1. Modern Day Satellite Photos
Weather Service began as part of the War Department September 1, 1872
Modern Day Satellite Photos
Geostationary Operational Environmental
Satellite (GOES) Program
Operated by National Weather Service (NWS)
Cloud charts, Thermometers, Wet Bulb shoelaces, Humidity charts. pinwheels

2. Density falls off with altitude; pressure is caused by impacts, dense air has more impacts

3. Buoyancy Lifting Start Local Heat Air Expands Dense Air Jiggles
Underneath
(heated atoms
speed up
paddle board
analogy)
Atoms close together (dense, high pressure)
“fall into” the empty (low pressure) area
forces warm
air up

4. Pressure differences cause WIND

5. WIND Wind is caused by differences in pressure
Pressure and Density of air are two aspects of the same thing
Atoms close together (in dense, high pressure areas ) “fall into” the empty (low pressure) area

6. H L The greater the contrast in pressure the faster the wind.
One atmosphere (pressure at surface) is about 1000 millibars
These red isobars are lines of equal pressure L H

7. Initially wind flows from high to low
Pressure Gradients
Coriolis turning
Initially wind flows from high to low
but Coriolis turns it nearly parallel
to lines of equal pressure (isobars)

8. Winds blowing parallel to isobars are called geostrophic winds
This occurs well above the surface where there is no friction
Notice pressure gradient force always from high to low pressure, but Coriolis perpendicular to actual flow direction

9. Winds blowing parallel to isobars are called geostrophic winds
Winds Aloft, maybe 3 km up
Wind speed feathers on the “from” end. Winds named for the “from” direction

10. Friction turns surface winds back toward the pressure gradient.
Near the surface, winds almost move from High to Low pressure
They spiral counterclockwise into a Low in Northern Hemisphere

11. L lowest local pressure
Near the surface, winds almost move from High to Low pressure
They spiral counterclockwise into a Low in Northern Hemisphere
L lowest local pressure
Blue arrows PGF
Red arrows Coriolis
Black arrows wind path L

12. Polar Cell
Ferrel Cell
Hadley Cell
Since earth’s rotation turns air, flows are broken into 3 cells per hemisphere

13. Horizontal temperature differences
Temperature effects density and pressure:
If you heat something it expands and gets less dense
A 500 millibar pressure level is much higher in hot air. Hotter air has lower density and greater volume
P = r R’ T so T = P /r R’
500 mb
700 mb
850 mb
1000 mb
Psurface
Warm
Cold

14. Polar Jet Formation
Steep gradients of Pressure cause higher velocity geostrophic winds.
This is the trigger for jet stream flow.
LOW _____________________HIGH

15. Polar air is denser, so it wedges under the low density warm air
Polar air is denser, so it wedges under the low density warm air. Rotation causes an eddy to form
For the Polar Jet, the eddy is in the Ferrel cell on the upper polar side, and so air flows from the west to the east, the “Westerlies”
Since the pressure difference is great at the boundary, the jet is a very fast wind

16. Air Masses
An air mass develops when the atmosphere is located above a relatively uniform land or water surface for several days.
The lower atmosphere assimilates some of the properties of the underlying surface.

17. Air Masses
Large regions (1,000s km2) of the lower troposphere with uniform characteristics (temperature, moisture content) originally defined by a source area.
Labels refer to temperature (arctic vs. polar vs. tropical) and source area (continental vs. maritime). The source area determines the moisture content.
Labels around here: cT cP mT mP

18.
Polar air enlarges and moves further south during winter and retreats northward during summer.

19. Typical Air Mass Changes
Heating (cP air moving south) will lead to instability (bouyancy) as warms.
Cooling (mT air moving
north) has the opposite effect, because colder air cannot rise
Orographic Lifting forces maritime mP and mT air upward over mountain ranges in the western U.S., leading to condensation and precipitation that converts the formerly humid air to a much dryer air mass.

20. Air Mass Interactions cP mT
Weather in any region is influenced by the interactions between air masses. Recall that the boundaries between contrasting air masses are called FRONTS

21. cP mT
Heavy rains can result from the interaction between the continental polar cP air mass and the maritime
tropical mT air mass

22. Mid-latitude Cyclones
ExtraTropical Cyclones are up to 2,000 km across & control the weather for three days to a week.
Winds circulate counter-clockwise (CCW) around the Low in Northern Hemis. L
Def: Synoptic Scale: space and timescales of
mid-latitude depressions
i.e. several thousand kilometers and timescales of several days

23. HIGH LOW HIGH LOW 1. ITCZ 8. Cold air sinks
Polar Cell
LOW
7. Mid-Latitude Cyclones are dominant where Ferrel meets Polar
4. Ferrel Cell
Mid-Latitude Cyclones north
Dry south 5.
HIGH
6. Gyre 2.
to E
0. Maximum
heat from
Sun
LOW
1. ITCZ

24. Dishpan Experiment Rossby Waves
Why are the boundaries between cold and hot not straight?
Camera rotates with dish
Cold Center hot edge
Large scale waves
and eddies form
They flow CW & CCW
Cold mass is denser and forces its way under the warmer mass
Rossby Waves

25. Mid-Latitude Cyclones
This clash between cP and mT air masses is the most common source of frontal systems in the U.S.
Weather conditions, and cloud types, change in a predictable sequence as warm and cold fronts pass over an area.
A front is a transition from one air mass air mass to another

26.
cP mT

27. DBZ is Decibels received back

28. Map Symbols These show surface positionscP mT

29. Warm Front
Ahead of a warm front, warm, humid air is transported upward over a distance of approximately 1,000 km (625 miles).
Rain may last longer than for a cold front because the warm front moves slowly and extends over a larger area.
Dangerous storms if cold air is very cold

30. Warm Front Clouds and Winds
First sign of warm front is sequence of clouds (cirrus, cirrostratus, altostratus).
If winds are right, up to 12 hours after the cirrus, the higher clouds will be replaced by lower nimbostratus with associated light to moderate precipitation.
Temperatures and humidity rise and winds typically shift direction (first from the south or southeast, then from the southwest) with the passage of the warm front.

31. Warm Front Changes
Expect increasing Temperature and Humidity as the warm front passes

32. Cold Front changes
Cold, dense continental polar air cP replaces moist, warm maritime tropical air mT across a cold front
Expect decreasing temperature and humidity and increasing atmospheric pressure with the passage of the cold front.

33. Warm Air Rises
Warm wet air has lowest density, so it will always rise over cooler air.
Both the cold and warm fronts are inclined toward the warm air mass.

34. Approaching Cold Front is behind Thunderstorms
Warm air is pushed up and over the advancing cold front, causing
relatively rapid cooling and condensation that results in the
development of tall cumulonimbus clouds. They host heavy but
relatively short-lived precipitation

35. Cold Front cloud bands are narrow
because cold front wedges are steep

36. because the edge of the cold air mass is steep
Cold Fronts are narrow
because the edge of the cold air mass is steep

37. A Cold Front Squall Line
Rapidly advancing cold fronts may be marked by the growth of a squall line of thunderclouds

38. Cyclogenesis 1 – Stationary Front A small scale wave formscP mT
These persist if the air masses have equal pressure
Then many small waves form, and storms are very frequent
Mid-Latitude Cyclones start as Stationary Fronts.

39. Cyclogenesis 2 – Warm and Cold FrontscP mT
In the Ferrell cell, winds have a strong westerly component, and storms move East.
As they mature and move, Warm Fronts pass over, followed by Cold Fronts.

40. Cyclogenesis 3 – Occluded Front forms
Where the cold front catches up, they are called Occluded Fronts. Note symbol. cP mT
Cold front catches warm, forcing warm air aloft. Broad precipitation area results.

41. Occluded Fronts
The cold front is faster than a warm front and will eventually close the gap between the fronts, forcing the intervening warm air upward generating additional rain mT
Nimbostratus cP
Rain then covers a wide area

42. Storm evolution mT cP 2. Wide precipitation band mT
1. Surface map, so winds cross
Isobars and spiral into the LOW
Along the occluded front warm moist air is force aloft, resulting in a broad band of rain
2. Wide precipitation band mT mT cP cP

43. Thunderstorms
Thunderstorms form where warm, humid air is forced upward to altitudes of up to 15 km (20 for supercells).
Condensation occurs as the air cools, releasing latent heat and ensuring that the rising air remains unstable (warmer so less dense than surrounding air).

44. Rising Warm air overshoots into stratosphere
Prevailing Wind Aloft
Freezing Line
Condensation
Line
Surface

45. Rain and Ice too heavy for updrafts
Type 1: Isolated Thunderstorms from unstable (hot, moist, low density) air
Rain, plus gusty winds caused by downdrafts
Droplets coalesce
15 minutes
Only updrafts.
No rain.
All downdrafts
Rain and Ice too heavy for updrafts
Lasts 15 to 30 minutes

46. Lightning
Most dangerous frequently encountered weather hazard that people commonly experience each year.
Second most frequent weather killer in the United States with nearly 100 deaths and 500 injuries each year,
after floods and flash floods

47. + marks concentrations of rising and falling ice
Lightning equalizes large charge differences between storm levels and the ground
Ice often positive
+ marks concentrations of rising and falling ice

48. 2. Synoptically Forced Thunderstorms
Most of the cloud formation associated with a cold front is actually in the warm moist air mass

49. Supercells
Severe thunderstorms, or supercells, are associated with frontal lifting along the cold front between the continental polar and maritime tropical air masses in mid-latitude cyclones. The lower portion of these storms, the mesocyclone, rotates. They often contain severe hail and sometimes tornadoes.
Most common during spring and early summer, when the contrast in temperatures and moisture between air masses is greatest.

50. Tropopause

51. 3. The strong updrafts lift falling ice and it gets covered with another layer of ice. If this happens enough times,
large hailstones are the result.
Tropopause
Hail Formation
2. Strong updrafts pull in more air from below.
1. Condensation heats the moist air, which accelerates upwards

52. Supercell Hail

53. Tornadoes
Funnel clouds that rotate at speeds of up to 500 km/hr beneath supercells.
Ranked from F0 (weakest) to F5 (strongest) using the Fujita Intensity scale.
Most move to the east or northeast at an average speed of approximately 50 km/hr.
Develop in association with mesocyclones

54. Wind Shear (different wind
directions at different altitudes)
causes rolling

55. Updrafts can pick up the roll

56. Jet Stream winds result from large pressure gradients at the boundary between mT and cP air, where a large difference in Tropopause height exists.
The Jet Stream N
The jet stream and tornadoes: pressure differences in the extreme

57. Tornados are narrow areas of extremely fast updrafts
A strong tornado often is associated with rapid removal of updraft air by the Jet Stream aloft

58. cT cP mT The Dry Line Tornado Conditions Jet Stream Aloft
Potential for wind shear with hot dry unstable cT
Similar temp but different density than mT which is moist and therefore “light” cT
The Dry Line
Dry, so dense
Tornado Conditions

60. Multiple vortices in an F5

61. May 3, 1999

62. Stecker

63. Supercell tops overshoot into stratosphere

64. A Stecker, Oklahoma Home
A Stecker, Oklahoma Home
“Only” F3
May 4th, 1999

65. A Perfect Hook

67. Hurricanes &Typhoons (Tropical Cyclones)
When extremely hot ocean surface temperatures (>26oC) cause hot, moist surface air, huge clusters of thunderstorms develop at sea. If uplift gets extreme, these can organize into a gigantic Low with spiral storm lines, and winds exceeding 74 mph, a Tropical Cyclone, aka Hurricane

68. Hurricanes are fueled by Latent Heat of Condensation release.
One day equals the energy production of US for a year

70. Jeanne
Hurricanes need hot moist air as fuel. This is why they weaken over land

71. Katrina

73. Trapped in House, swept away
Storm Surge

74. Storm Surge

75. Freshwater (rain) floods cause most fatalities