[BOOK: “The Atmosphere: an introduction to meteorology” by Frederick K. Lutgens, Edward J. Tarbuck.


“The nature of both weather and climate is expressed in terms of the same basic elements, those qualities or properties that are measured regularly. The most important of these are: (a) the temperature of the air, (b) the humidity of the air, (c) the type and amount of cloudiness, (d) the type and amount of precipitation, (e) the pressure exerted by the air, and (f) the speed and direction of the wind. These elements constitute the variables from which weather patterns and climatic types are deciphered.”


Humidity is the general term used to describe the amount of water vapour in the air.

At higher temperatures, more moisture is required for saturation. The amount of water vapour required for saturation at various temperatures is shown in Table 4-1.

Of the methods used to express humidity, absolute and specific humidity are similar in that they both specify the amount of water vapour contained in a unit of air.

Absolute humidity is stated as the weight of water vapour in a given volume of air (usually as grams per cubic meter). As air moves from one place to another, even without a change in moisture content, changes in temperature cause changes in volume and consequently the absolute humidity, thus limiting the usefulness of this index.

Specific humidity is expressed as the weight of water vapour per weight of a chosen mass of air, including the water vapour. Since it is measured in units of weight (usually grams per kilogram), specific humidity is not affected by changes in pressure or temperature.

The most familiar and perhaps the most misunderstood term used to describe the moisture content of air is relative humidity. Stated in an admittedly oversimplified manner, relative humidity is the ration of the air’s water vapour content to its water vapour capacity at a given temperature.  From Table 4-1 we see that at 25 C the capacity of the air is 20 grams per kilogram. If on a 25 C day the air contains 10 grams per kilogram, the relative humidity is expressed as 10/20 or 50 percent. When air is saturated, the relative humidity is 100 percent.”


“The result of the condensation process may be dew, fog or clouds. For any form of condensation to occur, the air must be saturated. Saturation occurs either when the air is cooled below the dew point, which most commonly happens, or when water vapour is added to the air. Second, there must generally be a surface on which the water vapour may condense. When dew occurs, objects at or near the ground serve this purpose. When condensation occurs in the air above the ground, tiny bits of particulate matter known as condensation nuclei serve as surfaces for the condensation of water vapour.


“In an actual situation the stability of the air is determined by examining the temperature of the atmosphere at various heights. Recall that this measure is called the lapse rate. You must not confuse the lapse rate, which is the temperature of the atmosphere as determined from observations made by balloons and air planes, with adiabatic temperature changes.  The latter measure indicates the change in temperature a parcel of air would experience as it moved vertically through the surface.”

“Stated quantitatively, absolute stability prevails when the lapse rate is less than the wet adiabatic rate. At the other extreme, air is said to exhibit absolute instability when the lapse rate is greater than the dry adiabatic rate. Another situation existing in the atmosphere is called conditional instability. This occurs when moist air has a lapse rate between the dry and wet adiabatic rates (between o.5 C and 1 C per 100 meters).”

“Since stable air resists upward movement, we might conclude that clouds would not form when stable conditions prevail in the atmosphere. Although this seems reasonable, processes do exist that force air aloft. When stable air is forced aloft, the clouds that form are widespread and have little vertical thickness in comparison to their horizontal dimension, and precipitation, if any, is light. By contrast, clouds associated with unstable air towering and are usually accompanied by heavy precipitation.”

“Any factor that causes the air near the surface to become warmed in relation to the air aloft increases instability. The opposite is also true; any factor that causes the surface air to be chilled results in the air becoming more stable.”

“Instability is increased by:

1. intense solar heating that warms the air from below

2. the heating of an air mass from below as it traverses a warm surface

3. forceful lifting of air, such as over an elevated land surface

4. upward movement of air associated with general convergence

5. radiation cooling from cloud tops

Stability is enhanced by:

1. radiation cooling of the earth’s surface after sunset

2. the cooling of an air mass from below as it traverses a cold surface

3. subsidence of an air column”


Sleet is a wintertime phenomenon and refers to the fall of small, clear to translucent particles of ice. In order for sleet to be produced, a layer of air with temperatures above freezing point must overlie s subfreezing layer near the ground.

On some occasions, when the vertical distribution of temperatures is similar to that associated with the formation of sleet, freezing rain or glaze results instead. In such situations, the subfreezing air near the ground is not thick enough to allow the raindrops to freeze.

Hail is precipitation in the form of hard rounded pellets or irregular lumps of ice. Hail is produced only in cumulonimbus clouds where updrafts are strong and where there is an abundant  supply of supercooled water.”


“Phycically, there is basically no difference between a fog and a cloud. The essential difference is the method and place of formation. While clouds occur when air rises and cools adiabatically, fogs (with the exception of upslope fogs) are the consequence of radiation cooling or the movement of air over a cold surface. In other circumstances, fogs are formed when enough water vapour is added to the air to bring about saturation (evaporation fogs)

Radiation fog, as the name implies, results from radiation cooling of the ground and adjacent air. Most common in the fall and winter, it is a nigh-time phenomenon that requires clear skies and a fairly high relative humidity.

Evaporation fogs: When cools air moves over warm water, enough moisture may evaporate from the water to produce saturation.”


“Fronts are defined are boundary surfaces separating air masses of different densities, one warmer and often higher in moisture content than the other. “


“Although hurricanes and tornadoes are, in fact, cyclones, the vast majority of cyclones are not hurricanes and tornadoes. The term cyclone simply refers to the circulation around any low-pressure center, no matter how large or intense it is.”


“Cyclones form along fronts and proceed through a somewhat predictable life cycle.”


“Any factor that destabilizes the air aids in generating a thunderstorm. Since many processes do affect the stability of the air, it should come as no surprise that thunderstorms can be triggered in a number of ways. For convenience of discussion, we will divide thunderstorms into three types: (1) isolated thunderstorms produced within tropical air masses, (2) thunderstorms produced by forceful lifting, either frontal or orographic, and (3) thunderstorms produced along squall lines”.


“Tornadoes are local storms of short duration that may be ranked high among nature’s most destructive forces. The lack of forewarning, the incredible fury of its winds, and the near total destruction of the stricken area have led many to liken its passage to a bombing raid during war.

Tornadoes, sometimes called twisters or cyclones, are intense centres of low pressure having a whirlpool-like structure of winds rotating around a central cavity where centrifugal force produces a partial vacuum.

Tornadoes are most often spawned along the cold front of a middle-latitude cyclone, in conjunction with cumulonimbus clouds and severe thunderstorms.”


“The whirling tropical cyclones that on occasion have wind speeds reaching 320 kilometres per hour are known in the United States as hurricane – the greatest storms on Earth.  These awesome storms form in all tropical waters (except those of the South Atlantic) between the latitudes of 5 degrees and 20 degrees (Figure 10-14).

A hurricane can be described as a heat engine that is fuelled by the energy liberated during the condensation of water vapour (latent heat).”


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