Lecture 10

CLOUDS AND PRECIPITATION

I. Water in the Atmosphere

A. Water vapor is the most important gas in the atmosphere. Water vapor is an odorless gas that mixes freely with other gases of the atmosphere. Also, water vapor is unique in that it changes from one state to another at temperatures and pressures experienced near the Earth's surface.

B. Latent heat is the heat energy not associated with temperature change. It is not available as heat until the liquid returns to the solid state when the heat is released.

C. The process of converting a liquid to a gas is termed evaporation. It takes about 600 calories of energy to convert 1 gram of water to vapor. The energy is absorbed by the water molecules during evaporation is used solely to give them the motion needed to escape the surface of the liquid and become a gas. This energy is called latent heat of vaporization.

D. Condensation is the process by which water vapor changes to the liquid state. When condensation occurs in the atmosphere, it results in the formation of fog and clouds. For condensation to occur, the water molecules must release heat energy (latent heat of condensation) equivalent to what was absorbed during evaporation.

E. Melting is the process by which a solid is changed to liquid. It requires absorption of about 80 calories of heat per gram of water (latent heat of melting)

F. Freezing is the reverse process of melting and releases about 80 calories per gram as latent heat of fusion.

H. Sublimation is the conversion of a solid directly to a gas without passing through the liquid sate. For example, dry ice will turn into white wispy vapor and generate smoke or fog.

I. Deposition is a term used when the reverse process of sublimation occurs. That is the conversion of vapor directly into a solid. frost is an example of deposition.

II. Humidity

A.      A.     Humidity is the term for the amount of water vapor in air. There are two types of humidity:

1.        1.      specific humidity and

2.        2.      2. relative humidity.

B. Saturation is "filling "the air with all the water vapor it can hold.

C. Vapor pressure is defined at that part of the total atmospheric pressure that can be attributed to the water vapor content.

D. Specific humidity is the weight of water vapor per weight of a chosen mass of air, including the water vapor. It is measured in units weight (gram per kilogram), specific humidity is not affected by changes in pressure or temperate. It is the measure of the actual quantity of water vapor in a given mass of air.

E. Relative humidity is the ratio of the air's actual water vapor content to its potential water vapor capacity at a given temperature. It is a comparison of the air's water vapor content (specific humidity) and its capacity, relative humidity can be changed in either two ways: 1. adding moisture to or subtracting moisture, and 2. air temperature. A decrease in temperature results in an increase in relative humidity.

F. Dew point is the temperature to which air would have to be cooled to reach saturation. When air in the atmosphere is cooled below its dew point, some of the water vapor condensed to form clouds. Thus, the clouds are made of liquid droplets and are no longer part of the water vapor content.

G. A psychrometer is used to measure relative humidity. The psychrometer consists of two identical thermometers mounted side by side. One thermometer is a dry bulb and the other one is a wet bulb.

III. The Basis For Cloud Formation

A. Fog and dew versus cloud formation

• Condensation may form dew, fog or clouds. Although these there forms are different, all require saturated air to develop.
• Near Earth's surface heat is readily exchanged between the ground and the air above. During evening hours, the surface radiates heat away, and the surface and adjacent air cool rapidly. This radiation cooling accounts for the formation of dew and some types of fog.

B. Adiabatic temperature changes result when air is compressed or allowed to expand. When air is allowed to expand, it cools, and when it is compressed, it warms.

C. When a particle of air moves upward, it passes though regions of successively lower pressure. As a result, the ascending air expands. As it expands, it cools adiabatically. Conversely, descending air comes under increasingly higher pressures, compresses and is heated. This cooling and heating of unsaturated air is called dry adiabatic rate.

D. Air will continue to cool after condensation begins, the released latent heat works against the adiabatic process, thereby reducing the rate at which the air cools. This is called wet adiabatic rate.

III. Stability of Air

A. The stability of air is determined by examining the temperature of the atmosphere at various heights.

B. A column of air is deemed unstable when the air near the bottom of this layer is significantly warmer than the air aloft, indicating a steep environmental lapse rate.

C. Conversely, the air is considered to be stable when the temperature drips gradually with increasing altitude.

D. The role of stability in determining our daily weather is important. To a large degree stability determines the type of clouds that develop and whether precipitation will be gentle or heavy downpours.

IV. Processes that Lift Air

A. Air that is stable will not rise on its own. Orographic lifting, frontal wedging, and convergence are mechanisms that will allow stable air to rise.

B. Orographic lifting occurs when elevated terrains, such as mountains, act as barriers to flowing air. As the air is pushed up slope adiabatic cooling generates clouds and precipitation. By the time the air reaches the leeward side of the mountain, much of the moisture has been lost resulting in a rainshadow desert.

C. Frontal wedging occurs when cool air acts as a barrier over which warmer, less dense air rises.

D. Convergence is said to be when air masses flow together. Such flow results in upward movement because when air converges, the height of the air column increases.

V. Condensation and Cloud Formation

A. Condensation occurs when water vapor in the air changes to a liquid. In order for condensation to occur the air must be saturated. Saturation will occur when air is cooled to its dew point or when water vapor is added to air.

B. There must be a surface on which the water vapor ma condense. This surface may consists of tiny bits of particulate matter, such as smoke, dust, salt, etc. These small particulates are called condensation nuclei. Some of these nuclei are water seeking, such as salt, and are called hygroscopic nuclei.

C. Clouds are a form o condensation which are visible aggregates of minute droplets of water or ice crystals. They provide a visible indication of what is going on in the atmosphere.

D. Clouds are classified on the basis of their form and height.

• Cirrus clouds are high, white, and thin.
• Cumulus clouds are globular individual cloud masses.
• Stratus clouds are sheets or layers that cover much or all of the sky.
• All other clouds reflect one of these three basic forms or a combination or modifications of them.

E. Three level of cloud heights are noted.

• High clouds are made up of three types and are above 6000 meter: cirrus, cirrostratus, and cirrocumulus.
• Middle clouds are made up of altocumulus and altostratus type clouds and are between 2000-6000 meters.
• Low clouds are made up of stratus, stratocumulus, and nimbostratus.
• Some clouds don not fit into any one of the three height categories mentioned. Such clouds have their base in the low height range but often extent upward in the middle or high altitudes. Thus, these clouds are referred to as cloud of vertical development. These clouds are associated with unstable air.

VI. Fog

A. Fog is defined as a cloud with its base at or very near the ground.

B. Fogs caused by cooling.

• Advection fog is when warm moist air is blown over a cool surface, the result is a blanket of fog.
• Radiation fog forms on cool, clear calm nights when Earth's surface cools rapidly by radiation.
• Upslope fog is created when relative humid air moves up a gradually sloping plain or steep mountains.

C. Fogs caused by evaporation.

• Steam fog develops when the rising water vapor meets the cold air, it immediately recondenses and rises with air that is being warmed from below.
• Frontal fog or precipitation fog occurs with frontal wedging. As warm air is lifted over colder air, the clouds yield rain and the cold air below is near the dew point, enough rain will evaporate to produce fog.

VII. Precipitation

A. Two mechanisms have been proposed to explain the formation of precipitation: ice crystal process and collision-coalescence process.

B. When ice crystals and supercooled water droplets coexist in a cloud, the stage is set to generate precipitation. I turns out that ice crystals collect the available water vapor at a much faster rate than liquid water. Thus, ice crystals grow larger at the expense of water droplets. Eventually, this process generates ice crystals large enough to fall as snowflakes. This is called the ice crystal process.

C. Precipitation can form in warm clouds that contain large condensation nuclei. The bigger droplets fall faster, they collide and join with smaller water droplets. After many collisions the droplets are large enough to fall to the grounds as rain. This is call the collision-coalescence process.

D. Forms of precipitation.

• Rain is the term restricted to drops of water that fall from a cloud and have a diameter of at least 0.5 mm.
• Drizzle is fine, uniform drops of water having a diameter less than 0.5 mm. Drizzle can be so fine that the tiny drops appear to float.
• Snow is precipitation in the form of ice crystals (snowflakes) or, more often, aggregates of crystals.
• sllt is a wintertime phenomenon and refers to the fall of small particles of ice that are clear to translucent. For sleet to be produced a layer of air with temperatures above freezing must overlie a subfreezing layer near the ground.
• Freezing rain or glaze results when the vertical distribution of temperatures is similar to that associated with formation of sleet.
• Hail is precipitation in the form of hard rounded pellets or irregular lumps of ice. Large hailstones consist of a series of nearly concentric shells of differing densities and degrees of opaqueness.

E. Measuring precipitation is done by using a standard rain gauge. A standard rain gauge has a diameter of about 20 cm at the top. Once the water is caught, a funnel conducts the rain into a cylindrical measuring tube that had a cross-sectional area only on-tenth as large as the receiver. When the amount of rain is less than 0.025 cm it is reported as a trace of precipitation. Snow is measured using depth and water equivalent.