Pressure fluctuations caused by the weather are very irregular. At one time people thought that pressure alone determines the weather. Therefore, the following inscriptions have been placed on barometers up to the present day: clear, dry, rain, storm. You can even find the inscription “earthquake”.

Changes in pressure really do playa big role in changing the weather. But this role is not decisive. Average or standard pressure at sea level is equal to 1013 millibars. Pressure fluctuations are comparatively small. The pressure rarely falls below 935–940 millibars or rises to 1055–1060. The lowest pressure—885 millibars—was registered on August 18, 1927, in the South China Sea. The highest—about 1080 millibars—was registered on January 23, 1900, at the Barnaul station in Siberia (all figures are taken with respect to sea level).¹

A map used by meteorologists analyzing changes in the weather is depicted on next page Figure 1. The lines drawn on the map are called isobars. The pressure is the same along each such line (its value is indicated). Note the regions of the lowest and highest pressures—the pressure “peaks” and “pockets”. The directions and strengths of winds are related to the distribution of atmospheric pressure.

Pressures are not identical at different places on the Earth’s surface, and a higher pressure “squeezes” air into places with a lower pressure. It would seem that a wind should blow in a direction perpendicular to the isobars, i.e. where the pressure is falling most rapidly. However, wind maps show otherwise. The Coriolis force interferes with air pressure and contributes corrections which are very significant.

As we know, a Coriolis force directed to the right of the motion acts on any body moving in the Northern Hemisphere. This also pertains to air particles. “Squeezed out” of places of higher pressure and into places where the pressure is lower, the particle should move across the isobar, but the Coriolis force deflects it to the right, and so the direction of the wind forms an angle of about 45° with the direction of the isobar.

A strikingly large effect for such a small force! This is explained by the fact that the obstacles to the action of the Coriolis force—the friction between layers of air—are also very insignificant.

The influence of the Coriolis force on the direction of winds at pressure “peaks” and “pockets” is even more interesting. Owing to the action of the Coriolis force, the air leaving a pressure “peak” does not flow in all directions along radii, but moves along curved lines—spirals. These spiral air streams twist in one and the same direction and create a circular whirlwind displacing air masses clockwise in a high-pressure area. Figure 3 in Section: Coriolis Forces clearly shows how a radial motion is converted into a spiral motion under the action of a constant deflecting force.

The same thing also happens in a low-pressure area. In the absence of the Coriolis force, the air would flow towards this area uniformly along all radii. However, along the way air masses are deflected to the right. In this case, as is clear from the figure, a circular whirlwind is formed moving the air counterclockwise.

Winds in low-pressure areas are called cyclones; winds in high-pressure areas are called anticyclones.

You shouldn’t think that every cyclone implies a hurricane or a storm. The passing of cyclones or anticyclones through the city where we live is an ordinary phenomenon related, it is true, more often than not to a change in weather. In many cases, the approach of a cyclone means the coming of bad weather, while the approach of an anticyclone the coming of good weather.

Incidentally, we shall not embark on the path of a weather forecaster.