Birth of a depression or low-pressure system

Figure 12

Although the mercury barometer was invented by Evangelista Torricelli in the early 17th century (Middleton, 1963), it was not until the early 19th century that routine observations of atmospheric air pressure began to be undertaken. With the advent of the UK Meteorological Office in the 1850s, it soon became apparent that there were fairly regular and repeating patterns of air pressure across the globe, and that weather systems were intricately linked to these pressure patterns.

The most common of all air pressure systems is the mid-latitude “depression” or “cyclone”, also known more simply as a “low-pressure system”. These are roughly concentric areas of lower than normal air pressure (see Figure 12), with a central pressure of about 5-10% less than global average sea-level air pressure (i.e. ~940-980hPa). Low pressure systems correspond to real “dips” in the pressure field of the atmosphere, hence the name “depression”.

The birth of a low-pressure system usually takes place on the Polar Front as “frontal wave” in a process known as “fronto-genesis”. Waves often develop on boundaries in the natural world, where there is a contrast in density between two opposing media. In the case of frontal boundaries, these two media are:

• The warm, light and moist tropical air on the equator-ward side of the front.
• The cold, heavier and denser polar air on the pole-ward side.

A typical frontal wave has a wavelength of a few hundred kilometres.