Distinction is made between free convection gravitational or buoyant convection , motion caused only by density differences within the fluid; and forced convection , motion induced by mechanical forces such as deflection by a large-scale surface irregularity, turbulent flow caused by friction at the boundary of a fluid, or motion caused by any applied pressure gradient. Free and forced convection are not necessarily exclusive processes. On a windy day with overcast sky, the heat exchange between ground and air is an example of forced convection.
On a sunny day with a little wind where the ground temperature rises, both kinds of convection take place. Or gravitational or buoyant convection. Motions that are predominantly vertical and driven by buoyancy forces arising from static instability , with locally significant deviations from hydrostatic equilibrium.
Atmospheric convection is nearly always turbulent. See slantwise convection. As specialized in atmospheric and ocean science, a class of relatively small- scale , thermally can be driven by salt concentration in the ocean direct circulations that result from the action of gravity upon an unstable vertical distribution of mass.
In the case of slantwise convection , though, the motions are larger scale, and are driven by a combination of gravitational and centrifugal forces acting at an angle to the vertical. Almost all atmospheric and oceanic convection is fully turbulent and is generally composed of a collection of convection cells , usually having widths comparable to the depth of the convecting layer.
In the atmosphere , convection is the dominant vertical transport process in convective boundary layers , which are common over tropical oceans and, during sunny days, over continents. In the ocean, convection is prominent in regions of high heat loss to the atmosphere and is the main mechanism of deep water formation.
As a result, the air around the equator becomes warmest. Take a few minutes to review the video below to help you understand Global Circulation a little better. In this animation, we're going to look at global wind patterns and talk about the reasons why the air circulates the way it does and also patterns of rising and sinking air and how that relates to precipitation. The engine that drives it all, I guess you could say, is the intense heating by the Sun that occurs only in the equator areas where the sun is shining is at a very high angle of incidence and this hot air near the equator being less dense Rises upward.
It rises up, going to move toward the poles and then it gradually sinks at about 30 degrees north and south latitude. So we create these big spinning circles of air that we call the Hadley cells near the equator where the air is rising it loses its ability to hold moisture and you get a band of high rainfall and low pressure because there's air leaving the equator where the air sinks.
In these, it belts at around 30 degrees north and south you get high pressure sinking air which creates areas of clear skies and desert climates now as this air circulates and tries to flow back toward the equator along the surface of the earth or as some of it heads toward the North Pole or toward the South Pole.
After this main process of convection is complete, there are a number of scenarios that could happen, each which forms a different weather type. The term "convective" is often added to their name since convection "jumps starts" their development. As convection continues, the air cools as it reaches lower air pressures and may reach the point where the water vapor within it condenses and forms you guessed it a cumulus cloud at its top!
If the air contains a lot of moisture and is quite hot, it will continue to grow vertically and will become a towering cumulus or a cumulonimbus.
Cumulus, towering cumulus, Cumulonimbus, and Altocumulus Castellanus clouds are all visible forms of convection. They are also all examples of "moist" convection convection where the excess water vapor in the rising air condenses to form a cloud.
Convection that occurs without cloud formation is called "dry" convection. Examples of dry convection include convection that occurs on sunny days when air is dry, or convection that occurs early on in the day before the heating is strong enough to form clouds.
If convective clouds have enough cloud droplets they'll produce convective precipitation. In contrast to non-convective precipitation which results when air is lifted by force , convective precipitation requires instability, or the ability for air to continue rising on its own. It is associated with lightning, thunder, and bursts of heavy rain. Non-convective precipitation events have less intense rain rates but last longer and produce a steadier rainfall. All of the rising air through convection must be balanced by an equal amount of sinking air elsewhere.
As the heated air rises, air from elsewhere flows in to replace it. We feel this balancing movement of air as wind. Examples of convective winds include foehns and sea breezes. Besides creating the above-mentioned weather events, convection serves another purpose -- it removes excess heat from the earth's surface. Only when the pocket of warm, rising air has cooled to the same temperature of the surrounding air will it stop rising.
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