Visualizing Thunderstorm Heat

When water vapor in the atmosphere condenses into liquid water we first get clouds, then rainfall.  But water vapor only condenses when heat is taken away from it.  So I got to wondering how much heat movement is involved in a rainstorm.

We occasionally have storms that drop rain at a rate of an inch per hour, and while that is unusually heavy for the area where I live, in wetter parts of the world they see that kind of rainfall, and heavier, on a regular basis. But since that is a rainfall rate that I have experienced and can visualize, and also because it is a nice round number, I decided to use that as the center point for this discussion of the heat release.One inch per hour rainfall means that a cubic inch of water would be falling in an hour for every square inch of surface area.  Employing a little arithmetic and using the density of water we can show that equates to about 0.016 kg/hr per square inch.  The latent heat of vaporization for water (at temperatures that we are interested in) is around 2450 kJ/kg.  This means that over an area that is receiving a rainfall of one inch per hour, there is a heat release of about 11 W/in².

 Now, heat fluxes are a little hard to visualize, sometimes, so I dug around a little bit and found various estimates for the heat output of a candle ranging from around 30-50 W for a “tea candle” to 40-80 W for a “standard candle”.  So, in a rough way, I think that we’d be safe to assume a “smallish” candle might provide heat in the neighborhood of 44 W. Another point of reference with which many people are familiar is the incandescent 100 W light bulb.

So imagine a large field laid out with a 2 inch by 2 inch grid, with one of our “smallish” candles at every grid point.  Or, alternatively, you can imagine a 3 inch by 3 inch grid with a 100 W light bulb at each grid point.  In either case, the heat output and the ensuing updraft would be phenomenal, and you wouldn’t want to float around above that field for long.

Now, the heat released in a storm isn’t quite as dramatic as that visualization because the heat output would be spread vertically over the thickness of the clouds—probably many hundreds of meters.  Nevertheless, it does inspire appreciation for the heat flows and consequent turbulent air movement associated with a rainstorm