“By way of definition, a jet in fluid dynamics is simply a core (or ‘stream’) of fluid moving at a higher velocity than the surrounding fluid. Although they are complicated to describe mathematically, the jet streams in the atmosphere are a straightforward, natural result of the meridional (that is, equator-to-pole) temperature gradient in the earth’s atmosphere. Analogous flows exist on other planets with substantial atmospheres having similar temperature gradients.
“The temperature gradient derives from the differential solar heating of the spherical surface of a planet: the surface is generally warmest at the equator and grows progressively cooler as one moves poleward. The centrifugal effects of the earth’s rotation, often called the Coriolis force, deflect the north-south transport of heat from the equator to the poles into the predominantly east-west motion of the jet stream. The relative strength, or velocity, of the jet stream is proportional to the intensity of this thermal gradient. During the winter months, when the equator-to-pole temperature disparity is at its greatest, the jet stream reaches its maximum velocity. During the summer months, when the temperature gradient between the equator and the pole is considerably less (only about half the winter value), the jet stream reaches its minimum velocity.
“As can be seen on high-altitude weather maps, the jet stream does not maintain a straight, zonal flow from west to east but rather takes on a more serpentine look, often with dramatic dips to the south or rises to the north. There are two major reasons for these nonzonal motions: the temperature gradient between the equator and the poles and the presence of land masses on the earth’s surface.
“The presence of land masses on the earth’s otherwise watery surface modifies the distribution of temperature, because continents heat and cool at a dramatically different rate than do the oceans. The topography of the land also influences the jet stream’s location. Mountain ranges and plains on large continents, for example, significantly affect the distribution of atmospheric temperature. And since the jet stream is a thermally driven phenomenon, the more complicated the three-dimensional temperature structure of the earth’s atmosphere, the more ‘wandering’ will take place in the course of the jet stream.
The 50°-60° N/S region is where the polar jet located with the subtropical jetlocated around 30°N. Jet streams vary in height of four to eight miles and can reach speeds of more than 275 mph (239 kts / 442 km/h).
The actual appearance of jet streams result from the complex interaction between many variables – such as the location of high and low pressure systems, warm and cold air, and seasonal changes. They meander around the globe, dipping and rising in altitude/latitude, splitting at times and forming eddies, and even disappearing altogether to appear somewhere else.