Heavy Weather

    One of the main influences on our weather is air pressure. Changes in pressure have caused some of the most exciting and dangerous weather on the planet, including tornadoes, hurricanes, thunder and lightning.

    Attempts to measure pressure date back to the 1600's, when the barometer was first invented. However, it was not until the nineteenth century that most sailors used this instrument to try to forecast storms. Though this alone could not give highly accurate forecasts, storms were predicted and lives saved. However, we have not always been so lucky. There are many examples of destruction caused by heavy weather.

    The mercury barometer measures atmospheric pressure. It works this way. Completely fill a long glass tube with mercury. Turn it upside down, and place the top below the surface of more mercury in an open basin. Rather than pouring out again, the mercury in the tube will only fall until the height of the column is about a meter. This is because the pressure of the air on the mercury in the basin is equal to the pressure of the mercury in the column. The gap at the top of the tube is not air but a vacuum. The difference in height between the top of the column and the top surface of the mercury in the basin is a measure of the weight of the air, which changes as the weather changes. The air pressure is then measured in units called millibars (mb) or hectoPascals (hP), both of which have the same scale.

    Because atmospheric pressure is the result of the weight of the atmosphere, pressure varies according to how high or low you are. The higher you are the lower the air pressure is. In Britain, at sea level the pressure is 1013 mb, but as you climb higher this decreases by 1 mb per 10 metres. Pressure also varies horizontally, though changes here are much more gradual: 1mb per 100km.

    Changes in air pressure also reflect potential changes in weather. If there is low pressure then a depression can develop which may cause stormy weather. High pressure is usually associated with dry and sunny weather.

    There are several different types of barometers within the Science Museum’s collection. These include marine, coastal, mountain and portable barometers. Perhaps one of the most interesting is the Admiral FitzRoy barometer, which was widely used at the time of its development and is still famous today.

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