Lecture 6
Weather forecasting
The Jet Stream
Jet stream is fast-moving upper-level winds
concentrated at the boundaries of the Hadley
cells,
where temperature difference ( or gradients )
are greatest, and produce a pressure gradient,
and this result in winds.
Jet activity found along pole-ward edge of well-
defined cirrus cloud
Wind is the mechanism used by nature to
move and redistribute heat and cold.
Jet stream speeds average over 50 knots
and often exceed 200 knots.
There are actually two jet streams,
Subtropical, and Polar Jets.
Subtropical Jet stream, concentrates around
30o degrees north and south on average,
Tending farther poleward in the hemispheric
summer.
The Polar Jet stream, concentrated around 60o
degrees north and south
Moving farther equator-ward in the hemispheric
winter.
Jet stream ridges indicate a pole-ward push of
warm air.
Jet stream Troughs indicate an equator-ward
push of cold air.
Jet Stream defines as a dynamic boundary
between warm and cold air, so the front below
is a “ weather maker, and
The storm bread there, are then steered in the
easterly direction by the Jet.
These weather disturbances, in which warm , moist
air mixes with cold, dry air, are in a sense the
pressure relief valve in the Earth’s heat engine.
Straight Jet much contrast (less mixing)
Strong Front
Wavy Jet less contrast (much mixing) weak
Front
500 hPa about 5000 meters about 18,000 feet is
merely an agreed-upon level near the base of
the jet stream from which many surface
forecast features can be derived.
Meteorologists used 500hPa wind flow and
pattern as controller of majority of synoptic-
scale and meso-scale weather events.
Imagine the 500hPa constant-pressure level
as a dynamic, transparent blanket in
atmosphere, but distorted by numerous
humps and hollows.
It humps up where atmospheric pressure is
locally higher,
It sinks where pessure is less
Air will flow from the Humps toward the
Hollows, deflected by the spinning of the
Earth.