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3.2.1 Evaporation. Evaporation is the phase transition by which a liquid is changed to a vapor (gas). In meteorology, the substance we are concerned about the most is water, and the primary source is the ocean. On average, about 120 centimeters (47 inches) 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.
Everyone has experienced evaporation personally. 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 you step out of a shower or swimming pool. The coolness you feel is from the removal of bodily heat used to evaporate the water on your skin.
3.2.2 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 1 percent is used in the growth process of the plant. The remaining 99 percent is passed into the atmosphere.
3.2.3 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).
3.2.4 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.
3.2.5 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.
3.2.6 Precipitation. Precipitation results when tiny condensation particles grow through collision and coalescence.
3.2.7 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.
3.2.8 Infiltration. Infiltration is the movement of water into the ground from the surface.
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3.2.9 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.
3.2.10 Plant Uptake. Plant uptake is water taken from the groundwater flow and soil moisture.
3.3 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 the water vapor it can hold, while unsaturated air means an air parcel can hold more water vapor.
3.4 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.
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 𝐻𝐻𝐻𝑅𝐻𝑅𝑅𝑦 = 𝑊𝑅𝑅𝑅𝑊 𝑅𝑅𝑣𝑣𝑊 𝑐𝑣𝑐𝑅𝑅𝑐𝑅𝑊𝑅𝑅𝑅𝑊 𝑅𝑅𝑣𝑣𝑊 𝑐𝑅𝑣𝑅𝑐𝑅𝑅𝑦 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 3-2 illustrates the concept below. An air parcel at sea level at a temperature of 30 °C has the capacity to hold 27 grams of water vapor.
If it actually held 8 grams, 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 grams 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 grams), 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.
3-4 Figure 3-2. Temperature Effects on Relative Humidity
3.5 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.
3.6 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., Aviation Routine Weather Reports (METAR)/Aviation Selected Special Weather Reports (SPECI)) 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 3-3 below illustrates the relationship between temperaturedewpoint 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.
3-5 Figure 3-3. Temperature-Dewpoint Spread Effect on Relative Humidity
Relative humidity (RH) depends on the temperature-dewpoint spread. In Figure 3-3, dewpoint is constant but temperature decreases from left to right. On the left panel, relative humidity is 50 percent, which indicates 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.
3.7 Change of Phase. Water changes from one state of matter (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 and ice crystals (visible as clouds) and as 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 2-4). That means 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.
3.7.1 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 3-4 below illustrates the latent heat transactions that occur when water undergoes phase transition.
3-6 Figure 3-4. 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 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 joules per gram due to the latent heat of fusion. Freezing, the reverse process, releases 334 joules per gram back to the environment.
Evaporation is the phase transition by which a liquid is changed to a vapor. During evaporation, water absorbs 2,501 joules per gram due to the latent heat of vaporization.
Condensation, the reverse process, releases 2,501 Joules per gram back to the environment.
Sublimation is the phase transition by which a solid is changed to a vapor. During sublimation, water absorbs 2,834 joules per gram due to the latent heat of sublimation.
Deposition, the reverse process, releases 2,834 joules per gram back to the environment.
3-7 Table 3-1. 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!
4-1 CHAPTER 4. EARTH-ATMOSPHERE HEAT IMBALANCES
4.1 Introduction. Weather is not a capricious act of nature, but rather the atmosphere’s response to unequal rates of radiational heating and cooling across the surface of the Earth and within its atmosphere. The absorption of incoming solar radiation causes heating, while the emission of outgoing terrestrial radiation causes cooling. However, imbalances in the rate of heating and cooling create temperature gradients.2 Atmospheric circulations and weather are the atmosphere’s never-ending attempt to redistribute this heat and achieve equilibrium.
4.2 The Earth-Atmosphere Energy Balance. The Earth-atmosphere energy balance is the balance between incoming energy from the sun (solar radiation) and outgoing energy from the Earth (terrestrial radiation), as seen in Figure 4-1. When solar radiation reaches the Earth, some is reflected back to space by air (8 percent), clouds (17 percent), or the surface (6 percent). Some is absorbed by water vapor/dust/ozone (19 percent) or by clouds (4 percent). The remainder is absorbed by the Earth’s surface (46 percent).
Figure 4-1. Earth-Atmosphere Energy Balance
100 units of incoming radiation from the sun is balanced by 100 units of outgoing radiation from the Earth.
2 A gradient describes the rate of change of a feature (in this case temperature) per unit of distance.
4-2 However, since the Earth is much cooler than the sun, its radiating energy is much weaker (long wavelength) infrared energy. We can indirectly see this energy radiate into the atmosphere as heat (e.g., rising from a hot road, creating shimmers on hot sunny days). The Earth-atmosphere energy balance is achieved as the energy received from the sun (solar radiation) balances the energy lost by the Earth back into space (terrestrial radiation). In this way, the Earth maintains a stable average temperature.
The absorption of infrared radiation trying to escape from the Earth back to space is particularly important to the global energy balance. Energy absorption by the atmosphere stores more energy near its surface than it would if there was no atmosphere.
The average surface temperature of the moon, which has no atmosphere, is -18 °C (0 °F).
By contrast, the average surface temperature of the Earth is 15 °C (59 °F). This heating effect is called the greenhouse effect.
Greenhouse warming is enhanced during nights when the sky is overcast (see Figure 4-2).
Heat energy from the Earth can be trapped by clouds, leading to higher temperatures as compared to nights with clear skies. The air is not allowed to cool as much with overcast skies. Under partly cloudy skies, some heat is allowed to escape, and some remains trapped. Clear skies allow for the most cooling to take place.
Figure 4-2. Greenhouse Effect on Nighttime Radiational Cooling
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4.3 Heat Imbalances Between Earth’s Surface and the Atmosphere.
The Earth-atmosphere energy balance numbers (see Figure 4-1) indicate that both sensible heat (7 percent) and latent heat (24 percent) processes transfer heat from the Earth’s surface into its atmosphere. Both processes are necessary to prevent the Earth’s surface from continually heating up and the atmosphere from continually cooling down.
4.3.1 Sensible Heating. Sensible heating involves both conduction and convection. It occurs due to differences in air density. Warm air is less dense than cool air.
On warm sunny days, the Earth’s surface is heated by incoming solar radiation or insolation. However, the heating is somewhat uneven because certain areas of the Earth’s surface absorb more heat from the sun than others. Heat is conducted from the relatively warm ground to the cooler overlying air, which warms a shallow layer of air near the ground. The heated air expands, becomes less dense than the surrounding cooler air, and rises. Through this process, a large bubble of warm air called a thermal rises and transfers heat energy upwards (see Figure 4-3). Cooler, denser air sinks toward the ground to replace the rising air. This cooler air becomes heated in turn, rises, and repeats the cycle.
Figure 4-3. Development of a Thermal
In this manner, convection transports heat from the Earth’s surface into the atmosphere.
Because air is a poor conductor of heat (see Table 2-3), convection is much more important than conduction as a heat transport mechanism within the atmosphere.
4-4 Figure 4-4. Example of Convection in the Atmosphere
4.3.2 Latent Heat. The phase transition of water and associated latent heat exchanges (see subparagraph 3.7.1) are largely responsible for transferring the excess heat from the surface of the Earth into its atmosphere. As the Earth’s surface absorbs radiation, some of the heat produced is used to evaporate (vaporize) water from oceans, lakes, rivers, soil, and vegetation. The water absorbs heat energy due to the latent heat of vaporization.
Some of this water vapor condenses to microscopic water droplets or deposits as ice crystals that are visible as clouds. During cloud formation, the water vapor changes state, and latent heat is released into the atmosphere. During this process, the excess heat is transferred from the Earth’s surface into its atmosphere.
4.4 Heat Imbalance Variations with Latitude. Global imbalances in radiational heating and cooling occur not only vertically between the Earth’s surface and its atmosphere, but also horizontally with latitude. Since the Earth is essentially spherical, parallel beams of incoming solar radiation strike lower latitudes more directly than higher latitudes (see Figure 4-5); that is, the solar zenith angle is lower, and the sun is more directly overhead in equatorial regions than at the poles. At higher latitudes, solar radiation is spread over a larger area and is less intense per unit surface area than at lower latitudes.
Thus, the Earth absorbs more solar radiation at lower latitudes than higher latitudes, which creates heat imbalances and temperature gradients between the Equator and the poles.
4-5 Figure 4-5. Solar Zenith Angle Variations with Latitude
The emission of terrestrial radiation also varies by latitude, but less so than the absorption of solar radiation. Terrestrial radiation emission decreases with increasing latitude due to a drop in temperature with latitude. Thus, at higher latitudes, the annual rate of cooling exceeds the rate of warming, while the reverse is true at lower latitudes.
Averaged over all latitudes, incoming solar radiation must equal outgoing terrestrial radiation. Otherwise, the Earth would be unable to maintain a constant average temperature. About 35° latitude in both hemispheres is where incoming and outgoing radiation is equal. This implies there is annual net cooling at higher latitudes and net warming at lower latitudes. However, we know this is untrue. The excess heat in the tropics must be transported polar by some mechanism(s). This poleward heat transport is accomplished by atmospheric circulations, weather, and ocean currents.
4.5 Seasons. Seasons are caused by the tilt of the Earth’s rotational axis as the Earth orbits the sun (see Figure 4-6). The Earth’s rotational axis is tilted by 23½° from the perpendicular drawn to the plane of the Earth’s orbit about the sun and points the same direction in space all year long. The North Pole is tilted most directly toward the sun Solstice (~December 22). Thus, in the Northern Hemisphere, the longest day of the year
4-6 (lowest solar zenith angle) occurs on the Summer Solstice, while the shortest day of the year (highest solar zenith angle) occurs on the Winter Solstice. Day and night are of equal length (12 hours) worldwide on the Vernal Equinox (~March 21) and the Autumnal Equinox (~September 23).
Figure 4-6. Solar Zenith Angle Variations with Northern Hemisphere Seasons
Figure 4-7 illustrates the average seasonal temperature variation in the Northern Hemisphere for both maritime and continental locations. Note that the warmest (coldest days) of the year occur after the summer (winter) solstice. This is due to the time lag necessary for heat flow processes to fully heat (cool) the surface of the Earth.
Figure 4-7. Average Seasonal Temperature Variation in the Northern Hemisphere