Meteorology Geography Department East China Normal University Shu Jiong
The Earth ’ s Global Energy Balance ◆ Solar radiation is the driving power source for wind, waves, weather, rivers, and ocean currents. ◆ The earth’s energy balance, which includes land and ocean surfaces and the atmosphere, controls the seasonal and daily changes in the earth’s surface temperature.
Electromagnetic Radiation ◆ Wavelength describes the distance separating one wave crest from the next crest. ◆ The unit to measure wavelength is the micrometer. ◆ Radiant energy can exist at any wavelength.
Radiation and Temperature ◆ There is an inverse relationship between the radiation that an object emits and the temperature of the object. ◆ Hot objects radiate more energy, and at shorter wavelengths, than cooler objects.
Solar radiation ◆ The Sun is a ball of constantly churning gases that are heated by continuous nuclear reactions. It has a surface temperature of about 6000 o C. Like all objects, it emits energy in the form of electromagnetic radiation.
Characteristics of solar energy ◆ The sun does not emit all wavelengths of radiation equally ◆ The intensity of solar energy is strongest in visible wavelengths
Longwave radiation from the Earth ◆ The earth’s surface and atmosphere are much colder than the sun’s surface,so the energy from the Earth has longer wavelength
The global radiation balance ◆ The earth constantly absorbs solar shortwave radiation and emits longwave radiation. ◆ The sun provides a nearly constant flow of shortwave radiation toward earth. ◆ The atmosphere, land, and ocean also emit energy in the form of longwave radiation.
Insolation over the Globe ◆ Insolation(incoming solar radiation) depends on the angle of the sun above the horizon.
The Path of the Sun in the Sky The sun’s path in the sky changes greatly in position and height above horizon from summer to winter. ◆ At equinox, the sun rises directly to the east and sets directly to the west. ◆ The noon sun is positioned at an angle of 50 o above the horizon in the southern sky.
◆ The sun is above the horizon for exactly 12 hours. ◆ At noon it will be 73.5 o above the horizon. ◆ The sun is above the horizon for about 15 hour
Daily Insolation Through the Year ◆ Daily insolation depends on two factors: ◆ the angle at which the sun’s rays strike the earth ◆ the length of time of exposure to the rays ◆ the equator has two periods of maximum daily insolation, these periods occur near the equinoxes.
◆ There are also two minimum periods near the solstices, when the subsolar point moves farthest north and south from the equator. All latitudes between the tropic of cancer 23.5 o N and the tropic of Capricorn 23.5 o S have two maximum and minimum values. ◆ Seasonal pattern of daily insolation is directly related to latitude.
Annual Insolation by Latitude ◆ Annual insolation varies smoothly from the equator to the pole ◆ The annual insolation value at the pole is about 40% of the value at the equator. ◆ The tilting of the earth’s axis redistributes a significant portion of the earth’s insolation from the equatorial regions toward the poles.
World Latitude Zones ◆ The equatorial zone encompasses the equator and covers the latitude belt roughly 10 o N to 10 o S ◆ Spanning the tropics of cancer and Capricorn are the tropical zones, ranging from latitudes 10 o to 25 o north and south
◆ Moving toward the poles from each of the tropical zones are transitional regions called the subtropical zones, ranging from latitudes 25 o to 35 o north and south ◆ The midlatitude zones lie between 35 o and 55 o latitudes in the northern and southern hemispheres
◆ Bordering the midlatitude zones on the poleward side are the subarctic zone and subantarctic zone, 55 o to 60 o north and south latitudes ◆ Astride the arctic and Antarctic circles from latitudes 60 o to 75 o N and S lie the arctic and Antarctic zones ◆ The polar zones, north and south, are circular areas between about 75 o latitude and the poles
Composition of the Atmosphere ◆ The earth’s atmosphere consists of air-a mixture of various gases surrounding the earth to a height of many kilometers. ◆ Almost all the atmosphere 97% lies within 30km of the earth’s surface.
The upper limit of the atmosphere is at a height of approximately 10,000km above the earth’s surface, a distance approaching the diameter of the earth itself.
◆ Pure, dry air consists largely of nitrogen, about 78% by volume, and oxygen, about 21%. Other gases account for the remaining 1% ◆ Water vapor is an important component of the atmosphere that varies in concentration from place to place and time to time
Ozone in the Upper Atmosphere ◆ Ozone (O 3 ) is found mostly in the upper part of the atmosphere, in a layer termed the stratosphere. Ozone in the stratosphere absorbs ultraviolet radiation from the sun as this radiation passes through the atmosphere
At both polar regions, climate and chemistry combine to deplete ozone during spring months. Dark blue indicates lowest ozone amounts. Arctic total ozone amounts seen by TOMS in March 2003 (above, left) were among the lowest ever observed in the northern hemisphere. The Antarctic ozone hole of 2003 (above, right) was the second largest ever observed.
Sensible Heat and Latent Heat Transfer ◆ Sensible Heat-the quantity of heat held by an object that can be sensed by touching or feeling ◆ When two objects of unlike temperature contact each other, heat energy moves by conduction from the warmer to the cooler. This type of heat flow is referred to as sensible heat transfer
The Global Energy System ◆ Insolation losses in the Atmosphere ◆ Albedo ◆ Counterradiation and the Greenhouse Effect ◆ Global Energy Budget of the Atmosphere and Surface ◆ Net Radiation, Latitude, and the Energy Balance
Winds and the Pressure Gradient Force ◆ Wind is air motion with respect to the earth’s surface, and it is dominantly horizontal. ◆ Barometric pressure falls with increasing altitude above the earth’s surface.
◆ The change in barometric pressure across the horizontal surface of a map constitutes a pressure gradient. The gradient is in the direction from higher pressure to lower pressure.
◆ Where a pressure gradient exists, air molecules tend to drift in the same direction as that gradient. This tendency for mass movement of the air is referred to as the pressure gradient force.
Sea and Land Breezes ◆ During the daytime, more rapid heating of the lower air layer over the land than over the ocean causes a pressure gradient from sea to land. Air moving landward in response to this gradient from higher to lower pressure constitutes the sea breeze.
At higher levels, a reverse flow sets in. Together with weak rising and sinking air motions, a complete flow circuit is formed.
◆ During the night, when radiational cooling of the land is rapid, the lower air becomes colder over the land than over the water. Higher pressure now develops over land and the barometric gradient is reversed. Air now moves from land to sea as a land breeze.
Cyclones and Anticyclones ◆ A center of low pressure is called a cyclone; a center of high pressure is an anticyclone. ◆ Winds in a cyclone in the northern hemisphere show an anticlockwise inspiral. In an anticyclone, there is a clockwise outspiral.
◆ The surface winds spiral inward on the center of the cyclone, so the air is converging on the center and must also rise to be disposed of at higher levels.
◆ For the anticyclone, by contrast, surface winds spiral out from the center. This motion represents a diverging of airflow and must be accompanied by a sinking of air in the center of the anticyclone to replace the outmoving air.
Figure 3 Surface winds in cyclones and anticyclones
Global Distribution of Surface Pressure Systems ◆ Over the equatorial zone is a belt of somewhat lower than normal pressure, between 1011 and 1008 mb, which is known as the equatorial trough.
◆ Lower pressure is conspicuous by contrast with belts of higher pressure lying to the north and south and centered at about lat.30°N and S. These are the subtropical belts of high pressure, in which pressure exceed 1020mb.
◆ In the southern hemisphere, south of the subtropical high-pressure belt, is a broad belt of low pressure, extending from the midlatitude zone to the arctic zone. The axis of low pressure is centered at about lat.65S. This pressure trough is called the subantactic low- pressure belt.
◆ Lying over the continuous expanse of Southern Ocean, this trough has average pressure as low as 984mb. Over the continent of Antarctica is a permanent center of high pressure known as the polar high.
The Global Pattern of Surface Winds ◆ From the two subtropical high-pressure belts the pressure gradient is equatorward, leading down to the equatorial trough of low pressure. Air moving from high to low pressure is deflected by the Coriolis effect. As a result, two belts of trade winds are produced.
◆ Meeting of the trades takes place within a narrow zone called the intertropical convergence zone. ◆ Along parts of the equatorial trough of low pressure at certain times of year, the trades do not come together in convergence. Instead, a belt of calms and varible winds, called the doldrums, forms.
◆ Between lat.35 and 60N and S is the belt of prevailing westerly winds, or westerlies. ◆ A wind system called the polar easterlies is characteristic of the arctic and polar zones.
Monsoon Winds of Southeast Asia ◆ In summer southern Asia develops a cyclone into which there is strong flow of air. From the Indian Ocean and the southwestern Pacific, warm, humid air moves northward and northwestward into Asia. This airflow constitutes the summer monsoon and is accompanied by heavy rainfall in southeastern Asia.
◆ In winter, Asia is dominated by a strong center of high pressure from which there is an outward flow of air reversing that of the summer monsoon. Blowing southward and southeastward toward the equatorial oceans, this airflow constitutes the winter monsoon and brings dry weather for a period of several months.
Mountain winds and valley winds ◆ During the daylight hours the air along the slopes of the mountains is heated more intensely than the air at the same elevation over the valley floor. This warm air glides up along the slope and generates a valley wind.
◆ After sunset the pattern is reversed. Rapid radiation heat loss along the mountain slopes results in cool air drainage into the valley below and causes the mountain wind.
Global Circulation ◆ The Hadley Cell Circulation In the zone between the equator and roughly 30° latitude, the surface flow is equatorward while the flow aloft is poleward. Near the equator the warm rising air that releases latent heat during the formation of cumulus towers is believed to provide the energy to drive this cell
◆ The circulation between 30 and 60 latitude is just opposite that of the Hadley cell. The net surface flow is poleward, and because of the Coriolis effect, the winds have a strong westerly component.
◆ About the circulation in the high latitudes, it is generally believed that subsidencenear the poles produces a surface flow that moves equatorward and is deflected into the polar easterlies of both hemisphere. The region where the cold polar winds and the warmer westerly flow of the midlatitudes clash has been named the polar front.
The global Circulation and Man ’ s Environment ◆ In low latitudes, the Hadley cell operates like a simple heat engine to transport heat from the equatorial zone to the subtropical zone.
◆ Upper-air waves take up the transport and move warm air poleward in exchange for cold air. ◆ The global atmospheric circulation also transports heat in the latent form held by water vapor.
◆ Winds carry a large amount of water vapor, which is deposited as precipitation on the coast. ◆ Winds also transport atmospheric pollutants, carrying them tens and hundreds of kilometers from the sources of pollution.
Air temperature ◆ Temperature is a measure of the level of sensible heat of matter, whether it is gaseous, liquid, or solid. ◆ Conduction describes the flow of heat from a warmer substance to a colder one when the two are touching.
◆ Evaporation, the process by which water changes from a liquid to a gas by absorbing heat, tends to lower the temperature of a wet surface.
Measurement of air temperature Air temperatures are now automatically recorded by thermometers at a uniform height above the ground.
The daily cycle of air temperature ◆ Because the earth rotates on its axis, incoming solar energy at a location can vary widely throughout the 24-hour period. ◆ Insolation is greatest in the middle of the daylight period, when the sun is at its highest position in the sky, and falls to zero at night.
Daily insolation and net radiation ◆ The daily cycle of temperature is controlled by the daily cycle of net radiation. ◆ At the equinox, insolation begins at about sunrise (6 a.m.),rises to a peak value at noon, and declines to zero at sunset (6 p.m.).
◆ At the June solstice, insolation begins about two hours earlier (4 a.m.) and ends about two hours later (8 p.m.). ◆ At the December solstice, insolation begins about two hours later than the equinox curve (8 a.m.) and ends about two hours earlier (4 p.m.).
◆ When net radiation is positive, the surface gains heat, and when negative, it loses heat. ◆ Net radiation begins the 24-hour day as a negative value-a deficit-at midnight. The deficit continues into the early morning hours. Net radiation shows a positive value- a surplus-shortly after sunrise and rises sharply to a peak at noon.
Daily temperature The minimum daily temperature usually occurs about a half hour after sunrise. Air temperature rises sharply in the morning hours and continues to rise long after the noon peak of net radiation. Air temperature rises as long as net radiation is positive. Temperatures are lowest just after sunrise and highest in midafternoon.
Urban and rural temperature contrasts ◆ Urban surfaces lack moisture and so are warmer than rural surfaces during the day. At night, urban materials conduct stored heat to the surface, also keeping temperatures warmer.
The urban heat island ◆ As a result of the above effects, air temperatures in the central region of a city are typically several degrees warmer than those of the surrounding suburbs and countryside. This is called a heat island.
◆ The heat island persists through the night because of the availability of a heat stored in the ground during the daytime hours. ◆ Another important factor in warming the city is fuel consumption. In summer, city temperatures are raised through the use of air conditioning.
Temperature structure of the Atmosphere Troposphere ◆ The troposphere is the lowest atmospheric layer, in which temperature decreases with increasing elevation. Everyday weather phenomena, such as clouds or storms, occur mainly in this layer.
◆ The troposphere contains significant amounts of water vapor and countless tiny dust particles.
Stratosphere ◆ Above the troposphere lies the stratosphere in which the air becomes slightly warmer as altitude increases. ◆ The stratosphere extends to a height of roughly 50km above the earth’s surface.
◆ It is the home of strong, persistent winds that blow from west to east. ◆ One important feature of the stratosphere is that it contains the ozone layer.
High-Mountain Environments ◆ At high elevations, air temperatures are generally cooler and show a greater day-to-night range.
Temperature Inversion and Frost ◆ In a temperature inversion, air temperature increases with altitude. ◆ Low-level temperature inversions often occur over snow-covered surfaces in winter.
◆ Inversions can also result when a warm air layer overlies a colder one. This type of inversion is often found along the west coasts of major continents.
Frost ◆ If the temperature of the lowermost air falls below the freezing point, for sensitive plants during the growing season, this temperature condition is called a killing frost.
The annual cycle of air temperature The annual cycle of net radiation, which results from the variation of insolation with the seasons, drives the annual cycle of air temperatures.
Land and water contrasts ◆ Land-water contrasts keep air temperatures at coastal locations more constant than at interior continental locations. ◆ Oceans heat and cool more slowly than continents.
◆ The surface of any extensive, deep body of water heats more slowly and cools more slowly than the surface of a large body of land when both are subjected to the same intensity of insolation.
The average daily cycle of air temperature for four different months shows the effect of continental and maritime location. Daily and seasonal ranges are great at El Paso, a station in the continental interior, but only weakly developed at North Head, Washington, which is on the Pacific coast. The seasonal effect on overall temperatures is stronger at El Paso.
Annual temperature cycle P63 figure 3.16 Annual cycles of insolation (a) and monthly mean air temperature (b) for two stations at lat. 50 ° N: Winnipeg, Canada, and Scilly Islands, England.
Insolation is identical for the two stations. Winnipeg temperatures clearly show the large annual range and earlier maximum and minimum that are characteristic of its continental location. Scilly Islands temperatures show its maritime location in the small annual range and delayed maximum and minimum.
World Patterns of air temperature ◆ Isotherms: Lines drawn to connect locations having the same temperature. ◆ Maps of isotherms show centers of high and low temperatures as well as temperature gradients.
Factors controlling air temperature patterns ◆ Global air temperature patterns are controlled primarily by latitude, coastal-interior location, and elevation.
World air temperature patterns for January and July P65 figure
◆ Temperatures decrease from the equator to the poles. ◆ Large landmasses located in the subarctic and arctic zones develop centers of extremely low temperatures in winter. ◆ Temperatures in equatorial regions change little from January to July.
◆ Isotherms make a large north-south shift from January to July over continents in the midlatitude and subarctic zones. ◆ Highlands are always colder than surrounding lowlands. ◆ Areas of perpetual ice and snow are always intensely cold.
The annual range of air temperatures ◆ The annual range increases with latitude, especially over northern hemisphere continents. ◆ The greatest ranges occur in the subarctic and arctic zones of Asia and North America.
◆ The annual range is moderately large on land areas in the tropical zone, near the tropics of cancer and Capricorn. ◆ The annual range over oceans is less than that over land at the same latitude. ◆ The annual range is very small over oceans in the tropical zone.
Global warming and the greenhouse effect ◆ The temperature of our planet is warming. Most scientists agree that the human-induced buildup of greenhouse gases has begun to affect global climate. However, natural cycles, such as variations in the sun’s output, still provide strong influences.