Water Vapor

6.1 Introduction

Water vapor is the gaseous form of water and one of the most important of all constituents of the atmosphere. It constitutes only a small percentage of the Earth’s atmosphere, varying from only trace amounts to 4 percent by volume, and its amount varies widely in space and time. Approximately half of all of the atmospheric water vapor is found below 2 km (6,500 ft) altitude, and only a minute fraction of the total occurs above the tropopause.

The development of clouds and precipitation can have potential impacts on flight operations. Water vapor is important, not only as the raw material for clouds and precipitation (e.g., rain and snow), but also as a vehicle for the transfer of heat energy and as a regulator of the Earth’s temperatures through absorption and emission of radiation, most significantly in the thermal infrared (i.e., the greenhouse effect). The amount of water vapor present in a given air sample may be measured in a number of different ways, involving such concepts as relative humidity and dewpoint. Before talking about these subjects, the process of water cycling through the Earth-atmosphere system will be discussed.

6.2 The Hydrologic Cycle

The hydrologic cycle (see Figure 6-1) involves the continuous circulation of water in the Earth-atmosphere system. Water vapor plays a critical role in the cycle.

6.2.1 Evaporation

Evaporation is the phase transition by which a liquid is changed to a vapor (gas). In meteorology, the substance meteorologists are concerned about the most is water, and the primary source is the ocean. On average, about 120 cm (47 in) is evaporated into the atmosphere from the ocean each year. For evaporation to take place, energy is required. The energy can come from any source: the Sun, the atmosphere, the Earth, or objects on the Earth, such as humans.

Humans experience evaporation in their body. When the body heats up due to the air temperature or through exercise, the body sweats, secreting water onto the skin. The purpose is to cause the body to use its heat to evaporate the liquid, thereby removing heat and cooling the body. The same effect can be seen when stepping out of a shower or swimming pool. The coolness felt is from the removal of bodily heat used to evaporate the water on the skin.

6.2.2 Transpiration

Transpiration is the evaporation of water from plants. In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Of the transpired water passing through a plant, only one percent is used in the growth process of the plant. The remaining 99 percent is passed into the atmosphere.

6.2.3 Sublimation

Sublimation is the phase transition by which a solid is changed into vapor (a gas) without passing through the liquid phase. In the atmosphere, sublimation of water occurs when ice and snow (solids) change into water vapor (a gas).

6.2.4 Deposition

Deposition is the phase transition by which vapor (a gas) is changed into a solid without passing through the liquid phase. In the atmosphere, deposition of water occurs when vapor (a gas) in a sub-freezing cloud changes into ice crystals (solid).

6.2.5 Condensation

Condensation is the phase transition by which vapor (a gas) is changed into a liquid. In the atmosphere, condensation may appear as clouds, fog, mist, dew, or frost, depending upon the physical conditions of the atmosphere.

6.2.6 Transportation

Transportation is the movement of solid, liquid, and gaseous water through the atmosphere. Without this movement, the water evaporated over the ocean would not precipitate over land.

6.2.7 Precipitation

Precipitation results when tiny condensation particles grow in the atmosphere through collision and coalescence and then fall to the Earth’s surface.

6.2.8 Runoff

Runoff occurs when there is excessive precipitation and the ground is saturated (i.e., cannot absorb any more water). This runoff flows into streams and rivers and eventually back into the sea. Evaporation of this runoff into the atmosphere begins the hydrologic cycle over again. Some of the water percolates into the soil and into the ground water, only to be drawn into plants again for transpiration to take place.

6.2.9 Infiltration

Infiltration is the movement of water into the ground from the surface.

6.2.10 Groundwater Flow

Groundwater flow is the flow of water underground in aquifers. The water may return to the surface in springs or eventually seep into the oceans.

6.2.11 Plant Uptake

Plant uptake is water taken from the groundwater flow and soil moisture.

6.3 Saturation

Saturation is the maximum possible quantity of water vapor that an air parcel can hold at any given temperature and pressure. The term “saturated air” means an air parcel has all of the water vapor it can hold, while “unsaturated air” means an air parcel can hold more water vapor.

6.4 Relative Humidity

Relative humidity is the ratio, usually expressed as a percentage, of water vapor actually in the air parcel compared to the amount of water vapor the air parcel could hold at a particular temperature and pressure. 𝑅𝑒𝑙𝑎𝑡𝑖𝑣𝑒 𝐻𝑢𝑚𝑖𝑑𝑖𝑡𝑦 = 𝑊𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑊𝑎𝑡𝑒𝑟 𝑣𝑎𝑝𝑜𝑟 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑦 While relative humidity is the most common method of describing atmospheric moisture, it is also the most misunderstood. Relative humidity can be confusing because it does not indicate the actual water vapor content of the air, but rather how close the air is to saturation. An air parcel with 100 percent relative humidity is saturated, while an air parcel with relative humidity less than 100 percent is unsaturated. An air parcel’s capacity to hold water vapor (at a constant pressure) is directly related to its temperature. It is possible to change an air parcel’s relative humidity without changing its water vapor content. Figure 6-2 illustrates this concept. An air parcel at sea level at a temperature of 30 °C has the capacity to hold 27 g of water vapor. If it actually held 8 g, its relative humidity would be 30 percent, and it would be unsaturated. However, if the air parcel’s temperature decreases to 20 °C, its water vapor storage capacity decreases to 15 g and its relative humidity rises to 53 percent. At 10 °C, the air parcel’s water vapor storage capacity decreases to equal the amount of water vapor it actually holds (8 g), its relative humidity increases to 100 percent, and it becomes saturated. During this cooling process, the air parcel’s actual water vapor content remained constant, but relative humidity increased with decreasing temperature.

6.5 Dewpoint

Dewpoint is the temperature an air parcel must be cooled at constant pressure and constant water vapor pressure to allow the water vapor in the parcel to condense into water (dew). When this temperature is below 0 °C (32 °F), it is sometimes called the frost point. Lowering an air parcel’s temperature reduces its capacity to hold water vapor.

6.6 Temperature-Dewpoint Spread (Dewpoint Depression)

The difference between an air parcel’s temperature and its dewpoint is the dewpoint depression, or commonly referred to as the spread. Surface aviation weather reports (e.g., METARs/SPECIs) provide observations of both temperature and dewpoint. The temperature greatly affects the air parcel’s ability to hold water vapor, while the dewpoint indicates the actual quantity of water vapor in the parcel. As the spread decreases, relative humidity increases. When the spread decreases to zero, relative humidity is 100 percent, and the air parcel is saturated. Figure 6-3 illustrates the relationship between temperature-dewpoint spread and relative humidity.

Surface temperature-dewpoint spread is important in anticipating fog but has little bearing on precipitation. To support precipitation, air must be saturated through thick layers aloft.

Relative humidity depends on the temperature-dewpoint spread. In Figure 6-3, dewpoint is constant, but temperature decreases from left to right. On the left panel relative humidity is 50 percent, indicating that the air parcel could hold twice as much water vapor as is actually present. As the air parcel cools, the temperature-dewpoint spread decreases while relative humidity increases. When the air parcel’s temperature cools to equal its dewpoint (11 °C), its capacity to hold water vapor is reduced to the amount actually present. The temperature-dewpoint spread is zero, relative humidity is 100 percent, and the air parcel is now saturated.

6.7 Change of Phase

Water changes from one state of matter, e.g., solid, liquid, or vapor, to another at the temperatures and pressures experienced near the surface of the Earth. Interestingly, water is the only substance on Earth that exists naturally in all three phases: as water droplets, ice crystals (visible as clouds), and water vapor. Water has some unique thermal properties, which make it a powerful heat transport mechanism. It has the highest specific heat capacity of any naturally occurring substance (see Table 5-4). This means that water has a much higher capacity for storing heat energy (with little resulting temperature change) than other substances. These properties make water an ideal heat transport mechanism and have important implications on weather and climate.

6.7.1 Latent Heat

Latent heat is the quantity of heat energy either released or absorbed by a unit mass of a substance when it undergoes a phase transition (change of state). Units are typically expressed in terms of joules per gram (J/g). Figure 6-4 illustrates the latent heat transactions that occur when water undergoes phase transition.

Heat is exchanged between water and its environment during phase transition. Although the temperature of the environment changes in response, the temperature of the water undergoing the phase transition remains constant until the phase change is complete; that is, the available heat, latent heat, is involved exclusively in changing the phase of water and not in changing its temperature. There are six phase transitions, three of which are associated with the absorption of latent heat by water from the environment (melting, evaporation, and sublimation), and three of which are associated with the release of heat energy by water to the environment (freezing, condensation, and deposition).

Melting is the phase transition by which a solid is changed to a liquid. During melting, water absorbs 334 J/g due to the latent heat of fusion. Freezing, the reverse process, releases 334 J/g back to the environment. Evaporation is the phase transition by which a liquid is changed to a vapor. During evaporation, water absorbs 2,501 J/g due to the latent heat of vaporization. Condensation, the reverse process, releases 2,501 J/g back to the environment.

Sublimation is the phase transition by which a solid is changed to a vapor. During sublimation, water absorbs 2,834 J/g due to the latent heat of sublimation. Deposition, the reverse process, releases 2,834 J/g back to the environment. Table 6-1 lists the latent heat exchanges of water.

The amount of energy associated with latent heat exchange should not be understated. An average hurricane releases 52 million trillion (5.2 x 1019) joules per day as water vapor condenses into clouds and precipitation. This is equivalent to about 40 times the total worldwide energy consumption per day in 2005!