Chapter 6, Water Vapor 6-1
6 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.
Chapter 6, Water Vapor 6-2
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.
Figure 6-1. The Hydrologic 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 w hen
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. I n 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 (solid s) change into
water vapor (a gas).
Chapter 6, Water Vapor 6-3
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 ga s) 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.
Chapter 6, Water Vapor 6-4
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 eq ual the amount of water vapor it actually holds (8 g), its rel ative humidi ty 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.
Figure 6-2. Temperature Effects on Relative Humidity
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). W hen 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
Chapter 6, Water Vapor 6-5
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.
Figure 6-3. Temperature-Dewpoint Spread Effect on Relative Humidity
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.
Chapter 6, Water Vapor 6-6
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 expres sed in terms of joules per
gram (J/g). Figure 6-4 illustrates the latent heat transactions that occur when water undergoes phase
transition.
Figure 6-4. Latent Heat Transactions 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 enviro nment (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.
Chapter 6, Water Vapor 6-7
Table 6-1. Latent Heat of Water at 0 °C
Latent Heat Type Energy Exchange
(J/g)
Latent heat of sublimation 2,834
Latent heat of vaporization 2,501
Latent heat of fusion 334
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!