A Hard Rain’s Gonna Chill

By Willis Eschenbach – Re-Blogged From http://www.WattsUpWithThat.com

I was re-reading an old post of mine entitled “How Thunderstorms Beat The Heat“. I say “re-reading” because I couldn’t remember writing some parts of it. Yes, it was only from two years ago … but during those two years, I’ve researched and written 83 other scientific posts here at WUWT, plus another 152 political and other posts at my own blog … so things can get lost in the flood.

In any case, I got to thinking about the following graphic from that post. It shows how much evaporative cooling occurs as a result of evaporating the observed amounts of rainfall.

scatterplot oceanic evaporation vs temperature

Figure 1. Scatterplot of evaporative cooling versus sea surface temperature. Evaporative cooling is calculated from TRMM satellite-observed rainfall data. Each dot is a 1° latitude x 1° longitude ocean surface gridcell. The conversion factor from rainfall to evaporation is that 80 watts per square metre (W/m2) of solar radiation applied for the period of one year will evaporate one cubic metre of seawater. Thus, one metre of annual rainfall is equivalent to 80 W/m2 of evaporative cooling.

Now, this graph shows the amount of rainfall evaporative cooling, or alternatively the amount of rainfall. I realized that there is an oddity … wherever ocean temperatures average above about 26°C or so, you will get rain. Might get more rain, might get less, but the area in the lower right has very, very few gridcells—not much ocean surface is both hot and rain-free.

I decided to take a closer look at just the tropical data in Figure1, and to show it in terms of the underlying rainfall data rather than evaporative cooling as in Figure 1 above. Figure 2 below shows that result:

scatterplot tropics rainfall vs sst.png

Figure 2. Scatterplot of tropical rainfall over the ocean versus tropical sea surface temperature (SST). Each dot is a 1° latitude x 1° longitude tropical ocean surface gridcell.

The oddity is the clearly defined minimum rainfall at the lower right in the graph. For example, the graph says if the average SST of a given gridcell is 28°C, that gridcell will get at least a half metre of rain. Might get more, might get a lot more, but other than a few outliers that is the minimum rainfall you’ll get at an average SST of 28°C.

What does this look like in the real world? Well, here’s the map of the average annual rainfall.

Annual Rainfall.png

Figure 3. Rainfall data from the TRMM satellite. The satellite only covers the area from 40°N latitude to 40°S latitude. Units are metres of rainfall per year.

You can see the clear evidence of the Inter-Tropical Conversion Zone (ITCZ) thunderstorms and associated tropical downpours just above the Equator in the Atlantic and the Pacific.

Next, Figure 4 shows a detailed temperature map of the warmest areas of the ocean …

areas w average ocean temps gt 27.png

Figure 4. Temperatures of the areas of the ocean that average 27°C (81°F) or warmer. Red area above Australia is the “Pacific Warm Pool”

When you compare the two figures, you can see the close relationship between average temperature and average rainfall.

To return to the question at hand, to me the oddity shown in Figure 2 is that there is a minimum rainfall for a given temperature. (There is also a maximum rainfall for a given temperature, following a similar curve, but it is not as well defined).

Not only is there a minimum rainfall for a given temperature, but the slope of the minimum rainfall continues to increase as the average temperature increases. Some experimentation yielded the following heuristic hyperbola (blue line) delineating the minimum temperature.

scatterplot tropics rainfall vs sst annotated.png

Figure 5. Same as Figure 2, with the addition of the hyperbola (blue line) which approximates the limit of the minimum values.

This lets us quantify the rate of increase of the minimum rainfall as temperatures warm. It’s shown in the second column in Figure 5 above, headed “Rain Slope”. For example, by the time the sea surface temperature gets up to 27°C, minimum rainfall is increasing at the rate of an additional 146 mm of rainfall per degree C of surface warming.

Finally, let me return to where I started. This was a discussion of evaporative cooling as measured by rainfall. As I mentioned above, a metre of rain per year requires 80 W/m2 over the year to evaporate that amount of precipitation. So we can interconvert between amounts of rain and the equivalent amount of evaporation needed to provide the water for that amount of rain.

As a result of this interconversion ability, Figure 5 also lets us look at how fast evaporative cooling is increasing with each additional degree C of sea surface temperature. This is shown in the third column in Figure 5, headed “Evap Slope”. At 27°C, for example, the cooling is going up by 12 W/m2 per degree C …

One of the results of this relates to the oceanic temperature maximum. It has been noted for some years that in the open ocean, almost nowhere is the average temperature 30°C or greater. You can see this in Figures 4 and 5 above, and I discuss this temperature maximum in a post linked in the endnotes below. Only 1.5% of the individual tropical ocean temperature gridcells shown in Figure 5 are above 29.5°C, and only 0.15% of the gridcells are above 30°C.

To explain the existence of this oceanic temperature maximum, let me suggest that there is plenty of cooling inherent in the minimum rainfall data shown in Figure 5 to put a solid upper limit on ocean temperatures. By the time you get up to 28°C or 29°C, the evaporative cooling is increasing at a remarkable rate. In practice, this means that at ocean temperatures up near 30°C, any extra incoming solar energy merely increases evaporation, with only a minimal increase in the sea surface temperature. This keeps the average sea surface temperature under 30°C everywhere in the open ocean.


Here, we’re in a two-week spell of no rain. It looks as though after a very wet last winter, this winter may be very dry. There’s a technical name for that kind of thing. They call it “weather” …

That’s the downside of that dang weather stuff. I say abolish the environment, it takes up too much room …

The upside of the weather is that it is a lovely calm sunny day today, about 75°F (24°C), with the tiny bit of visible ocean glittering and winking in the far distance. The cat outside the front door gives the peaceful sunshine two thumbs up. Or it would if it had thumbs …

cat in the sun.png

Best of this wonderful life to everyone,


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