See Figure 4.1

Hydrogen bonds are the attractive forces that exist between hydrogen atoms in one water molecule and oxygen atoms of another water molecule. Water is denser in its liquid phase than in its solid phase due to the rigid hexagonal structure of ice, which contains a lot of empty spaces (and therefore has greater volume and lower density). This allows ice to float on top of water, ensuring that lakes freeze from the top down. This has a major effect on the global energy balance and, consequently, the weather. In addition, during heating some of the energy is used to break the hydrogen bonds (rather than increase the molecular motion), resulting in water heating up and cooling down more slowly than other common substances.

Whenever water changes state, energy is exchanged between water and its surroundings; this energy is referred to as latent heat. This latent heat is the energy absorbed by ice to convert it to water and to water to convert it to vapor. It’s the same heat released to the environment when vapor condenses or water freezes.

Evaporation is the conversion of water to water vapor. Condensation is the conversion of vapor back to water. Freezing is the conversion of water to ice and melting is the conversion of ice to water. Sublimation is the conversion of ice directly to vapor and deposition is the conversion of water vapor directly back to ice.

Absolute humidity is the mass of water vapor in a given volume of air. 

Mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of air. 

Vapor pressure is that part of the total atmospheric pressure attributable to its water vapor content.

Saturation is the atmospheric state when evaporation reaches an equilibrium with condensation. The pressure exerted by the motion of the water vapor molecules at that point is referred to as saturation vapor pressure.

Relative humidity can change whenever water vapor is added to, or removed from the atmosphere, and relative humidity can change whenever the air temperature changes.

Dew point is the temperature at which saturation occurs. High dew-point temperatures equate to moist air and, conversely, low dew-point temperatures indicate dry air.

Warm air has a greater capacity to hold water vapor

Hygrometers calculate the quantity of water vapor in the air by measuring either the relative humidity or dew point.

An adiabatic temperature change in an air mass occurs not because there is any exchange of heat but due compression or expansion of the air mass.

The dry adiabatic rate is 10°C/1000 m (i.e., dry, rising air cools at 10°C/1000 m due to expansion or sinking air warms at the same rate due to compression). The wet adiabatic rate is between 5–9°C/1000 m (i.e., moist, rising air cools at 5–9°C/1000 m due to expansion). They wet adiabatic rate is lower due to the latent heat released upon condensation, which slows the rate of cooling.

The Lifting Condensation Level is the altitude where condensation occurs, the Level of Free Convection is the altitude where air will rise due its own buoyancy, and the Equilibrium Level is the altitude at which, after a period of free convection, the temperature of a rising air parcel again becomes equal to its surroundings, causing it to lose buoyancy.

Orographic lifting occurs when air is forced to rise over a physical barrier, such as a mountain. Frontal lifting forces warm, less dense air over cooler, denser air. Convergence lifting occurs when the wind pattern near the surface is such that more air is entering an area than is leaving. Local convection lifting occurs when air near the Earth’s surface is heated, becomes less dense and rises.

Rain shadow refers to the dry area on the leeward side of a mountain which receives little rainfall due to the mountain barrier.

Absolute stability occurs when the environmental lapse rate is less than the wet adiabatic rate. Absolute instability occurs when the environmental lapse rate is greater than the dry adiabatic rate and conditional instability occurs when the environmental lapse rate falls between the wet adiabatic rate and the dry adiabatic rate.

The environmental lapse rate refers to the temperature change of the atmosphere as altitude increases. It is variable and is determined by measurements (vertical soundings). Adiabatic cooling, on the other hand, applies only to the cooling by expansion that occurs in vertically ascending air.

Instability is enhanced by the following: warming of the lowermost layer of the atmosphere by solar radiation during daylight hours; heating of a cold air mass from below as it passes  over a warm surface or bringing in warm air near the surface (warm air advection); upward movement of air caused by processes such as orographic lifting, frontal lifting, or convergence, and radiation cooling of cloud tops.

Stability is enhanced by the following: radiation cooling of Earth’s surface after sunset; the cooling of an air mass from below as it traverses a cold surface or the arrival of cold surface air (cold air advection), and subsidence within an air column.

When stable air is forced aloft, relatively thin widespread clouds typically form, and any precipitation that results is light to moderate. In contrast, as unstable air rises, towering clouds are generated that are usually accompanied by heavy precipitation.