What set the atmosphere in motion?

Review of last lecture Thickness of the atmosphere: less than 2% of Earth’s thickness Thickness of the atmosphere: less than 2% of Earth’s thickness Definition of temperature and its unit. Definition of temperature and its unit. Four layers of the atmosphere, what separate them? Four layers of the atmosphere, what separate them? Definition of pressure and its unit. Definition of pressure and its unit. Definition of pressure gradient. Pressure gradient sets the air in motion. Definition of pressure gradient. Pressure gradient sets the air in motion. Equation of state (Relationship between P, ρ, and T) Equation of state (Relationship between P, ρ, and T) Vertical Pressure Distribution. How does pressure change with height? What is the hydrostatic equilibrium? Vertical Pressure Distribution. How does pressure change with height? What is the hydrostatic equilibrium? Horizontal Pressure Distribution. Most common atmospheric circulation structure Horizontal Pressure Distribution. Most common atmospheric circulation structure

The most common atmospheric circulation structure L H H L Heating Cooling or No Heating Imbalance of heating  Imbalance of temperature  Imbalance of pressure  Wind

Horizontal Pressure Gradients and wind The pressure gradient force initiates movement of atmospheric mass, wind, from areas of higher to areas of lower pressure Horizontal wind speeds are a function of the strength of the pressure gradient Pressure and winds plotted on same chart  Notice the strong winds in Ohio – due to tight pressure gradient

Forces Affecting the Speed and Direction of the Wind Horizontal pressure gradients responsible for wind generation Horizontal pressure gradients responsible for wind generation Three factors affect wind speed and/or direction (velocity): Three factors affect wind speed and/or direction (velocity): 1. Pressure Gradient Force (PGF) 2. Coriolis Effect (CE) 3. Friction Force (FF)

1. Pressure Gradient Force (PGF) pressure gradient: high pressure  low pressure pressure differences exits due to unequal heating of Earth’s surface spacing between isobars indicates intensity of gradient flow is perpendicular to isobars

Video: Coriolis effect

2. The Coriolis Effect objects in the atmosphere are influenced by the Earth ’ s rotation objects in the atmosphere are influenced by the Earth ’ s rotation Rotation of Earth is counter-clockwise looking down from N. Pole. Rotation of Earth is counter-clockwise looking down from N. Pole. results in an ‘ apparent ’ deflection (relative to surface) results in an ‘ apparent ’ deflection (relative to surface) deflection to the right in Northern Hemisphere (left in S. Hemisphere) deflection to the right in Northern Hemisphere (left in S. Hemisphere) Greatest at the poles, 0 at the equator Greatest at the poles, 0 at the equator Increases with speed of moving object and distance Increases with speed of moving object and distance CE changes direction not speed CE changes direction not speed

Winds in the upper air: Geostrophic Balance Now the wind speed/direction is simply a balance between the PGF and CE. This is called GEOSTROPHIC BALANCE. Upper air moving from areas of higher to areas of lower pressure undergo Coriolis deflection Air will eventually flow parallel to height contours as the pressure gradient force balances with the Coriolis force Friction is very small in the upper air:

Winds near the surface Friction slows down wind speed and reduces Coriolis deflection Friction slows down wind speed and reduces Coriolis deflection Friction is important for air within ~1.5 km of the surface (the so- called planetary boundary layer). It varies with surface texture, wind speed, time of day/year and atmospheric conditions. Friction above 1.5 km is often small (often called the free atmosphere), except over regions with storms and gravity waves. Friction is important for air within ~1.5 km of the surface (the so- called planetary boundary layer). It varies with surface texture, wind speed, time of day/year and atmospheric conditions. Friction above 1.5 km is often small (often called the free atmosphere), except over regions with storms and gravity waves. The third term (friction) must be considered:

Effect of frictional force Upper air w/out Friction (geostrophic balance) Near surface w/ Friction Counterclockwise in NH (opposite in SH)characterized by ascending/diverging air which cools to form clouds/precipitation Clockwise airflow in NH (opposite in SH) Characterized by descending/converging air which warms creating clear skies

Upper air: isobars usually not closed off Troughs (low pressure areas) Troughs (low pressure areas) Ridges (high pressure areas) Ridges (high pressure areas) Near surface: isobars usually closed off due to surface friction Cyclones (Low pressure areas) Cyclones (Low pressure areas) Anticyclones (High pressure areas) Anticyclones (High pressure areas) Cyclones, Anticyclones, Troughs and Ridges

Summary Know 3 Forces that affect wind speed /direction Know 3 Forces that affect wind speed /direction Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force? Especially work on Coriolis force, as this is the hardest to understand. Which direction is air deflected to by Coriolis force? What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds What is the geostrophic balance? At which level is it valid? Difference between upper level and surface winds Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter- clockwise around them? How about anticyclones? Does cyclones correspond to high or low surface pressure? Is the air moving clockwise or counter- clockwise around them? How about anticyclones?