Measuring Temperature and Air Pressure

Temperature

In our everyday language weather often connotes such qualities as warm or cold, and from these descriptive terms we decide whether we want to wear trousers or shorts. For most people these attributes are fairly adequate, but meteorology – the study of weather – involves more quantitative assessments, such as measurements of temperature and air pressure.

Temperature is defined as a measure of the thermal state of the air, and can be measured with a thermometer, most commonly with a working liquid in glass thermometer that usually contains mercury or alcohol. Requiring no further equipment beyond the human eye, the volumetric thermal expansion of the liquid makes it simple to measure temperature. Most temperature scales today are expressed in degrees Celsius (ºC), with the notable exception of the United States where one will encounter Fahrenheit (ºF) in use. A conversion from Celsius to Fahrenheit is as follows: T(°F)=T(°C)*1,8+32.

To ensure valid data, the thermometer must be shaded from sunlight, and held in an upright position with nothing touching the bulb.

Air Pressure

Although we are not aware of it, the air exerts certain pressure on everything. This force, or weight, exerted on a surface is called air pressure, and meteorologists typically measure this state in hectopascals (hPa), or quote it in millibars (mb) as 1 hPa is equivalent to1 mb.

Due to differences in pressure from one location to another, wind blows from areas of high pressure towards areas of low pressure. As elevation increases, atmospheric pressure decreases, because there is less ‘atmospheric mass’. A so-called pressurealtimeter allows pilots to measure their altitude by subtracting the actual barometric pressure at its location from the current pressure at sea-level.

An aneroid barometer consists of a box of partially evacuated air which either expands or contracts as the pressure falls or rises. If there is any change in air pressure, this will be indicated by a dial that is connected to a system of levers.

Barometers should similarly not be exposed to direct sunlight. On occasion, it is possible that a barometer will not measure small changes in pressure; if this occurs, tap its glass face until it commences working properly.