Prevailing westerlies blow high above the middle latitudes. Especially strong currents within them are called jet streams. Japan's weather is greatly influenced by the prevailing westerlies.
Basic Knowledge
Global atmospheric circulation occurs as air heated at the equator rises and air cooled at the poles sinks. Ideally, it is divided into three circulations: the Hadley circulation (equator–30 degrees), Ferrel circulation (30–60 degrees), and polar circulation (60–90 degrees).
Near Earth's surface, winds blow as the northeast trade winds (0–30 degrees), the prevailing westerlies (30–60 degrees), and the polar easterlies (60–90 degrees). Because Japan is in the middle latitudes, it lies in the belt of prevailing westerlies.
A jet stream is a narrow, strong westerly wind flowing through the upper troposphere (about 10km altitude), and its wind speed can reach 50–100m per second. It becomes stronger in winter and controls the weather when it meanders near Japan.
Prevailing westerlies (winds blowing from west to east high above the middle latitudes)
Jet stream (strong prevailing westerlies in the upper troposphere; maximum wind speed over 100m/s)
Global atmospheric circulation (large-scale air flow over the entire Earth; three circulations)
Hadley circulation (equator–30 degrees; air rises at the equator and sinks at 30 degrees)
Migratory high-pressure system (a high-pressure system moving west to east carried by the prevailing westerlies)
Meandering of the prevailing westerlies (Rossby wave) (a north–south wavering flow of the prevailing westerlies)
Further Study (Background and Significance)
Japan's weather changes from west to east because of the prevailing westerlies. Therefore, looking at regions to the west in a weather forecast is like seeing the future.
Meandering jet streams produce winter cold waves and warm winters. When the jet stream bulges northward toward the pole, warm air enters Japan; when it dips southward toward the equator, cold air moves south. In recent years, it has been suggested that global warming has intensified the meandering of the prevailing westerlies and increased extreme weather events.
The difference in flight times between flights from Tokyo to Los Angeles and return flights is also caused by tailwinds and headwinds from the jet stream. Eastbound flights with tailwinds are shorter, while westbound flights with headwinds are longer.
- Prevailing westerlies = above the middle latitudes; west → east
- Jet stream = strong prevailing westerlies in the upper troposphere
- Global atmospheric circulation = Hadley + Ferrel + polar circulation
- Japan's weather changes from west → east
- Meandering prevailing westerlies cause cold waves and warm winters
- Jet streams are stronger in winter
- The east–west difference in aircraft speed is influenced by jet streams
Points of Caution (Easily Confused)
① Prevailing westerlies blow from west to east (they are winds from the west), not easterly winds. ② Trade winds are low-latitude northeasterly winds (in the Northern Hemisphere), opposite to the prevailing westerlies. ③ A jet stream is part of the prevailing westerlies, not a separate phenomenon. ④ Global atmospheric circulation consists of three circulations and is different from ocean circulation (deep circulation).
Practice
- What are the winds called that blow from west to east high above the middle latitudes?
- What is the especially strong flow within the prevailing westerlies called?
- Why does Japan's weather generally change from west to east?