Anticyclones

An anticyclone does almost nothing, and that is precisely why it dominates the weather. It produces no fronts, no rain bands, no organised lift. Its entire influence comes from air sinking slowly over an area hundreds of miles across — a motion far too gentle to feel, and strong enough to erase clouds, cap thunderstorms, hold pollution against the ground and set both the hottest and the coldest days of the year.

The word itself defines the thing by what it isn't: an anticyclone is the opposite of a cyclone, the counterpart to an area of low pressure. The naming has aged well. Nearly every property of a high is the mirror image of a low, including the one that matters most — the direction air moves in the vertical.

What the pressure actually means

Standard air pressure at sea level is 1013.25 millibars, equivalent to 29.92 inches of mercury. Pressure is nothing more exotic than the summed force of air molecules striking a surface, and it falls with height because there are fewer molecules above you. That makes raw station readings useless for comparison between a mountain town and a coastal one, so every observation is converted to the value the instrument would read if it sat at sea level. When a map shows a high, it is showing a region where that corrected value is elevated relative to its surroundings.

Isobars — lines of equal pressure — wrap around the centre, with values increasing inward. Their spacing is the pressure gradient, and the speed of the wind is directly proportional to it. This is the first thing worth reading on any surface chart: widely spaced isobars over a high mean the light winds that let air stagnate; tightly packed ones on the flank of a strong high mean it is driving a real airstream somewhere.

Three forces set the flow. The pressure gradient force pushes air outward from high toward low. The Coriolis force, a consequence of the Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern. Together those two would send the wind circling parallel to the isobars. Friction with the rough surface of the Earth slows the wind and breaks the balance, so the flow spirals slightly outward across the isobars rather than simply orbiting. That outward spiral — divergence — is the whole story.

Why divergence means sinking

Mass cannot be created or destroyed in a given area, so air leaving the base of a high has to be replaced. It cannot come in sideways; it is already leaving sideways. It comes from above. Air descends over the entire breadth of the system to feed the outflow beneath it.

Descending air is compressed by the increasing pressure around it, and compression warms it. Warming air lowers its relative humidity, so evaporation begins to exceed condensation and existing cloud dissolves. This is the mechanism behind the phrase "fair weather high," and it works in reverse under a low, where converging surface winds are forced to rise, cool, and condense into cloud and precipitation.

Sinking over a wide area also builds a subsidence inversion: a layer aloft where temperature increases with height instead of decreasing. That is an unusual arrangement, and it acts as a lid. A parcel of air rising from the surface into an inversion finds itself colder — and therefore denser — than its surroundings, and stops. Nothing gets through without help.

The lid, visible

The recognisable version is a field of fair-weather cumulus whose tops all stop at the same altitude, as though sheared by a ruler. Those clouds are rising thermals, and the flat line across their tops is the base of the inversion. On days when the lid is stronger, the same layer becomes the cap that separates warm moist air below from cooler dry air above, suppressing thunderstorms even when the atmosphere underneath has enormous potential energy. The cap is one of the most important ingredients in severe weather episodes, because while it holds, instability keeps accumulating. If it is removed or weakened, the release can be explosive.

The other visible signature is the horizon. Under subsidence, whatever is emitted at the surface has nowhere vertical to go, and the air develops a milky, brownish edge where it meets the sky.

Same mechanism, opposite comfort

In summer, a persistent high aloft is the engine of heat waves. Subsidence warms the lower atmosphere by compression while the inversion simultaneously traps heat rising from the surface. Skies stay clear because the downward motion prevents cloud, and clear skies mean unfiltered sunshine on ground that never gets a night's break. Precipitation is shunted around the periphery of the high, which is why a stalled high produces drought at its centre and flooding wherever the displaced storm track ends up.

Winter reverses the sign without changing the physics. Clear skies radiate heat to space efficiently at night, and the immediate surface cools much faster than the air a few hundred feet above it. That produces a radiational inversion overnight — a shallow cold layer pinned to the ground beneath warmer air. On an ordinary morning it erodes quickly after sunrise. Under a strong winter anticyclone, with a subsidence inversion sitting above the radiational one and a low sun that never delivers much energy, it may not erode at all. Fog, haze and cold pool together in valleys and stay there. The same suppression of vertical motion that gave July a cloudless sky gives January a lid of grey.

Winter highs over continental interiors also arrive already cold. Continental arctic and continental polar air masses form over high-latitude land, take on the character of the frozen surface beneath them, and are very cold and very dry. Combine that air mass with clear-sky radiational cooling and light winds, and the resulting minimum temperatures are the extreme end of what a region can produce.

What ends one

Blocking highs are stubborn by construction. They divert other systems around themselves and can hold position for several days, producing long stretches of unchanging weather. They weaken when a shortwave — a disturbance in the mid or upper atmosphere — moves across the top of the ridge and erodes it. Until that happens, forecasting under a high is mostly a matter of stating that nothing will change, which is accurate and, depending on whether your reservoir is full, either reassuring or not.