Atmospheric Heating
The atmosphere is in constant motion and certain factors combine to set the atmosphere in motion, a major one being the uneven heating of the Earth’s surface. This heating upsets the equilibrium of the atmosphere, creating changes in air movement and atmospheric pressure. The movement of air around the surface of the Earth is called atmospheric circulation. Heating of the Earth’s surface is accomplished by several processes, some of which are described below:
Absorption of Solar Radiation
The ozone layer absorbs much of the incoming solar radiation. The remaining solar radiation that does penetrate the ozone layer is either absorbed by other constituents within the atmosphere, reflected back to space or absorbed by the earth’s surface.
Absorption of Terrestrial Radiation
Some outgoing infrared radiation from the earth’s surface is absorbed by carbon dioxide and water vapour in the atmosphere. Clouds also absorb a significant amount of this type of terrestrial radiation and re-radiate it either back to Earth, or into space. The combined effect of terrestrial radiation and conduction causes the lower several thousand feet of the atmosphere to be heated from below.
Convection
Convection refers primarily to atmospheric motions in the vertical direction. As the earth is heated by the sun, its surface begins to warm. As the surface warms, it heats the overlying air, which gradually becomes less dense than the surrounding air and begins to rise. The bubble of relatively warm air that rises upward from the surface is called a “thermal”. As air cools, the molecules pack together more closely, becoming denser and heavier than warm air. As a result, cool, heavy air tends to sink and replace the warmer rising air.
Advective Warming
An air mass that is moved from a cold portion of Earth’s surface to a warmer portion will have its lower levels heated by conduction. This heat is then distributed vertically via convection as earlier described.
Turbulent Mixing
Wind causes turbulent air motion that mixes the surface layer of air that has been heated by conduction with the cooler air aloft thereby spreading heat upwards.
Conduction
Conduction is a process in which, when two objects are touching, heat is transferred from the hotter object to the colder one. For example, a cold layer of air near earth’s surface will be heated via conduction if it overlies a warmer surface. Air however, is a very poor conductor of heat and atmospheric heating via conduction is generally confined to a very shallow layer near the surface of the earth, unless it is distributed aloft via convection.
Axis of Incidence
Earth’s axis of rotation is tilted. This means that one pole (and the associated hemisphere of Earth) will be directed away from the Sun at one side of the orbit, and half an orbit later (half a year later) this pole will be directed towards the Sun. This is the cause of Earth’s seasons. Summer occurs in the Northern hemisphere when the north pole is directed toward the Sun.
Reflection
Not all incoming solar energy is absorbed by earth’s surface. Some of it is reflected and therefore does not contribute to the heating of the earth. The degree of reflection is dependant on the angle at which the incoming solar energy strikes the surface of the earth (angle of incidence) and the type of surface it strikes. In general, a brighter surface is a better reflector than a dark surface and a larger angle of incidence results in increased reflection. A snow, or white covered surface can reflect 90% of incoming solar radiation. That is why, on a bright and sunny winter day in the parries, air temperature may remain as low as -30°C throughout the entire day.
Maritime Effects
The surface temperature of lakes or oceans do not change as rapidly as land surfaces. Water, when heated, is able to distribute the heat to great depths whereas the land maintains heat within the first few inches of the surface. It is for this reason that during the day, land surfaces heat up much more readily than water surfaces and subsequently also cool much more during the night. In other words, land areas will be characterized by large diurnal and seasonal temperature ranges and water surfaces will be characterized by very small diurnal and seasonal temperature ranges.
Latent Heat
As water vapour is carried upwards through the atmosphere it may begin to condense. As it condenses latent heat is released.
