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 km) in July.  Distance not the only factor impacting seasons.

The elliptical path (highly exaggerated) of the earth about the sun brings the earth slightly closer to the sun in January than in July.

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.

As the earth revolves about the sun, it is tilted on its axis by an angle of 231⁄2 o. The earth’s axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun.

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.

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, March 20, Sun directly above Equator, all locations have a 12 hour day

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

The apparent path of the sun across the sky as observed at different latitudes

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.

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

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

On a sunny, calm day, the air near the surface can be substantially warmer than the air a meter or so above the surface.

Vertical temperature profiles above an asphalt surface for a windy and a calm summer afternoon.

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.

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

On On a clear, calm night, air near surface can be much colder than air above. Increase in air temperature w/increasing ht above surface is called a radiation temperature inversion.

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.

Vertical temperature profiles above an asphalt surface for a windy and a calm summer afternoon.

Average air temperature near sea level in January ( o F).

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

Average air temperature near sea level in July ( o F).

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

Air temperature data  Special topic: What’s normal? Climate normal is the 30 year average for a given temperature variable.  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

Mean annual total heating degree-days across the United States (base 65 o F).

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.

Mean annual total heating degree-days across the United States (base 65 o F).

The elliptical path (highly exaggerated) of the earth about the sun brings the earth slightly closer to the sun in January than in July.

As the earth revolves about the sun, it is tilted on its axis by an angle of 231⁄2 o. The earth’s axis always points to the same area in space (as viewed from a distant star). Thus, in June, when the Northern Hemisphere is tipped toward the sun, more direct sunlight and long hours of daylight cause warmer weather than in December, when the Northern Hemisphere is tipped away from the sun.