Wet bulb temperature

A very important point is made in this Facebook comment:

"It's not like there's a reservoir of "Polar Vortex" heat that is likely to send temperatures in Miami or Dallas to 140 degrees F this summer, but there's a fine line between relative humidity and heat. Oh, I know Americans are exceptional, and that they might well survive a 120 degree heatwave in Tucson. But they will die at that same temperature in a much more humid climate. Americans, we know, haven't been told that fact. Based on the near-vertical delta to the curve of novelty and change, I wouldn't be surprised if parts of America got nailed that way this summer, and that it added at least another zero or two to the death toll in Australia this January. Temperatures wouldn't need to approach 140 degrees to be lethal in some coastal communities. At what point, I wonder, would Americans begin to become cognizant of the scope of the problem. The death toll would probably have to be over a hundred thousand, maybe even as high as a million, for anything to really register on the emotional Richter Scale for most Americans."

Not only that but how will people survive, even in very dry areas without all the benefits of technology, such as airconditioning – when they are exposed to the elements?

This is an interesting article from last year - 

Ocean Heat Dome Steams Coastal China: Shanghai to Near Very Dangerous 35 Degree Celsius Wet Bulb Temperatures This Week

Shanghai, southeast China swelter under Ocean Heat Dome.

(Image source: NASA/Lance-Modis)

Robertscribbler,

6 August, 2013

An ocean heat dome that formed over a broad area of the Pacific Ocean, the South and East China seas, and a large stretch of coastal China during late July continues to create a dangerous combination of record hot temperatures and high humidity.

According to reports from AccuWeather, the sweltering coastal China town of Shanghai hit a new all-time record high temperature of 105.8 degrees Fahrenheit (41 degrees C) on Tuesday. But this marker may just be a milepost to what is predicted to be a 107-108 degree scorcher on Wednesday and Friday. With humidity predicted to be around 50% and barometric pressure readings expected to hit 1005 millibars, these represent extraordinarily dangerous conditions.

The human wet bulb limit: 35 C

Recent climate papers by former NASA scientist James Hansen have issued warnings of the potential for wet bulb temperatures on the surface of the Earth to start to exceed levels that are lethal for humans at 35 degrees C  for longer and longer periods as humans continue to warm the atmosphere. Hansen notes:

One implication is that if we should “succeed” in digging up and burning all fossil fuels, some parts of the planet would become literally uninhabitable, with some time in the year having wet bulb temperature exceeding 35°C. At such temperatures, for reasons of physiology and physics, humans cannot survive, because even under ideal conditions of rest and ventilation, it is physically impossible for the environment to carry away the 100 W of metabolic heat that a human body gene rates when it is at rest.

Different from direct air temperature, wet bulb readings measure what air feels like on the surface of the skin. The measure simulates the cooling effect caused by human sweat evaporating from the skin surface. In very dry, hot conditions, human skin temperature can remain below this lethal level as the rate of evaporation increases due to dryness. Since most of the world’s hottest regions are very dry, humans can withstand air temperatures of 120 degrees (Fahrenheit) and above. Thankfully, it is very rare that extraordinarily hot and humid conditions occur in the same locations. This is generally due to a cooling affect provided by an adjacent ocean mass — as most damp regions are also near or surrounded by cooler ocean air.

The Ocean Heat Dome

Enter the weather conditions forecast for Shanghai tomorrow and Friday…

A massive heat dome high pressure system has settled, not just over land areas of China, but directly over a large region of the Pacific Ocean and adjacent China seas. The result is that sea surface temperatures are now ranging 1-4 degrees Celsius above the already warm 1971-2000 average with a large area showing temperatures above 30 degrees Celsius (86 Fahrenheit). This large region of hot water and corresponding hot ocean air is pumping both heat and humidity into the Shanghai region. Hot ocean air is being pumped over Southeast Asia where it mixes with the already baking land mass air to form a brutal brew of very high heat and high humidity. The clockwise flow of the heat dome then pulls this mixture of record hot and humid air over the highly populated regions of Shanghai.

These ocean-based heat dome conditions are not normal, with typical heat dome conditions usually forming over land. The danger in this particular set of conditions is that very high heat combines with higher than usual humidity to result in much greater heat injury risks for humans.

Sea surface temperatures under Ocean Heat Dome

(Image source: Weather Online)

Forecasts for tomorrow and Friday are showing Shanghai temperatures will probably reach at least 107-108 degrees Fahrenheit (42 C) in an area where relative humidity is forecast to be 50% and where barometric pressures are forecast to remain around 1005 millibars of mercury. This brings us to the extraordinarily dangerous high wet bulb temperature of 33 degrees Celsius. And should thermometers crack 113 degrees (F) under those same temperature and pressure conditions, Earth will have achieved a new and very ominous wet bulb temperature record of 35 degrees Celsius.

Please do your best to stay safe

In such instances, the best defense is to find a cool, shaded location and limit exposure to heat during the hottest times of the day. Drinking cold fluids can also aid in reducing core body temperatures. A common heat mitigation aid is freezing a bottle of water and carrying it in a pocket next to your thigh. The cold bottle will contact the skin near the femoral artery, cooling blood there and transporting this cooler blood throughout the body. If extreme heat is still too much, placing the bottle in direct contact with the large veins in the neck will provide even more efficient cooling. This simple cooling pack also doubles as a means to replenish vital fluids.

Under such conditions, it is also important to be alert for signs of heat exhaustion and heat stroke to include:

• Confusion
• Dark-colored urine (a sign of dehydration)
• Dizziness
• Profuse sweating
• Weakness
• Muscle cramps
• A lack of sweating

China has also activated emergency operations facilities and is providing information and aid in the hardest hit regions.

Unfortunately, record heat is expected to continue over Shanghai through at least next Wednesday with only one day expected to see below 100 degree (F) readings. With so many already dealing with heat stress, our best hopes are that all there will have the means to remain safe.

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