Atmosphere

TEMPERATURE INVERSION: What ? Norrmally temperature decreases with height increase but due to certain conditions this does not occur and temperature increase with height. This is called temperature inversion.

Why happen? (i) Surface inversion: in low level, or surface inversion, might take place is on a clear night, when the earth's surface radiates heat away rapidly. If the air is clear, the ground, and the air directly above it, can be cooler than the air at higher altitudes. (ii) Another type of inversion, called an advectional inversion, involves a horizontal inflow of cold air. This might be air blowing in from cold water to a coastal area. Along the California coast, winds frequently blow onshore, passing over the cold ocean waters before reaching land. When this occurs, the air at ground level may be colder than the air above it, and the air is stable. (iii) Surface inversion takes place at night in valleys, when cold, dense air flows downslope under the influence of gravity, draining off the slopes and uplands, and into the valleys. The air in the valley bottoms is colder than the air above. Other types of inversions may also develop under various conditions. Terms: 1. sensible heat transfer: heat transfer either by convection or conduction 2. Latent heat transfer: changes in state e.g. condensation and evaporation. * Nov 07 – day and night model. Global energy budget (Why hotter at equator than poles) The amount of solar radiation received and lost depends on a number of factors: 1. Distance from the sun according to the earth’s elliptical orbit. 1

At equator earth closer to sun , at poles away from sun. 2. Latitude – lower latitude directly at equator, higher latitude (poles) further distance. 3. Distance from sea 4. Altitude Global circulation models. Lower latitudes (at equator) are warmer than higher latitude (poles) this is due to air rises over the equator due to strong heating. Air then moves polewards to sink which is then drawn back to low pressure. The three cell models. Halley (1686) first suggested and Hadley (1735) explained, the convective cell, seen above, that transfers heat from the equator to the poles by means of convection. This model was adapted by Ferrel in 1856 when he dicovered that there were infact three cells and Rossby (1941) made further refinements to the model. The tricelular model found by Ferrel is still the best model we have. Air that cross warm oceans in the trade winds and become warm and moist arrives at Equator (at ITCZ) is heated and rises. The unstable air rises to form very high cumulo-nimbus clouds and afternoon thunderstorms and low-pressures are found. (At equator-area with very gentle winds called doldrums) As it rises it cools and moves away from the Equator. Further cooling occurs and increasing density and diversion by the coriolis force cause the air to slow down and subside, bringing an area of high pressure. The latitude at which this occurs is about 300. This area has clear skies and stable weather conditions. When the air reaches the ground the Hadley cell is completed and some of the air returns back to the Equator as the NE trade winds (SE if looking at the southern hemisphere) and some continues towards the poles. The air which continues towards the poles forms the bottom of the Ferrel cell. This picks up moisture as it crosses the seas and meets cold polar air at a latitude of about 600, this is known as the polar front. This air is then forced upwards and this causes an area of low pressure and brings unstable conditions which produce cyclonic rainfall. The rising air at this stage goes one of two ways. It either travels back towards the equator along the top of the Ferrel cell, or travels up towards the poles where, having cooled down, it descends forming an area of high pressure. The air then returns to the polar front as cold Easterlies.

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The three cell model.

New circulation models changes into new models influences by: 1. Jetstream: strong regular winds which blow in the upper atmosphere about 10 km above the surface, they blow between the poles and tropics (100-300 km/hr) 2. Rossby waves: meandering rivers of air formed by westerly winds; three to six waves in each hemisphere; formed by major relief barriers, thermal differences; uneven land-sea-land interface.

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World map showing distribution of isoterms/ isobars in July and Jan. World map showing distribution pressure in July.

In Winter (Jan) -areas in sub-tropical zone such as in central Asia –temperature are lower thus lead to high pressure. -wind blows from asian high pressure area to more intense low pressure Australia. In summer (July) - upper westerly winds begin to move north. - ITCZ move north. - Low pressure areas developed over Asia - Land much warmer than sea thus create high pressure on land. Air masses AIR MASSES. What is an air mass?  It is an area or mass of air which has similar properties of temperature (hot or cold, warm or cool) and humidity (moist or dry)  An air mass may be very large spreading over hundreds of kilometres; small and local.

 An air mass as mobile: Air masses move from one part of the atmosphere to another part.

 As they move, their temperature and humidity properties can change.  The temperature and humidity of an air mass comes from the region from which the air mass originated. For example: (i) A polar air mass is very cold of it originates from the poles. (ii) A tropical air mass is warm and originates from the tropics.

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(iii) (iv)

A maritime air mass has taken up moisture from an ocean of its warm. A continental air mass is dry, having begun over large areas of land (or continents)  However the temperature and humidity of an air mass can change over long travel.  As an air mass moves up vertically it cools. As it descends it warms air masses. Air masses move horizontally too.  An air mass that originated over the interior of Australia is dry and warm. It is called a continental air mass.  It may rise and cool and continue to move eastwards to the Australian coast and Tasman sea. Once it has descends over the Tasman sea it is able to absorb moisture from the sea (evaporation). It becomes a warm, moist air mass.  The movement of air in the atmosphere is wind.  No air can move vertically (rise or fall) and horizontally.  Winds result from differences in air pressure.

MESO-SCALE: LOCAL WINDS. The three meso-scale circulations: (i) Land and sea breezes (caused by local temperature differences) (ii) Mountain and valley winds (caused by local temperature differnces) (iii) The fohn ( results from pressure differences on either side of a mountain side)

(i) Land and sea breezes: Sea breeze.

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 The land heats rapidly, reaching peak temperatures in the late afternoon, especially in summer.  The air above is heated and begins to rise.  A local area of low pressure forms over land.  The sea heats less rapidly and the cooler surface maintains a low air temperature.  Air flows from high to low pressure (from sea to land) as a cool, moist sea breeze. Land breeze.

 During the night the land radiates heat rapidly and cools the air above.  The colder air sinks to form a local area of high pressure.  The sea retains heat gained slowly during the day and the air above remains relatively warm.  A local area of low pressure forms over the sea.  Air flows from high to low pressure (from land to sea) as a cool land breeze, most apparent in the early morning. (ii)The mountain and valley wind. Read your textbook Waugh, pg. 240-242 describes and draw mountain and valley winds. Air stability ELR Environmental Lapse Rate DALR Dry Adiabatic Lapse Rate Saturated Adiabatic Lapse SALR Rate

Actual lapse rate measured in the air Theoretical lapse rate for dry air (ie air below dew point) Theoretical lapse rate for saturated air, ie with vapour condensing

Lapse rate: is the rate of temperature decrease with altitude. The environmental lapse rate (ELR) is on average 6.5 C per 1000 m. It varies with local conditions such as: (i)height (lapse rate lower near ground level), (ii) humidity (lower on wet (humid) air, (iii)type of air mass (iv)time –(hight and day)

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Adiabatic lapse rate (ALR): internal change. Air loses or gains heat as a result of rising (expanding) or sinking (contracting). No heat is gained from external sources. Dry Adiabatic lapse rate (DALR): rate at which unsaturated (dry) air cools, 9.8C per 1000 m. (rounded up to 10C/1000m) Saturated adiabatic lapse rate (SALR): this is the rate at which saturated air cools. Saturated air releases heat through condensation (cloud formation) this offset cooling process. The SALR cools at a rate of between 4C/km for warm air, 9C/km for cold air. Dew point: the temperature at which air is saturated. Condensation level:is the altitude at which dew point is reached. Saturation :the air is holding the maximum amount of moisture. Absolute humidity: the amount of moisture g/m held in the air. Relative humidity: is the amount contained compared to the saturated vapour pressure. Relative humidity:

Absolute humidity X 100 Saturated vapour pressure

Stability occurs when a rising parcel of air cools more quickly than air surrounding it (ELR). As it is colder than the ELR it is denser, and therefore sinks back to position. Stable air is associated with calm (high pressure) dry conditions.

Instability.

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Instability is common on hot days. Localised heating raised the temperature of air above it. The air begins to rise as it is warmer and less dense than the surrounding air it continues to rise. If it rises to sufficient height, condensation cloud development and rain may occur. Conditionally instability.

Conditionally instability occurs when the ELR is lower than the DALR become coller than surrounding air and should sink down to the ground. However they may force to rise for eg over hill. This may cause the air to cool to its dew point. Once saturation occurs, condensation takes place. Thus air begins to cool at the SALR. If it becomes warmer than the surrounding air. It will continue to rise. The air is unstable on the condition that dew point is reached, and it cools at the SALR. The differecnes between lapse rates in the atmosphere enable different weather to occur and different clouds form. See stability and instability

Green house. What Causes the Greenhouse Effect? Life on earth depends on energy from the sun. About 30 percent of the sunlight that beams toward Earth is deflected by the outer atmosphere and scattered back into space. The rest reaches the planet’s surface and is reflected upward again as a type of slow-moving energy called infrared radiation.

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As infrared radiation is carried aloft by air currents, it is absorbed by “greenhouse gases” such as water vapor, carbon dioxide, ozone and methane, which slows its escape from the atmosphere. Although greenhouse gases make up only about 1 percent of the Earth’s atmosphere, they regulate our climate by trapping heat and holding it in a kind of warm-air blanket that surrounds the planet. What can we do? 1. 2. 3. 4. 5.

energy efficient: reduce our use of oil, gasoline and coal. Reduce use of electricity around the home (electricity power by fossil fuel such as oil) Recycling car pooling. planting more trees and avoid cutting trees.

Urban Heat islands An urban heat island (UHI) is a metropolitan area which is significantly warmer than its surroundings. Urban area hotter than countryside area. Causes: surface covered with concrete, building and lack of vegetation covered & lack of evapotranspiration. Tall building block wind-inhibit cooling by convection. Human activity-car pollution, industry-create local greenhouse effect. Heat islands form as vegetation is replaced by asphalt and concrete for roads, buildings, and other structures necessary to accommodate growing populations. These surfaces absorb - rather than reflect - the sun's heat, causing surface temperatures and overall ambient temperatures to rise. The lesser-used term heat island refers to any area, populated or not, which is consistently hotter than the surrounding area. Problems: Cause death-excessive heat. Increase energy required for refrigeration and air con. Aside from the obvious effect on temperature, UHIs can produce secondary effects on local meteorology, including the altering of local wind patterns, the development of clouds and fog, the number of lightning strikes, and the rates of precipitation. How to mitigate? The heat island effect can be counteracted slightly by using white or reflective materials to build houses, pavements, and roads, thus increasing the overall albedo of the city. This is a long established practice in many countries. A second option is to increase the amount of well-watered vegetation. These two options can be combined with the implementation of green roofs.

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