Water Vapor – Atmospheric Processes and Phenomenon (2022)

Alison Nugent and Shintaro Russell

Learning Objectives

By the end of this chapter, you should be able to:

  1. Compute saturation vapor pressure using the Clausius-Clapeyron equation;
  2. Convert between humidity variables. Differentiate between relative humidity, specific humidity, absolute humidity, wet-bulb temperature, mixing ratio, and dew point;
  3. Describe the conditions for saturation to occur;
  4. Apply the moist adiabatic lapse rate;
  5. Use the principles of phase change and latent heating to describe why the moist adiabatic lapse rate is less than the dry adiabatic lapse rate.

Water can exist as a solid, liquid, or gas at typical conditions found on Earth. As we learned, the process of liquid water becoming water vapor is called evaporation and this process absorbs or requires energy. The opposite process is called condensation, where water vapor becomes liquid water, releasing energy. Condensation is especially important in atmospheric science because this is the process that allows clouds to form.

Water Vapor – Atmospheric Processes and Phenomenon (1)

Clouds are composed of millions and billions of tiny liquid water droplets. How do they form? Why are they there?

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Before we can understand clouds in the atmosphere, we need to explore concepts like how humidity is defined and what saturation means.

In general, humidity is the amount of water vapor in the air. You’ve likely heard of relative humidity and dew point temperature, but what do these quantities mean physically?

Imagine a closed jar filled halfway with water. At the initial time, more water molecules evaporate from the water surface than the number that return. However, after some time, the number of molecules evaporating from the surface will be equal to the number of molecules condensing back into the water surface. When condensation and evaporation are equal, this is called saturation.

Saturation occurs when air contains the maximum amount of water vapor possible for its given temperature. That is why condensation equals evaporation. If evaporation occurs, the air cannot contain more water vapor, so some must condense. Now let’s get quantitative.

Vapor pressure at saturation

Every gas in the atmosphere exerts pressure, for example, vapor pressure makes up a fraction of the total atmospheric pressure. In the following equation, all of the gases in Earth’s atmosphere contribute to the total atmospheric pressure Patmosphere.

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Specifically for water vapor, the more water vapor that is added to the atmosphere, the higher the vapor pressure PH2O. The units for vapor pressure are the same as pressure and can be in Pascals, hectoPascals, or kiloPascals. Because we are staying consistent with Roland Stull’s Practical Meteorology textbook, we will use kiloPascals (kPa) throughout this chapter.

The amount of water vapor that the atmosphere can contain depends on temperature. Lower temperature air cannot contain as much water vapor as higher temperature air. If we think of this quantitatively in terms of pressure,saturation vapor pressure refers to the pressure exerted by the movement of water vapor molecules exerted over a surface of liquid water. When the partial pressure exerted by water vapor is equal to the saturation vapor pressure, the air is saturated.

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The Clausius-Clapeyron equation gives the approximate relationship between saturation vapor pressure (es) and temperature in the atmosphere

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where the water-vapor gas constant ℜv is 461 J·K–1·kg–1, T0is 273.15 K, e0is 0.6113 kPa, and Lv is the latent heat of vaporization, 2.5×106 J·kg–1. Thisresults in Lv/ℜv being equal to 5423 K. In this equation, units for temperature must be in Kelvin. Note that in the equation above, exp[x] implies the exponential function ex, but it is written on one line for visual purposes.

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The image shows the relationship between temperature and saturation vapor pressure based on the Clausius-Clapeyron equation. Lower temperatures are saturated with respect to water vapor at lower vapor pressures, while higher temperatures need higher vapor pressures to be saturated. Temperature is the primary factor determining water vapor saturation.

In the graph of saturation vapor pressure vs. temperature notice the saturation vapor pressure value at the boiling temperature, 100°C. The saturation vapor pressure value es(100°C)=101.325 kPa, is the same value as the atmospheric surface pressure. Water boils at the Earth’s surface when the saturation vapor pressure is equal to the atmospheric pressure, which is why water boils at 100°C. Will water boil at the same temperature at the top of Mount Everest?

Vapor pressure is one way of defining humidity, but there are many others. Here is a non-comprehensive list of humidity variables and their typical units.

e = vapor pressure (kPa)
r = mixing ratio (g·kg–1)
q= specific humidity (g·kg–1)
ρv = absolute humidity (g·m-3)
RH = relative humidity (%)
zLCL = lifting condensation level (km)
Td = dew point (temperature) (°C)
Tw = wet-bulb temperature (°C)

Vapor Pressure

We’ve already discussed saturation vapor pressure, es, but you can also compute vapor pressure, e. However, because Tdis often unknown, the easiest way is usually through relative humidity.

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Again, e0is 0.6113 kPa,Lv is 2.5×106 J·kg–1, ℜv is 461 J·K–1·kg–1, T0is 273.15 K, and Td is dew point temperature, which will be defined later.

Mixing Ratio

Mixing ratio, r, is the ratio of the mass of water vapor to the mass of dry air. It is typically expressed as grams of water vapor per kilogram of air (g·kg–1).

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Water Vapor – Atmospheric Processes and Phenomenon (8)

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Pressure (P) should be in the same units as vapor pressure (e).The constantε is 0.622, is the ratio between the gas constant for dry air and the gas constant for water vapor.

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Saturation mixing ratio, rs, is computed the same way as the mixing ratio but with saturation vapor pressure, es, instead of e.

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When calculating mixing ratio, the pressure units on the top of the fraction will cancel with the pressure units on the bottom of the fraction. While it appears unit-less, its technically not based on its definition of mass of water vapor as compared to mass of dry air. See the Pro Tip below for more information.

Pro Tip: Many units of moisture are given in g·kg-1 or kg·kg-1so technically the units could cancel and it could be unitless! Don’t let this fool you. It is important to remember that the mass in the numerator and denominator are different. In the case of mixing ratio, the value is given in mass of water vapor proportional to the mass of dry air.

Specific Humidity

Specific humidity, q, is the ratio of the mass of water vapor to the total mass of air (dry air and water vapor combined). It is expressed as grams of water vapor per kilogram of air (g·kg–1).

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Again, saturation specific humidity, qs, is computed with es instead of e.

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Absolute Humidity

Absolute humidity,ρv, is the ratio of the mass of water vapor to the volume of air. It is expressed as grams of water vapor in a cubic meter of air (g·m-3). It is effectively water vapor density.

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Again, saturation absolute humidity,ρvs, uses es instead of e.

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Relative Humidity

Relative humidity, RH, is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor needed for saturation at a certain pressure and temperature. It is typically multiplied by 100 and expressed as a percent. Relative humidity shows how close the air is to being saturated, not how much water vapor the air contains. For this reason, RH is not a good indicator of the quantitative amount of water vapor in the air. It is only a relative measure that is highly dependent on the air temperature. Relative humidity greater than 100% is called supersaturation.

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or

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Imagine two parcels of air with the same volume, pressure, and relative humidity. Parcel 1 has an air temperature of 20°C while Parcel 2 has an air temperature of 30°C. Which parcel contains more water vapor?

Dew Point Temperature

The dew point temperature, Td, is the temperature to which the air must be cooled to reach saturation, without changing the moisture or air pressure. It measures the actual moisture content of a parcel of air. Saturation occurs when the dew point temperature equals the air temperature.

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When the dew point temperature is lower than the freezing point of water, it is also called the frost point.

Wet-Bulb Temperature

Wet-bulb temperature, Tw, is the lowest temperature that can be achieved if water evaporates within the air. When the relative humidity is 100%, the wet-bulb temperature is equal to the air temperature because there is no evaporation.

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The wet-bulb temperature is difficult to calculate but easy to measure. To measure the wet-bulb temperature, all you need is a thermometer with a wet cloth wrapped around the bulb. Typically this thermometer is attached to an apparatus called a sling psychrometer to make it easy to spin around in the air to create lots of airflow over the wet cloth on the thermometer. The evaporation from the wet cloth cools the temperature measured, hence the wet-bulb temperature is always lower than the air temperature (or dry-bulb temperature) when relative humidity is less than 100%.

You can also estimate the wet bulb temperature using lines on a graph. Normand’s Rule is used to calculate the wet-bulb temperature from the air temperature and the dew point temperature. The wet-bulb temperature is always between the dew point and the dry-bulb temperature (Td TwT). This can be implemented on thermodynamic diagrams, such as the Skew-T log P, which is discussed in more detail in the next chapter.

Take note of this description for later. To find the wet-bulb temperature on a Skew-T log Pdiagram, follow the dry adiabatic lapse rate line upward from the air temperature. Next, use the dew point temperature and follow an isohume (line of constant relative humidity) upward. The point where these two lines meet is called the lifting condensation level (LCL). From the meeting point, follow the moist (saturated) adiabatic lapse rate back down to obtain the wet-bulb temperature value. This is probably confusing at this point because we have not discussed the LCL or the moist adiabatic lapse rate, but don’t worry, we’ll repeat this logic again in the next chapter to make sure this is clear.

You may be wondering at this point why we care so much about moisture and why we need so many definitions of (almost) the same thing. The reason is that moisture is an extremely important atmospheric property. Water can exist in three phases (vapor, liquid, ice) within the atmosphere at typical pressures and temperatures. It has an especially large impact on the human experience—think about a humid day, foggy conditions, rain, snow, or even hail! Less obvious is its impact on atmospheric stability, which drives the aforementioned conditions.

For now, let’s think about the process of water vapor condensing to form liquid water. There is one final definition of humidity that will be helpful.

Lifting Condensation Level

The lifting condensation level, zLCL, is the altitude where clouds form. At the LCL, temperature equals the dew point temperature, resulting in saturation and therefore condensation. The height (z) of the LCL is

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where a is 0.125 km°C-1. We can also define the temperature at the LCL as follows.

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Moist Adiabatic Lapse Rate

In the last chapter, we discussed how temperature changes as a dry parcel of air is lifted in the atmosphere. You will recall that as an air parcel is lifted, the temperature drops by 9.8 K every kmdue to the work the air parcel must do to the environment as it expands. Let’s add moisture to the discussion and see how this changes things.

If the air parcel reaches saturation (100% relative humidity) and water vapor condenses to liquid water within the parcel, latent heat will be released. In the case of a rising air parcel that is cooling from adiabatic expansion, this added heat from condensation counterbalances some of the cooling. Hence, the air parcel will no longer cool at the dry adiabatic lapse rate but at the smaller moist adiabatic lapse rate (Γm). Unlike the dry adiabatic lapse, the moist adiabatic lapse rate is not constant and varies based on the temperature and moisture of the air parcel.

We will approximate the moist adiabatic lapse rate with the following value.

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The difference between the dry adiabatic lapse rate (Γm) and the moist adiabatic lapse rate (Γm)is significant and has profound influence on atmospheric stability, the topic of the following chapter.

Chapter 4: Questions to Consider

  1. Explain the conditions needed for saturation to occur.
  2. What is the saturation vapor pressure of air at 26°C?
  3. Explain the difference between specific humidity and relative humidity.
  4. If the temperature is 10°C and the pressure is 700 hPa, calculate the saturation specific humidity and the saturation mixing ratio.
  5. Explain why the moist adiabatic lapse rate is less than the dry adiabatic lapse rate.

Selected Practice Question Answers:

FAQs

Water Vapor – Atmospheric Processes and Phenomenon? ›

Water can exist as a solid, liquid, or gas at typical conditions found on Earth. As we learned, the process of liquid water becoming water vapor is called evaporation and this process absorbs or requires energy. The opposite process is called condensation, where water vapor becomes liquid water, releasing energy.

What is atmospheric water vapor? ›

Atmospheric water vapor is the absolute amount of water dissolved in air. When measured in linear units (millimeters, mm), it is the height (or depth) the water would occupy if the vapor were condensed into liquid and spread evenly across the column.

How water vapors are formed in an atmosphere? ›

Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Water vapor is transparent, like most constituents of the atmosphere. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation.

What effect does water vapor have on atmospheric stability? ›

Moist Stability

The effects of moisture change the lapse rate of the air parcel and, therefore, affects stability.

What happens to air and water vapor in the atmosphere? ›

If a volume of air contains its maximum amount of water vapor and the temperature is decreased, some of the water vapor will condense to form liquid water. This is why clouds form as warm air containing water vapor rises and cools at higher altitudes where the water condenses to the tiny droplets that make up clouds.

What is the importance of water vapour in atmosphere? ›

Water vapor is also the most important greenhouse gas in the atmosphere. Heat radiated from Earth's surface is absorbed by water vapor molecules in the lower atmosphere. The water vapor molecules, in turn, radiate heat in all directions. Some of the heat returns to the Earth's surface.

Why is water vapor important in the atmosphere? ›

Water vapor is the most prevalent greenhouse gas. It causes roughly half of the warming of the planet. Like other greenhouse gases, it lets almost all of the sunlight reach the earth's surface but absorbs heat radiated upward from the earth.

What is an example of water vapor? ›

Water vapor is steam. An example of water vapor is the floating mist above a pot of boiling water. Water in a gaseous state, especially when diffused in the atmosphere. Steam.

How much of the atmosphere is water vapor? ›

Averaged throughout the entire atmosphere, water vapor makes up about 0.4% of the total.

What are the three types of atmospheric stability? ›

Three characteristics of the sounding then determine the stability of the atmospheric layer in which the parcel of air is embedded. These are: (1) The temperature lapse rate through the layer; (2) temperature of the parcel at its initial level; and (3) initial dew point of the parcel.

How long does water vapor stay in the atmosphere? ›

Water vapour will generally stay in the atmosphere for days (before precipitating out) while other greenhouse gases, such as carbon dioxide or methane, will stay in the atmosphere for a much longer period of time (ranging from years to centuries) thus contributing to warming for an extended period of time.

Where does most of the water vapor in the atmosphere originate? ›

About 90 percent of water in the atmosphere is produced by evaporation from water bodies, while the other 10 percent comes from transpiration from plants.

What does water vapor do to the temperature of the atmosphere? ›

Instead, it's a consequence of it. Increased water vapor in the atmosphere supercharges the warming caused by other greenhouse gases. It works like this: As greenhouse gases like carbon dioxide and methane increase, Earth's temperature rises in response. This increases evaporation from both water and land areas.

What are the properties of water vapor? ›

When water is heated, it remains a liquid until quite a high temperature, finally boiling at 100°C to give an invisible vapor. Water vapor behaves much like any other gas. It is easily compressible. Even when quite modest pressures are applied, the volume decreases noticeably.

What is water vapor also known as? ›

(Also called aqueous vapor, moisture.) Water substance in vapor form; one of the most important of all constituents of the atmosphere.

What is water vapor made of? ›

Water vapour is water in gaseous instead of liquid form. It can be formed either through a process of evaporation or sublimation. Unlike clouds, fog, or mist which are simply suspended particles of liquid water in the air, water vapour itself cannot be seen because it is in gaseous form.

Where is water vapor in the atmosphere? ›

Most of the water vapor in the atmosphere, along with dust and ash particles, are found in the troposphere—explaining why most of Earth's clouds are located in this layer. Temperatures in the troposphere decrease with altitude. The stratosphere is the next layer up from Earth's surface.

What is atmospheric stability explain in detail? ›

Atmospheric stability is a measure of atmospheric status which determines whether or not air will rise, sink, or be neutral. In general stability refers to air tendency to rise or to resist vertical motion (Salby 1996; Houghton 2002; Hewitt and Jackson 2003; Lutgens and Tarbuck 2009; Hantel 2013).

What are the four mechanisms that can cause air to rise? ›

Lifting mechanisms are forms of lift that cause air to rise. In this topic we cover orographic lift, frontal lift, convergence, and convective lift.

Why is atmospheric stability important? ›

Concepts: Atmospheric stability determines whether or not air will rise and cause storms, sink and cause clear skies, or essentially do nothing. Stability is dependent upon the Dry and Saturated Adiabatic Lapse Rates and the Environmental Lapse Rate.

Where is water vapor in the atmosphere? ›

Most of the water vapor in the atmosphere, along with dust and ash particles, are found in the troposphere—explaining why most of Earth's clouds are located in this layer. Temperatures in the troposphere decrease with altitude. The stratosphere is the next layer up from Earth's surface.

What are 3 types of atmospheric moisture? ›

Water in the atmosphere, as on the Earth's surface and in its subsurface, occurs in three forms: as a gas (water vapor), a liquid (liquid water), and a solid (ice). In this scene, liquid water in the atmosphere is expressed as tiny drops in the clouds and as any raindrops falling from the clouds.

What is an example of water vapor? ›

Water vapor is steam. An example of water vapor is the floating mist above a pot of boiling water. Water in a gaseous state, especially when diffused in the atmosphere. Steam.

How much of the atmosphere is water vapor? ›

Averaged throughout the entire atmosphere, water vapor makes up about 0.4% of the total.

What is water vapor also known as? ›

(Also called aqueous vapor, moisture.) Water substance in vapor form; one of the most important of all constituents of the atmosphere.

What are the properties of water vapor? ›

When water is heated, it remains a liquid until quite a high temperature, finally boiling at 100°C to give an invisible vapor. Water vapor behaves much like any other gas. It is easily compressible. Even when quite modest pressures are applied, the volume decreases noticeably.

How long does water vapor stay in the atmosphere? ›

Water vapour will generally stay in the atmosphere for days (before precipitating out) while other greenhouse gases, such as carbon dioxide or methane, will stay in the atmosphere for a much longer period of time (ranging from years to centuries) thus contributing to warming for an extended period of time.

What is the main source of atmospheric moisture? ›

Although the oceans are the principal source of atmospheric moisture, transpiration from plant, is also important. But in and areas, transpiration adds little moisture to the atmosphere.

What are the different forms of water in the atmosphere? ›

Water is present in the atmosphere in three forms namely – gaseous, liquid and solid. The moisture in the atmosphere is derived from water bodies through evaporation and from plants through transpiration.

What happens when water evaporate? ›

Evaporation happens when a liquid turns into a gas. It can be easily visualized when rain puddles “disappear” on a hot day or when wet clothes dry in the sun. In these examples, the liquid water is not actually vanishing—it is evaporating into a gas, called water vapor.

What is water vapor made of? ›

Water vapour is water in gaseous instead of liquid form. It can be formed either through a process of evaporation or sublimation. Unlike clouds, fog, or mist which are simply suspended particles of liquid water in the air, water vapour itself cannot be seen because it is in gaseous form.

What is the uses of water vapour? ›

Excess of water vapour in the atmosphere minimizes the rate of evaporation. Also, excess of water vapour produces rain, dew and mist. It also seves as a natural source of water for plants and animals.

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