Seasons Why we have them And some local effects

The Earth tilts The Earth travels around the sun in an ellipse –The tilt of the North of the Earth is away from the sun during northern hemisphere winter –The Earth is further from the sun in northern hemisphere summer

Seasons © 1997 Prentice-Hall. From Moran and Morgan, Meteorology, 5 th Ed.

Seasons -- Equinox © 1997 Prentice-Hall. From Moran and Morgan, Meteorology, 5 th Ed.

Solstice & Equinox Figure 3.3 Earth's tilt of 23.5° and revolution around the sun creates seasonal solar exposure and heating patterns. A solstice tilt keeps a polar region with either 24 hours of light or darkness. A solstice tilt keeps a polar region with either 24 hours of light or darkness. A equinox tilt perfectly provides 12 hours of night and 12 hours of day for all non-polar regions. A equinox tilt perfectly provides 12 hours of night and 12 hours of day for all non-polar regions.

Seasons -- Summer Solstice © 1997 Prentice-Hall. From Moran and Morgan, Meteorology, 5 th Ed.

Seasons -- Winter Solstice © 1997 Prentice-Hall. From Moran and Morgan, Meteorology, 5 th Ed.

Variation in Solar Radiation that Strikes the Earth Solar radiation at the surface is most intense when the sun is directly overhead. Solar radiation at the surface has a lower intensity when the sun is not directly overhead. © 1997 Prentice-Hall. From Moran and Morgan, Meteorology, 5 th Ed.

Seasons & Solar Intensity Figure 3.2 Solar intensity, defined as the energy per area, governs earth's seasonal changes. A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly. A sunlight beam that strikes at an angle is spread across a greater surface area, and is a less intense heat source than a beam impinging directly.

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Latitude Effects Temperature © 1998 Prentice-Hall. From Lutgens and Tarbuck, The Atmosphere, 7 th Ed. 99% of Overhead 89% of Overhead 60% of Overhead 28% of Overhead

Earth's Tilt & Atmosphere Figure 3.5 Earth's atmosphere reduces the amount of insolation striking earth's surface. Earth's atmosphere and tilt combine to explain variation in received solar radiation. Earth's atmosphere and tilt combine to explain variation in received solar radiation. Figure 3.6

Why are cities warmer than forests? Albedos Evaporation Turbulence Energy sources

Heating of the Ground Factors that can effect ground heating –Type of ground (color and texture) Black asphalt is typically hot Lighter concrete is typically not as hot as asphalt –Vegetation –Clouds –Advection –Water –Slope of Ground

Surface Albedo IRIR R/I = 0.5R/I = 0.1

Heat Islands The average temperature of a city is typically slightly higher than that of the surrounding countryside. Some days the city temperature may be as much as 10 o C higher. This “island” of warmer temperatures is called a heat island.

Heat Islands Why is this so? –More heat sources in the city People, Cars Industry, Furnaces –Better absorption of solar radiation –Less foliage and evaporative cooling Heat island effect causes a slightly elevated layer of pollution above the city. Stronger winds may form a “plume” of pollution from the city.

How hot is that surface? What color is it? –Light colors reflect more –They absorb less –Don’t get as hot How rough is it? –Rough surfaces reflect less light –They absorb more –They get hotter

Examples Hot surfaces –Tarmac –Slate roofs –Natural grass Cold surfaces –Ice!! –Concrete paving

Surfaces also emit Dark surfaces –Emit well –Cool down Rough surfaces –Emit poorly –Stay warm

Cities are warmer than forests because: Their surfaces are dark and absorb more solar energy –The dark surfaces also re-radiate the heat into the lower atmosphere well There is little open water or vegetation to cool the surface by evaporation (latent heat loss) Cities have an abundance of energy sources (buildings, vehicles) that add to the warmth

Summary Summer is hot because: –Your part of the Earth us tilted toward the sun –You receive more direct solar radiation Your day is longer There is less angle of incidence for the incoming radiation

Wintery You get cold in winter –Sun is lower in the sky –Days are shorter –Earth is tilted away In the southern hemisphere these effects are increased because the Earth is further from the sun in winter and closer in summer

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