Atmospheric Thermodynamics

1. Atmospheric Thermodynamics Kyle Imhoff Spring 2016

2. Thermodynamics • The study of thermodynamics focuses on the physics of molecules and their movements • In atmospheric science, thermodynamics play a key role in understanding exactly how the physical processes that drive the weather work • Fundamental in meteorology

3. The Thermal Connection • Physical Principle • Temp proportional to kinetic energy (1/2*mv2) • Thermometers measure themselves • must be protected/shade - at six feet AGL (2m) • type of thermometers: • liquid in glass (Hg,alcohol) • resistance based (change in current flow prop to T) • bimetallic (thermostats) • Placement is critical! • Temperature drift is almost always upward

4. The Thermal Connection • Insolation warms the ground • The ground heats the air • by conduction (direct contact) • by convection (bubbles of warm air) • The air is heated from below (by IR out) • maximum temp at surface • The air absorbs UV from above • second max in temp in stratosphere • see vertical profile of atmospheric temperature

5. The Thermal Connection

6. The Thermal Connection

7. Daily Temperature diurnal cycle min in am / max in pm amplitude of cycle amount of insolation sun angle/day length latitude (low - constant) (mid & hi - varies) continent vs. maritime soil/vegetation type lag due to balance of incoming vs. outgoing Annual Temperature varies with seasons Low latitudes - small changes (due to wet/dry season) Mid latitudes - larger changes (due to coast vs inland) Hi latitudes - largest changes where most continental Lag due to heat capacity differences The Thermal Connection

8. The Pressure is On! • Definition • Pressure = Force / unit area • weight of the air above us • Pressure always decreases with height • Measurements • Barometers • mercury, water, alcohol • units: inches, millibars, hecta-pascals (Hpa)

9. Ideal Gas Law PV=nRT at constant V: P is proportional to T but inverse to density V is not constant adiabatic - no energy change expands as rises -cools contracts as descends and warms Pressure Gradients Wind is driven by PGF Always from Hi to Lo Horizontal PGF 980 to 1030 = 50/1000 surface winds: 5-25m/s Vertical PGF 500 to 1000 = 1/2 vertical winds: 5-25cm/sec The Pressure is On!

10. Pressure change in Warm Air High Temperature, lower density (Tropics) Pressure decreases more slowly with height Therefore; higher pressure aloft at the same elevation Pressure change in Cold Air Low Temperature, higher density (Polar) Pressure decreases more quickly with height Therefore; lower pressure aloft at the same elevation The Pressure is On!

11. The Pressure is On!

12. The Pressure is On! • High pressure aloft over Tropics • Low pressure aloft over the Polar Regions • PGF goes from Hi to Lo pressure, so the wind blows from South to North (NH) • Include the spinning Earth Force (Coriolis) and the wind turns to the right - Westerlies!

13. The Pressure is On!

14. The Pressure is On! • Coriolis Force - an apparent force on a rotating sphere • air at equator is moving at 22,000 miles/day • or nearly 1,000 miles an hour • air at the poles is not spinning at all • or has no velocity • when air travels from the equator toward the poles it is bringing with it this ‘apparent’ speed • causes air to turn to ‘right’ in NH, ‘left’ in SH

15. The Pressure is On!

16. Geostrophic Balance • Geostrophic balance refers to a balance of forces between the PGF and the Coriolis force – this leads to winds flowing parallel to isobars (lines of equal pressure)

17. Geostrophic Balance • This theoretical balance of forces is a very good approximation at the upper levels of the atmosphere • Does not work as well at the surface • Friction plays a role here

18. The Force of Friction always acts against the velocity proportional to speed primarily in the boundary layer of the atmosphere Friction turns the wind turns the wind toward low pressure away from high pressure usually at 20-40 degree angle The Pressure is On!

19. Resulting Balance • As a result of surface friction, winds blow clockwise away from centers of high pressure (divergence) and blow counter-clockwise and toward centers of low pressure (convergence) • When air converges at the surface, it must go somewhere – in this case, up

20. Vertical Motion • Vertical motions associated with low pressure centers are the key ingredient to clouds and precipitation • As a parcel (block) of air rises, its temperature will cool. Assuming the moisture content of the air remains relatively constant, the RH of that parcel increases. Once RH gets to 100%, clouds form and precipitation can fall out of those clouds • Understanding and accurately assessing the magnitude of vertical motions in the atmosphere is crucial to accurately predicting the weather – it is not easy to do, unfortunately!

21. Summary • Thermodynamics is key to understanding atmospheric motions • Pressure always decreases with height in the atmosphere • Temperature, however, does not • Winds blow clockwise and away from centers of high pressure, counter-clockwise and toward centers of low pressure • Low pressure at the surface is associated with convergence, rising motion, and precipitation