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/