Chapter 3 Air Temperature Daily temperature variations n

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

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 n forced convection: strong wind n water vapor effects n • Cumulus clouds are markers of convection.

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

Stepped Art Fig. 3 -2, p. 57

Fig. 3 -5, p. 60

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

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

The Controls of Temperature

The Controls of Temperature Latitude: control daylight period and solar radiation n land water distribution: water moving n ocean currents n elevation n specific heat: energy needed to raise T of 1 kg n 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

July sea level Ta Fig. 3 -9, p. 64

Air Temperature Data

Daily, Monthly and Yearly Temperatures n n n 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

Fig. 3 -11, p. 65

What is normal T (30 -yr average) for a particular day (March 15 over southwestern U. S. )? Figure 2, p. 69

The Use of Temperature Data heating degree-days: daily mean T below 64 o. F n cooling degree-days: daily mean T above 65 o. F n growing degree-days: daily mean T 1 degree n 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 n wind chill index Shaded area: frostbite could occur in 30 min or less

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

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

Precip, wind, T/Td, rain type, lightning, cloud base, freezing rain, visibility