1. Measuring air pressure
Modern Barometers
Small chamber, partially emptied of air, which is sealed and connected to a mechanism attached to a needle on a dial. As air pressure increases, it presses on the chamber; as air pressure decreases, it relieves pressure on the chamber – changes in air pressure move the needle.

2. Earth’s gravitational force
Atmospheric Pressure
As the atmosphere is held down by gravity, it exerts a force upon every surface (pressure = force per unit area)
At sea level the force is the weight of 1 kg of air that lies above each square centimeter of the surface (around 15 lbs per in2)

4. Differences in air pressure = a pressure gradient
The pressure gradient force acts at right angles to the isobars (90 degrees)
This exists because the earth is unevenly heated
820
830
840
850
860
820
830
840
850
860
870
880
890
weak pressure gradient
strong pressure gradient

5. Wind: Description and Movement
Anemometer
Wind vane

6. Naming Wind
Figure 6.5

7. Pressure Gradient Force
Figure 6.6a

8. Chapter 6, Figure 6.7c Labeled

9. Chapter 6, Figure 6.7d Labeled

10. Chapter 6, Figure 6.7g Labeled

11. Upper air pressure gradient and
geostrophic wind
Note that once air is motivated to move by the pressure gradient force it is deflected to the right in the Northern Hemisphere and eventually ends up flowing parallel to the isobars rather than across them. After a while under idealized conditions, when the coriolis force is exactly equal and opposite to the pressure gradient, the air flow is said to be in geostrophic balance. Figure 6-13 Lutgens and Tarbuck 2001.

12. SUN But heat is transported from the Equator to the Poles - how? Earth
Cold
High Pressure 0o
30oN
60oN
30oS
60oS
90oN
Warm
Low Pressure
SUN
Earth

13. L warm air rises at the equator producing low pressure (Intertropical
Convergence Zone,
ITCZ) and flows
towards the poles 0o
30oN
60oN
30oS
60oS
90oN L

14. H L H Cold air sinks at 30o N and S latitude Creating high pressure
30oS
60oS
90oN
Cold air sinks at 30o
N and S latitude
Creating high pressure
(subtropical high
pressure, STH) H L H

15. H L H Northeasterly and southeasterly surface winds flow from the
subtropical high pressure belts
(30o N and S) to the low
pressure belt (ITCZ) at
the equator (calm winds:
doldrums)
westerly surface winds
flow from the subtropical
high pressure belts towards
higher latitudes 0o
30oN
60oN
30oS
60oS
90oN H L H

16. L H L H L westerly surface winds are forced
to rise around 60o N and S latitude
when they encounter cold polar
easterly winds from the poles
resulting in Subpolar Low
pressure (SPL) belts 0o
30oN
60oN
30oS
60oS
90oN L H L H L

17. cold air sinks at the poles producing
polar high (PH)
pressure regions 0o
30oN
60oN
30oS
60oS
90oN L H L H L H

18. Figure 5.17, p. 163

19. H L H L H L H Jet streams are streams of fast moving air aloft
polar jet stream
Jet streams are
streams
of fast moving
air aloft
that occur where
atmospheric
temperature gradients
are strong 0o
30oN
60oN
30oS
60oS
90oN L H L
subtropical
jet streams H L H
polar jet stream

20. Chapter 6, Figure 6.9a Labeled

21. Chapter 6, Figure 6.9b Labeled

29. Phase Changes of Water, Latent Heat
Heat is consumed in evaporation, melting; heat is released in condensation, freezing (sublimation).
Schematic view of the molecular structure of water in its three physical states and heat-energy exchange among those states. The latent heat-exchange numbers between the arrows are explained in the text (values are for 0°C).

30. Measurements of water vapor
Vapor pressure is the pressure exerted by water vapor molecules
Saturated vapor pressure is maximum pressure of water vapor at that temperature
Dew point is the temperature the air must be cooled to reach saturation
Relative humidity is ratio of water vapor in air
to maximum the air can hold at that
temperature.
RH is usually highest when daily temp-
erature is lowest.
Specific humidity is the mass of water
Vapor in the air per unit mass of air.
Mixing ratio is the mass of water vapor in
The air to the mass of dry air containing the
Water vapor.
Variation of saturation vapor pressure (mb) with temperature (C). The curve is nearly a pure exponential. At temperatures below 0C saturation values over supercooled water are greater than over ice.

33. “The Mountain Problem”
Effect of orographic lifting
Think of the Cascades as an example;
Relationship between humidity, saturation,
DALR, SALR (or WALR), and Lifting Condensation Level

34. Fig 12.2
Hydrologic cycle. The numbers attached to the stages express each value as the volume of water divided by Earth surface area. Thus the values shown represent the depth of water (centimeters per year) associated with each mass transfer. All can be directly compared to the global average precipitation rate, which is about 100 cm/year.

35. Water in the Hydrosphere
Most of the water is salt water in the ocean
Most of the fresh water is locked up in ice sheets and glaciers
Most of the liquid fresh water is in the ground
Fig 12.3
Distribution of water in the hydrosphere. The middle and lower bars show the percentage distribution of the 2.8 percent of total hydrospheric water that is fresh. Of that freshwater component, only about one-tenth is easily available to humans.

36. Water Usage in the United States, 2005
Which states use the most water?
Total water withdrawals (millions gallons per day) for the United States in 2005.

37. Condensation near the ground forms fog, a cloud in contact with the ground.
As the ground and surface air cools to the dewpoint, water vapor condenses into a radiation fog (cooling by longwave radiation overnight) here in East Africa.

38. Cloud Types
Schematic diagram of the different cloud types – stratus, cumulus, and cirrus, arranged by their typical altitude.

39. Precipitation Processes
The Ice-Crystal Process-requires the coexistence of ice and super-cooled water droplets in the cloud. Ice grows at the expense of water droplets that evaporate water molecules which adhere to the ice until they are large enough to fall as snow. If the air is warm enough, the snow melts and rain occurs.
The Coalescence Process-requires different sizes
of water droplets within warm clouds. Larger
droplets grow by falling faster and sweeping up
smaller droplets by coalescing until they are
large enough to fall as rain.
Source:

40. Types of Precipitation
Besides rain and snow, there is:
Sleet-melting ice that refreezes before reaching ground
Freezing rain-melting ice that freezes on contact with a frozen surface
Hail-ice particles that grow within
clouds that have strong updrafts
Graupel-soft, partially melted hail
Source:
Four major forms of precipitation. (A) A rainstorm douses the Ponderosa Pine Forest near Flagstaff, Arizona. (B) Falling snow accumulates in south-central Alaska. (C) Freezing rain forms an icy coating on pine needles on a golf course in Wawona, near Yosemite National Park, California. (D) Golf-ball-sized hailstones litter the countryside following a storm in northern Texas.

41. Four Forms of PrecipitationB A
A-rainstorm
B-snow
C-freezing rain
D-hail C D

42. Water Balance for Different Climates
Range of water balance conditions found at the surface of Earth. (A) Baghdad, Iraq, experiences a constant deficit because potential evapotranspiration normally exceeds precipitation. (B) At Tokyo, Japan, the situation is reversed, and a constant water surplus is recorded. (C) At Faro, Portugal, the intermediate situation occurs, with a combination of surplus and deficit at different times of the year.

43. Water Balance Averages by Latitude
Average annual latitudinal distribution of precipitation, evapotranspiration, and runoff in cm per year. The arrows show the direction of the water vapor flux by the atmospheric circulation.

44. Global Distribution of Annual Evaporation, Evapotranspiration
Fig 12.10?
Global distribution of annual evaporation and evapotranspiration in centimeters, with land elevations adjusted to sea level. Red isolines show the pattern over land; blue isolines over the oceans.

45. Global Distribution of Annual Precipitation
Source:
Global distribution of annual precipitation in millimeters/day.

46. Air Masses
An air mass is a large body of air with relatively homogeneous character of temperature and humidity.
Classification by latitude, moisture
mT, mP, cA, cP, cT
Movement and transitions-character
of air masses can change as moves over
different land surfaces, or crosses
mountains, ie mP can become cP after
crossing the Rockies, cP can become mT
when moving over warm water.
Source:

47. North American Air Masses & Sources
Source regions and common paths of the principal air masses that affect the continental United States.

48. Lifting Mechanisms That Produce Precipitation
Convergence-when similar air masses
converge, they are forced to rise,
forming clouds
Convection-surface heating causes
warm air parcel to rise and form clouds
Orographic-mountains and large
obstacles force air to rise, forming clouds
Frontal-when warm and cold air masses
meet, cold air lifts warmer air forming clouds
Source:

49. Lifting by Convergence
Along the ITCZ warm trades meet and rise. Precipitation along the ITCZ shifts with season, into the summer hemisphere.
Average daily rainfall rates (mm/day) for January and July based upon measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite for the years

50. Frontal Precipitation
Vertical cross-section through a warm front (top) and cold front (bottom). The vertical scale is greatly exaggerated. Warm fronts typically have a 1:200 vertical-to-horizontal ratio; the ratio for cold fronts is approximately 1:70.

51. Convectional Precipitation
Convection often leads to isolated showers from building cumulus clouds. Occasionally, these clouds grow large enough to form thunderstorms. Each thunderstorm cloud, cumulonimbus, has a life cycle of 3 stages.
The three stages of the life cycle of an air mass thunderstorm: cumulus, mature, and dissipating. Updrafts dominate the developing stage, both updrafts and downdrafts are found during the mature stage, while only downdrafts are found during the dissipating stage. The approximate width at each stage of the thunderstorm is shown at the bottom.

52. Cumulonimbus grows near the Canadian Rockies in an afternoon thunderstorm.
Source:

53. Severe Thunderstorms
Conceptual model of a severe thunderstorm producing sizeable hailstones. The red dashed lines represent the warm updrafts, the blue lines show the cold downdrafts, and the green lines represent the movement of hailstones. The thunderstorm is moving from left to right.

54. Orographic Precipitation
Fig 13.7
Orographic precipitation on the upper windward slope of the Cascade Mountains in west-central Oregon. Note the significant temperature and moisture differences between the windward and leeward sides of this major mountain barrier. (Vertical scale is greatly exaggerated.)

55. Fig 13.8
Oregon’s precipitation pattern, with the distribution of isohyets exhibiting the results of the orographic effect as westerly winds off the Pacific are forced across the north-south-trending Cascade Mountains. The transect across the Cascades between Eugene and Bend, diagrammed in Fig , is marked by a red line.