A thermometer measures air temperature. Thermometers work because matter expands when heated. Most thermometers are closed glass tubes containing liquids such as alcohol. When air around the tube heats the liquid, the liquid expands and moves up the tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the temperature.
A barometer measures the pressure of the air pushing on it. This is important because air pressure affects the weather; storms more or less follow certain patterns of high and low pressure systems. In general, falling air pressure means that clouds and precipitation are likely. Rising air pressure signals that clear weather is likely.
A weather vane, also called a wind vane, is used to measure wind direction. Very often these are in the shape of cockerels and are called weather cocks. They are often attached to an elevated object such as a roof.
The weathervane, or weathercock must be balanced so that half its weight is on either side of its axis, but also designed so that the areas exposed to the wind are unequal. This unequal area causes the vane to rotate to minimize the force of the wind on its surface. The design of the vane causes the end with the smallest area to turn into the wind, pointing to the source of the wind.
A psychrometer measures relative humidity. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.
The bulb or sensing area of one of the thermometers either is covered by a thin piece of clean muslin cloth wetted uniformly with distilled water or is otherwise coated with a film of distilled water. The temperatures of both the bulb and the air contacting the bulb are lowered by the evaporation which takes place when unsaturated air moves past the wetted bulb. An equilibrium temperature, termed the wet-bulb temperature, will be reached; it closely approaches the lowest temperature to which air can be cooled by the evaporation of water into that air.
The weather station displays a variety of weather conditions: temperature and humidity (both inside and out), atmospheric pressure, rain and snow, wind direction and speed, dew point, wind chill, and heat index – even solar and ultraviolet radiation. A basic home weather station offers features such as a clock, inside /outside temperature, humidity and atmospheric pressure. Complete weather stations can also monitor wind and rainfall, as well as calculate dew point, heat index, and wind chill. Many are also capable of calculating evapotranspiration, an important factor for agricultural concerns.