Weather Studies Introduction to Atmospheric Science American Meteorological Society

1. Weather StudiesIntroduction to Atmospheric ScienceAmerican Meteorological Society Chapter 8 Wind and Weather Credit: This presentation was prepared for AMS by Michael Leach, Professor of Geography at New Mexico State University - Grants

2. Case-in-Point • This Case-in-Point talks of the sinking of the Edmund Fitzgerald in Lake Superior in 1975 • At the time, it was the largest ore carrier in the Great Lakes at 222 m (729 ft) • An intense low-pressure system moved over the Great Lakes • Wind speeds were estimated to be 95 km/hr (58 mph) gusting to 137 km/hr (85 mph) with waves of 3.5 to 5 m (12 to 16 ft) • This is the shipwreck that was memorialized in a song by Gordon Lightfoot • The Case-in-Point serves as a reminder that even the largest ships are vulnerable to wind and weather

3. Driving Question • What forces control the speed and direction of the wind? • Different weather systems bring different types of weather depending on the air circulation (wind) that characterizes each system • Wind is the local motion of air measured relative to the rotating Earth • In this chapter we will: • Investigate the various forces that either initiate or modify atmospheric circulation • First, each force will be examined separately • Then, they will be combined to show how together they drive atmospheric circulation

4. Monitoring Wind Speed and Direction • Wind velocity is a vector quantity • Meaning it has both magnitude (speed) and direction • Wind is usually distinguished between horizontal and vertical components • The magnitude of vertical air motion is typically only 1% to 10% of the horizontal wind speed • The most common instruments only measure the horizontal wind • A wind vane consists of a free rotating and counterweighted arrow that points into the wind • Wind direction is the direction it is coming from, not blowing to • A windsock stretches downwind • Wind speed can be estimated by its effect on water using the Beaufort scale

5. Beaufort Scale of Wind Force

6. Monitoring Wind Speed and Direction • Wind speed is measured using a cup anemometer • The speed at which the cups spin is translated into wind speed • A hot wire anemometer measures the loss of heat from a heated wire, and translates that into wind speed • Aerovanes measure wind speed and direction • A 3 or 4 blade propeller spins at a rate proportional to the wind speed and a fin on the back turns it into the wind, indicating direction. Electronic sensors are connected to a recording computer or digital display. • A sonic anemometer consists of 3 arms that send and receive ultrasonic pulses. Sound wave travel times are translated into wind speed and direction • Scheduled to replace cup anemometers in NWS ASOS • Instruments should be mounted at 10 m (33 ft) above the ground. Rooftop locations should be avoided. • Radiosondes, satellites, and wind profilers measure winds aloft

7. Monitoring Wind Speed and Direction Sonic Anemometer Time variations in wind speed and direction over a six-hour period Cup Anemometer

8. Forces Governing the Wind • A force is a push or pull that can cause an object at rest to move, or that alters the movement of an object already in motion • Force has both direction and magnitude (vector quantity) • It is useful to apply each force governing the wind to a parcel that is a unit mass (e.g., single kilogram) or air • Newton’s second law of motion • Force = mass x acceleration • Acceleration (a change in velocity) is a response to a force • Forces that act on wind are a consequence of: • An air pressure gradient • Centripetal force (occurs as a consequence of other forces) • Coriolis Effect (an apparent, but not a true force) • Friction • Gravity

9. Forces Governing the Wind • Pressure Gradient Force • A gradient is a change in some property over distance • Air pressure gradients exist whenever air pressure varies from one place to another • A horizontal pressure gradient refers to air pressure change along a constant altitude surface • They can be determined on weather maps from isobar patterns. By U.S. convention, isobars are drawn at 4-mb (4-hPa) intervals and interpolation between stations is always necessary. • A vertical air pressure gradient exists over a certain point and is a permanent feature of the atmosphere

10. Pressure Gradient Force • Example: Sloshing water back and forth in a tub creates pressure gradients along the tub bottom, analogous to a horizontal air pressure gradient in the atmosphere. In response to a pressure gradient, water (or air) flows from an area of higher pressure to an area of lower pressure.

11. Forces Governing the Wind • Centripetal Force • Isobars plotted on a surface weather map are almost always curved; the wind blows in curved paths. Curved motion indicates the influence of the centripetal force. • Center-seeking force; the string exerts a net force on the rock by confining it to a curved path • Increasing the rotation rate or shortening the string requires a large centripetal force • Not an independent force; the tension in the string is responsible for the centripetal force • If the string is cut, the centripetal force no longer operates and the rock flies off in a straight line as described by Newton’s first law of motion (an object in straight-line, unaccelerated motion remains that way unless acted upon by an unbalanced force). • Results from an imbalance in other forces operating in the atmosphere

12. Forces Governing the Wind • Coriolis Effect • Frame of reference example: In looking at the Earth from space, a storm system appears to move in a straight line at constant speed. Meanwhile, an observer on Earth observes the storm center following a curved path. • Curved motion implies that a net (or unbalanced) force is operating and unaccelerated, straight motion implies a balance of forces • A net force operates on the Earthbound rotating coordinate system whereas forces are balanced in the non-rotating system fixed in space • The net force responsible for curved motion is the Coriolis Effect

13. Coriolis Effect • The familiar north-south, east-west frame of reference rotates eastward in space as Earth rotates on its axis. Rotation of the coordinate system gives rise to the Coriolis Effect.

14. Forces Governing the Wind • Coriolis Effect, continued • Deflection is to the right in Northern Hemisphere and to the left in the Southern Hemisphere • Deflection is strongest at the poles, decreases moving away from poles, and is zero at the equator • Fast-moving objects are deflected more than slower ones because faster objects cover greater distances. The longer the trajectory, the greater is the shift of the rotating coordinate system with respect to the moving air parcel • Coriolis Effect only significantly influences the wind in large-scale weather systems

15. Forces Governing the Wind • Friction • The resistance an object or medium encounters as it moves in contact with another object or medium • The resistance of fluid (liquid and gas) flow is termed viscosity • Two types: • Molecular viscosity: the random motion of molecules in the fluid • Eddy viscosity (more important): arises from much larger irregular motions, called eddies • Atmospheric boundary layer: the zone to which frictional resistance (eddy viscosity) is essentially confined • Above 1000 m (3300 ft), friction is a minor force • Turbulence: fluid flow characterized by eddy motion • We experience turbulent eddies as gusts of wind

16. Examples of Eddy Viscosity Stream Example: Rocks in a streambed cause the current to break down into eddies that tap some of the stream’s energy so that the stream slows Snow Fence Example: A snow fence taps some of the wind’s kinetic energy by breaking the wind into small eddies. Wind speed diminishes, causing loss of snow-transporting ability

17. Forces Governing the Wind • Gravity • The force that holds objects to the Earth’s surface • Net result of gravitation and centripetal force • Gravitation is the force of attraction between the Earth and some object • It’s magnitude is directly proportional to the product of the masses of Earth and the object • It is inversely proportional to the square of the distance between their centers of mass • The much weaker centripetal force is caused by the Earth’s rotation • Gravity always acts directly downward • It does not influence horizontal wind • It only influences air that is ascending or descending • Accelerates a unit mass downward toward Earth’s surface at 9.8 m per sec each second

18. Forces Governing the Wind • Summary • Horizontal pressure gradient force is responsible for initiating almost all air motion • Accelerates air parcels perpendicular to isobars, away from high pressure and toward low pressure • Centripetal force is an imbalance of actual forces • Exists when wind has a curved path • Changes wind direction, not wind speed • Always directed inward toward center of rotation • Coriolis Effect arises from the rotation of Earth • Deflects winds to the right in the Northern Hemisphere • Deflects winds to the left in the Southern Hemisphere • Friction acts opposite to the wind direction • It increases with increasing surface roughness • Slows horizontal winds within about 1000 m (3300 ft) of the surface • Gravity accelerates air downward • It does not modify horizontal winds

19. Joining Forces • Newton’s first law of motion • When the forces acting on a parcel of air are in balance, no net force operates, and the parcel either remains stationary, or continues to move along a straight path at a constant speed • Interaction of forces control vertical and horizontal air flow through: • Hydrostatic equilibrium • The geostrophic wind • The gradient wind • Surface winds, horizontal winds within the atmospheric boundary layer

20. Joining Forces • Hydrostatic equilibrium • Air pressure always declines with altitude • Vertical pressure gradient force is upward • Were this the only force, air would accelerate away from Earth • Counteracting downward force is gravity • Balance between the two forces is hydrostatic equilibrium • Slight deviations from hydrostatic equilibrium cause air parcels to accelerate vertically

21. Joining Forces • Geostrophic wind • Winds blowing at a large scale tend to parallel isobars with low pressure on the left in the Northern Hemisphere • Geostrophic wind is a horizontal movement of air that follows a straight path at altitudes above the atmospheric boundary layer • Caused by a balance between the horizontal pressure gradient force and the Coriolis Effect • Develops only where the Coriolis Effect is significant (i.e., in large-scale weather systems)

22. Joining Forces • Gradient Wind • Shares many characteristics with the geostrophic wind • Large-scale, frictionless, and blows parallel to the isobars • The path of the gradient wind is curved • Forces are not balanced because a net centripetal force constrains air parcels to a curved trajectory • Occurs around high and low pressure centers above the boundary layer • High (anticyclone) in N. Hemisphere • Coriolis Effect is slightly greater than the pressure gradient force giving rise to an inward-directed centripetal force • Wind is clockwise • Low (cyclone) in N. Hemisphere • Pressure gradient force is slightly greater than the Coriolis Effect giving rise to an inward-directed centripetal force • Wind is counterclockwise

23. Joining Forces • Surface Winds • Friction slows the wind and interacts with the other forces to change wind direction • Friction combines with the Coriolis Effect to balance the horizontal pressure gradient force • Friction acts directly opposite the wind direction whereas the Coriolis Effect is always at a right angle to the wind direction • Winds now cross isobars at an angle, which depends on roughness of Earth’s surface • Angle varies from 10 degrees or less to 45 degrees

24. Joining Forces • Surface Winds, cont. • The closer to the Earth’s surface the winds are, the more friction comes into play • For the same horizontal air pressure gradient, the angle between the wind direction and isobars decreases with altitude in the atmospheric boundary layer

25. Joining Forces • Surface winds in the Northern Hemisphere • Surface winds blow clockwise and outward in a high (anticyclone) • Surface winds blow counterclockwise and inward in a low (cyclone) • In the Southern Hemisphere, surface winds in a cyclone blow clockwise and inward; in an anticyclone winds blow counterclockwise and outward

26. Joining Forces On a typical surface weather map, isobars exhibit clockwise curvature (ridges) and counterclockwise curvature (troughs)

27. Continuity of Wind • Horizontal and vertical components of the wind are linked • Surface winds follow Earth’s topography • Uplift occurs along frontal surfaces • In a surface high, horizontal winds diverge from the center. A vacuum does not develop because air slowly descends to replace diverging air at the surface. Aloft, horizontal winds converge above the center of the surface high. • Anticyclones typically have clear skies and a weak horizontal pressure gradient Warming & evaporation H

28. Continuity of Wind • In a surface low, horizontal winds converge toward the center. Air does not pile up at the center, but ascends in response to converging surface winds and diverging winds aloft. • Cyclones are typically stormy weather systems with cloud and precipitation development Cooling & Condensation L

29. Continuity of Wind Surface winds accelerate and undergo horizontal divergence when blowing from a rough surface to a smooth surface (e.g., from land to water). Surface winds undergo horizontal convergence when blowing from a smooth to a rough surface (e.g.’ from water to land). Divergence of surface winds causes air to descend, whereas convergence of surface winds causes air to ascend.

30. Scales of Weather Systems • Planetary-scale systems: large-scale wind belts encircling the planet (e.g., midlatitude westerlies, trade winds) • Synoptic-scale systems: continental or oceanic in nature (e.g., migrating cyclones, hurricanes, and air masses) • Mesoscale-scale systems: circulation systems that influence weather in part of a large city or county (e.g., thunderstorms, sea breeze) • Microscale systems: weather system covering a very small area such as several city blocks (e.g., weak tornado)

31. Conclusions • Unequal rates of radiational heating and cooling within the Earth-atmosphere system are responsible for temperature gradients • The atmosphere circulates in response and heat energy is converted to kinetic energy • Various forces studied in this chapter shape atmospheric circulation (the wind) • This chapter built a realistic model of atmospheric motion that demonstrates why and how winds circulate around high and low pressure systems