1. Chapter 3: Air Temperature
Daily temperature variations
The controls of temperature
Air temperature data
Air temperature and human comfort
Measuring air temperature

2. Daily Temperature Variations
Record high T: 58 C or 136 F (Libya)
Record low T: -89 C or -129 F (Antarctic)

3. Daytime Warming Thermals: calm wind condition
forced convection: strong wind
water vapor effects
Cumulus clouds are markers of convection.

4. Nighttime Cooling radiational cooling nocturnal inversions
Inversions tend to occur on
clear, calm, dry nights:
strong winds would mix
air;
clouds or water vapor would
absorb longwave radiation

5. Figure 3.2: The daily variation in air temperature is controlled by incoming energy (primarily from the sun) and outgoing energy from the earth’s surface. Where incoming energy exceeds outgoing energy (orange shade), the air temperature rises. Where outgoing energy exceeds incoming energy (blue shade), the air temperature falls.
Stepped Art
Fig. 3-2, p. 57

6. Figure 3.5: An Idealized distribution of air temperature above the ground during a 24-hour day. The temperature curves represent the variations in average air temperature above a grassy surface for a mid-latitude city during the summer under clear, calm conditions.
Fig. 3-5, p. 60

7. Cold Air Near the Surface
inversions
thermal belts
Drainage winds: cold air that slides downhill.

8. Protecting Crops from the Cold Night Air
orchard heaters
wind machines
Freeze: Ta below
freezing for a long
enough time
To protect small
plants:
Use straw, cloth,
or plastic bag
to cover

9. The Controls of Temperature

10. The Controls of Temperature
Latitude: control daylight period and solar radiation
land and water distribution: water moving
ocean currents
elevation
specific heat: energy needed to raise T of 1 kg
air by 1 K
Average weather conditions in the interior of large continents are much different than average conditions in coastal areas.

11. January sea level Ta Fig. 3-8, p. 64
Figure 3.8: Average air temperature near sea level in January (°F).
January sea level Ta
Fig. 3-8, p. 64

12. Figure 3.9: Average air temperature near sea level in July (°F).
July sea level Ta
Fig. 3-9, p. 64

13. Air Temperature Data

14. Daily, Monthly and Yearly Temperatures
diurnal temperatur range: Tmax – Tmin
large range occurs for dry, clear sky over desert
clouds and humidity effects: reduce daytime T but increase nighttime T
proximity to large bodies of water
Urban heat island:
annual temperature range:
max monthly T -- min monthly T

15. Figure 3.11: (a) Clouds tend to keep daytime temperatures lower and nighttime temperatures higher, producing a small daily range in temperature. (b) In the absence of clouds, days tend to be warmer and nights cooler, producing a larger daily range in temperature.
Fig. 3-11, p. 65

16. What is normal T (30-yr average) for a particular day
(March 15 over southwestern U.S.)?
Figure 2: The high temperature measured (for 30 years) on March 15 in a city located in the southwestern United States. The dashed line represents the normal temperature for the 30-year period.
Figure 2, p. 69

17. The Use of Temperature Data
heating degree-days: daily mean T below 64oF
cooling degree-days: daily mean T above 65oF
growing degree-days: daily mean T 1 degree above base T

18. Air Temperature and Human Comfort
Human body stabilizes its T primarily by converting food
into heat (metabolism)
The stronger the wind, the faster the body’s heat loss
High winds in below-freezing air can remove heat from exposed
skin so quickly that the skin may actually freeze (called frostbite)
Dry heat: perspire to feel cool; heat and humidity: feel hotter

19. Air Temperature and Human Comfort
wind chill index
Shaded area: frostbite could occur in 30 min or less

20. Measuring Air Temperature
While T is as high as 500oC
in the upper atmosphere of
300 km, astronauts still feel
very cold. Why?

21. Measuring Air Temperature
liquid-in-glass thermometers: mercury or alcohol
maximum and minimum thermometers
Infrared sensor or radiometer
Electrical thermometers
instrument shelters

22. Precip, wind, T/Td, rain type, lightning, cloud base,
freezing rain, visibility
Figure 4: Instrument shelters such as the one shown here serve as a shady place for thermometers. Thermometers inside shelters measure the temperature of the air more accurately than a thermometer held in direct sunlight.