Post on 30-Dec-2015
description
CHAPTER 3
TEMPERATURE AND HUMIDITY EXTREMES
CHAPTER 3
TEMPERATURE AND HUMIDITY EXTREMES
Each day is like a tiny season!◦ Daytime heating,
nighttime cooling Why do the high and
low temperatures happen when they do?
Daily temperature range near the ground is much greater than that above the ground!
Latitude Land and water distribution Ocean currents Elevation
Fig. 3.15, p. 78
Fig. 3.14, p. 77
Tale of three citiesTale of three cities
Fig. 3.8, p. 70
Fig. 3.4, p. 66
Fig. 3.5, p. 67
Fig. 3.9, p. 71
Fig. 3.10, p. 72
(Note: these are kind of confusing!) “Average temperature”: the average of the
high and low temperature for the day “Normal temperature”: a 30-year average
Not once since 1952 has the high been exactly 62!
So, interpret the “normal” high with caution – what’s normal is for the temperature to be different each year!
Standard deviation = 11.5
Mean +/- 2 standard deviations gives a range of about 39 to 85 – the high temperature will fall between these two values 95% of the time
Body’s Perception: Sensible temperature◦ How we exchange heat energy with environment
Wind and cold◦ Body, like the planet, must also have a
heat/energy balance if temperature is to be maintained.
◦ Thin layer of warm molecules next to skin Wind interferes with this, rapidly removes heat
◦ Sensible temperature for cold weather: Wind Chill
Frostbite: Skin actually freezes and discolors
Cold, Damp Weather◦ A cold rainy day often feels colder than a “dry”
one because water on exposed skin conducts heat away from the body better than air does
◦ Hypothermia – body temp drops below normal Most hypothermia occurs between freezing and 50°
F, snow vs. rain, the wet really matters! “Died of Exposure”
Fig. 3.18, p. 81
The maximum amount of water vapor that can be in the air is a function of temperature◦ The hotter it is, the greater amount of moisture that
can be in the air If this maximum amount of water vapor is
actually in the air, the air is saturated Usually the air near the ground is not
saturated (but what might happen if it is?) Most of the time, the amount of water vapor
in the air stays about the same from day to night, even though the temperature goes up and down quite a bit
Two types of humidity measures: absolute and relative
Absolute measures describe the actual amount of moisture in the air◦ Examples: absolute humidity, dewpoint
temperature, specific humidity, mixing ratio Relative humidity compares the amount
of moisture in the air with the amount needed for saturation
Mass of water vapor divided by mass of air Must add or subtract water vapor to the
parcel to change the specific humidity
Another absolute measure Indicates how much you would have to cool the
air to get to saturation. The moisture needed for saturation at 60° is
much less than at 80°. If the temperature is 80° and the dew point is 60°, you would have to cool the air 20° to have it be saturated
Mixing Ratio – mass of water vapor over mass of dry air (how is this different from specific humidity?)
Vapor Pressure – The actual pressure only due to water vapor.
Both of these have actual values, and saturation values for the current temperature.
Relative Humidity: (actual water vapor/saturation water vapor)*100◦ Can use either mixing ratio or vapor pressure
◦ RH can be changed two ways: Change vapor content Change saturation point (how?)
Fig. 3.20, p. 83
Dew PointDew Point (usually a better way to compare humidity)(usually a better way to compare humidity)
http://weather.unisys.com/surface/sfc_con_dewp.html
Extreme Dew Points◦ Record high Td in the US occur during heat waves
of summer◦ Td > 80°F rare in the US ◦ Highest Td in the US (90°F): New Orleans, LA;
Melbourne, FL; Appleton, WI Extreme Dew Points
◦ Highest Td in the world: Middle East near large bodies of extremely warm water Red Sea: surface temperature of 98°F Sharjah, United Arab Emirates, Td 93°F Dhahran, Saudi Arabia, Td 95°F
Fig. 3.21, p. 83
Body cools by emitting infrared energy and perspiration evaporation
Less evaporation causes people to feel hotter (perception, remember!)
Air with 100% relative humidity is saturated because the air is holding as much water in the vapor state that it can hold (at that temperature)
The higher the relative humidity, the less evaporation
Fig. 3.24, p. 87
Heat index – factors in both Heat index – factors in both temperature and humiditytemperature and humidity
Table 3.6, p. 88
On average, the deadliest weather in U.S. Prolonged period of abnormally hot (&
humid, usually) weather Almost 50,000 died in Europe in 2003
Table 3.5, p. 87
Table 3.7, p. 89
“Air was so heavy and thick because of the humidity” (Yankees getting off the plane in Houston)
Humid Air and Dry Air Do Not Weigh the Same◦ At the same temperature and level in the atmosphere,
hot, humid air is lighter (less dense) than hot, dry air A molecule of water vapor (H2O) weighs appreciably less
than a molecule of either nitrogen (N2) or oxygen (O2)
Still, it indeed does feel thick and heavy, related to perception and effective temperature.
Standards for a Cooperative Station thermometer:◦ Placed in a shelter 2
m above ground◦ Shelter must be
painted white◦ Doors must open to
the north (in the N. Hemisphere)
◦ Vented sides◦ Placed on a grassy
area where possible
(photo from surfacestations.org)
Generally good siting; station has been in same location for 100+ years
Orland, California(photo from surfacestations.org)
Not so good!
Marysville, California(photo from surfacestations.org)
Now most stations have automated equipment which uses electronic means
http://128.194.168.16/