Moisture and Humidity

For liquid water to evaporate, water molecules must absorb enough energy to break bonds between each other. To do this, the liquid water must absorb energy and heat from the surrounding environment. This release of energy is called latent heat. If the water vapor absorbs enough energy, they will begin to vibrate fast enough to break their molecular bonds and become individual water molecules or gas. Evaporation is a cooling process because it takes heat from the surrounding environment. The concept of latent heat is essential to understand and will be revisited later when cloud formation and severe weather are discussed.

The opposite must occur for water vapor to condense into liquid water. For fast vibrating water molecules to condense into liquid, it must release latent heat to the surrounding environment. Releasing energy allows the water molecules to slow down their vibration and attach to other water molecules to become liquid. However, one step is missing. For water vapor to become liquid, it needs something to condense onto condensation nuclei. Condensation nuclei consist of microscopic dust, smoke, salt particles, or even bacteria that float in the air. It is believed that bacteria make up nearly half of all condensation nuclei. To summarize, for water vapor to condense into small liquid or ice cloud droplets, condensation nuclei must be present.

Humidity is defined as the amount of water vapor in the atmosphere. There are several ways to classify humidity, but we will focus on relative humidity for this course. Relative humidity is the ratio of the atmosphere’s actual water vapor content divided by the amount of water vapor required for atmospheric saturation at that temperature; it is usually expressed as a percentage. If the relative humidity is 25 percent, the atmosphere is only holding a quarter of what it could hold. If the relative humidity is at 100 percent, the atmosphere is saturated.

There are two ways to change relative humidity: moisture content and temperature. If the air temperature stays the same, but the amount of water vapor increases or decreases, relative humidity will change. Next, it should first be noted that warm air can “hold” more moisture than cooler air. If the water content stays the same, but temperature increases, relative humidity will decrease. If the water content stays the same, but the temperature decreases, relative humidity will increase.

Relative humidity is just as the name implies; it is a relative measurement. A more direct measurement and analysis of humidity is dew point; the atmospheric temperature air must cool down to in order for it to condense into liquid water or solid ice crystals. So if the dew point is 42 degrees Fahrenheit for a particular geographic location at a particular time, then the current temperature must fall to 42 degrees for the air to become saturated. The higher the dew point reading, the less air must cool to become saturated and condense. The lower the dew point reading, the more air must cool to become saturated; thus, the air is quite dry. Dew point analysis is vital for weather forecasting in the summer to determine the likelihood of afternoon thunderstorms. If the humidity is high, providing a high dew point measurement, then afternoon convection does not require the unstable moisture to rise as high in order for condensation and thunderstorms genesis to occur. Recall that condensation from water vapor to liquid water or ice crystals releases latent heat is a crucial ingredient for the formation of thunderstorms.

Types of Precipitation

For precipitation to occur, a variety of atmospheric conditions must occur. This includes a moisture source and an atmospheric lifting mechanism. Next, atmospheric instability needs to occur so that cloud formation may develop. Finally, moisture in the air needs to condense onto condensation nuclei so that the condensed moisture can become large enough to fall from the clouds in the form of precipitation. The atmosphere can produce a variety of precipitation types.

Rain is liquid water falling from nimbostratus or cumulonimbus clouds. Many times, in the midlatitudes, precipitation will fall from clouds in the form of ice or snow, which then melts on its way down toward the ground.

Snow is precipitation in the form of ice crystals. The temperature the snow forms will determine the size, shape, and concentration of snowflakes. When temperatures are freezing, snowflakes tend to be small, dry, to produce “powder” and powdery. When the temperatures are warmer, the snowflakes are larger.

Sleet is precipitation that falls as ice pellets. It occurs when precipitation falls from the base of a cloud in the form of snow. As the snow falls, it enters a region of warm air and melts into rain. However, as the rain continues to fall, it enters a layer of cold air and refreezes in the form of ice pellets.

Freezing rain (also called glaze) is similar to sleet except for the last step. As the rain falls, it enters a layer of cold air. However, the rain is not in this cold region long enough to freeze. Instead is stays as supercooled raindrops. However, once the supercooled raindrops reach the ground, they freeze instantly on any object it touches.

Hail is precipitation in the form of hard pellets of ice and only forms in cumulonimbus clouds where the lower region of the cloud contains liquid water and is above freezing, while the upper region is below freezing. When an ice pellet falls within the cumulonimbus cloud, it enters the warm, liquid region and picks up moisture. Then the updrafts through the ice pellet back up above the freezing point hardening the newly gathered water. The ice pellet will fall again to collect liquid water and thrown back up to refreeze. This process will continue until the hailstone’s weight becomes too heavy for the updrafts to hold it up. Once the hail becomes too heavy, the hail will precipitate from the cloud. (Dastrup, 2014)