If you’ve ever sat in a car on a sunny but cold day, or been outdoors when the air temperature is low but in sunshine, one may feel quite warm and even hot.
So using air temperature alone is far from a true measure of comfort.
The other day, after a customer demo, we left one of our wireless temperature loggers on our smoko table, where the sun shone brightly. Now it was cold; crazy cold, but the logger read 38°C. How come? The answer is of course, radiated heat from the sun.
Humidity, sunshine, wind and air temperature all combine to create feelings of comfort or discomfort
This may appear obvious, but sometimes things that are obvious, are worthy of further contemplation.
WGBT sensors (Wet Globe Bulb Temperature) take all of the above parameters into consideration.
WGBT was developed by the military as a way to determine the effect of weather on the ability of soldiers to perform because as we’ve already discussed, both sunshine and humidity* greatly affect how one feels regardless of the air temperature and air temperature was not sufficient measurement for people.
Wind-speed can make one seem much colder and hence the Metservice forecasts display not only air temperature but the ‘feels like’ numbers
At Homershams, we have a black radiant heater above our packing bench and it’s very effective, but only when one stands directly under it; it has no effect of air temperature. Similarly, if one stands outside a pub on a cool evening, we are warmed by the glowing heaters, but there’s little or no effect on the air temperature. Why is this?
Note in the setup in the image above, we see 4 parameters being
2. Relative Humidity*
3. Air temperature
4. Wind Speed and direction
*See our article on humidity and psychometric tables here https://homershams.co.nz/Articles/Humidity.html
The combination of these 4 measurements will determine how a
At high school, we all learned about different forms of heat transfer, but when I come across examples in real life, they are sometimes resonant and it’s occasionally worthy of a reminder.
Radiation is the heat emitted by our black light heater, the outdoor pub heater and by sunshine. It’s an electromagnetic wave (mostly infrared) and travels in straight lines, without heating the air directly.
Convection is the movement of heat through a medium such as air or water,
And Conduction is the movement of heat along a solid object such as the metal rod shown.
Many times, air temperature will tell us what we need to know, but when the other forms of heat energy are involved we may need to consider these too.
Especially outdoors, but anywhere direct sunshine exists, one needs to consider solar radiation effects. A solar shield as shown left, is common for outdoor thermometers.
It’s also important to consider Solar radiation when doing things like Infrared evaluation of power lines, where the vast energy of the sun overwhelms any local heating effects.