Adiabatic Process
By KD1LD

Under certain conditions currents may be set up in the atmosphere in such a way that air will travel upward or downward. Air pressure decreases with rising altitude. Air is constantly undergoing a change in pressure. Within this working system of air moving upward and downward no heat is gained or lost to the surrounding air. However, within the system itself there is a temperature change of cooling and heating which is independent of the surrounding air. This is know as the adiabatic process.

Air does not come in parcels, but in this demonstration it does. In order to study the adiabatic process we must see this parcel lifted vertically by some form of lifting action. This movement is called convection. One form of lifting action can be accomplished mechanically. The parcel of air is first moved horizontally. This movement is called advection. When an air parcel encounters an obstruction such as a mountain, it rises. This movement is called orographic lifting. The advection and orographic lifting together forces air up the mountain slop. This process is know as mechanical lifting. The force pushes the air over the mountain and the air descends to it's original level on the opposite side. This air is stable because it resist displacement.

Let's make some examples. On the surface the temperature is twenty five degrees centigrade. When it is lifted the pressure decreases as it expands and cools to twenty two degrees at the one thousand foot level. For each thousand feet of rise the air cools three degrees centigrade. This constant cooling of the air is called the dry adiabatic rate. When a parcel of dry air descends the same adiabatic process takes place, but in reverse. With each thousand feet of decent the pressure on the air increases and the air warms at the rate of three degrees centigrade per thousand feet. This parcel of dry air has gone aloft and returned to the surface through the adiabatic processes without a gain or loss of heat. Therefore the temperature at the surface will be the same as when it started. To plot the dry adiabatic rate on the rayob the surface pressure is located and then the surface temperature. From this starting point temperatures are marked off decreasing three degrees centigrade for each thousand feet of altitude. These points are connected forming a dry adiabatic rate line.

Now lets make an example with saturated air. But first what is meant by saturated air. Water vapor is a gas present in the atmosphere. When a parcel of air contains some water, but not the maximum it can absorb at a given temperature, we would have moist air. When a parcel is packed full of the maximum vapor it can carry, at that particular temperature we would have saturated air. It's relative humidity would be one hundred percent. While the moist air could be anything from ninety nine percent or less relative humidity Air that is not saturated is referred to as dry air. Now lets talk about saturated air. One hundred percent relative humidity doesn't mean that the saturated air is solid water. The water vapor could be as much as four percent by volume. If the parcel of saturated air is heated it ceases to be saturated. The more a parcel of air is heated the more water vapor it can hold. But if it is cooled it returns to saturation and one hundred percent relative humidity. With further cooling the excess water vapor is squeezed out. In this way clouds are formed. The temperature at this point where the air is saturated and condensation will occur is called the dew-point temperature. Let's make an example. Advection moves a parcel horizontally. Advection plus orographic lifting forces the air over a mountain slope, resulting in mechanical convection. As the dry air cools it finally reaches the dew-point temperature. The air is now saturated and now has one hundred percent relative humidity. Further lifting squeezes out the excess water vapor making it visible as a stratus type cloud. The force pushes the air over the mountain and the air descends to it's original level on the opposite side. The air in this system was stable because it resisted displacement and had to be forced over the mountain. Now lets make an example. Starting at the surface with a parcel of dry air we have lifted mechanically, the pressure decreasing with rise in altitude, and the air expanding. The air cools at three degrees centigrade for each thousand feet of rise. As long as the air is dry, it cools at the dry adiabatic rate, however, when the air reaches a level where the air is equal to the dew-point temperature at that level the air has become saturated. Any further lifting beyond this point starts condensation and a straits type cloud is formed. The water vapor is undergoing a change of state from a vapor to liquid water drops. This change releases what is know as laten heat of condensation which warms the parcel. Because of this added heat the parcel is now cooling at a slower rate. Two degrees per thousand feet centigrade. This is know as the moist adiabatic rate. It only applies to saturated air. The dew-point remains equal to the temperature of the air parcel throughout this condensation. At five thousand feet the air parcel has reached the top of the mountain and crosses to the other side. As the air descends on the lee side the pressure increases and the air warms again. It is no longer saturated. All the way down the air warms at the dry adiabatic rate three degrees centigrade per thousand feet of decent. Back at the surface on the original level this parcel is two degrees warmer then when it started aloft. Because moisture was lost on the windward side of the mountain, the relative humidity is less on the lee side of the mountain. On the rayob the values for the moist adiabat have been pre calculated. The rate of cooling varies with the temperature and altitude. With colder temperatures the laten heat of condensation is of much smaller value and less heat is released. With cooler temperatures the moist adiabat appears more like the dry adiabat. With warmer temperatures the difference becomes greater.