How Thunderstorms Beat The Heat

By Willis Eschenbach – Re-Blogged From

I got to thinking again about the thunderstorms, and how much heat they remove from the surface by means of evaporation. We have good data on this from the Tropical Rainfall Measuring Mission (TRMM) satellites. Here is the distribution and strength of rainfall, and thus evaporation, around the middle of the planet.

trmm annual average evaporative coolingFigure 1. Evaporation in W/m2 as shown by rainfall data from the TRMM. It takes about 80 watt-years of energy to evaporate a cubic metre of water, so a metre of rainfall per year is equivalent to an average surface cooling of 80 watts per square metre. The TRMM satellite only covers from 40° North to 40° South.

I have held for some time that the global surface temperature is restricted to a fairly narrow region (e.g. ± 0.3°C over the 20th century) by the action of emergent phenomena (see references at the end of the post). Chief among these emergent phenomena are tropical thunderstorms. My hypothesis says that when the tropical surface temperature goes over a certain threshold, that thunderstorms emerge to put a firm cap on the temperature by cooling the surface.

Thunderstorms cool the surface in a number of ways, but the main cooling method uses the exact same mechanism used by the refrigerators that keep our food cold. Thunderstorms use a standard evaporation/condensation cycle. In one part of the cycle the working fluid evaporates, cooling the surroundings. In another part of the cycle in another location, the working fluid condenses. For a refrigerator, the working fluid used to be some form of Freon, nowadays it’s some other fluid. For thunderstorms, the working fluid is water. When it evaporates at the surface, it cools the local area, and the heat is moved from the surface to the clouds and on upwards.

Now, for my hypothesis to be correct, the number and intensity of thunderstorms needs to increase quickly as temperatures go above a certain temperature threshold. In addition, the change in the resulting evaporation needs to be quite large in order to successfully control the system.

With that in mind, I made a scatterplot of sea surface temperature versus thunderstorm evaporative cooling. Figure 2 shows that result.

scatterplot oceanic evaporation vs temperatureFigure 2. Scatterplot of 1° x 1° gridcell annual average ocean-only thunderstorm evaporative cooling on the vertical axis, in watts per square metre (W/m2) versus 1° x 1° gridcell annual average sea surface temperature on the horizontal axis. 

As you can see, the thunderstorms are clearly functioning to cap the temperature. When the ocean surface gets hot, thunderstorms form and exert immense cooling power. There are some points worth noting about Figure 2.

First, the red area shows the tropics. This part of the earth is important because it is not only about 40% of the planet’s surface. In addition, just over half of all the energy absorbed by the surface of the earth is absorbed in the tropical regions of the planet. As a result, the regulation of this large amount of incoming energy by albedo control is crucial to the overall energy balance of the planet.

Next, these are annual averages. However, they are also daily averages. But during the day/night cycle in the tropics, the evaporation is by no means constant. At night the evaporation is small, some tens of watts per square metre. During the day, on the other hand, evaporation is quite large, hundreds of watts per square metre, because the strong tropical sunshine evaporates the water directly, plus the thunderstorms are largely a daytime phenomenon. This means that the peak hourly thunderstorm evaporative cooling is on the order of twice the average values shown above, up to about 600 W/m2.

Next, the cooling effect of the thunderstorms is not applied blindly or randomly. The thunderstorms only form as and when the local area is above the temperature threshold. This means that the cooling effect, which can be up to 500-600 w/m2, is always located where it is most needed, on the local hotspots.

Finally, here is the most important consideration. The timing and the amount of thunderstorms are NOT a function of greenhouse gas forcing, or of solar forcing, or of volcanic forcing, or of any other kind of forcing. As Figure 2 shows, they are a function of temperature. As long as the surface is sufficiently hot, thunderstorms will form even at night when the radiative forcing is quite small.

This means that their effect will be to maintain the same temperature, regardless of reasonable-sized fluctuations in the amount of forcing. Clouds don’t know about forcing, they form and disappear based on local conditions.

And this is simply one more piece of observational support for my hypothesis that emergent phenomena regulate the temperature and maintain it within a fairly narrow range.

My best to everyone. Here in California, we have about 150% of the usual snowpack in the Sierra Nevada mountains. When it was drought, it was said to be the result of global warming … and of course, now that there is heavy snow, that is also said to be the result of global warming.

Buckle your seatbelts and keep your hands inside the vehicle, it’s gonna be a long, uphill struggle to get rid of this madness …


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s