METEOROLOGIST JEFF HABY
One of a forecaster's first thoughts when confronted with the
300 / 200 mb chart is the
jet stream. The jet stream
is a high velocity river of air that flows completely around the Earth at the mid-latitudes. During winter, the jet
core is located generally closer to 300 millibars since the air is more cold and dense in the vicinity of the jet
stream during the cool season. The 200 millibar chart is used for the jet stream in the warm season but either chart
in most instances will suffice.
Many hot air balloonists have tried to ride this river of air around the world with
not much success for most. The river of air is not continuous. Embedded within the jet stream are higher velocity
jet streaks. Jet streaks are segments of faster wind speed within the jet stream. At 300 mb, the
air density is much
smaller than near the surface. A 100-knot wind at the 200 / 300 millibar level does not feel as strong as a 100 knot
wind at the surface. Even though the density is smaller, these air currents have the power to drive the movement of
storm systems and build
troughs and ridges.
The jet stream is useful for the prediction of temperature. The jet
stream divides colder air to the north from warmer air to the south. The transition between temperatures on each side
of the jet is very abrupt. Heights are higher to the south of the jet and lower to the north. In the upper levels,
this creates relatively high heights to the south of the jet and relatively low heights to the north. The
Pressure Gradient Force flows from a southerly to northerly direction. However, the
Coriolis force shifts the wind flow to
the right of the path of motion. Therefore, the jet stream flows from the west to east. When a trough builds over a
region it often indicates cooler temperatures due to
CAA which is sometimes combined with cloudier weather (especially
on the right side of the trough). A ridge builds by low level (between the surface and 700-mb) warm air advection
and upper level forcing (negative vorticity). Air in a ridge is sinking and is thus expanding and creating higher
heights. Therefore, temperatures are warmer than normal in a ridge due to warmer temperatures and sunnier weather.
This is especially true when a ridge occurs in high latitudes.