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.

Complex thermal comfort data needs colorful solutions!

Operating a bedroom air conditioner to get the best comfort at the least cost is a lot more complicated and non-intuitive than most of us (including me) have imagined. Netatmo plus the visual SQL display tools from Tableau are the only way to really understand what is going on here in this "simple" display of thermal comfort data in a single night in steamy Bangkok March 2017 which we will examine in more detail later.

We thought that keeping the bedroom door closed to "keep the heat out" and "not waste electricity" was the best policy but the data here suggest otherwise!

Truly, the delight is in the detail!

Bangkok destined to be unliveable? Ominous trend from Netatmo

I am neither a "warmist" or a "denier" as they call those who are inclined to a group ideological approach to the subject of global warming. And I know from paleoclimatic studies that global changes occur naturally and plausibly via contemporary and excess human energy use. But I find reason for worry with the data shown here. Notice the daily high temperatures for the hot season (April and May) do not seem to have increased much if at all over the past 60 years of record from the Don Muang airport. But I have never thought that daily highs are of great significance. after all, Tucson gets a lot hotter in the day.

But look at daily Bangkok low temperature, usually at or about dawn, over the span of the last sixty years. In contrast to maximum daily hot season temperatures which everyone talks about, daily minima are not much recognized as being of importance, though I claim that these daily low numbers represent, at least in the tropics, the best index of thermal stress on human and perhaps other life. The coincidence of high dew point temp, surface sea temp in the Gulf of Thailand, and thermal comfort level through the day really determine the level of discomfort and danger, and these indices follow closely the daily Tmin during the hot season (April and May) in Bangkok. We see here a marked trend for hot season Tmin (only April and May data shown here) with values rarely exceeding 26-27 back in the fifties, but now commonly at 30 or more. Big difference!

As I write, Bangkok residents, Thai and non-Thai, have found conditions barely tolerable in the past few weeks of early May, 2016, many people remarking that it has been "the worst ever". Those of us who have lived through it would not disagree with my claim, backed by theory and experience,  that a daily morning min of about 35 degrees would be intolerable to human life without some kind of air conditioning. So our thinning margin of safety might be said to be half of what it was when I first came to Thailand in the 1960s.

But we should not rush to claim that this is due mostly to greenhouse gasses. Temperatures here and many places are airport temps, and airports have progressively become embedded in hundreds of square miles of mostly concrete, creating the well-understood effect of urban heating. The other morning, with 7 am temps of the pavement of 34 degrees in my neighborhood on Thonglor, you might say we were already having a local doomsday, though the minimum official airport temp for that day did not exceed 31.

How to distinguish between heating from urbanization and from global warming? I think I see away to do that and will make a project of it.

Temporary heat blips from direct sun

You probably don't want to place your Netatmo outdoor module where it will get direct sun. This yellow anomaly below is caused by direct morning sun for and hour or two on the outdoor module, and probably to a lesser degree from heating of the dark brown soil on which the module is set in this case. 



Poor location! Better put the module out of the sun and on a less heat absorbing surface.

Radiant heat: invisible but deadly

On a warm afternoon with full sun a darkish thin surface (such as a metal roof or a tile roof) heats up to 50 degrees C or more, with the heat being almost immediately transferred undiminished to the underside, as you can see from the temp reading of the underside of this rubberized canvas tent which is supposed to provide cooling shade, but really provides heating shade, which will radiate on the order of (52-32)*5=100 watts per square meter to your head, plus perhaps as much again convected to the air below. Not as bad as the 700 watt sun, you might say, but not comfortable either – and suffocating if the underside of the roof is not radically ventilated. So next time you are sitting under one of these tents at a Thai wedding or funeral, be warned -- it's the heat, not your emotions, that is causing you to feel like you are going to faint.


Here is an excerpt from the manual of the Australian company Bluescope Steel who make (and promote) steel roofing in Thailand. This indicates a reduction in heat transmission through thin roofs (steel, cement tile, etc) if they have foil insulation, properly installed (air space above and below foil, and foil with shiny side down.) For a dark roof, the thermal transmission is said to drop from 165 to 22 watts, almost 90 percent. But I can say from my own experience that there are many qualifications to this supposed effectiveness of thin foil, more specifically that bubble-backed foil that comes in rolls, to be discussed later with a real example.