Friday, April 29, 2016

Dangerous Wet Bulb Temperatures in India

It's not even May yet, but at least one city in India has reached dangerous wet bulb temperatures.

A WBT of 30-35°C is dangerous. If you're engaged in physical activity, 27°C or above can be dangerous. 33°C is dangerous even for if you're naked and lying in the shade. Wikipedia says 35°C. At that temperature you literally cannot cool off (without A/C) -- perspiration can't evaporate fast enough; you start to be cooked. Not good if you're poor or don't have electricity (300 M people do not), or you can't afford an A/C.
A sustained wet-bulb temperature exceeding 35 °C (95 °F) is likely to be fatal even to fit and healthy people, unclothed in the shade next to a fan; at this temperature our bodies switch from shedding heat to the environment, to gaining heat from it.[7] Thus 35 °C is the threshold beyond which the body is no longer able to adequately cool itself.
Given the temperature, humidity and surface pressure, this NOAA site returns the WBT. I wrote about WBTs for last year's heat wave in the Persian Gulf here.

I used recent data from Wunderground for Titlagarh, India, in southeastern India, which I read was having a heat wave. (People cannot go out in the street during the day; social events have to take place in the evenings. 99 deaths so far.) On April 24th, their high temperature was 119°F (48.3°C), and their wet bulb temperature reached about 32.4°C. On April 26th it was 113 F° (45.0°C) but more humid, with a WBT of 32.8°C.

And it's only April.

N.b. You really need to know temperature, relative humidity and surface pressure as functions of time; the weather site only gives daily highs, daily lows, and daily aveages. In the above I used the high relative humidities for the day, and the high surface pressure.

With the high temperaure and average RH and average pressure, the numbers are 29,2°C and 31.4°C respectively -- still dangerous.


steve said...

Note that diurnal max T usually corresponds with min RH, because dew point doesn't usually change much through the day. So yes, dangerous, but the wet bulb temperatures were very probably on the lower end of your ranges.

Jay Alt said...

That is bad. Though this region expects their highest temps in April & May.