Once the data is gathered, it needs to be measured and read in order to forecast the weather.
1 Atmospheric Pressure
Atmospheric pressure is the weight of the atmosphere pushing down on the earth. It is measured in units called millibars (mb) by an instrument called a barometer (figure 1).
How a Barometer Works
Marker needle
Main needle
A very sensitive vacuum chamber inside the barometer responds to atmospheric pressure.
Lever
As pressure increases or decreases, the main needle on the face moves accordingly: clockwise as pressure increases, anticlockwise as pressure decreases.
Vacuum chamber
1. A barometer Key Words
Atmospheric pressure
Millibars (mb) Barometer Barograph Isobars
High atmospheric pressure
The second needle is only a marker. It can be moved by rotating the stud in the centre of the face. By aligning the marker with the main needle, one can ‘mark’ existing pressure. This can be useful for observing pressure changes later.
In a weather station, a barograph (a barometer that records its readings on a moving graph; figure 2) measures and
records atmospheric pressure.
3
The pen moves up and down as pressure changes.
Low atmospheric pressure
1 The barometer is squeezed as air pressure increases and expands when pressure decreases.
2 Levers transfer the barometer’s
movement to a pen.
4
The pen’s movement draws a line on paper on a slowly rotating cylinder.
