Clouds and Rain

Clouds and Rain

Abstract

Using simple 'rules of thumb' and equations, the mixing ratio, relative humidity (RH), saturation specific humidity and the lifting condensation level (LCL) of particular air parcels can be determined. By doing this an understanding of the amount of water in the atmosphere and that in clouds by region, an appreciation of the factors that determine the height and depth of clouds and a relation to how weather systems influence moisture, clouds and rain can be established.

Introduction

The hydrologic cycle or the water cycle is comprised of processes such as precipitation, evaporation and condensation and it is affected by the varying climatic conditions. The cycle describes the continuous movement of water on, above and below the surface of the Earth where the thermodynamic properties of water permit water molecules to readily change phase through the earth-atmosphere system. The aim of this laboratory activity is explore the thermodynamics of cloud formation to become familiar with how different factors such as mountain ranges and weather systems influence moisture, clouds and rain.

Method

Using Table 1 (Saturation Mixing Ratio at Sea-level Pressure as a Function of Temperature), the values given and the equation: specific humidity=RH x saturation specific humidity / 100, calculate the specific humidity and dewpoint of the two parcels. If the density of air is 1.1kg/m^3, calculate the perceptible water over 1m^2 15km from the surface and over the average Australian house (1000m^2).

Assuming no condensation or evaporation calculate the temperature and mixing ratio when parcel A and parcel B are mixed evenly. Calculate the RH, temperature and mixing ratio of the new air parcel formed when 35oC at 75% RH is mixed into 2oC and 50% RH.

Calculate the lifting condensation (LCL) for parcel A and parcel B.

Determine the temperature, dewpoint and mixing ratio at the top of the mountain. Using the difference in mixing ratio estimate the amount of liquid water in the clouds (assuming cloud density of 0.8kg/m^3). Calculate how much rain would fall on the mountain per hour if it takes 5mins for the air parcel to rise through the cloud.

Determine the temperature, dewpoint temperature, RH and mixing ratio of the downward wind at ground level.

Discussion and Conclusion

Regardless of stability, mountains force air up vertically. Air parcels can also be forced upwards by an increase in temperature and by the convergence of air. Warmer temperatures cause air parcels to expand and therefore rise into the atmosphere. Also, when winds come together the air parcels are forced to concentrate.