4. Atmospheric Moisture and precipitation

4.1. The hydrologic cycle and balance

About 97.2 percent of the water on the earth is in the oceans, the remaining 2.8 percent is fresh water. The next largest reservoir of fresh water is stored as ice or glaciers, which account for 2.15 percent of total global water. Water is held in deep storage called ground water and account for 0.63 percent of global water. Among the remaining, about 0.005 precent of global water is stored in the soil and 0.001 percent in the atmosphere and 0.017 in the lakes and only 0.0001 percent in the river channels.

The hydrological cycle refers to the unending circulation of our planet’s water supply. Its essential feature is that liquid water (primarily from the oceans) evaporates into the air, condenses to liquid (or solid) state, and returns to Earth as some form of precipitation.

It is noted that evaporation from the ocean is six times as much as form the continents because (1) ocean covers most of the planet (2) ocean always has enough water for evaporation. Precipitation over oceans is nearly four times greater than precipitation over land. More water evaporates from ocean than precitation; More precipitation falls than evaporation on the land.

Now let’s calculate global water balance:

On the ocean,

In: Precipitation + runoff = out: evaporation

On the land:

In: precipitation = out: evaporation + runoff

When compare ocean and land, we find the difference between evaporation and precipitation is equivalent to the runoff flowing from land to oceans.

4.2 Evaporation and condensation

the convesrion of moisture from liquid to gas-in other words, from water to water vapor- is called evaporation. tHis process involves molecular escape: moleculars of water escape from the liquid surface into the surround air.

The amount and rate of evaporation from a water surface depend on three factors: the temperature (of both air and water), the amount of water vapor already in the air, and whether the air is still or moving.

Evaporation can take place at any temperature , but higher temperatures cause molecules to move faster. Like if you cook water, with the burner off, only very few molecules evaporate from the surface. As you turn on the burner, with the temperature rises, more and more water molecules become agitated and escape from the water becoming water vapor.

Each gas in the atmosphere exerts a pressure, and the sum of all the pressures exerted by the individual gases is what we call atmospheric pressure. The pressure exerted by water vapor in the air is called vapor pressure.

At any given temperature, there is a maximum vapor pressure, we say that air is saturated in this stage. When this maximum vapor pressure is exceeded, some water vapor molecules must leave the air become liquid. The higher the air temperature, the higher the maximum vapor pressure. iN other words, the warmer the air, the more water vapor it can hold before becoming saturated.

If the air overlying a water surface is almost saturated with water vapor, very little further evaporation can take place. If the air remains calm and the temperature does not change, there is no net evaporation. However, if the air is in motion, the moist air may be blow away and some dry air is brought in so evaporation could keep going.

In the land, some moisture escape from plants to atmosphere, which is called transpiration. The combination of evaporation and transpiration in the land is called evapotranspiration.

The amount of water vapor in the air is referred to the humidity. A direct measurement of the water vapor content of air is absolute humidity. Absolute humidity has a limiting value that depends on temperature. The actual quantity of water vapor held by a parcel of air is called specific humidity.

Another way of describing the water vapor content of air is by its dew-point temperature. If air is slowly chilled, it eventurally will reach saturation. At this temperature, the air holds the maximum amount of water vapor possible. If furhter colling continues, condensation will begin and dew will form. The temperature at which saturation occurs is therefore known as the dew-point temperature.

The most familiar of humidity measures is relative humidity, which is the amount of water vapor in the air at a given temperature compared with the amount that could be there if the air were saturated at that temperature.

Two ways could change relative humidity: first, direct gain or loss of water vapor; second, change of temperature.

Figure 4.7 indicate that the relative humidity decreases during the daytime due to the increase of temperature, and increase during the nighttime. The dew-point temperature, which indicates the true moisture content, remains nearly constant.

Condensation is the opposite of evaporation. It is the process whereby water vapor is converted to liquid water. In other words, it is a change in state from gas to liquid. For condensation to take place, the air must be saturated. However, saturation alone is not enough to cause condensation. The characteristic of water called surface tension makes it virtually impossible to grow droplets of pure water. Thus it is necessary to have a surface on which condensation can take place. If no such surface is available, no condensation occurs. In this situation, the air becomes supersaturated.

Normally, plenty of surfaces are available for condensation. At ground level, availability

of a surface is obviously no problem. In the air above the ground, tiny dust, smoke, salt, and other compounds can be used as condensation nuclei.

4.3.Adiabatic processes

One of the most significant facts in physical geography is that the only way in which large masses of air can be cooled to the dew point is by air masses rises.

As a parcel of unsaturated air rises, it cools at the relatively steady rate of 5.5 F per 1000 feet (10 C per kilometer). This is known as the dry adiabatic lapse rate. (the term is easily confused: the air is not necessarily dry; it simply is not saturated). It the air mass rises high enough, it cools to the dew point, condensation begins, and clouds form.

As soon as condensation begins, latent heat is released. If the air continues to rise, cooling continues but release of the latent heat slackens the rate of cooling. This diminished rate of cooling is called saturated adiabatic lapse rate and depends on temperature and pressure but averages about 3.3 F per 1000 feet (5 C per kilometor).

Adiabatic warming occurs when air descends. The increase in temperature increases the capacity of the air for holding moisture and thus causes saturated air become unsaturated. Therefore, any descending air warms at the dry adiabatic lapse rate.

4.4 Clouds, fog and dew.

Clouds are collections of minutes droplets of water or tiny crystals of ice. At any given time, about 50 percent of Earth is covered by clouds.

Clouds can be grouped into two major classes on the basis of form-stratiform (latin “ stratus, “spread out”), or layered clouds, and cumuliform (latin cumulus, ”mass” or “pile”). Stratusform clouds are blanket-like and cover large areas. Cumuliform coulds are massive and rounded that are associated with small to large parcels or rising air.

Fog is simply a cloud on the ground. There is no physical difference between a cloud and fog, but there are important difference in how each forms. Most clouds develop as a result of adiabatic cooling in rising air. Most fogs are formed either when air at Earth’s surface cools to below its dew point or when enough water vapor is added to the air to saturate it.

Dew: night time radiation cools objects (grass, pavement, automobiles, or whatever) at Earth’s surface, and the adjacent air is in turn cooled by conduction. The air is cooled enough to reach saturation, tiny beads of water collect on the cold surface of the object, which is called dew.

4.5 Precipitation

All precipitation originates in clouds, but most clouds do not yield precipitation. There are three types of precipitation: orographic, convectional and frontal.

Orographic precipitation.

Topographic barriers that block the path of horizontal air movements are likely cause large masses of air to travel upslope. This kind of forced ascent can produce orographic precipitation if the ascending air is cooled to the dew point.. As in figure 4.16. moist air arrives at the coast after passing over a large ocean surface. As the air rises on the windward side of the range, it is cooled at the dry adiabatic rate. When cooling is sufficient, the lifting condesation level is reached,condensation occurs, and clouds begin to form. Cooling now proceeds at the wet adiabatic rate. Eventuraly, precipitation begins.

After passing over the mountain summit, the air begins to descend down the leeward slopes. As it descends, it causes the adiabatic warming at always dry adiabatic rates, develop less precipitation in the leeward side, called rainshadow areas.

Convectional precipitation

Because of unequal heating of different surface areas, a parcel of air near ground may be warmed by conduction more than the air around it. So it will be less dense and rise upward. As the air rises, it is cooled adiabatically and it temperature will decrease as it rises. we know that the temperature of the surrounding air will normally decrease with altitude as well. However, as long as the bubble is still warmer than the surrounding air, it will be less dense and will therefore continue to rise.

If the bubble remains warmer than the surrounding air and uplift continues, adiabatic cooling chills the bubble below the dew point. Condensation occurs and precipitation may develop. Convective precpitation typically is hsowery, with large raindrops falling fast and furiously but for only a short duration. It is particularly associated with the warm parts of the world and warm season.

Frontal precipitation

When a warm air mass meets a cold air mass, a boundary between them is called front. The warmer air, since is less dense, rises over the cooler air. As the warmer air is forced to rise, it may be cooled to the dew point with resulting clouds and precpitation. Precipitation that results from this process is referred to as frontal precpitation.