Where to Put Your Thermometers

The thermometers need to take readings in the shade for good results. Other weather watchers will be taking their readings in the shade and you want your readings to be able to be compared to theirs. The bulbs of the thermometers should be approximately 1.2m (120cm) above the ground.
The best place to put your thermometers is in a Stevenson screen. If you have not got a Stevenson screen, and really cannot make one using these instructions, you need to make sure that you put your thermometers on a north facing wall or post. This will shade them as much as possible.
The photograph on the right shows a weather station made from one plank of wood. It has holes to allow the air to circulate. The door opens to the north so that even when it is open the sun will not shine directly into it.

Units of Measurement

All temperatures are recorded in degrees Celsius (ºC). This is the standard unit in Europe and most of the world. When reading the scale check first to see what each mark represents. On this scale, from the wet thermometer, each line represents one degree Celsius. You can then count on or back from the nearest number.
This thermometer is reading just above 17ºC.

Recording tenths of a degree

Digital thermometers give readings to the nearest tenth of a degree. Traditional thermometers sometimes have half degrees marked. However as a rule you will have to get use to estimating how many tenths of a degree is shown on a thermometer. Helpful pointers to think about are in the calibrating document. With practice you will become more confident in recording tenths of a degree.

What is the difference between the readings of these two thermometers?

Data Checking

All temperature reading numbers should be reasonable. Some high numbers should be regarded as suspicious in the winter time. Would you expect 15ºC in winter? Very low numbers should also be looked at closely in summer. Would you expect 5ºC in summer? You should ask yourself if your readings are sensible.
When the temperature drops below 0ºC water will freeze. This causes various types of frost. The frost on the rain gauge in the picture on the right was very pretty.

Parallax

When reading thermometers you must have your eye level with the top of the liquid in the thermometer. If you look down on it, or look up to it, you will get an incorrect reading. Click on the picture on the right to see how important this is. The central thermometer is taken with the camera lens level with the top of the indicator liquid.

Calibrating thermometers

When you buy some expensive thermometers they come with a certificate which tells you exactly how close to the actual temperature they measure. The certificate might say that the readings are perhaps 0.1ºC below the actual temperature. Then when you take readings with this thermometer you would have to add a tenth of a degree onto your readings. Finding out how accurate the thermometer is is called calibrating it. To do this the thermometers are arranged in a location where the temperature is even, and they are left for a while to settle before being read. Find out more in the calibrating document.

A selection of thermometers have been set up in the same location so that they can be calibrated.

Maximum and minimum temperature

Maximum temperature is the warmest (largest number) since the last reading. The Minimum temperature is the coldest (smallest number) since the last reading. The maximum temperature should never be smaller than the minimum temperature. A good way to check this is to graph the maximum temperature and the minimum temperature on the same graph. If the maximum and minimum temperatures cross something is wrong. You could use the Intermediate Spreadsheet to do this.
The thermometer on the right is a special one for recording these temperatures. This is called a Six's thermometer because it was invented by a man called James Six. See more about these useful thermometers on the Six's Thermometers page.

NB. These readings cover 24 hours, from 09.00 to 09.00 GMT. The maximum reading may have occurred on the previous day. You have the choice of recording it for the day it was read, or recording it in the previous day's observations. Using the 'thrown back' method will make it easier to compare your data with others.

Dial Maximum and Minimum Thermometer

This thermometer shows the temperature by a needle pointing to a number on a dial. There are also two pointers which get pushed by the black pointer. The red one shows the maximum temperature and the blue one shows the minimum temperature. These thermometers do not have mercury or spirit to react to the temperature, they have a special metal strip.

Wet and Dry Temperatures

The current temperature is taken from the dry temperature reading. The purpose of the wet and dry temperatures is to allow you to work out the humidity of the air. See the Humidity page for more details.

Checking Wet and Dry Readings

The wet temperature reading should not be bigger that the dry temperature. This is because when things are wet they are colder. On very dry sunny days these temperatures will be further apart. It is usual for the two temperatures to be fairly close together when it is raining.

Maintenance

Make sure that the reservoir of water is topped up with clean distilled water. If it runs dry you will have two dry thermometers! The little bottle unscrews so that you can fill it.

Using ICT

There are many cheap digital thermometers these days. The instrument in the image can measure the current, maximum and minimum temperatures in two locations. One reading is taken from an internal sensor, the other uses the probe on the end of the cable.
See more information on the ICT page.