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In regard to outside temperature, I have read a couple articles describing how temperature is typically taken outside in a ventilated box hovering over the grass ground for true temperature. Is there some base temperature difference between that measurement and direct sunlight? I can imagine there is a variation, but if it is 100 degrees outside, no clouds, mid day and no wind at all, what would the difference likely be? Is there a ratio or general variation?


The temperature of a thermometer or temperature sensor can properly reflect the air temperature only when the sensor is in the shade and is ventilated. The reason is that sunlight shining on a temperature sensor will warm it up to well above the actual air temperature.

So, there is usually a large difference between the ventilated temperature measurement and the temperature of an object exposed to sunlight. Depending on the characteristics of the object that is heated by the Sun, the temperature difference between a ventilated temperature sensor at 100 degrees Fahrenheit and an object in the Sun can easily be 40 degrees Fahrenheit or more. This is easily seen if you place a piece of metal in bright sun for a short In fact it may be too hot to touch without burning your skin. This is critically important for people who are exposed to sunlight, as the Sun can warm your skin to well above your normal skin temperature, causing drying of the skin and eventually a sunburn, as well as the possibility of dehydration and sunstroke.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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