What's the temperature of rain?

1009 reads

By Steven Schmidt on July 13, 2012, 8:32am

Use your ← → (arrow) keys to browse more stories.

I’m sure that this won’t be the last time you hear me say this, but if you ask me, rain is one of the most intriguing and exciting topics in meteorology. The multidisciplinary aspect it carries, with its ability to pull together the various sciences such as biology or chemistry make its study extremely flavorful for anyone with an appetite to learn.

Thinking back to when I was an undergraduate at WCSU, I recall the fact which first got me thinking critically about rain. It was the fact that a rain drops temperature as it falls is that of the wet-bulb temperature.

Gilbert D. Kinzer and Ross Gunn of the U.S. Weather Bureau published an article in the Journal of Meteorology back in 1951 which discussed in detail the evaporation, temperature and thermal relaxation-time of free-falling water drops, and it was in this article that I found the insightful statement below…

“A freely falling drop evaporates and cools, so that it is invariably at a lower temperature than its environment. In fact, such a drop is an ideal, ventilated wet-bulb thermometer, having negligible corrections, and it may be used to determine the relative humidity of the environmental air.”

From a modeling stand point, this statement is beautiful. With a measurement of the wet-bulb temperature or of the dewpoint temperature with a slight mathematical conversion to the wet-bulb, rain’s potential thermodynamic impacts can be modeled in various scenarios.

Here’s a simplified example.

Let’s assume we have two five gallon buckets, one within the other making somewhat of an insulated cylinder. The bucket is left outside overnight with a gallon (3.785 x 10^-3 m³) of water in it at thermal equilibrium with its environment. The temperature of the water is T_B.

Now, a thunderstorm comes along and drops a half inch of rain within a 30 minute window. We can now determine what’s known as the rain’s heat input, H_r (the heat transferred as a result of the rain’s input) using equation 1 below where M_R is the mass added by the rain, T_R is the temperature of the rain drops (the wet-bulb temperature), and C is the specific heat of water.

H_R = C x M_R x (T_R – T_B) (1)

Additionally, we can determine the final temperature, T_F, of the water in the bucket after the rain event using equation 2, and then, finally, the temperature change of the water in the bucket (∆T ) using equation 3.

T_F = M_B x T_B + M_R x (T_R – T_B) (2)

∆T = T_F – T_B (3)

It’s funny really, if you understand what I’ve written, you will probably stop and stare at a puddle of water a little differently next time it rains. I know I did when this all ‘clicked’ for me nearly three years ago.

References:

Flament, P., M. Sawyer, 1995: Observations of the effect of rain temperature on the surface heat flux in the intertropical convergence zone. J. Phys. Oceanogr., 25, 413–419.

Kinzer, Gilbert D., Ross Gunn, 1951: The evaporation, temperature and thermal relaxation-time of freely falling waterdrops. J. Meteor., 8, 71–83.

Acknowledgements:

Thanks to WXedge.com contributor Stephen Barabas for the cover photo.

Use your ← → (arrow) keys to browse more stories.