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.

Scale Example Duration

Global ( Rossby waves, 50o-120o longitude)

Jet Stream Weeks to months

Synoptic ( short waves, less than 50o 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, 30o to 60o 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.