Highs, Lows and Cold Air: Ten Patterns That Steer the Weather

A polar high is a persistent area of high pressure over each pole, fed by cold air that sinks. The Antarctic high is markedly stronger than the Arctic one because land exchanges heat with the air more efficiently than ocean, and the regions beneath these highs are so dry that they count as polar deserts.

Anticyclogenesis is the development or strengthening of a high-pressure circulation, the opposite of cyclogenesis. The entry places it where air sinks through the troposphere, preferentially beneath the western side of upper-level troughs, so a high is traced back to motion in the air above it.

A cold-core low is a cyclone aloft with a pool of cold air at high altitude and no fronts. It grows stronger with height because colder air has less room between pressure surfaces, and once it is cut off from the main westerly winds it moves erratically and can spawn tornadoes near its center.

A cut-off low is a closed upper-level low that has separated completely from the main westerly current, typically between about 20 and 45 degrees of latitude. It forms when a trough pinches off, and because it no longer moves with the flow it can linger in a blocking pattern for days or even weeks.

A cold wave is a rapid fall in air temperature, defined by the United States National Weather Service as one that happens within 24 hours and calls for substantially increased protection for agriculture, industry, commerce and social activity. Cold periods often cause more temperature-related deaths than heat waves, and an unexpected freeze in the growing season can destroy crops outright.

A cold pool is a pocket of dense cold air made when rain evaporates beneath a thunderstorm, and it typically spreads outward at around 10 metres per second for two to three hours. When its gust front passes a place, wind speed jumps while temperature and humidity both drop suddenly.

This ring of fire has nothing to do with volcanoes. It is a setup in which thunderstorms form along the rim of a strong upper-level high-pressure ridge, usually in the warmer months alongside conditions like a heat dome, and in the United States it plays a major role in warm-season derecho events.

The Meiyu front, also called the plum rain front, is a persistent stationary front that brings the East Asian rainy season to China, Taiwan, Korea and Japan in late spring and early summer. It forms where moist Pacific air meets cooler continental air and lasts until the subtropical ridge strengthens enough to push it north of the region.

A post-tropical cyclone is a former tropical cyclone that no longer has a warm core or tropical energy source, usually after extratropical transition has given it the frontal structure of a mid-latitude storm. Losing tropical status does not mean the danger has passed, since these systems and their remnants can keep producing high winds and heavy rain.

Clear-air turbulence is rough air with no cloud or other visible warning, found where air masses moving at very different speeds meet, most often at roughly 7,000 to 12,000 metres near the tropopause and within jet streams. Most encounters cause only passenger discomfort, but some have caused serious injury.