Lecture 4 Weather forecasting

What Makes the Weather? Our earth’s surface consists of Land and Water, with Water being thermally stable substance ( inverse Land ) and weather over water tends to be more Predictable and stable than over land.

Equatorial regions receive more solar heat than Polar Regions

Heat must be transfer to redistributed over the whole earth’s surface

1 2

On regions 1 and 2 in the previous figure the winds are not strong because air motion is vertical, and horizontal temperature differences on the surface are slight.

Predicting the Earth’s Complex atmospheric interactions is a combination of ART and SCIENCE. Good analysis and predictions depend upon a sound grasp of meteorological concepts weather operates through multiple levels of TIME and SPACE.

ScaleExampleDuration Global ( Rossby waves, 50 o -120 o longitude) Jet Stream Weeks to months Synoptic ( short waves, less than 50 o longitude) Air masses, High- and Low-pressure system Days / weeks Mid-latitude and tropical Surface troughs and ridges, Hurricanes, typhoons, monsoon troughs week or less Meso ( intermediate ) Squall lines Several hours / day Small Thunderstorms, hailstorms A few hours or less Micro Donwbursts, waterspouts, tornadoes Less than an hour Scales of weather development

To analyze and forecast weather properly, one must begin looking at the Earth’s atmosphere as a whole, not only Micro / Meso / Synoptic, or others.

Upper-level waves are characterized by undulations in upper atmosphere’s wind flow with north-south axes, results from the cold air in northern latitudes moving south and warm air from southern latitudes moving north, and its shape effected by the amount of warm and cold air with the topography underlying.

On average, there are three to seven Rossby waves circling the Earth at any given time, in response to the continuous north-south movement of warm and cold air within the Earth’s atmosphere. Rossby waves have their greatest influnce in Mid-latitudes, 30 o to 60 o degrees north and south.

Embedded within the long Rossby waves a smaller synoptic scale ( short-wave troughs ), which extend over large areas and exist a few days to about a week. Synoptic-scale waves control the development and movement of surface high- and low- pressure systems.