### Overview

The atmosphere’s most abundant chemicals are molecular nitrogen (N_{2}), molecular oxygen (O_{2}), and Argon (Ar). These are all only in the gas phase. Water vapor, the next most abundant, can exist as vapor, liquid, or solid. The phase changes of water have a major role in weather and in climate. In the atmosphere, water is always trying to achieve a balance between evaporation and condensation while never really succeeding. In this lesson, you will discover the conditions under which the phases of water are in balance and will see that they depend on only two quantities—the amount of water and the temperature. Equilibrium conditions, often called saturation, are expressed mathematically by the Clausius–Clapeyron Equation. We will see that phase changes of water create weather, including severe weather, and that we can use the 1^{st} Law of Thermodynamics to do many calculations involving situations where there are phase and temperature changes. Combining the Clausius–Clapeyron equation with the equations of thermodynamics, we can construct a diagram called the skew-T. The skew-T is useful in helping us understand both the atmosphere’s temperature structure and the location and behavior of clouds.

### Learning Objectives

By the end of this lesson, you should be able to:

- differentiate among the different ways that moisture can be expressed and choose the correct one for finding an answer
- explain the meaning of the lines and spaces on a water vapor phase diagram
- calculate relative humidity using the Clausius–Clapeyron Equation
- solve energy problems related to temperature and phase changes
- demonstrate proficiency with using the skew-T to find the lifting condensation level (LCL), potential temperature, relative humidity, wetbulb temperature, dry and moist adiabats, and equivalent potential temperature

### Questions?

If you have any questions, please post them to the Course Questions discussion forum. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.