Explain the forces that generate winds Explain why there is well-developed westerly flow in the upper troposphere Differentiate between high and low pressure systems Compare and contrast surface and geostrophic winds Describe/read maps of constant pressure surfacesIdentify and discuss wind types and the forces that generate them Recognize surface and upper-level atmospheric maps and identify general patterns of windflow Explain monsoonal flow and land-sea breezes

Air pressure and atmospheric motion

Air pressure and atmospheric motion

Q: What makes the wind blow?

A: Air pressure differences.

Air pressure

• Force exerted by molecules in atm due to gravity and temperature

Air pressure

Surface air pressure variability

• Average sea level pressure 1013 mb

Mercury barometer1013 mb = 29.91 inches

Aneroid barometer

Pressure systems

• Two types: high and low

• Low: associated with clouds and instability.

• High: associated with clear conditions and stability

Low pressure systems

• Cyclone• Converging rising air at

surface• Diverging air aloft• Winds rotate

counterclockwise in NH• Areas of “light” atmosphere;

air is forced into these locations

• Unstable surface conditions

High pressure system

• Anticyclone• Converging air aloft• Diverging sinking air at

surface• Winds rotate clockwise in

NH• Areas of “heavy”

atmosphere; air is forced out of these locations

• Stable surface conditions

Is the location for these pressure systems the northern or the southern hemisphere?

High and low pressure systems

• Occur on a variety of spatial and temporal scales– Some pressure systems may be stationary for a

long period of time, others may migrate rapidly around the planet

– Some pressure systems are closed, others are more belt-like and open

Low pressure systems

• Types of low pressure systems: tornados, thunderstorms, hurricanes….

Low pressure systems

….., midlatitude cyclones, the ITCZ, thermal lows

Dust devil versus ITCZ

Another secret of weather forecasting

• The atmosphere is a heterogeneous collection of pressure systems in three dimensions.

• Weather forecasting involves looking at surface conditions as well as upper level conditions (aloft, in the upper troposphere)

• http://weather.unisys.com/

• A midlatitude cyclone is a low pressure at the surface coupled to a low pressure aloft in the upper troposphere

• A hurricane is a high pressure aloft and a low pressure at the surface

“Deep” tropospheric phenomena

Midlatitude cyclones

• Strong, “deep” interaction between surface and upper levels

• May travel large distances around the globe

Midlatitude cyclone

“Shallow” tropospheric phenomena

• Thermal low (warm)• Thermal high (cold)

– Weak interaction between surface and upper levels

– May occur on a daily basis or persist over many months

L H

Pressure differences initiate…..

…….winds• Named according to the

direction they blow from• Winds can blow at different

directions at different altitudes in the atm

• Forces that act on winds:– PGF– CF– Surface friction

1. PGF: Pressure gradient force – winds blow from high to low

Where are winds the fastest?

2. Coriolis Force (CF)• Apparent deflection of the winds due to

rotation of the Earth – NH winds deflected right– SH winds deflected left

Coriolis Force (CF)

• CF is not a true force; it is an apparent force arising from the effect of the Earth’s rotation

• Deflection is strongest at poles and zero at the Equator

• CF acts perpendicular to the direction of motion

• CF deflects to the right in the Northern Hemisphere

Bart versus Australia

Misconceptions about the Coriolis Force

• The CF does not determine the rotation in a drain.

3. Surface friction (SF)

• Topography (mnts, elevated plateaus) deflect winds

Two major types of winds

1. Geostrophic (upper troposphere winds)– Influenced by PGF and CF only– Wind flow is parallel to

isobars/geopotential heights– Geostrophic flow is westerly

(west to east) in NH– Shown on geopotential height

maps

Geostrophic winds on geopotential height map

Two major types of winds

2. Surface winds– Influenced by

PGF, CF, and SF

– Winds cross isobars

Types of surface winds• Monsoonal flow

– Creates extreme wet and dry seasons

Location ofthermal highsand lows associatedwith monsoonalflow

Monsoon in India

Weak monsoonal flow in southwestern US

Winter Summer

Types of surface winds

• Land-sea breeze circulation• Many other different types of

surface winds based on local physiography and arrangement of pressure cells. Many have unique names.

Sea breeze blowingfrom ocean to land