Wind: Small Scale and Local Systems

The tiny microscale motions constitute a part of the larger mesoscale motions, which, in turn, are part of the much larger synoptic scale. Notice that as the scale becomes larger, motions observed at the smaller scale are no longer visible.

Scales of atmospheric motion.

The scales of atmospheric motion with the phenomenon’s average size and life span. (Because the actual size of certain features may vary, some of the features fall into more than one category.)

Winds flowing past an obstacle. (a) In stable air, light winds produce small eddies and little vertical mixing.(b) Greater winds in unstable air create deep, vertically mixing eddies that produce strong, gusty surface winds.

Satellite image of eddies forming on the leeward (downwind) side of the Cape Verde Islands during April, 2004. As the air moves past the islands, it breaks into a variety of swirls as indicated by the cloud pattern.

(The islands are situated in the Atlantic Ocean, off Africa’s western coast.)

Under stable conditions, air flowing past a mountain range can create eddies many kilometers downwind of the mountain itself.

Small-scale winds interacting with the environment Observations: Eddies & Air Pockets

Eddies on leeward side of solid objectRoll eddies, mountain wave eddy (clear air

turbulence)Increase wind speed/shear deforms layer

into wave and air pocket.

Force of the WindBridges and hills or rises can modify wind,

increasing the force at specific locations.

Determining wind speed and direction

Wind characterized by direction, speed, and gustiness

Wind direction describes the direction from which it is blowing

An onshore wind blows from water to land; whereas an offshore wind blows from land to water.

Wind direction can be expressed in degrees about a circle or as compass points.

Determining wind speed and direction Wind Measurements

Wind vanePressure plate anemometerCup anemometerAerovaneRawinsondeWind soundings

Determining wind speed and direction Influence of Prevailing Winds

Prevailing most frequently observed direction during a given time period

Impact human and natural landscape

Local Winds

Thermal circulationHeating and cooling of the atmosphere

above the ground create cold, core high and warm, core low pressure cells.

Wind travels from high to low and rises until it cools and begins to sink.

Local Winds

Sea and Land BreezeUneven heating of land

and waterDay: land hot, water cold

= sea breezeNight: water hot, land

cold = land breezeSea breeze front, sea

breeze convergence

Development of a sea breeze & a land breeze. (a) At the surface, a sea breeze blows from the water onto the land, whereas (b) the land breeze blows from the land out over the water. Notice that the pressure at the surface changes more rapidly with the sea breeze. This situation indicates a stronger pressure gradient force and higher winds with a sea breeze.

Local Winds

Local Winds and WaterLocal winds will

change speed and direction as they cross a large body of water due to less friction, greater speed and greater Coriolis effect

Typically, during the summer over Florida, converging sea breezes in the afternoon produce uplift that enhances thunderstorm development and rainfall. However, when westerly surface winds dominate and a ridge of high pressure forms over the area, thunderstorm activity diminishes, and dry conditions prevail.

Stepped Art

Fig. 9-25, p. 241

The convergence of two lake breezes and their influence on the maximum temperature during July in upper Michigan.

Sinking air develops where surface winds move offshore, speed up, and diverge. Rising air develops as surface winds move onshore, slow down, and converge.

Local Winds

Seasonally Changing Winds: The MonsoonArabic for seasonalWinds change direction seasonably causing

extreme dry and wet seasonEastern and southern Asia, North America

Changing annual wind-flow patterns associated with the winter and summer Asian monsoon.

Local Winds Mountain and Valley Breeze

On mountain slopes, warm air rises during the day creating a valley breeze; during night nocturnal drainage of cool air creating a mountain breeze

Associated with cumulus clouds in the afternoon

Katabatic windsCold wind rushes down elevated slopes,

usually 10 kts or less but can reach hurricane strength

Valley breezes blow uphill during the day; mountain breezes blow downhill at night. (The L’s and H’s represent pressure, whereas the purple lines represent surfaces of constant pressure.)

Strong katabatic winds can form where cold winds rush downhill from an elevated plateau covered with snow.

Local Winds

Chinook/Foehn WindsDry warm descending on the leeward side of

a orographic barrierEastern slope of Rockies (chinook), Europe

(foehn), Argentina (zonda)

Focus: Snow EatersThirsty wind on east side of Rockies that eat

or melt snow due to rapid change in temperature

A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind sides.

Local Winds

Santa Anna WindsWarm dry that blows from east or northeast

down canyons into S. CaliforniaVery fast, desiccates vegetation, providing

fuel for fires

Desert windsDust storms, sand storms, dust devil,

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