# 2: Thermodynamics

• 2.1: Gas Laws
Understanding atmospheric thermodynamics begins with the gas laws that you learned in chemistry. Because these laws are so important, we will review them again here and put them in forms that are particularly useful for atmospheric science. These laws will be used again and again in many other areas of atmospheric science, including cloud physics, atmospheric structure, dynamics, radiation, boundary layer, and even forecasting.
• 2.2: The Atmosphere’s Pressure Structure - Hydrostatic Equilibrium
The atmosphere’s vertical pressure structure plays a critical role in weather and climate. We all know that pressure decreases with height, but do you know why?
• 2.3: First Law of Thermodynamics
The First Law of Thermodynamics tells us how to account for energy in any molecular system, including the atmosphere. As we will see, the concept of temperature is tightly tied to the concept of energy, namely thermal energy, but they are not the same because there are other forms of energy that can be exchanged with thermal energy, such as mechanical energy or electrical energy.
• 2.4: The higher the temperature, the thicker the layer
Consider a column of air between two pressure surfaces. If the mass in the column is conserved, then the column with the greater average temperature will be less dense and occupy more volume and thus be higher. But the pressure is related to the weight of the air above the column and so the upper pressure surface rises. If the temperature of the column is lower, then the pressure surface at the top of the column will be lower.
• 2.5: Adiabatic Processes - The Path of Least Resistance
So far, we have covered constant volume (isochoric) and constant pressure (isobaric) processes. There is a third process that is very important in the atmosphere—the adiabatic process. Adiabatic means no energy exchange between the air parcel and its environment: Q = 0.
• 2.6: Stability and Buoyancy
We know that an air parcel will rise relative to the surrounding air at the same pressure if the air parcel’s density is less than that of the surrounding air. The difference in density can be calculated using the virtual temperature, which takes into account the differences in specific humidity in the air parcel and the surrounding air as well as the temperature differences.

Thumbnail: A constant pressure balloon stays aloft for weeks at an altitude of 100,000 ft so that the instruments in the attached gondola can make long-term measurements. Credit: National Scientific Balloon Facility, Palestine TX