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 30 o degrees north and south on average, Tending farther poleward in the hemispheric summer. The Polar Jet stream, concentrated around 60 o 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.