1. Transformations of Moist Air 1.1 Description of Moist Air * The equation of state for vapor: where the specific gas constant for vapor: the ratio of molar weights of vapor and dry air:

* Absolute humidity (density): * Specific humidity: * Mixing ratio: * Relation between the mixing ratio and vapor pressure:

* What is the value of specific gas constant for moist air (a mixture of dry air and vapor) ? * The equation of state for moist air (a mixture of dry air and vapor): The virtual temperature: * Specific heats for moist air:

An indication of how far the system is from saturation * Saturation Properties: An indication of how far the system is from saturation Relative humidity: Dew point temperature: The temperature to which the system must be cooled isobarically to achieve saturation If the saturation is with respect to ice, this temperature is called the frost point temperature Dew point spread: What is the relation between the relative humidity and dew point spread?

1.2 Implications for the Distribution of Water Vapor Saturation values represent the maximum amount of water vapor that can be supported by air for a given temperature and pressure. Saturation vapor pressure depends exponentially on temperature. * Condensation in the atmosphere by isobaric cooling Radiation fog Advection fog * Global distribution of water vapor

1.3 State variables of the two-component system 6.3.1 Unsaturated moist air Three state variables are required to specify the moist air. Pressure, temperature and moisture Because there is only trace amount of vapor, the thermodynamic processes for moist air are very much similar to those for dry air. The virtual potential temperature:

1.3.2 Saturated moist air A mixture of dry air and two phases of water (vapor and condensate) The chemical equilibrium requires therefore

1.4 Thermodynamic behavior accompanying vertical motion 1.4.1 Condensation and the release of latent heat Saturation mixing ratio: where is a reference saturation mixing ratio The saturation mixing ratio increases with decreasing pressure, but decreases with decreasing temperature.

Lifting Condensation Level (LCL): As an air parcel is lifted it can cool until condensation begins at the cloud base or LCL. Below LCL: Above LCL:

1.4.2 The Pseudo-adiabatic process The pseudo-adiabatic process is nearly identical to a reversible saturated adiabatic process, i.e., an isentropic process. * The condensate does not precipitate out. * The heat transfer with the environment is negligible. The equivalent potential temperature is defined as which is constant during the pseudo-adiabatic process.

* 1.4.3 The saturated adiabatic lapse rate The saturated parcel’s temperature decreases with height is defined as the saturated adiabatic lapse rate, i.e., Because of the release of latent heat,

1.5 The Pseudo-Adiabatic Chart The pseudo-adiabatic chart is one of many different thermodynamic charts, and is used to describe the adiabatic process in the atmosphere. * Adiabats: (dark solid lines) For the dry and unsaturated conditions, temperature changes with height at the rate of * Pseudo-adiabats: (dashed lines) For the saturated conditions, temperature varies with height at the rate of * Isopleths of saturation mixing ratio: (thin solid lines)