Class #2: Seasonal and daily variations in temperature Chapter 3 Class #2 July 8, 2010

Seasonal and Daily temperatures Chapter 3 Seasonal and Daily temperatures Class #2 July 8, 2010

Why the Earth has seasons Earth revolves in elliptical path around sun every 365 days. Earth rotates counterclockwise or eastward every 24 hours. Earth closest to Sun (147 million km) in January, farthest from Sun (152 million lm) in July. Distance not the only factor impacting seasons. Class #2 July 8, 2010

FIGURE 3.1 The elliptical path (highly exaggerated) of the earth about the sun brings the earth slightly closer to the sun in January than in July. Class #2 July 8, 2010

ACTIVE FIGURE 3.2 Sunlight that strikes a surface at an angle is spread over a larger area than sunlight that strikes the surface directly. Oblique sun rays deliver less energy (are less intense) to a surface than direct sun rays. Visit the Meterology Resource Center to view this and other active figures at academic.cengage.com/login Class #2 July 8, 2010

FIGURE 3.1 The elliptical path (highly exaggerated) of the earth about the sun brings the earth slightly closer to the sun in January than in July. Class #2 July 8, 2010

Why the Earth has seasons The amount of energy that reaches the Earths surface is influence by the distance from the Sun, the solar angle, and the length of daylight. When the Earth tilts toward the sun in summer, higher solar angles and longer days equate to high temperatures. Class #2 July 8, 2010

Why the Earth has seasons Seasons in the Northern Hemisphere Summer solstice: June 21, Sun directly above Tropic of Cancer, Northern Hemisphere days greater than 12 hours Winter solstice: December 21, Sun directly above Tropic of Capricorn, Northern Hemisphere days less than 12 hours Autumnal and Spring Equinox: September 22, Marc 20, Sun directly above Equator, all locations have a 12 hour day Class #2 July 8, 2010

FIGURE 3.5 The relative amount of radiant energy received at the top of the earth’s atmosphere and at the earth’s surface on June 21 — the summer solstice. Class #2 July 8, 2010

FIGURE 3.6 During the Northern Hemisphere summer, sunlight that reaches the earth’s surface in far northern latitudes has passed through a thicker layer of absorbing, scattering, and reflecting atmosphere than sunlight that reaches the earth’s surface farther south. Sunlight is lost through both the thickness of the pure atmosphere and by impurities in the atmosphere. As the sun’s rays become more oblique, these effects become more pronounced. Class #2 July 8, 2010

FIGURE 3.8 The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21), the December solstice (December 21), and the equinox (March 20 and September 22). Class #2 July 8, 2010

Figure 2.24: The apparent path of the sun across the sky as observed at different latitudes on the June solstice (June 21), the December solstice (December 21), and the equinox (March 20 and September 22). Stepped Art Class #2 July 8, 2010 Fig. 3-8, p. 63

Figure 3.4 Land of the Midnight Sun. A series of exposures of the sun taken before, during, and after midnight in northern Alaska during July. Class #2 July 8, 2010 Fig. 3-4, p. 60

TABLE 3.1 Length of Time from Sunrise to Sunset for Various Latitudes on Different Dates in the Northern Hemisphere Class #2 July 8, 2010

Why the Earth has seasons Special Topic: First day of winter December 21 is the astronomical first day of winter, sun passes over the Tropic of Capricorn; not based on temperature. Class #2 July 8, 2010

Why the Earth has seasons Seasons in the Southern Hemisphere Opposite timing of Northern Hemisphere Closer to sun in summer but not significant difference from north due to: Greater amount of water absorbing heat Shorter season Class #2 July 8, 2010

Local temperature variations Southern exposure: warmer, drier locations facing south. Implications for Vegetation Viniculture Ski slopes Landscaping Architecture Class #2 July 8, 2010

FIGURE 3.10 In areas where small temperature changes can cause major changes in soil moisture, sparse vegetation on the southfacing slopes will often contrast with lush vegetation on the northfacing slopes. Class #2 July 8, 2010

Local temperature variations Environmental Issues: Solar Heating In order to collect enough energy from solar power to heat a house, the roof should be perpendicular to the winter sun. For the mid-latitudes the roof slant should be 45°- 50° Class #2 July 8, 2010

Daily temperature variations Each day like a tiny season with a cycle of heating and cooling Daytime heating Air poor conductor so initial heating only effects air next to ground As energy builds convection begins and heats higher portions of the atmosphere After atmosphere heats from convection high temperature 3-5PM; lag in temperature Class #2 July 8, 2010

FIGURE 3.11 On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface. Class #2 July 8, 2010

FIGURE 3.12 Vertical temperature profiles above an asphalt surface for a windy and a calm summer afternoon. Class #2 July 8, 2010

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 Class #2 July 8, 2010 Fig. 3-13, p. 67

Daily temperature variations Properties of soil affect the rate of conduction from Earth to atmosphere Wind mixes energy into air column and can force convection. Class #2 July 8, 2010

Daily temperature variations Nighttime cooling As sun lowers, the lower solar angle causes insolation to be spread across a larger area Radiational cooling as infrared energy is emitted by the Earth’s surface Radiation inversion: air near ground much cooler than air above Thermal belt Class #2 July 8, 2010

FIGURE 3.14 On a clear, calm night, the air near the surface can be much colder than the air above. The increase in air temperature with increasing height above the surface is called a radiation temperature inversion. Class #2 July 8, 2010

FIGURE 3.15 Vertical temperature profiles just above the ground on a windy night and on a calm night. Notice that the radiation inversion develops better on the calm night. Class #2 July 8, 2010

Daily temperature variations Protecting crops from cold Cover Smudge pots Fans Sprinklers Class #2 July 8, 2010

FIGURE 3.19 Wind machines mix cooler surface air with warmer air above. Class #2 July 8, 2010

FIGURE 3.20 Average air temperature near sea level in January (oF). Class #2 July 8, 2010

The controls of temperature Latitude: solar angle and day length Land & water: specific heat Ocean currents: warm and cold currents Elevation: cooling and increase range Class #2 July 8, 2010

FIGURE 3.20 Average air temperature near sea level in January (oF). Class #2 July 8, 2010

FIGURE 3.21 Average air temperature near sea level in July (oF). Class #2 July 8, 2010

Air temperature data Daily, monthly, yearly temperature Range: maximum minus minimum Mean: average of temperature observations Maximum: highest temperature of time period Minimum: lowest temperature of time period Class #2 July 8, 2010

FIGURE 3.25 Temperature data for San Francisco, California (37oN), and Richmond, Virginia (37oN) — two cities with the same mean annual temperature. Class #2 July 8, 2010

Air temperature data Special topic: What’s normal? Climate normal is the 30 year average for a given temperature variable. Class #2 July 8, 2010

Air temperature data The use of temperature data Heating degree-day: people heat when temperature below 65°F Cooling degree-day: people cool when temperature above 65°F Growing degree-day: temperature above of below base temperature for specific crop Class #2 July 8, 2010

FIGURE 3.25 Temperature data for San Francisco, California (37oN), and Richmond, Virginia (37oN) — two cities with the same mean annual temperature. Class #2 July 8, 2010

FIGURE 3.26 Mean annual total heating degree-days across the United States (base 65oF). Class #2 July 8, 2010

TABLE 3.2 Estimated Growing Degree-Days for Certain Naturally Grown Agricultural Crops to Reach Maturity Class #2 July 8, 2010

Air temperature and human comfort Body heats through metabolism wind-chill index Hypothermia Body cools through emitting infrared energy and evaporation of perspiration Class #2 July 8, 2010

Air temperature and human comfort Observation: 1000 degrees Thin air at the top of the atmosphere does not have enough molecules to create a high temperature as measured by a thermometer. Class #2 July 8, 2010

Measuring air temperature Thermometers: liquid-in-glass, maximum, minimum, electrical resistance, bimetallic ASOS Thermistors Infrared sensors Class #2 July 8, 2010

Measuring air temperature Observation: Thermometers in the shade Radiant energy from the Sun in direct sunlight increases the temperature recorded by a sensor. True air temperature measured in the shade. Class #2 July 8, 2010