1. Atmospheric Moisture
Atmospheric moisture is a very important topic under the theme of climatic system.
In this presentation, you can make use of photos and diagrams to explain atmospheric stability and moisture processes in air within the system.
Students can also appreciate the beauty of the nature from this presentation.

2. Condensation Condensation takes place
a. when the temperature of the air is reduced but its volume remains constant and the air is cooled to dew point;
b. if the volume of the air is increased without addition of heat; this cooling takes place because adiabatic expansion causes energy to be consumed through work;
c. when a joint change of temperature and volume reduces the moisture-holding capacity of the air below its existing moisture content.

3. Water Vapour Liquid Solid
Water is a liquid compound which is converted by heat into vapour (gas) and by cold into a solid (ice).
Solid

4. Air Stability
Atmospheric stability is one major factor affecting weather conditions and it is related to heat transfer and moisture processes in air. Students are expected to relate the concept of atmospheric stability to the explanation of condensation and precipitation processes.

5. Adiabatic Processes
No heat is added / removed from the moving air parcel by the external environment because of fast movement of the air parcel.
All temperature changes within the air parcel are due to expansion / contraction.
Lapse rates
The environmental lapse rate (ELR) is the decrease in temperature usually expected with an increase in height through the troposphere. The ELR is appropriately 6.5°C per 1000m, but varies according to local air conditions.
The adiabatic lapse rate (ALR) describes what happens when a parcel of air rises and the decrease in pressure is accompanied by an associated increase in volume and a decrease in temperature.
The dry adiabatic lapse rate (DALR) is the rate at which an unsaturated parcel of air cools as it rises or warms as it descends, remains constant at
9.8 °C per 1000m.
For air in which condensation is occurring, the saturated adiabatic lapse rate (SALR) pertains at 6 °C per 1000m. This has a lower value because latent heat released in the condensation process offsets the adiabatic temperature loss.

6. Adiabatic process No heat exchange air parcel air parcel cooler 1000m
ground

7. Unstable air cloud Stable air
Cooler than
surrounding
Condensation level
precipitation
Energy loss
The rate and maintenance of any vertical uplift depend upon the temperature-density balance between the rising parcel and the surrounding air.
The state of stability is when a rising parcel of unsaturated air cools more rapidly than the air surrounding it.
Instability arises when the rising air remains warmer and lighter than the surrounding air.
Air stops
rising
Stable
air
Warm air

8. Causes of Cooling

9. Orographic Cooling
Orographic cooling results when warm maritime air is forced to rise where confronted by a coastal mountain barrier.

10. Rising air

11. Frontal Cooling
Frontal cooling occurs when two air streams meet in areas of low pressure. Within the tropics, the trade winds, blowing towards the Equator, meet at the inter-tropical convergence zone or ITCZ. The air is forced to rise and produces the heavy afternoon thunderstorms associated with the equatorial climate. In temperate latitudes, depressions form at the boundary of two air masses. At the fronts, warm, moist, less dense air is forced to rise over colder, denser air, giving periods of prolonged and sometimes intense rainfall.

12. Condensation
Front
Rising air

13. Thermal Cooling
Convectional rainfall occurs when the ground surface is locally overheated and the adjacent air, heated by conduction, expands and rises. During its ascent, the air mass remains warmer than the surrounding environmental air and it is likely to become unstable with towering cumulo-nimbus clouds forming.

14. Rising air

15. Precipitation
This is a full-motion video showing precipitation. Students can have clear visual impression of how the process works.

16. Prepared by Humanities Unit Curriculum Development Institute Education Department