Weather observations

• Thermometers should be placed in the shade. Ideally, they should be in a white slatted box (thermometer screen) with the door facing north. If this is not available they could be hung in a permanently shaded area on a wall or fence. Care should be taken, since even a north-facing aspect will get sunshine in summer. There should be a large area of shading around the thermometer because bricks or wood can retain heat, which would spoil the readings if there is little shading.
• Rain gauges should be away from walls, fences, bushes, etc. as these affect the amount of rain caught in the rain gauge.
• Wind instruments should be well clear of walls, fences and houses as these cause eddies which would give unrepresentative readings of wind speed and direction.

Official meteorological observations are made every hour. Observations from associated stations are usually made less frequently - every three or every six hours during the daytime only. However, making an observation just once a day, say at 9 a.m. GMT (referred to by meteorologists as 0900 GMT as a 24-hour clock is used), can be a useful exercise in taking measurements and provides useful and interesting information over a period of time.

If 0900 GMT is not possible, another time can be chosen, but it is important that observations are made at the same time each day. Recordings can be entered in a suitably ruled exercise book, or into a tabular computer file. A column for 'remarks' should be included. Plotting daily temperature and pressure readings on a graph can produce interesting results over a period of time. Daily, weekly, monthly and annual totals of rainfall are also of great interest.

There are two basic types of thermometer which are readily available.

• The liquid-in-glass thermometer (Fig 4, left) depends upon the fact that a liquid expands by an amount proportionate with the temperature. Consequently, as the temperature increases, the liquid (usually alcohol or mercury) in the bulb of the thermometer expands and pushes a column of liquid up a capillary tube. There is a scale marked on the side of the capillary tube and the temperature is read from the position of the end of the liquid column. Temperature measurements are read directly from this type of thermometer.
• The other type of thermometer (Fig 5) is based on a bimetallic strip, which changes shape according to the temperature, due to the different amounts of expansion and contraction of the two types of metal. These changes cause a pointer to move in such a way that the temperature is indicated on a semicircular scale. Temperature measurements are made remotely using this type of thermometer, which means that the bimetallic strip is outside while the gauge is inside a room.

Figure 8 above shows a typical set up of official thermometers. The dry bulb can be seen as the left-hand vertical thermometer, whilst the wet-bulb is the right-hand vertical thermometer.

If the relative humidity of the air is low there is a lot of evaporation from the wet-bulb. The resulting cooling causes the wet-bulb temperature to be much lower than that of the dry-bulb temperature. However, if the air is very moist there will be little difference between the wet- and dry-bulb temperatures. Consequently the larger the difference between the wet- and dry-bulb temperatures, the lower the relative humidity.

Instead of using wet- and dry-bulb thermometers, simple dial humidity hygrometers can be used. One relatively cheap type makes use of the expansion and contraction of a strip of paper with rises or falls in humidity. The instrument can be 'calibrated' by leaving it wrapped in a damp cloth for a while, then setting the dial to 100%. It is not as accurate as using the thermometers, but is a lot more convenient. A simple hygrometer can also be made using a strand of human hair.

Fig 10 (right): A simple dry- and wet-bulb thermometer © Crown

The standard instrument for measuring wind speed is an anemometer (see figure 13). Ideally, this should be sited in the open at a height of 10 metres above the ground.

The measurements are usually made remotely on a dial or LCD device linked to the anemometer.

Hand-held anemometers can be obtained which provide a direct reading of wind speed when held up to the wind.

 
If an anemometer is not available, wind speeds can be estimated using the Beaufort wind scale (see Table 2 above).

A simple cup anemometer can be made by mounting four ice cream or margarine cartons on a metal, plastic or wooden cross, which rotates horizontally about a vertical spindle at the top of a mast. If one of the cups is of a different colour from the rest, the wind speed can be assessed by counting the number of revolutions per minute. Alternatively, a counter could be attached, which would give a measure of the number of revolutions over a given time - or run-of-wind.

A cheap and reliable alternative is a ventimeter (figure 15) in which wind blows into a hole in the side of the instrument and causes a disc to rise - the height reached by the disc is a measure of the wind speed.

Fig 15 (right): A simple ventimeter © Crown

Cloud classification can be very interesting. There are ten types of cloud which can be divided into high, medium or low cloud according to the height of their base.

• High - cirrus, cirrostratus and cirrocumulus
• Medium - altostratus, altocumulus and nimbostratus
• Low - stratus, stratocumulus, cumulus and cumulonimbus