1. TOPIC 7
WEATHER

2. Weather
Defn: The condition of the atmosphere in a given place and time.
Weather Variables

3. Temperature Amount of heat energy in the atmosphere.
Instrument: Thermometer
Thermometers work due to expansion.

5. Air Pressure: The weight of the overlying air Instrument: Barometer
Millibars and Inches of Mercury ( see ref. tables page 13)

8. Pressure Conversions Convert to inches 996.0 mb = 1033.0 mb =
Convert to millibars
29.50 in =
30.06 in =
28.79 in =

9. Factors Affecting Air Pressure:
Altitude
As altitude increases, air pressure decreases.
Temperature
As temperature increases, air pressure decreases.
Humidity (water vapor)
As the humidity increases, air pressure decreases.
All 3 are indirect relationships!!!

10. Moisture:
As the air temperature increases, it’s ability to hold water vapor increases. OR
Warm air can hold more water vapor than cold air.

11. Relative Humidity%-
The amount of water vapor the air is holding compared to what it can hold.
Dew Point Temperature-
The temperature at which the air will become saturated (rel. humidity = 100%)
Condensation occurs when the air becomes saturated.
Examples:

12. Instrument- Sling Psychrometer (wet bulb/dry bulb)
See page 12 in Ref. Tables

13. Dew Point Chart

14. Relative Humidity

15. As the temperature and the dew point get closer…
The relative humidity increases.
The chance of clouds and/or precipitation increases.

16. Which city has the greatest chance of clouds and precip.?
Which city has the lowest relative humidity?
City
Temperature
Dew Point
Syracuse
25o
20o
Rochester
19o
10o
Utica
Albany
24o
22o

17. Wind Defn: the horizontal movement of air
Wind is caused by a difference in air pressure (caused by the uneven heating of the Earth)
Air moves from regions of high pressure towards regions of low pressure

18. CIRCULATION H L

19. Instrument: Vane (direction)
Anemometer (speed)
Wind speed is determined by the pressure gradient.
As the gradient increases, the wind speed increases.
The closer the isobars, the higher the wind speeds.

20. Wind Vane
Anemometer

22. Station Model Map

24. Station Model 27 040 3 = +12/ 24 .75 Wind Speed (25 knots) Temp. (oF)
Wind Direction (NE) 27
040
Visibility (3 miles)
Present Weather (rain)
Air Pressure ( mb) 3 =
+12/
Dew Point (o F)
Barometric trend (1.2 mb increase) 24
.75
Precip. (.75 inches in past 6 Hours)
Cloud Cover (50%)

25. Practice Station Model
Temp = 22o F
Dew Point= 18o F
Winds 8 mph
Cloudy
Snow showers
Air Pressure = mb
Visibility 4 miles

26. 974 32 7 * 30

27. Water Cycle

28. Evaporation Liquid to gas (water vapor) Requires the addition of heat
Transpiration- water vapor entering the atmosphere through plants

29. Factors Affecting Evaporation
Temperature
As temp increases, evap rates increase.
Wind
As wind increases, evap rates increase.
Humidity
As humidity increases, evap rates decrease.
Surface area
As surface area increases, evap rates increase.

30. Condensation Gas to liquid Heat is released to the atmosphere.
Condensation occurs when…
The air becomes saturated with water vapor and
Condensation nuclei are present.

31. Cloud Formation Clouds usually form in air that is rising! AIR RISES
EXPANDS
COOLS
CONDENSES

32. Precipitation: When water returns to the Earth’s surface.
Occurs when the water droplets or ice crystals become heavy enough to fall.
A method by which the atmosphere cleanses itself.
Ice crystals will form when the temp = the dew point and they are at or below 32o F

33. Air Masses
A “blob” of air with similar moisture and temperature characteristics.
Air masses get their characteristics from the surface over which they form. (source region)

34. cP Continental Polar – Dry and cold
Temperature
Polar – Cold (P)
Tropical – Warm (T)
Moisture
Continental – Dry (c)
Maritime – Moist (m)
cP Continental Polar – Dry and cold
cT Continental Tropical – Dry and Warm
mP Maritime Polar – Moist and cold
mT – Moist and warm
cA Continental Arctic- Dry and Very Cold

35. cP mP cT mT mT

36. L H WEATHER SYSTEMS Cool/ Dry air Clear/ Fair weather
Air is sinking and diverging
Warm/ Moist air
Clouds & Precip.
Air is converging and rising. L H

37. More Pressure
Less Pressure

38. The air inside this balloon is heated to make it less dense so it rises

39. Why is dry air heavier than moist air?
PINGPONG BALL = WATER VAPOR
GOLF BALL = AIR MOLECULES
“MOIST AIR”
“DRY AIR”

40. H L Weather Systems High Pressure Low Pressure Cold/ Dry Warm/ Moist
Clear/ Fair
Sinking & Diverging
Low Pressure
Warm/ Moist
Clouds/ Storms
Rising & Converging H L

42. Cold Front – cold air pushes into warm air.
Conditions:
Temp decreases
Precip is heavy and short (t-storms).
Vertical cloud development
Cumulonimbus clouds

43. Warm Front - warm air displacing cold air.
Conditions
Temp increases.
Precip. is steady light and long.
Horizontal cloud development
Stratus clouds

45. Stationary Front - the boundary between warm and cold air remains in the same place.
Conditions:
Weather similar to a warm front.

46. Occluded Front:
Occurs when a cold front catches up to a warm front in a wave cyclone.
Wave Cyclone L L

47. Winds blow parallel to a front ahead of it and perpendicular to the front behind it !
When a front passes…
Temperature changes
Air pressure changes
Wind direction changes
Usually clouds and/or precip. occurs

49. Localized Weather Factors
Lake Effect Snow
Heavy snowfall that occurs on the leeward side of large lakes.
Warm lake water, cold air & wind

50. Nearness to Large Body of Water
Cooler summers and warmer in winters.
Water heats and cools slower than land.
Increase in precipitation

51. Orographic Effect
Windward
Leeward

53. Land and Sea Breezes

59. Hazardous Weather Thunderstorms
Severe storms associated with warm air.
Usually occur along cold fronts.
Conditions
Heavy rain Winds
Lightning Hail(?)
Thunder

60. Precautions (Lightning)
Don’t be the highest object in the area.
Don’t stand under a tree or near a window.
Stay off the phone.
Don’t touch plumbing or electrical devices.
Stay inside a car or crouch down.

62. Tornadoes
Small scale, short lived violent storms associated with severe t-storms.
“Twisters”
Fujita Scale
0 – 5
Weak Strong

67. lat

70. Precautions (Tornadoes)
Get to the interior lowest level of the building.
Cover your head.
Get out of your car and lay in a ditch.

71. Hurricanes Large scale, severe low pressure systems.
Stages of Development

74. Hurricane Season
Late summer/ Early fall
Conditions:
High winds Flooding
Heavy precip Storm Surge
T-storms
Storm surge-
wall of ocean water that invades low coastal areas caused by the high winds and low pressure.

75. Bonus:
What is the name given to the “wall of ocean water that invades low coastal areas caused by the high winds and low pressure.”?

79. Atlantic Hurricane Track

82. Hurricanes form and intensify over warm ocean water!
Precautions
Early evacuation
Move to higher ground
Stock up on non-perishable food, water radio & batteries
Board up windows.

86. Blizzards
Storm with winds greater than 35 mph and considerable amounts of falling and/or blowing snow.
shut down transportation
Disrupt telephone and electrical service
Prevent people from obtaining fuel, food and needed services.

87. Precautions Have food, fuel and other supplies on hand. Stay indoors.
Do not travel.

90. BONUS:
The air pressure is currently inches of mercury. How would this air pressure be shown on a station model?