Wind: Small-Scale and Local Systems RECAP Coriolis Force: • ♦ Results from the rotation of the planet. ♦ Maximum at the poles and no effect at the equator. ♦ Acts perpendicular to the direction of motion: changes the direction of the wind but not the wind magnitude. ♦ The force is proportional to the wind velocity: larger force on stronger winds and vice versa ♦ In the NH deflects the wind to the right. ♦ In the SH deflects the wind to the left. RECAP Geostrophic winds • ♦ the pressure force balanced by the Coriolis force. ♦ the wind is parallel to the isobars. ♦ westerly winds in both NH and SH Gradient winds • ♦ pressure force not balanced by the Coriolis force. ♦ Cyclones (L), around centers of low pressure • Counterclockwise in NH, clockwise in SH ♦ Anticyclones (H), around centers of high pressure • Clockwise in NH, counterclockwise in SH Surface winds (today): • ♦ Balance between the pressure gradient force, the Coriolis force and the air friction. ♦ The wind crosses the isobars. • Vertical air motion (today): convergences and divergences. Surface Winds-a balance of three forces In the boundary layer (~1km thick) friction is important! • Friction is acting opposite the direction of the velocity -> friction • reduces the wind speed -> the Coriolis force becomes weaker -> it cannot balance the pressure force. The wind starts to blow across the isobars towards the low pressure • The angle between the direction of the wind and the isobars is on • average 30 deg (Buys-Ballot’s law). It depends on the topography. Buys-Ballot’s Law • Turn your back to the wind, then W. Ferrel Buys Ballot turn clockwise 30 deg. The center James Coffin of low pressure is on your left. Is this a surface or a high-altitude map? Which hemisphere is this? Surface map in the Northern hemisphere Vertical Air Motion: Convergences and Divergences Near a center of low surface pressure there is a • convergence of air -> the air is forced to rise and then diverge at higher altitudes. The opposite takes place near a center of high surface pressure. Hydrostatic Equilibrium On average gravity is balanced by the pressure gradient • force -> hydrostatic equilibrium Small deviations from hydrostatic equilibrium result in • small vertical winds (a few cm/s) Begin Chapter 9 Scales of motion Microscale: short-lived eddies, form around obstructions • Mesoscale: a few – 100 km in size, may last last minutes to hours. • Local winds, thunderstorms, tornadoes, small tropical storms… • Synoptic scale: thousands of kms, may last days or weeks. • Planetary (global) scale. • Macroscale: synoptic + planetary scales. Friction and turbulence Friction of air flow mostly due to • turbulence. Mechanical turbulence: eddy motions • due to obstructions. Wind gusts. ♦ Strong wind speeds ♦ Rough or hilly landscape Thermal turbulence: due to thermals • ♦ Steep lapse rate, unstable atm. Eddies: big and small Size and shape of eddies depend on the size and shape • of the obstacle and the speed of the wind Eddies downwind from a mountain • ♦ Roll eddies (rotors) ♦ Mountain wave eddy The force of the wind Strong crosswinds create dangerous traffic conditions • ♦ Near hills parallel to the road ♦ On high exposed bridges • Especially hazardous for tall vehicles (trucks, SUVs) Microscale winds: some examples sand dune snow roller ripples ripples desert pavement wind-sculptured trees snow snow dune fence Determining wind direction and speed Wind direction: the direction from • which the wind is blowing. Other names: • ♦ Onshore versus offshore winds ♦ Upslope versus downslope winds Expressing wind direction: • ♦ In degrees ♦ In terms of compass points The influence of prevailing winds Prevailing wind: the wind most commonly observed at a • given location over some time period. Importance for city planning, building a house… • Representation: wind rose (the percentage of time the • wind blows from a particular direction). Wind instruments Measuring wind speed: anemometer • Measuring wind direction • ♦ Wind sock ♦ Weather cock ♦ Wind vane Thermal circulations Due to uneven heating of • the surface. Example: ♦ South area heats up, North area cools ♦ Warmer southern air aloft moves north towards low pressure ♦ It then cools and sinks ♦ Surface pressure to the North increases ♦ Surface wind from N to S ♦ The surface air warms up and rises. ♦ The process continues Sea breeze Land breeze Valley breeze Mountain breeze Seasonally changing winds: the monsoon The monsoon is a periodic sea/land breeze on a very large scale • Surface winds in a thermal circulation: from Cold to Warm • ♦ Dry season (winter monsoon, December – February): winds from the cold land towards the warmer seas ♦ Wet season (summer monsoon, June-September): winds from the colder seas towards the warmer continent Desert winds Dust storms and sandstorms • Dust devils (whirlwinds) • ♦ Form on clear hot days over a dry surface (need some convection) ♦ No preference for cyclonic or anticyclonic ♦ Begin at the surface (tornadoes “touch down”) .