Why would a mercury thermometer give the wrong temperature?

Question

I bought it a few days ago. It's a cheap indoor/outdoor thermometer that you hang on the outside of the window. This morning it said 99 degrees and now it says 90. Its definitely only 80 or less and this morning it was probably in the low 70s. What would cause this? The only thing I can think of is that this morning the sun was shining on it, but I didn't think that actually increased the temperature.

Answer 1

Hanging a glass thermometer in direct sunlight will cause errors in the reading of temperature.  There are two main reasons:

1.  the curved glass acts as a lens and creates a concentration of heat energy inside the tube, where the mercury (or other fluid) is located;

2.  the heating of the glass itself creates a change in the shape and magnitude of the hollow inside the glass tube, so that the mercury will adopt an unpredicatble position, unrelated to the actual outside temperature.

Hang the thermometer in the shade and you'll get the termperature in the shade.  That's a better measurement of the environmental condition anyway.

Hope this is useful.

Answer 2

The sun shining on it includes the same UV that burns your skin. It is a faster moving wavelength and can cause excitation of materials generating some extreme heat.

I did several thousand hours worth of work on solar interaction with buildings and development. Solar radiation and the same UV that burns us is causing buildings to burn. The buildings aren't designed, insulated or insured for the temperatures and is contributing to low air pressure. Go to the following link and see where atmospheric heat is coming from. http://www.thermoguy.com/globalwarming-h...

For fun, leave the thermometer in the sun and it will heat up.

Answer 3

It is because of the sun.  If the sun is shining directly on the thermometer, it raises the temperature on the thermometer about ten to fifteen degrees

Answer 4

To begin with, many indoor/outdoor thermometers are not accurate. They are merely only for giving approximations.

But some factors for why it would give inaccurate readings are: it may have fallen and cracked, you placed it in an area where heat gathers/builds (kind of like a closed car), or you touch it (hold it) while reading it...there are more possibilities.