Air Pressure

Air Pressure is a very important feature of weather systems. Monitoring its state and its changes gives an insight into what the weather will be like in the near future. Air pressure is the current weight of the air around us. It can be different in different places, and can change. On a weather forecast the air pressure can be shown by black lines called issobars. Weather forecasters often use the words 'high' or 'low' to describe the air pressure.

To see more about this topic go to the More About Air Pressure page.

When you have finished reading about this you may want to return to Collecting Data.

Air Pressure

The barometer is a delicate instrument for measuring air pressure. This is best located somewhere where it can be looked at and where it will not get damaged. It needs to be indoors, not in the Stevenson screen. They are often located by an outside door.


The scale which weather watchers use is calibrated in millibars (mb) or hectopascals (hPa). One hPa = one mb. The pressure readings in this country can range between 950mb and 1050mb. For the barometer to go to these extremes is unusual. The readings are more likely to go between 970m.b. and 1030m.b. The brass knob in the middle controls the short shiny indicator. This pointer can be moved to align with the indicator. When the needle then moves, you will be able to see if the pressure has gone up or down.

Taking Readings

On this barometer the inner scale is the millibar calibration. The outer readings are not being used. Tap the barometer gently and note the direction of movement (up or down). It is the amount of movement in a short time that indicates the type of weather that is to come. Take the reading in millibars after tapping the glass.

Rising or Falling?

Generally the speed of change in air pressure is of interest. If the instrument shows a sudden fall in air pressure then it is likely that the weather will be stormy. If the instrument remains constantly high, then it indicates little change, especially in the summer.
Some barometers are ornamental as well as useful. This one hangs in a hall, so that it can be tapped in the morning.