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

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

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

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

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

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

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

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

The Controls of Temperature

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.

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

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

Air Temperature Data

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

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

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

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

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

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

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

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

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.