Seasonal & Daily Temperatures. Seasons & Sun's Distance. Solstice & Equinox. Seasons & Solar Intensity

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1 Seasonal & Daily Temperatures Seasons & Sun's Distance The role of Earth's tilt, revolution, & rotation in causing spatial, seasonal, & daily temperature variations Please read Chapter 3 in Ahrens Figure 3.1 Earth is 5 million kilometers further from the sun in July than in January, indicating that seasonal warmth is controlled by more than solar proximity. Seasons & Solar Intensity Solstice & Equinox Solar intensity, defined as the energy per area, governs Earth's seasonal climate 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. Earth's tilt of 23.5 and revolution around the sun creates seasonal solar exposure and heating patterns At solstice, tilt keeps a polar region with either 24 hours of light or darkness At equinox, tilt provides exactly 12 hours of night and 12 hours of day everywhere 1

2 Midnight Sun NH summer June 21 Equinox March 20, Sept 22 NH winter The region north of the Arctic Circle experiences a period of 24 hour sunlight in summer, where the Earth's surface does not rotate out of solar exposure Daily Sunshine at Top of Atmosphere 75º N in June gets more sun than the Equator! Compare N-S changes by seasons Very little tropical seasonality Dec 21 Energy In, Energy Out Incoming and outgoing energy must balance on average But there are huge differences from place to place Way more solar heating in tropics Some places (deserts) emit much more than others (high cold clouds over rainforests) IN OUT 2

3 Net Accumulation of Energy Incoming solar minus outgoing longwave Must be balanced by horizontal transport of energy by atmosphere and oceans! Questions to Think About Since polar latitudes receive the longest period of sunlight during summer, why aren t temperatures highest there? Why aren t temperatures highest at the summer solstice? What would happen if we changed the tilt of the earth? Would we get a more/less pronounced seasonal cycle in the NH if the tilt was increased? What would happen if the tilt was 90 degrees? 0 degrees? Seasons Seasons are regulated by the amount of solar energy absorbed at Earth s surface The solar energy received at the top of the atmosphere depends on: angle at which sunlight strikes Earth s surface. how long the sun shines per day Seasons are NOT due to the elliptical shape of the earth s orbit. Earth's annual energy balance between solar insolation and terrestrial infrared radiation is achieved locally at only two lines of latitude A global balance is maintained by transferring excess heat from the equatorial region toward the poles Earth's Energy Balance 3

4 Regional Seasonal Cycles Regional differences in temperature, from annual or daily, are influenced by geography, such as latitude, altitude, and nearby water or ocean currents, as well as heat generated in urban areas San Francisco is downwind of the Pacific Ocean Richmond, VA is downwind of North America! Daily Temperature Variations Each day is like a mini seasonal cycle Sun rays most intense around noon As is the case with the seasons, the maximum temperatures lag the peak incoming solar radiation. An understanding of the diurnal cycle in temperature requires an understanding of the different methods of atmospheric heating and cooling: Radiation Conduction Convection What Controls Daily High Temperatures? T max depends on Radiation (Cloud cover) Surface type Absorption characteristics Strong absorbers enhance surface heating Vegetation/moisture Available energy partially used to evaporate water Wind Strong mixing by wind will mix heated air near ground to higher altitudes Local Solar Changes Northern hemisphere sunrises are in the southeast during winter, but in the northeast in summer Summer noon time sun is also higher above the horizon than the winter sun 4

5 Landscape Solar Response Atmospheric Heating by Convection Sunlight warms the ground Ground warms adjacent air by conduction Poor thermal conductivity of air restricts heating to a few cm Hot air forms rising air bubbles (thermals) leading to convection heats the air, but cools the surface! Mechanical mixing due to wind enhances this mode of heat transport South facing slopes receive greater insolation, providing energy to melt snow sooner and evaporate more soil moisture. North and south slope terrain exposure often lead to differences in plant types and abundance. Daytime Warming Solar radiation heats the atmosphere from below by soil conduction and convection. Winds create forced convection vertical mixing that diminishes steep temperature gradients. Temperature Lags Radiation Earth's surface temperature is a balance between incoming solar radiation and outgoing terrestrial radiation. Peak temperature lags after peak insolation because surface continues to warm until infrared radiation exceeds insolation. 5

6 Nighttime Cooling Cold Dense Air Drains Drains Downhill Radiational cooling creates a temperature inversion near the surface that may be diminished by winds. Nighttime radiational cooling increases air density. Cold air near the ground is heavy negatively buoyant takes energy to stir this air with warmer air aloft On hill slopes, denser air settles to the valley bottom, creating a thermal belt of warmer air between lower and upper cooler air. Protecting Crops from Below Protecting Crops from Above When T is below freezing aloft, crops are not helped by convection or mixing, but by spraying water. Latent heat of freezing of water warms air, protecting crops Impacts of radiational cooling near the surface can be mitigated by wind machines mixing warmer air from above. 6

7 Controls of Temperature Air temperature is governed by length of day and intensity of insolation, which are a function of: 1) latitude Additional controls are: 2) land and water 3) ocean currents 4) elevation July Isotherms Southern hemisphere has fewer land masses and ocean currents that encircle the globe, creating isotherms that are more regular than those in the northern hemisphere. Figure 3.22 January Isotherms Air temperature is warmer at the equator than at the poles, but land and water, ocean currents, and elevation create additional variations. Daily Temperature Range Surface absorbs solar energy and efficiently radiates infrared energy, creating a large diurnal temperature range (max min) in the lower atmosphere. 7