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.