hurricanes in the next few years. Its predictions do not make for pleasant reading.

The exponent of 3.5 in Ehrlich’s power law means that numbers of hurricanes should increase sharply as the world warms and much more dramatically than climatologists have been expecting. His prediction is that a 2 degree C increase in average temperature will lead to an 11-fold increase in the number of hurricanes.

And the increase in numbers of hurricanes is only part of the story, he says. “An eleven-fold increase in hurricanes at a particular location would just be one part of the story, which would include (1) a potentially larger increase in the total number of hurricanes given the increase in the size of the basin as temperatures rise, (2) an increase in the destructive potential of each hurricane, and (3) an increase in the height of the storm surge due to rising sea levels that would invariably occur in a warmer world.”

Frightening stuff.

Ref: arxiv.org/abs/1002.3291: A Universal Hurricane Frequency Function

It turns out that the fucnt exactly matches the distribution his model predicts