Precipitation, Wind velocity, Atmospheric Stability, Adaibatic Lapse Rate

Precipitation

The precipitation denotes all the form of the water that reaches the earth surface from the atmosphere. The usual form of the precipitation is rainfall, snowfall, hail stone, dew, frost etc. In the tropical country, the major portion of the precipitation is in the form of the rain. So that’s the reason why the precipitation and rainfall are always similar or used as synonymous. There are mainly four conditions which are to be satisfied for the precipitation to occur i.e,

1: The atmosphere must have the moisture.

2: There must be the sufficient presence of the nuclei to aid the condensation.

3: The weather concentration must be good for the condensation of water.

4: The process of the condensation must reach the earth.

The precipitation occurs in many forms. For example; drizzle, rain, glaze, sleet, snow, hail, and dew etc.

Wind direction and its velocity

The horizontal movement of the air relative to the earth is known as the wind. The wind is caused by the unequal distribution of pressure. The direction of the flow of the wind is from the higher pressure to the low pressure. The wind has been essential for the thermodynamics mechanism of the atmosphere which transfer heat, moisture from one part of the earth to the other and also to balance the global pressure distribution.

The wind velocity is the vector quantity having the both magnitude and the direction. The magnitude of the wind velocity is called the wind speed and the direction is regarded as the direction from which it is blowing. The velocity and the direction of the wind are measured by an instrument called as Anemometer whereas the wind wave respectively measures the wind direction.

The most important factor which affects the direction and speed of wind are described below:

1: Horizontal pressure gradient:

The difference between the atmospheric pressure of the two adjacent areas is called as the horizontal pressure gradient. A pressure gradient is the immediate cause of all the air movements. The direction of the flow for this pressure gradient is from the high pressure to the low pressure and volume of air flow is directly related to the rate of change of pressure with distance. A pressure gradient is steep then the flow of wind is very rapid.

2: Rotation of earth (Coriolis force):

The deflective force due to the rotation of the earth on its own axis is known as the Coriolis force which helps to influence the direction of the wind, it causes all the winds in the northern hemisphere to move upwards right in the southern hemisphere towards the left. The Coriolis force at the equator is zero and this Coriolis force is maximum at the poles. This force acts as an angle of 90 degrees in the horizontal direction of the wind and is directly proportional to the horizontal wind velocity. However, the velocity of wind is not influenced by the Coriolis force.

3: Frictional force:

The force which is developed when the air movement occurs between the atmosphere and the earth surface or between adjacent layers of air. This force acts in the opposite direction to that of relative movement. The frictional force is large at near the earth surface, so the air below a height of about 1km of an atmosphere is said to be a frictional layer. However, above this layer, a frictional force is negligible and the region is called the free atmosphere.

4: Centrifugal force:

The pressure gradient force tends to move air in the straight line but Coriolis force causes it to move in the curved path. If the motion of the air is curved, centrifugal force is then developed. However, centrifugal force is not actually forced, it is due to the imbalance of the other forces when the isobaric curve imbalances change the direction of the flow of wind.

Atmospheric stability

The most of the weather phenomenon depends on whether the air masses are stable or unstable. When the air masses are stable, vertical motion is suppressed whereas when the air masses is unstable vertical current developed. The air masses at the high levels are then too heavy to be supported by the warmer air masses. So, this results in the vertical current.

Definition:

To test whether the atmosphere is stable or unstable, we let a unit of air be placed a small distance up or down. If the unit after being thus displaced has a tendency to return its original level then the atmosphere is said to be stable.

However, if the unit after being displaced has the tendency to move further away from its original level then the atmospheric is said to be unstable.

The limiting case may occur that is when the displaced unit tends neither to return nor to move further away from the original level. In this case, the atmosphere is said to be neutral or indifferent.

Since any minute disturbance will upset the unstable system and bring them to a stable state. It follows that unstable system cannot exist for any applicable length of time. The transition from the unstable state to a state of the equilibrium involves the reduction of the potential energy.

The principle of stability and instability that hold far solid bodies is also applied to the atmosphere but the conditions are a here more complicated outing to the fact that the air is compressible and therefore changes its density as it moves upward and downward. A second complication arises when the air becomes saturated with the moisture. The latent heat of the vaporization is then liberated and is used to heat the air. This process of heating affects the density and therefore the stability of atmosphere is affected.

In order to derive the convenient criteria for the stability and instability of air masses or atmosphere, we shall discuss the adiabatic charts and discuss non-saturated and saturated air separately.

Adiabatic Lapse Rates

The rate of the change of the temperature in an ascending air mass through the adiabatic process is called the adiabatic lapse rate. But the most of the important point to remember is that the lapse rate of saturated air parcels is not the same. Moreover, the adiabatic rate of the temperature changes in ascending or descending air parcels is also controlled by the differences in the initial air temperature of such parcels. The initial temperature brings about the wide variation in the heights if the condensation levels.

1: Dry adiabatic lapse rate:

When the ascending or the descending air parcel is dry or unsaturated its temperature changes at the constant rate. In the meteorology, the dry adiabatic rate of change in the temperature with the height is referred as the dry adiabatic lapse rate. This dry adiabatic lapse rate is always 10 degrees centigrade per 1000m. The dry adiabatic lapse rate is defined as the rate of decrease in the temperature with the height experienced by an air parcel being lifted adiabatically through an atmosphere in hydrostatic equilibrium. While the environmental lapse rate is characterized by the temporal as well as spatial variations, the dry adiabatic lapse rate always remains as constant.

2: Wet adiabatic lapse rate:

When in an ascending air mass, the process of the condensation starts after the temperature has come down to saturation point. The latent heat is returned to the ascending parcel of air during the process of condensation. The rate of the cooling of the rising air is reduced. Thus, the reduced rate of the temperature change caused by the addition of the latent heat of condensation is called the wet or the moist adiabatic lapse rate.

The wet adiabatic lapse rate varies from the 0.5 degrees to 0.9 degree Celsius per 100 meters. Higher moisture content in a rising air lowers down, the rate of temperature change whereas in the relatively drier air mass, there is a higher rate of temperature changes.

References:

Miller, Jr. G.T. Living in the Environment. Wadsworth Publication, 2003.

S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

T., Richard. Environmental Science Towards a Sustainable Future. India: PHI (P) Ltd., 2008.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)