A fiery April ascendancy of the sun

A noon without shadows, April 26

Now at the end of April last week the sun reached its noon zenith over the city of Bangkok. The morning 6 am temperature never dropped below 30 C -- possibly a record high in an era of ascending thermal discomfort. The temperature on my bedroom air conditioning thermostat, hardly ever different than the 29 C that drives me to flip the switch on at night, actually now tends to read 30 C, a level I never remember seeing before. My neighbor's walls are slightly cool to the touch, about 27, indicating that they, to my benefit, are using their air conditioning, holding room temperatures to about 25 which is the usual here in Thailand (though the government is trying to get people to raise their set temperatures to 26.)

Creeping heat shown by Netatmo module station 17th floor condo.

6 am temperatures on Sukhumvit,
Bangkok over the past 10 days

Based on a 50-year stay in Thailand and study of long-term records I have concluded that the dawn (6 am) temperature is the best indicator of hot season discomfort for the coming day. This just broke through the 30 degree C level this Sunday morning, a rare event, for the first time this year, portending some major hot weather during the peak of the hot season over the coming three weeks. (Maximum sun exposure will be this coming Friday but the greatest discomfort arrives a couple of weeks later, unless we have a strong and early onset of rains). Over the past few decades there has been a very fast rise in hot season dawn temperature, 2 degrees since the late 1970s, reflecting both urban heat island effect from pavements etc but also probably an ominous long term land warming throughout Thailand. The consequences of a continuing trend at this rate over the next 20 years are almost unthinkable —  the doom temperature is said to be about 35.

Meanwhile those of us living in our Bangkok condos and apartments must cope with this difficulty. Even now for some, this means full air conditioning 24/7, typically two to four split unit compressors whirring away, 12 hours a day, as much as 50,000 baht a month. For many systems this produces an unpleasant clammy coolness because the on-again, off-again operation of the typically oversized condo units set to temperatures of 25 or less accumulates interior humidity reaching as much as 80 percent or more. Not comfortable!

A moderate amount of adaptation by owners and smart use and scheduling of the machinery can cut this power use and cost by as much as three quarters or more. My own bill for a typical month is less than 1500 baht and our condo is just as comfortable as our neighbors' who have the big bills of 15k baht or more.

Given the current rate of increased warming in the city, this is an excellent time to experiment a bit, both to economize and to set better standard practices for the coming generation who are expanding the use of air conditioning. I will follow up later this week with another post setting forth specific steps aimed at improved comfort at much reduced cost — with continuing use of air conditioning when it is really needed.

Netatmo map this morning predicts daily discomfort

We approach (in 7 days) the maximum direct solar heating of the year at latitude 13.75 (BKK) with the prospect of encountering this year the "most sweltering year in Bangkok ever", which case I will argue later. But for now, notice today that the dawn temperature, which I claim is the singlemost important indicator of perceived heat for the day, is about 29-30C( 84F) according to the excellent Netatmo weather map, which includes my own dawn reading of 29C (84F). Today is likely to be "the worst heat" for a long time from the human point of view. Although it has been claimed in the scientific literature that the hot season daily minimum temperature is higher in the big city than in rural areas, this fresh data from this morning suggest otherwise.

If you dread tropical discomfort, look only at the 6 am temperature. More than 29C (84F) and you are not going to be happy unless you can find either aircon or more wisely local spots of radiant cooling at say T=25 (79F) or less.

Only when we go far afield and higher elevation do we get the reduced early am temperature, as at Nakhorn Ratchasima, where I have my farm. The data suggest (to my surprise) that heat island effects may not be as important as often believed.

Fan at low setting cuts heat discomfort by four degrees

This graphic is from a study by Joseph Khedari done in Thailand with Thai students. With humidity in Bangkok in the hot season at 70 percent or more, we see that comfortable temperatures range from about 27C with no fan, up to 32 degrees with a fan air velocity of about 1 m/s. This applies to a person sitting at work with no shirt and a fan blowing on the face at about 1 meter distance, which I find refreshing -- more so than air conditioning. An overhead fan works too (I am sitting under one as I type this entry) but only if there is not a pocket of hot air lurking above near the ceiling.

Take a bath in front of the whole neighborhood

That's what my wife does in her Southern Thai village, doesn't think a thing of it, carrying on several conversations at once. The Thai bathe at least twice a day, and in all the years -- more than 50 -- I've been here, I've never smelled a single case of B.O., even on crowded buses in the heat of the afternoon in Bangkok. Remarkable how at the end of a hot day, everyone's shirt seems as freshly pressed as in the morning.

It was a shock to move from Thailand in 1963 to England, where many people bathed once a week. Standing in line next to nice English ladies one day at Sainsbury market, I actually thought I might be downwind from the cheese department.

Bangkok hot season: coolest spot around

Almost everyone is amazed at the coolness of the sky, shown here as 16 degC (61F) on a hot humid Bangkok day (dew point 25C) when the air temperature is in the nineties (34 C). If we go up country a bit we will find it's even hotter but the water vapor will be lower and the corresponding radiant temperature of the day and night sky as low as freezing point,  0 deg C! Our sense of how hot it is where exposed to the sky (but preferably in shade) will be a compromise between radiant temperature of all the objects around us (sky above at 16 degrees and land below the horizon which here is about 34 degrees) and the air temperature. In other words clear sky exposure, out of the direct sun, helps comfort.

But the main benefit of a cool sky is to cool the ground surface at night time. Locally the moisture in the air and cloudiness are the main factors that affect this night ground cooling, with dry clear sky promoting cooling by roughly 100 watts per square meter. By comparison, the effect of raised CO2 levels in the atmosphere is less than 2 watts per square meter, though this effect is near-global and not easily reversed. The great uncertainty about human-caused global warming is the interaction between CO2 and water vapor. If CO2 promotes higher atmospheric humidity then its greenhouse effect is much amplified. This is difficult to model, hence the uncertainty in estimated impacts.