Thermal Convection
By KD1LD

There are only two ways by which surface air can be carried aloft to operate by the adiabatic process. Mechanical convection and thermal convection. Thermal convection is the result of instability in the atmosphere. Mechanical convection is caused by advaction and orographic lifting. Thermal convection is caused by heating from below. The basic principle of thermal convection is that heating from below causes surface air to expand because of it's lesser density. It is displaced upward by the surrounding air of greater density. This incoming air is in turn heated, expanded, and forced aloft.

There are two methods of starting thermal convection. When a parcel of cold air is moved over a warm surface the heat radiating from the surface warms the parcel causing it to expand and move aloft, or when the earth's surface is heated by the sun's energy. The air immediately above the surface is intern heated by radiation from the warm surface. It expands and is forced
aloft by heavier colder air moving in. The sun's energy is heating the earth's surface causing incredible amounts of warm service air to rise into the upper air. This rising warm air cools adiabaticly. Dry air by the dry adiabatic rate and saturated air by the moist adiabatic rate

. Let us use a parcel of dry air for an example. As long as the rising parcel of air remains warmer than the surrounding air, it will continue to rise. When a parcel of air is displaced and continues to rise we have unstable air. If a parcel of dry air is lifted by some form of convection, the change in temperature would be represented on the rayob by the dry adiabat. The temperature of the colder air surrounding the parcel being lifted would be represented on the rayob by the assisting lapse rate line. When the rayob shows us that the existing lapse rate line representing the existing or surrounding air slants more sharply to the left than the dry adiabat the parcel of air being lifted is unstable. With saturated air the existing lapse rate line would be compared to the moist adiabat. We see that the existing lapse rate line slants more sharply to the left, thus unstable air is again indicated. For thermal convection to take place the existing air must be unstable in the lowest level of the troposphere. Now let us study how a thermal convection system works adiabaticly. First with dry air. Heated from below it moves aloft with the pressure on it steadily decreasing. The air cooling at the dry adiabatic rate, three degrees centigrade per thousand of rise. More air is drawn in at the surface. It is heated and is forced aloft. The column of rising air continues it's ascent as long as it is warmer than the surrounding air. When it reaches the level where it is cooled adiabaticly to the same temperature as the surrounding air it's ascent is checked. How ever the momentum of the ascending air generally carries it passed the point of equilibrium perhaps as much as a thousand feet. Lets see how a thermal convection system works adiabaticly with dry air that will become saturated as it goes aloft. We start with dry air with the temperature of twenty five degrees centigrade and the dew-point temperature of nineteen degrees centigrade. The parcel cools at the dry adiabatic rate until it has cooled to it's saturation point. One hundred percent humidity. Now lets see this on the rayob. With further lifting and cooling condensation begins and a cumulus cloud is formed. The saturated air now cools at the new rate of two degrees centigrade per thousand feet, the moist adiabatic rate. Let us look at the rayob again. From the point of saturation we are following the moist adiabat. The column of warm air continues rising and forming a cumulus cloud until it reaches a level where it has cooled to the same temperature as the surrounding air. It's decent is checked but it's momentum carries it another thousand feet or so.

What will stop this thermal convection system. Since it started with surface heating, it will stop when the heating from below ceases. This occurs daily when the sun sets. It also occurs when a layer of air above a warm surface is moved over a cold surface.

Now with mechanical convection the parcel of air was stable because it was colder and heavier than the surrounding air. It had to be forced over the mountain by a combination of advaction and oragrafic lifting. It is also possible for unstable air to be lifted mechanically. We know that unstable air has a lapse rate greater than the dry adiabatic rate. When a parcel of unstable air is lifted mechanically it will cool at the adiabatic rate. Thus as the parcel goes aloft it is warmer than the surrounding air. Being warmer it is lighter and now acts like a parcel going under thermal convection. If this is a moist parcel a cumulus cloud would form on the windward side of the mountain. Heating from below promotes thermal convection. Thermal convection works by the adiabatic process and turbulence is the result. It is possible to have a combination of mechanical and thermal convection. Learning to recognize the causes of thermal convection will help you to anticipate instability and turbulence.