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Cold spots and hot spots
Cold spots and hot spots

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Analysing cold spots
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Evidence is everything!

Warm spots ....

In the picture above the arrows show the flow of radiative heat, which, unlike convection, travels in straight lines. At the top of the picture there is a hot surface radiating heat and causing a 'warm spot'; anyone standing there would feel heat coming from the direction of the hot surface.

Below there is a cold surface (such as an uncurtained window on a cold night). Because it is cold, any nearby warm body (like a human) will radiate extra heat towards it. The effect is a feeling of cold in that direction. It operates like a radiative heater in reverse. The effect could be interpreted, by someone standing in that area, as a 'cold spot' since there would be no obvious cause for it.

It is not a particularly obvious concept. The best way to think of it might be like a heater working in reverse. It is drawing heat away from the room. Luckily, it doesn't operate over a huge distance.

   

Radiative heat loss

Though it is less obvious than convection, cold spots can also be created by radiative heat loss.

When you stand directly in front of an electric fire or radiator, you will feel heat. Less well known is that people can LOSE heat in the same way. If you stand directly in front of a cold object, such as an uncurtained window on a cold night, you will feel colder. Your body is radiating heat in all directions. However, it will radiate more, to maintain its temperature, in the direction of cold objects. This additional loss of heat will be felt as cooling. Generally, you need to be quite close to a cool object to get the radiative loss. Like a heater, if there is anything between you and cool object, you may not feel the effect. Like convection, a conventional thermometer will not register this apparent temperature drop.

Measuring a cold spot

The most obvious cause of a cold spot is a drop in air temperature. So the first thing you need on a vigil is a thermometer. The ideal set up would be an array of small thermal sensors placed in a grid to precisely locate any cold spot.

To spot convection you'll need an anemometer to measure wind speed. Some instruments can measure windchill directly.

To measure the temperature of cold surfaces you can use a 'point and shoot' infra-red thermometer. However, be aware that there are various issues with these instruments that can cause inaccurate readings. For instance, different surfaces can appear to have different temperatures purely due to the colour they are painted. This is called emissivity. Also, most IR thermometers indicate the surface they are measuring with a laser dot. However, the area measured is wider than this and varies according to the distance of the surface.

For more information on the use of IR (laser) thermometers, see here.

You might also try using thermal imaging to measure cold surfaces.

© Maurice Townsend 2007

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