Calm weather and dry conditions are the hallmarks of an anticyclone -- a weather phenomenon which most people probably know better as a "high pressure area." These zones of generally tranquil conditions are created by dry air masses. Dry air is heavier than a similar volume of wet air, so it tends to sink and compress, forming an area of high pressure. The fact that that air is sinking means that winds flow outwards from the high pressure area's center at ground level, where the Earth's surface itself prevents the heavy, dry air from sinking any more. These winds spiral outwards in a clockwise direction in the northern hemisphere and in a counterclockwise pattern in the south, due to the effects of the earth's spin.
Since air needs to be dry in order for an anticyclone to exist in the first place, clear, sunny days and starry nights are the rule when a high pressure area is in control over a region of the planet. In the middle latitudes or temperate zones, high pressure areas give way to low pressure areas after a few days, or sometimes weeks, as different air masses jostle against each other. In some latitudes, however, permanent anticyclones form, ensuring everlasting clear weather -- and creating some of the Earth's mightiest deserts.
Anticyclones are an important part of the weather cycle, both on a local scale and as part of the world's whole air circulation system. From pleasant days for a stroll in the park to huge circulation systems lasting for millions of years, anticyclonic air masses are a familiar, important part of the Earth's weather scene.
Summer anticyclones: pleasant days or the dog days of August?
High pressure areas that arrive in summer in the temperate latitudes are marked by clear skies both day and night, which means that solar heat isn't blocked by clouds during the daytime, nor is it held in by an umbrella of clouds after nightfall. A summer anticyclone brought in by a cold front can be pleasantly mild for several days until the sun and ground heat warm the air enough for conditions to start getting hotter -- and these fresh, brilliantly sunny days are among the most beautiful of the year.
Summer anticyclones can also bring in hot days full of glaring sunlight in the middle latitudes. This is especially true if the anticyclone is large enough or persistent enough to remain over the same area for longer than a few days. The temperature will steadily rise, since not only does the lack of cloud cover mean that the sun's rays can strike the ground with few of them being reflected back into space, but the steady sinking of dry air means that solar heat in the atmosphere flows towards the ground and builds up there.
On the other hand, anticyclonic nights in summer tend to cool off rapidly since there are no clouds to hold the heat in. The downward pressure of air may cause moisture that is released by the ground as it cools to remain close to ground level, so that fog may form on nights of high pressure. Obviously, if there's a lot of water at the surface -- in the form of rivers, lakes, or the ocean -- there's a much higher chance of fog forming, since more moisture will be 'exhaled' by the planet's surface in such conditions.
Vivid images of huge, roiling dust clouds from the years of the Dust Bowl still haunt the memory of the United States, and everywhere around the world, there are localized droughts every year. Droughts can be caused by many factors, but one that interests us here is that an anticyclone that remains in control of region too long can cause a drought in that area. Such an anticyclone is called a "blocking high."
Blocking highs are anticyclones which occupy an area of the Earth's surface and stay there for weeks. They are large and strong enough to deflect low-pressure systems around their edges -- hence the name 'blocking.' Most blocking highs don't last long enough to cause droughts, but the ones that do show that even as tranquil a weather form as an anticyclone can have dramatic impacts on our lives and the climate of the world. This is especially true when a blocking high is combined with the heat of summer -- and the major desert belts of the world are caused by what are basically eternal blocking highs.
Winter anticyclones: days as bright and cold as steel
Where a summer anticyclone can often bring hotter weather after a period of cool, rainy conditions, winter anticyclones turn up the deep freeze to maximum more often than not. Brilliant, frosty blue skies by day and diamond-sharp stars at night are the signs of a winter anticyclone.
Many of the anticyclonic traits of the summer are the same for the winter season, of course. A winter anticyclone is also caused by dry, sinking air and can produce fog as well as clear weather. However, the sun is in the sky for far fewer hours each day, so solar heating doesn't really occur. Instead, the air mass tends to lose heat steadily to space -- more slowly by day and more quickly by night, but there is no period when it actually becomes warmer. In winter, an anticyclone can lead to dangerous levels of cold, and fog can be persistent for a good part of the day since the low-angle sunlight isn't warm enough to disperse it quickly.
Eternal anticyclones -- the womb of deserts
Nearly everyone is familiar with images of the Sahara or the even more desolate Empty Quarter in the Arabian Peninsula -- endless rippling waves of sand beneath a starkly clean blue sky. These deserts and others at around the same latitudes both north and south of the equator are caused by what are effectively planet-girdling rings of anticyclonic weather -- known technically as "Hadley cells."
The forces that make these permanent anticyclones are the vast heat of the sun and the unending summer of the equator. Air at the equator is always being heated by the sun, since there is no winter and the sun is always overhead. This air rises, and because air is always rising beneath it, it's forced outwards in the only way it can go -- northwards, away from the equator. Rising also cools the air, condensing the moisture in it into colossal tropical thunderstorms of a height and violence almost never seen in the northerly regions. This dries the air, which streams northwards.
As it moves north, the air loses more and more moisture. Eventually, it becomes heavy enough to sink, and where it sinks, it forms two bands of permanent high pressure, one north and one south of the equator at about 30 degrees. It is on these lines that most of the world's major desert areas occur. Steady winds known as the "trade winds" and the "westerlies" also blow outwards as the sinking air hits the ocean surface, just as winds flow out from the center of a temperate zone high pressure area. These winds are also important to the circulation of the whole planet's atmosphere.
So, ultimately, it is the unchanging heat and moisture of the equator that pumps air north -- air that eventually dries and sinks steadily over the Sahara, the Namib, and other desert areas. Anticyclonic weather, it can be seen, not only brings in periods of sunny weather but changes the topography of the world itself when it persists long enough. The anticyclone phenomenon is one that we are all familiar with, and a crucial piece of the meteorological mosaic that makes up the weather of planet Earth.