1. Chapter 1 Properties of the Atmosphere How is the atmosphere characterized?

2. Preliminaries 90% of the atmosphere’s mass is between mean sea level (MSL) and 12 km (7.5 mi) above MSL Atmospheric compostion: –N 2, 78%; –O 2, 21%; –water vapor is variable but very important; –aerosols, clouds, precipitation Forms of water in the atmosphere –Vapor (gas) –Liquid (cloud droplets and rain drops) –Solid (ice crystals and ice precipitation)

3. Temperature Defined as a measure of the average speed (energy of motion) of molecules in a substance. Well, what about liquids and solids? –Molecules in solids experience vibrations –Molecules in a liquid have limited path lengths over which no collisions take place

4. Temperature scales

5. Metric system Length: meters (m) or kilometers (km) Time: seconds (s) –Speed: m s -1 ; acceleration: m s -2 Mass: kilogram (kg) –Density: kg m -3 Force: Newton (N, kg m s -2 ) Pressure: Pascal (Pa, N m -2 = kg m -1 s -2 ) Energy: Joule (J, N m = kg m 2 s -2 )

6. Distribution of surface (2 m AGL) temperature in January and July T = temperature What causes temperature changes? - Daily variation? - Annual variation?

7. T pattern over the U.S. today

8. Annual variation of T

9. Seasons (tilt of earth’s axis of rotation

10. Vertical variation of T (standard atmosphere)

11. Tropopause variation vs latitude

12. Pressure Force per unit area (Pa = N m -2 ) Weight of a column of air above a unit area –All molecules are summed in that column

13. Columns with different weights

14. Vertical profile of pressure

15. Mercury barometer

16. p and wind over the U.S. today

17. Moisture Water vapor Clouds Precipitation Water vapor’s benefit Measurements of water vapor

18. Measures of water vapor Vapor pressure –Partial pressure (e) due to water vapor –A direct measure of the total number of H 2 O molecules Dew point temperature –Temperature at which saturation is attained Saturation  100% relative humidity (e = e s ) RH = e/e s (see p. 11) –Commonly plotted on surface weather maps –Depends on (a) amount of water vapor in the air, and (b) the amount of water vapor that the air can “hold” at a given temperature

19. Average vapor pressure (e) and dew point temperature (D) in Jan. & July Note the relationship between e and D highest lowest highest lowest

20. Saturation vapor pressure and T RH = Relative humidity RH = (vapor pressure / saturation vapor pressure) x 100% or RH = e/e s The curve is exponential, meaning that the rate of increase in es with increasing temperature increases as T increases. e s = Ae- B/T

21. Daily variation of T and RH What is the relation between T and RH? If the absolute amount of water vapor does not change, then at low T, RH is high, and at high T, RH is low. Look at the behavior of RH and T today here in Huntsville.

22. T d pattern over the U.S. today

23. Integrated water vapor Precipitable water (PW) –The depth of liquid water that would result if all of the water vapor in the column between the surface and the “top of the atmosphere” were condensed. Refer to NSSTC web site (SuomiNet) http://vortex.nsstc.uah.edu/mips/data/cu rrent/surface/ Water vapor TOA sfc Condensed water PW Extra!!

25. Phase changes and latent heating Latent heating is associated with a change of phase in water: water vapor liquid (cloud drops, rain drops) ice (ice crystals, ice precip.) Latent heating is the primary source of energy of thunderstorms Latent heating has profound effects on many atmospheric systems

26. Air density (  ) Number of air molecules per unit volume –Mass per unit volume: kg m -3 Cannot directly measure density Equation of state is used to calculate  –p  RT or r = p/RT –R is the gas constant Density is low on a hot day in Denver, CO Density is high on a cold day in International Falls, MN Advanced concept

27. Wind Wind is the movement of air Wind is measured with anemometers Doppler radar/lidar/sodar Direction – defined as the direction from which the wind blows Speed – mph, knots, or m/s Symbols The importance of wind: Transports temperature and water vapor horizontally Strong winds produce damage

28. Relation between wind speed (V) and the pressure gradient (PG): V  1/PG Strong wind Weak wind Winds are stronger over the water surface (lower friction)

29. Automated Surface Observing System (NWS) Fig. 2A from Ch. 2

30. A question for thought Is there a relation between temperature and pressure?

31. Homework Test your problem solving skills, p. 17 –Numbers 2, 3