What causes convection in the atmosphere?

Answer 1

One Sentence Answer:

In the generic meaning of the term, convection in fluids (liquid or gas) is the movement of the material (mass) of the fluid itself and it occurs because some force has been applied to the fluid to cause fluid flow.

Deconstructing:

Convection is mass transfer in a fluid and in its generic form is the same as fluid flow. The term convection is however the preferred term when describing the transport of heat by fluid flow. Even then, there is a distinction between "natural convection" and "forced convection."

Heat transport by convection requires mass transport and refers to the movement of a warm region of fluid into a region of cold fluid or vice versa. Thermal energy is transported by the motion of the fluid. Another term would be "convective heat transport". The motion can be through natural convection or forced convection.

Natural convection refers to convection that results from density variations in a fluid and hence buoyant forces acting to move less dense region up and more dense regions down. This is usually associated with differences in temperature in different regions of the fluid, but it can be caused by differences in physical or chemical make up of the fluid.

Forced convection merely means that some artificial force, such as a fan, is acting to move the fluid to cause motion. Some would say this is not "true convection," and the term heat advection is then used to describe the force movement of heated fluids to transport thermal energy.

Examples:

1. Weather is largely driven by natural thermal convection. Air at the surface of the Earth is warmed by the Sun's energy and buoyant warm air rises because it has a lower density than cold air. The broad global circulation patterns of weather are however driven by forced convection with the driving force being the coriolis force that results from Earth's rotation.

2. A convection oven, common in many households, is actually a forced convection oven and hat air is circulated by a fan.

3. When a pot of water is placed over the burner or heating element of a stove, it is easy to observe the change from motionless water to a convecting fluid as the water at the bottom of the pot becomes warm and less dense and pushed upward by buoyant forces.

4. Most houses which have central heating employ a fan to move the air. This is called forced air convection. (A century or more ago, most heated houses relied on natural convection to distribute warm air since electric fans were generally unavailable.)

5. Lake turnover is a convection process where the water at the bottom of a lake is swapped with the water on top when the lake freezes or thaws. This occurs when the surface reaches 39 degrees Fahrenheit (4 degrees Celsius), the temperature when water density is maximum. In spring, ice melt and the water on top warms until 4 C, then sinks and in the process does the important tasks of bringing up nutrients and taking down oxygen. The mass transport and thermal transport are both important. (Smaller lakes constantly exchange water as a result of water currents created by wind at the surface which is forced convention and not natural thermal convection.)

Answer 2

Convection is caused by differences in air density. These differences in air density can be caused naturally or by man-made forces.

Answer 3

Solar heating.