7.2 Factors affecting wind speed and direction
4 min read•july 31, 2024
Wind speed and direction are crucial elements in understanding atmospheric dynamics. These factors are influenced by a complex interplay of forces, including pressure gradients, Earth's rotation, and surface . Grasping these concepts is key to comprehending global wind patterns and local weather phenomena.
The chapter on Atmospheric Pressure and Wind explores how these factors work together to create wind systems. From the gentle sea breeze to powerful jet streams, understanding wind behavior helps us predict weather patterns and explain climate variations across the globe.
Factors Affecting Wind
Atmospheric Forces and Wind Generation
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- Pressure gradients drive wind formation
- Created by differences in atmospheric pressure between locations
- Wind flows from high to areas
- Earth's rotation causes
- Deflects wind to the right in Northern Hemisphere
- Deflects wind to the left in Southern Hemisphere
- Friction with Earth's surface impacts wind
- Affects speed and direction in lower atmosphere levels
- Varies based on terrain roughness (smoother over water, rougher over land)
Environmental Influences on Wind Patterns
- shapes local and regional winds
- Mountains create barriers and channeling effects
- Valleys funnel winds and create thermal circulations
- Large-scale atmospheric circulation contributes to global winds
- Hadley cells (tropics)
- Ferrel cells (mid-latitudes)
- Polar cells (high latitudes)
- Temperature differences generate local wind systems
- Sea breezes (daytime flow from cool water to warm land)
- Land breezes (nighttime flow from cool land to warm water)
- Upper-level jet streams affect surface winds
- Narrow bands of strong winds in upper troposphere
- Influence weather systems and surface wind patterns
Pressure Gradients and Wind
Pressure Gradient Fundamentals
- Pressure gradients measure atmospheric pressure change over distance
- Typically expressed in millibars per 100 kilometers
- Steeper gradients produce stronger winds
- Wind flows perpendicular to isobars on weather maps
- Isobars represent lines of equal pressure
- Closer isobar spacing indicates stronger pressure gradients
- balances pressure gradient and Coriolis forces
- Theoretical wind above friction layer
- Flows parallel to straight isobars
Pressure Systems and Wind Behavior
- Cyclones (low-pressure systems) influence wind patterns
- Create counterclockwise rotation in Northern Hemisphere
- Generate clockwise rotation in Southern Hemisphere
- Anticyclones (high-pressure systems) affect wind flow
- Produce clockwise rotation in Northern Hemisphere
- Cause counterclockwise rotation in Southern Hemisphere
- Pressure tendency indicates potential wind changes
- Rate of atmospheric pressure change over time
- Falling pressure suggests approaching low-pressure systems
- Rising pressure indicates high-pressure systems moving in
Friction's Role in Wind
Surface Friction Effects
- Surface friction resists air movement near ground
- Reduces wind speed in lower atmosphere
- Most pronounced in planetary boundary layer (surface to 1-2 km altitude)
- Friction alters wind direction relative to isobars
- Causes wind to cross isobars at an angle towards lower pressure
- Typically 10-20 degrees over water, 30-40 degrees over land
- Surface roughness determines frictional force magnitude
- Smoother surfaces (water, ice) exert less friction
- Rougher surfaces (forests, urban areas) create more friction
Vertical Wind Structure
- Vertical wind shear develops due to friction
- Wind speed increases with height above surface
- Wind direction changes with altitude (veering)
- Diurnal variations in friction affect wind patterns
- Stronger friction during daytime due to increased turbulence
- Weaker friction at night leads to nocturnal low-level jets
- Ekman spiral forms in boundary layer
- Wind direction changes with height due to friction and Coriolis effect
- Typically extends up to 1-2 km in atmosphere
Earth's Rotation and Wind Patterns
Coriolis Effect Fundamentals
- Coriolis effect results from Earth's rotation
- Deflects moving air to right in Northern Hemisphere
- Deflects moving air to left in Southern Hemisphere
- Coriolis force varies with latitude
- Strongest at poles
- Zero at equator
- Influences wind direction but not speed
Global Wind Systems
- form in tropical regions
- Northeast trades in Northern Hemisphere
- Southeast trades in Southern Hemisphere
- dominate mid-latitudes
- Prevailing westerly winds between 30° and 60° latitude
- Stronger in Southern Hemisphere due to less land mass
- Polar easterlies occur in high latitudes
- Easterly winds poleward of 60° latitude
- Generally weaker than other global wind systems
Large-Scale Atmospheric Circulation
- Rossby waves develop in upper-level winds
- Large-scale meanders in jet streams
- Influence global heat and moisture transport
- Affect formation and movement of weather systems
- Hadley, Ferrel, and Polar cells create global circulation
- Hadley cells drive trade winds and tropical convergence
- Ferrel cells influence mid-latitude weather patterns
- Polar cells contribute to polar easterlies and high-latitude climate
Key Terms to Review (16)
Anemometer: An anemometer is an instrument used to measure wind speed and, in some cases, wind direction. It plays a crucial role in meteorological observations, helping scientists understand atmospheric conditions and the dynamics of the atmosphere, as well as informing various applications such as aviation and weather forecasting.
Coriolis effect: The Coriolis effect is the apparent deflection of moving objects, such as air or water, due to the rotation of the Earth. This phenomenon influences global wind patterns, storm systems, and ocean currents, leading to the characteristic rotation of weather systems and variations in local wind behavior.
Friction: Friction refers to the force that opposes the motion of air as it moves across the Earth's surface, resulting from the interaction between the wind and the surface features. This force plays a crucial role in shaping local and global wind patterns by altering wind speed and direction, ultimately influencing weather systems and climate. The impact of friction varies depending on surface roughness, land use, and other geographic factors, which together affect how winds behave at different scales.
Geostrophic wind: Geostrophic wind is the theoretical wind that results from the balance between the pressure gradient force and the Coriolis effect, causing it to flow parallel to isobars rather than directly from high to low pressure. This wind is most significant in the upper atmosphere, where friction is minimal, and it helps explain how large-scale weather patterns develop and move.
Gradient wind: The gradient wind is a theoretical wind that flows along the curved isobars of pressure, balancing the forces of pressure gradient and Coriolis effect. It represents an idealized state of wind flow in the atmosphere, allowing meteorologists to understand how wind behaves in relation to pressure systems. Understanding the gradient wind helps explain how winds develop around high and low-pressure areas, influencing weather patterns and systems.
High pressure: High pressure refers to regions in the atmosphere where the pressure is higher than that of the surrounding areas. These areas are typically associated with descending air, leading to clear skies and stable weather conditions. High pressure systems play a significant role in influencing wind patterns and overall atmospheric dynamics.
Jet stream: The jet stream is a fast-flowing ribbon of air located high in the atmosphere, typically between 6 to 12 miles above the Earth's surface, that plays a crucial role in shaping weather patterns and influencing the movement of air masses. These narrow bands of strong winds can impact temperature and precipitation across regions, connecting different layers of the atmosphere and affecting various weather phenomena.
Katabatic winds: Katabatic winds are cold, dense winds that flow down a slope due to the force of gravity, typically occurring in areas with significant elevation differences like mountains or ice sheets. These winds are influenced by the cooling of air at higher altitudes, which becomes denser and moves downwards, often leading to strong gusts at lower elevations. The presence and behavior of katabatic winds can significantly affect local weather conditions and are particularly notable in polar and mountainous regions.
Low pressure: Low pressure refers to a region in the atmosphere where the atmospheric pressure is lower than that of surrounding areas. This phenomenon typically leads to cloud formation and precipitation, making it essential for understanding weather patterns. In areas of low pressure, air rises and cools, which can create unstable weather conditions, influencing both local climates and broader meteorological systems.
Thermal turbulence: Thermal turbulence refers to the chaotic movement of air caused by variations in temperature, primarily resulting from uneven heating of the Earth's surface. This phenomenon plays a significant role in shaping wind patterns and can influence local weather conditions by causing changes in air pressure and density, leading to the development of vertical currents. Understanding thermal turbulence is essential for grasping how temperature differences drive air movement and contribute to atmospheric dynamics.
Topography: Topography refers to the arrangement of the natural and artificial physical features of an area. It includes the landscape's elevation, slope, and landforms, which can significantly influence climate patterns and wind behavior in a given region. Variations in topography can create microclimates and affect how wind flows, leading to diverse climatic conditions even within small geographical areas.
Trade winds: Trade winds are steady, prevailing winds that blow from east to west in the tropics, originating from high-pressure areas near the subtropical regions and moving towards the equator. These winds play a crucial role in global wind patterns and influence various local weather systems, ocean currents, and climate phenomena.
Tropical cyclones: Tropical cyclones are intense circular storms that originate over warm tropical oceans and are characterized by low atmospheric pressure, high winds, and heavy rain. They play a significant role in global weather patterns and can have devastating effects on coastal regions through storm surges and flooding.
Urban Heat Island Effect: The urban heat island effect refers to the phenomenon where urban areas experience higher temperatures than their rural surroundings due to human activities and the built environment. This effect is driven by factors such as heat absorption by buildings and roads, reduced vegetation, and waste heat from vehicles and air conditioning, leading to significant impacts on local weather patterns and climate.
Westerlies: Westerlies are prevailing winds that blow from the west to the east in the mid-latitudes of both the Northern and Southern Hemispheres, typically between 30° and 60° latitude. These winds play a crucial role in shaping weather patterns and ocean currents, influencing both global wind systems and local climates. The westerlies are affected by the Earth's rotation and the Coriolis effect, making them an essential component of the atmospheric circulation system.
Wind vane: A wind vane is a meteorological instrument used to indicate the direction of the wind. It consists of a rotating pointer or arrow mounted on a vertical axis, which aligns itself with the wind's direction. Understanding how wind vanes work is essential in observing global wind patterns and local wind systems, as they help determine how air moves across different regions. Wind vanes are also critical in surface and upper-air observation systems, providing real-time data about wind direction that is vital for accurate weather forecasting.