Wind: Small-Scale and Local Systems

Wind: Small Scale and Local Systems

Overview

• Quote by Lewis Fry Richardson:

  • "Big whirls have little whirls, that feed on their velocity; and little whirls have lesser whirls, and so on to viscosity."

Scale of Wind Systems

Definition of Scale

• Wind systems can be classified by their spatial scales, which include:

  • Macroscale: Global scale (approximately 5000 km)

  • Synoptic scale: Regional scale (approximately 2000 km)

  • Mesoscale: Localized scale (approximately 20 km)

  • Microscale: Very small scale (approximately 2 m)

Synoptic Scale Characteristics

• Composed of:

  • Small turbulent eddies

  • High and low-pressure areas

  • Weather fronts

  • Specific weather phenomena (e.g., thunderstorms, tornadoes, waterspouts, dust devils, land/sea breeze, Chinook wind, Santa Ana wind, hurricanes, tropical storms)

Mesoscale Characteristics

• Duration of phenomena:

  • Ranges from seconds to minutes to hours

  • Longwaves in the westerlies can last days to a week or more

Friction and Turbulence

Properties of Air

• Air is a physical substance and experiences friction:

  • Frictional Drag: Decreases with height from the Earth's surface

  • Impacts Wind Speed: Wind speed generally increases with height

Types of Turbulence

• Mechanical Turbulence: Caused by obstructions leading to eddies

• Thermal Turbulence: Resulting from vertical motion, surface heating, and atmospheric instability

Boundary Layer Influence

Planetary Boundary Layer (PBL)

• The section of the troposphere affected by friction:

  • Average height: approximately 1000 m (3300 ft)

  • Height influenced by:

  • Strong winds

  • Surface terrain variations

  • Surface heating

  • Instability in the atmosphere

Influences on Friction in the Boundary Layer

1. Surface Heating: Causes steep lapse rates and strong thermal turbulence

2. Wind Speeds: Strong winds contribute to mechanical turbulence

3. Topography: Rough or hilly landscapes generate strong mechanical turbulence

Eddies - Big and Small

Formation and Characteristics

• Eddies form on the leeward side of objects in the wind:

  • Size of eddies depends on:

  • Obstacle size and shape

  • Wind speed

• Effects of Wind:

  • Light winds produce small stationary eddies

  • Smooth surfaces lead to fewer eddies; rough surfaces create more

  • Roll eddies (rotors) can occur in mountainous regions, producing hazardous conditions for flying

The Strong Force of Wind

Wind Force Characteristics

• The force acting on an object due to wind is proportional to the square of the wind speed:

  • Small increases in wind speed can significantly amplify wind force

  • Effects of strong winds include:

  • Uprooting trees and moving vehicles

  • Supporting birds and hang gliders in upward air motions

  • Wind speeds increase when flowing over ridges, making mountain winds stronger than at lower levels

Wind and Soil

Erosion and Landscape Formation

• Wind shapes landscapes by blowing over exposed soil:

  • Lifts loose particles such as sand, silt, and dust, as well as larger rocks.

  • Leading to the formation of:

  • Sand dunes (with gentle windward slopes and steeper leeward slopes)

  • Sand ripples (formed perpendicular to wind direction)

Wind and Snow

Effects of Wind on Snow

• Wind over snow-covered areas can lead to:

  • Creation of snow ripples, drifts, or dunes

  • Formation of snow rollers, which are clumps rolling across the ground

• Snow Fences: Constructed in open areas to promote snow retention and protect highways

Wind and Vegetation

Effects on Plant Life

• Winds laden with sand can:

  • Damage or destroy crops

  • Increase evapotranspiration in plants

• Dried vegetation poses fire risks

• Vegetation can help reduce wind erosion and soil moisture loss

• Shelterbelts: Combinations of conifer and deciduous trees used to protect agricultural lands

Wind and Water

Wave Formation

• Friction from wind over water generates waves:

  • Energy transfer depends on:

  • Wind speed

  • Duration of wind effect

  • Distance over open water (fetch)

• Changes in wind speed and direction occur over large bodies of water due to:

  • Reduced friction

  • Increased wind speeds

  • Coriolis effect causes turning to the right

Convergence and Divergence

• Changes in friction as air moves from water to land slow it down, leading to convergence (accumulation) and divergence (dispersal)

Thermal Circulations

Definition

• Thermal circulations are driven by temperature variations causing warmer air to rise and cooler air to sink:

  • Thermal Highs: Regions of high pressure from cooling air in the atmosphere

  • Thermal Lows: Regions of low pressure from warming air in the atmosphere

Mechanism

• Warming of air above the surface leads to pressure dispersion away from the source, resulting in thermal lows; cooling leads to pressure accumulation and thermal highs

Sea and Land Breezes

Description of Breezes

• Sea Breeze: Flows from sea (or lake) to land due to temperature and pressure differences, strongest near the beach in the afternoon.

• Land Breeze: Flows from land to water, predominantly at night when the land is cooler.

Scale of Atmospheric Motion

• Sea breezes exemplify which atmospheric motion scale? Options include:

  • A. Synoptic Scale

  • B. Mesoscale

  • C. Macroscale

  • D. Microscale

  • E. Fishscale

Mountain and Valley Breezes

Development and Effects

• Valley Breeze: Occurs during the day as heated, less dense air in valleys rises.

• Mountain Breeze: Occurs at night when cooler, denser air flows down into valleys.

Katabatic and Chinook Winds

Katabatic Winds

• Downslope breezes stronger than mountain breezes.

• Initiated by very cold, denser air that creates local high pressure leading to movement.

Chinook Winds

• Warm, dry downslope winds caused by strong westerly winds over North American mountain ranges (e.g., Rockies or Cascades).

• Rapid temperature increases and decreases in humidity are notable characteristics.

Desert Winds

Types of Desert Winds

• Dust storms may arise from larger deserts and can traverse oceans.

• Sandstorms occur over sandier deserts.

• Haboob: Dust storms associated with thunderstorm downdrafts.

• Dust Devils: Small, spinning air columns formed by intense local heating.

Monsoons

Definition and Characteristics

• Term "monsoon" derives from Arabic, meaning "seasonal".

• Change dramatically between seasons, driven by thermal circulation.

• In Asia, land cooling in winter leads to high pressure and dry conditions; in summer, land heats more than the ocean, resulting in low pressure and wet conditions.

Comparison to Sea Breeze

• Monsoons function similarly to sea breezes on a much larger scale, influencing significant weather patterns and precipitation.

Wind Direction and Speed

Wind Direction Explanation

• Wind direction is defined as originating from a specific compass point:

  • Wind from 360° = "north wind"

  • Wind from 270° = "westerly wind".

Importance of Prevailing Winds

• Prevailing Wind: The most frequently observed wind direction, crucial for:

  • City planning (e.g., location of landfills, airports)

  • Residential planning (e.g., energy efficiency)

  • Predicting the spread of diseases, plant spores, dust, and volcanic eruptions.

Wind Rose Representation

• A wind rose visually represents these prevailing winds, displaying frequency and direction across the compass.

Instruments to Measure Wind

Common Instruments

• Wind Vane: measures wind direction and points into the wind, typically arrow-shaped; cones (wind socks) are also used.

• Anemometer: measures wind speed; types include:

  • Three-cup anemometer

  • Sonic anemometer (provides direction as well).

• Combined Instruments: such as Wind Monitors or Aerovanes, measure both speed and direction

Homework Assignments

1. Find the wind rose for a city of interest.

2. Identify an aspect of that city unrelated to its airport that reflects prevailing winds.

3. Explain the directional origins of the prevailing winds in the chosen city.