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 231⁄2o. 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 HemisphereCloser 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 (oF).

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 (oF).

Air temperature data

Daily, monthly, yearly temperatureRange: maximum minus minimumMean: average of temperature observationsMaximum: highest temperature of time

periodMinimum: 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 dataHeating degree-day: people heat when

temperature below 65°FCooling degree-day: people cool when

temperature above 65°FGrowing degree-day: temperature above of

below base temperature for specific crop

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

Measuring air temperature Observation: Thermometers in the

shadeRadiant 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 65oF).

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 231⁄2o. 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.