Chapter 4 Water Cycle

Water is essential to weather and climate on Earth. The sun supplies energy to Earth in the form of electromagnetic waves. This energy is unevenly distributed over the globe. The atmosphere and ocean transport the excess heat from the tropics to the poles. Much of the heat transported poleward by the atmosphere cannot be measured with a thermometer, but is in the form of latent heat. Solar energy evaporates water from the oceans, the water vapor is transported poleward and when it condenses to form a cloud heat is added to the air.

Water continually cycles through the atmosphere as evidenced by the appearance of clouds and precipitation. The formation of clouds and precipitation only occurs under the correct weather conditions, which depend on the wind and water vapor content of the atmosphere. The ability for water to change phase makes it an important energy source for atmospheric storms.

Heavy precipitation affects our everyday lives by determining what we wear, hindering transportation, influencing the health of our economy, and even affecting our moods. Every day millions of people listen to weather forecasts to determine what to wear and to plan activities. Certain types of precipitation, heavy snows and freezing rain, are a safety concern in for all modes of transportation.

When you complete this chapter you will understand how clouds and precipitation form, how clouds are classified, and the different types of fog.

Evaporation - the Source of Atmospheric Water

Evaporation is the process by which water in its liquid state is converted into water vapor. Evaporation is important to weather and climate because evaporation is the primary route for water molecules to enter the atmosphere. Evaporation occurs because water molecules at the surface of the water have enough energy to escape the liquid. When we heat the water, the molecules move faster, increasing the number of molecules that have enough kinetic energy to escape the main body of water. Eventually, with continued heating, all the molecules in the liquid will have enough energy to enter the vapor phase. Evaporation is an important process in weather and climate studies.

Evaporation is greatest when the evaporating surface is much warmer than the air above, the atmospheric pressure is low, the wind speed is high, and there is relatively little water vapor already in the air.

Saturation is the condition at which equal numbers of water molecules cross a flat surface between air and water, or air and ice.

Consider the following example of evaporation. A sealed beaker is partially filled with liquid water and kept at a constant temperature (Figure 4.1). If the water and the liquid are at the same temperature, the average kinetic energy of water molecules in both the air above and in the liquid is the same. Some individual molecules will have more and some less kinetic energy than the average. For instance, a water molecule in the liquid phase might gain kinetic energy considerably above the average because of several rapid collisions with neighboring molecules. Now imagine this molecule at the liquid's surface, the boundary between the water and the air. If it has enough kinetic energy to overcome the attractive force of nearby molecules and is moving toward the air, it may escape from the liquid. Molecules that escape from the surface form a vapor above the liquid. Vapor molecules may collide with the surface of the liquid. If the kinetic energy of one of these molecules is sufficiently below the average, the molecule may be captured and become part of the liquid.

At the liquid surface, at any given time, some water molecules will be escaping, or evaporating, and others will be captured, or condensing. If the number evaporating is greater than the number condensing, then the water level in the container lowers. If condensation is greater, the water level rises. Eventually, because the beaker is sealed, the number of molecules leaving the surface of the liquid will be the same as the number captured. There will be no net change in the number of molecules in the liquid phase. A situation in which there is no net change is described as being in equilibrium. When the number of molecules leaving the liquid is in equilibrium with the number condensing, the air above the surface is saturated.

Counting the number of molecules in the beaker above the water is one way to measure the amount of water in the beaker's air. There are several methods of specifying the amount of water vapor in the atmosphere.

Measuring Water Vapor in the Air

Specifying the amount of water vapor in the atmosphere is important for several reasons:

Water is the only substance that can exist in all three phases (vapor, liquid and ice) in the atmosphere. The change of phase of the water is an important energy source for storms and atmospheric circulation patterns. The change of phase of water also affects the buoyancy of air, which is important in cloud and precipitation formation.

Water vapor is the source of all clouds and precipitation. The potential for cloud formation and dissipation depends on the amount of water vapor in the atmosphere.
The amount of water in the atmosphere determines the rate of evaporation. Rates of evaporation are important for weather and to many forms of plant and animal life.
Water vapor is a principle absorber of shortwave and longwave radiant energy. It is the most important greenhouse gas.

News reports of current weather conditions often include the dew point temperature and the relative humidity. These are just two of several ways to express the amount of water vapor in the atmosphere. Each is a method that has advantages and disadvantages. In this section we will discuss four different methods of representing the amount of water vapor in the atmosphere: mixing ration, vapor pressure, relative humidity and dew point/frost point. In addition to memorizing the definitions associated with these methods, it is important to learn how these methods of describing the amount of water in the atmosphere change when temperature and pressure conditions vary.

Mixing Ratio

Mixing ratio expresses the amount of water in the atmosphere in terms of the mass of water vapor per unit mass of dry air.

One way of expressing the amount of water vapor in the atmosphere is the ratio of the weight of water vapor to the weight of the other molecules in a given volume of air. This is the mixing ratio. The unit of mixing ratio is grams of water vapor per kilogram of dry air (g/kg). Typical values of the mixing ratio near the surface of the earth range between less than 1 g/kg in polar regions to over 15 g/kg in the tropical regions.

Since the surface of the Earth is a source of water vapor for the atmosphere, the mixing ratio generally decreases the farther you get from the surface (Figure 4.2). Adding or removing water vapor molecules from a fixed volume of air changes its mixing ratio. Evaporating water into the volume increases the mixing ratio. Since missing ratio has to do with weight, as opposed to temperature or volume, cooling the air or expanding the air has no effect on the value of the mixing ratio, since the total mass and total number of molecules remain unchanged.

As you learned in Chapter 2, the amount of solar energy absorbed by the atmosphere is related to how many water molecules are present relative to other molecules. For this reason, when discussing radiative energy transfer in the atmosphere, the amount water vapor is often expressed in terms of the mixing ratio.

Vapor Pressure

Vapor pressure measures how much water vapor is in the atmosphere in terms of its pressure.

Gas molecules exert a pressure when they collide with objects. The pressure the water molecules exert is another useful method of representing the amount of water vapor in the atmosphere. There is always water vapor in the atmosphere. The pressure exerted by these water vapor molecules is the vapor pressure. Atmospheric vapor pressure is expressed in millibars (mb). The number of water vapor molecules in the atmosphere is always small compared to the number of nitrogen and oxygen molecules, so the vapor pressure is small compared to the total atmospheric pressure. Near the surface of Earth the vapor pressure is typically less than 40 mb while the average atmospheric pressure is approximately 1013 mb.

A variety of things can change the vapor pressure. The higher the temperature, the greater the average kinetic energy of the molecules and the higher the vapor pressure. Increasing the number of water vapor molecules for a specific volume of air will also raise the vapor pressure. If more water evaporates into a volume of air, both the vapor pressure and the mixing ratio increase. However, if we cool the air, the vapor pressure decreases, but the mixing ratio remains constant. Atmospheric scientists use vapor pressure to express the amount of water in the atmosphere when they discuss the formation of cloud particles.

Saturation vapor pressure is the vapor pressure at which the number of molecules leaving a flat liquid, or ice, surface equals the number of molecules entering the liquid or ice. It is a function of temperature.

When air is saturated (as in Figure 4.1), the pressure exerted by the water vapor molecules is called the saturation vapor pressure. Remember that the ability of a molecule to escape from the intermolecular forces in the liquid is a function of its kinetic energy. As the temperature of water increases, the number of molecules with enough kinetic energy to evaporate from the water surface increases. Increasing the temperature increases the number and speed of the water molecules in the vapor phase. More molecules moving at greater speeds exert more pressure. Therefore, the saturation vapor pressure increases as the temperature increases. Saturation vapor pressure in the atmosphere is reached whenever the atmospheric water vapor exerts a pressure equal to what the saturation vapor pressure would at that particular temperature in a closed container.

Relative Humidity

Supersaturated conditions represent relative humidities of greater than 100%. Relative humidities of greater than 100.4% are rare.

Relative humidity indicates how close the air is to saturation.

Neither the vapor pressure nor the mixing ratio tells us how close the air is to being saturated. The ratio of the actual vapor pressure exerted by molecules of water vapor to the saturation vapor pressure at the same temperature is an indication of how close the air is to saturation and is called the saturation ratio. Multiplying the saturation ratio by 100% yields the relative humidity. Saturated air has a relative humidity of 100%, since the vapor pressure equals the saturation vapor pressure. A relative humidity of 50% tells us the vapor pressure is half that required for saturation. Relative humidity can exceed 100% by a few tenths of a percent. This is referred to as supersaturation.

Relative humidity describes how far the air is from saturation. It is specifically used to express the amount of water vapor when discussing the amount and rate of evaporation. Relative humidity is also commonly mentioned during weather reports because it is an important indicator of the rate of moisture and heat loss by plants and animals (See Box 4.1).

Changing the vapor pressure changes the relative humidity. Adding water molecules to a fixed volume of air increases the vapor pressure but has no effect on the saturation vapor pressure. Adding water molecules to a volume of air increases the relative humidity. A dehumidifier lowers the relative humidity of the air by removing water vapor molecules from the air.

Changing the saturation vapor pressure also changes the relative humidity. The saturation vapor pressure decreases if the temperature of the air decreases. Therefore, a decrease in temperature results in an increase in the relative humidity.

Dew Point/ Frost Point

Dew point is the temperature to which air must be cooled at constant pressure to become saturated.

So, one way to approach saturation, a relative humidity of 100%, is to cool the air. In order to do that we need to know how much the air needs to be cooled to reach saturation. When air near the ground is saturated, water condenses on objects to form dew (Figure 4.3). So, the temperature to which air must be cooled to become saturated without changing the pressure is called the dew point. The dew point temperature is determined by keeping the pressure fixed because changing the pressure affects the vapor pressure and therefore the temperature at which saturation occurs.

The dew point temperature tells us nothing about how many water molecules are in the atmosphere or how close the air is to a relative humidity of 100%. To know how close the air is to saturation, we need to know the dew point and the air temperature. The dew point temperature can never be greater than the air temperature. When the dew point equals the air temperature, the air is saturated. The closer the dew point is to the air temperature, the closer the air is to saturation. The temperature difference between the air and the dew point temperature is called the dew point depression.

Frost point is the highest temperature at which atmospheric moisture will form frost.

If the temperature to which air must be cooled at a constant pressure to become saturated is below 0C (32F), that temperature is called the frost point.

Whether or not a blade of grass cools below the frost point is a function of its energy gains and losses. On clear nights, objects (such as blades of grass) loose energy by radiative processes. Grass loses energy by emission of longwave radiation while gaining energy by absorbing the longwave radiation emitted from surrounding objects. Under clear sky conditions, more radiation is emitted by the objects on the ground than the sky, and so the blades of grass cool. If the temperature of a grass blade falls below the frost point, frost will form on the grass. There are many occasions when frost forms in an open field but not under a tree (see Figure 4.4.) This is because trees emit more radiation towards the ground than the clear sky. Energy losses of the grass in the open field are greater than the grass under the tree. The grass in the open field cools faster and reaches the frost point before the grass blades under the tree.

Dew is water that condensed onto a surface near the ground that has fallen below the dew point. Frost forms if the dew point is below freezing.

Certain conditions are favorable for dew and frost to form. First, dew and frost form in air close to the ground. Radiative processes cool the surface to the dew point temperature. The object at the surface on which the dew will form must be effective at emitting longwave radiation. As the surface is cooling it must be insulated from receiving heat from the soil. A high relative humidity in the surface air layer and a low specific humidity of the air above the surface is favorable for dew formation as these conditions permit sufficient cooling of the object. Dew forms when air reaches the dew point. Dew may form and then freeze if the temperature falls below freezing forming frozen dew. Frozen dew is different from frost.

The dew point is useful in forecasting minimum temperatures, forecasting the formation of dew and frost, and predicting fog. Formation of frost and dew are examples of phase transitions between the gas phase of water and its solid and liquid states. Cloud formation is another example of a phase transition.

Condensation and Deposition - Cloud Formation

Clouds form when the water vapor condenses into small particles that can either be liquid or solids. Liquid particles suspended in the atmosphere are referred to as cloud droplets and the solid particles are often called ice crystals. This section discusses how vapor can change phase to form cloud droplets and ice crystals.

Water vapor molecules are always condensing onto surfaces. They are also always leaving these surfaces. If condensation is greater than evaporation a thin film of water will form on an object. Comparing the rate of condensation relative to evaporation is another way to think of relative humidity. A relative humidity of 100% means that condensation equals evaporation, while a 90% relative humidity means that condensation is less than evaporation. A relative humidity of greater than 100%, which often occurs in clouds, means that condensation is greater than evaporation!

As a volume of unsaturated air cools, its relative humidity increases. If sufficiently cooled, the relative humidity becomes 100%, the temperature equals the dew point and it seems that condensation and cloud formation should occur. But forming cloud droplets can actually occur at relative humidities of more than 100% and less than 100%! Why? Because of the opposing forces of the curvature effect and the solute effect.

Relative humidity is measured with respect to a flat surface. In our previous discussion of evaporation we discussed a single molecule near the edge of this flat surface of still water. This molecule is attracted by its neighbors, which attempt to keep it part of the water. For the surface molecule to escape the water it must have enough energy to overcome the attractive forces of the surrounding water molecules. But what if the surface is curved, like that of a water droplet? A molecule on the surface of a drop of water has fewer neighbors to attract it (Figure 4.5) and can therefore escape the fluid more easily. The smaller the droplet, the fewer the neighbors and the easier it is for a water molecule on the surface to escape. If the air is saturated with respect to a flat surface of water, it is unsaturated with respect to a curved surface. This is called the curvature effect. It opposes the formation of small droplets by condensation.