In this section, cloud formation is explained and typical clouds types that are associated with midlatitude cyclones are described. The cloud features within a mature cyclonic disturbance are typically organized in a comma form. Specific cloud types can be identified with polar orbiter images and, to a lesser extent, GOES images.
Air is comprised mainly of nitrogen and oxygen, but also contains a small amount of water vapor. Clouds form when a parcel of air is cooled until the water vapor that it contains condenses to liquid form. Another way of saying this is that condensation (clouds) occur when an air parcel is saturated with water vapor.
The amount of moisture in a parcel of air is expressed in a variety of ways. The standard scientific measure is the partial pressure of water vapor. Partial pressure simply refers to the pressure exerted by only the water vapor part of the air parcel. The standard unit of measure is millibars (mb) and is typically a small fraction of total atmospheric pressure. The water vapor content can also be expressed as a mass mixing ratio, that is, the mass of water vapor per total mass of air. Mixing ratio is usually expressed as grams H20 per kilograms air.
The partial pressure of water vapor at the point of condensation is termed the saturation pressure (es). The saturation pressure of any air parcel is proportional to temperature and is described by the Clausius-Clapeyron equation, figure 28.
|Figure 28.||Clausius-Clapeyron Equation indicates the dependence of saturation vapor pressure on temperature. It is derived fromt he first law of thermodynamics.|
An example illustrating cloud formation is given in figure 29. The starting point for the parcel of air is Point A. At this temperature (T1) and water vapor pressure (e1), the parcel of air is not saturated with respect to water vapor. That is, it is positioned below and to the right of the saturation line (es). If the parcel is cooled with no change in moisture, it will move along the line A-B. When it reaches point B, its vapor pressure (e1) is equal to the saturation vapor pressure (es) for that temperature (Td) and condensation occurs. The temperature at point B is known as the dew point temperature or dew point.
The ratio of vapor pressure at Point A to the saturation vapor pressure for the initial temperature (Point C) -- expressed in percent -- is the relative humidity. As the parcel cools along the line A-B, its relative humidity increases. When temperatures cool in the evening, with little change in local moisture levels, relative humidity increases and reaches a peak just before sunrise.
For a given temperature (T1):
vapor pressure at Point A
___________________________________ = relative humdiity (%)
saturation vapor pressure for Point C
Clouds may occur when air is cooled to near its dew point. There are three ways to cool air to its dew point:
The horizontal transfer of any atmospheric property by the wind.
Cloud Condensation Nuclei|
In the atmosphere, clouds can form at relative humidities of less than 100%. This is due to the presence of minute (0.1 - 2 micrometers in radius) water- attracting (hygroscopic) particles. Water vapor will stick to, and condense on, these particles to form clouds -- hence the particles are termed cloud condensation nulcei (CCN).
CCN occur naturally in the atmosphere. Major sources of CCN are:
CCN can also result from man's activities. In particular, CCN occur as a byproduct of any combustion process. This includes motor vehicles emissions, industrial activity, and controlled fires (slash and burn agriculture).
The effect of CCN concentrations on climate is an area of continuing research. For example, if greenhouse-gas-induced-global warming occurs, sea surface temperature (SST) will increase. Will this result in increased emission of sulfates from phytoplankton? If so, will this significantly affect CCN concentrations over the oceans? Will increases in CCN concentrations result in increased cloud cover? Will this in turn lead to a cooling effect that will modulate the warming trend?
The most common ways to lift a parcel of air are: buoyancy, topographic lifting, and convergence. Buoyant lifting results from surface heating. This is a common manner of cloud formation in the summer. Buoyancy lifting is also called convection and occurs when local warm areas heat the air near the surface (fig 31a). The warm air is less dense than the surrounding air and rises. This rising air will eventually cool to its dew point and form a fair-weather cumulus cloud.
Air that is forced into, or over, a topographic barrier will also rise and cool to form clouds (figure 31b). This occurs near mountain ranges. For example, warm and moist air from the Gulf of Mexico can be pushed northwestward and up the eastern slope of the Rockies to form extensive cloud decks.
Finally, lifting occurs where there is large scale convergence of air. Cold fronts are a location of strong convergence as cold, dense southward moving air displaces warmer air. Convergence can also occur on smaller scales along the leading edge of the sea or bay breeze boundaries.
The formation of clouds is an application of the First Law of Thermodynamics. According to the First Law, a change in the internal energy of a system can be due to the addition (or loss) of heat or to the work done on (or by) the system. In the atmosphere system, the change of internal energy is measured as a change in temperature and the work done is manifested as a change in pressure. Because air is a relatively poor conductor of heat energy, the assumption is made that the parcel of air upon which work is being done is insulated from the surrounding environment. This is the adiabatic assumption. For a rising air parcel, the change in internal energy is therefore due entirely to pressure work with no addition or loss of heat to the surrounding environment. A simple relationship for temperature change for a rising parcel of air can then be determined. This change of temperature with height is the dry adiabatic lapse rate of -9.8oC per kilometer.
The process without transfer of heat, compression results in warming, expansion results in cooling
Air is, of course, not entirely dry and always contains some water vapor which can condense as the air parcel rises and cools. Condensation creates clouds and affects the temperature and vertical motion of the parcel. During condensation, heat is released
The process by which clouds are formed adiabatically can be summarized using buoyancy clouds as an example. In figure 33, a parcel of air (point A) is heated by the surface and its temperature increases (point B). Because it is warmer than the surrounding measured air temperature, the air parcel cools dry adiabatically as it rises (line BC). At the height (Z1) at which the parcel cools to its dew point (Td) temperature, condensation occurs and heat is released. Because the parcel remains warmer than the environment temperature (line AE) it continues to rise but cools at a slightly slower rate (moist adiabatic lapse rate). The parcel will continue to rise until its temperature is less than the measured air temperature that surrounds it (Z2). At this point, vertical motion ceases and the cloud top height (Z2) is attained.
Many of the clouds formed by the processes noted above can be observed by satellite. The mid-latitude cyclones that are the focus of this chapter contain a subset of cloud types. These clouds are organized into common patterns which are described below.
Clouds are intially classified into types based on their height. They are then subclassified based on their shape. While the shape of a given cloud type can often be adequately observed by satellite, determination of cloud height can be difficult. In order to fully determine cloud shape and height, both visible and infrared satellite images are useful. Shape or appearance of clouds can be determined by a visible image, but temperature -- and, by inference, height -- are best determined by infrared images.
GOES and polar orbiting satellites return both visible and infrared (IR) images. Visible images are created by sunlight reflected from cloud tops. Smooth cloud tops will give a much different reflected signal than clouds that are irregularly shaped. However, two layers of smooth, thick clouds will reflect sunlight in a similar manner making relative height dtetermination difficult. In some cases, if the layers overlap and the sun angle is aligned properly, shadows will reveal the height differences. In most cases, the best way to determine cloud top height is by the use of infrared imagery. Infrared sensors detect the radiation emitted by clouds. Because temperature decreases with height in the troposphere, higher clouds will appear colder (or whiter) on the satellite images. If image enhance software is available the differences can be accentuated.
Some types of clouds are not observed well by satellites. Small clouds, such as fair weather cumulus, are simply too small to be resolved by the satellite sensors. Clouds which are thin or scattered also may not be observed well (figure 34). For a thin or scattered cloud, a GOES infrared detector will receive infrared radiation from both the colder cloud fragments, and in the clear spaces -- from the warmer Earth. When the total radiation is averaged, the satellite will see clouds that appear warmer due to this heterogeneous field of view.
Prior to looking at images it is important to be familiar with the clouds. Clouds most often associated with mid-latitude cyclones are listed below and discussed in the following paragraphs.
Upper Level Clouds (6-12 km): Cirrus (Ci), Cirrostratus (Cs), Cirrocumulus (Cc), Cumulonimbus (Cb)
Mid Level Clouds (2-6 km): Altostratus (As), Altocumulus (Ac)
Low Level Clouds: Stratus (St), Stratocumulus (Sc), Cumulus (Cu)
The highest clouds are cirroform clouds. These clouds are made up of ice crystals and are found at 6-12 km. This group includes cirrus clouds, which are observed from the surface as thin hooks and strands. While cirrus clouds are easily observed from the surface, they are usually so thin that they are difficult to detect by satellite. In strong thunderstorms, however, strands of thicker cirrus clouds are often visible as outflow at the top of the thunderstorm. Cirrus clouds are very helpful in determining the direction of upper-level winds. The cloud strands, when visible, are oriented parallel to the upper level winds. Dense cirrus decks can be observed in visible images as streaks or bands and can be distinguished from lower clouds by the shadow they cast below. In the infrared image, the denser cirrus are very bright because of their cold temperature, but can be subject to the effects of a heterogeneous field of view.
|Figure 36a||GOES infrared image, November 5, 1994
image courtesy of M. Ramamurthy, University of Illinois, Urbana/Champaign
|Figure 36b||enhanced GOES infrared image, November 5, 1994
image courtesy of M. Ramamurthy, University of Illinois, Urbana/Champaign
The clouds typically associated with extratropical cyclones are illustrated in figure 35. Clouds that make up the bulk of the comma cloud seen in satellite images are the cirrostratus clouds. As shown in figure 35, the mature comma cloud has an extensive deck of cirrostratus clouds. The GOES IR image in figure 36a is an example of the illustration in figure 35. The western limit of the cirrostratus deck typically marks the position of the surface cold front. In this case, it is found in Missouri, eastern Arkansas, and central Louisiana. The northern limit of the cirrostratus typically marks the southern edge of the jet stream. This is found across Minnesota and Lake Superior. In figure 36b, the IR image is enhanced to show the cirrostratus cloud region in black. Note that there are whiter regions embedded within the cirrostratus deck, particularly in central Alabama. These are very high cirrus clouds associated with cumulonimbus clouds that have formed along the cold front.
The final form of upper level clouds are cirrocumulus. These small puffy clouds are usually too small to be resolved by the satellite or subject to contamination effect. If the cirrocumulus are large and extensive enough, they are distinguished from cirrostratus by a lumpy texture.
Mid-level clouds, which are found at heights of 2-6 km, frequently resemble the upper level clouds although they tend to be composed of liquid water droplets rather than ice. Altostratus clouds, like cirrostratus, are usually found in association with midlatitude cyclones. Often the only way to distinguish mid-level from upper level clouds is by using software to enhance infrared images, as in figure 36b. In the visible, altostratus is quite similar to higher or lower stratiform clouds and may only be distinguished if shadows are present. Altocumulus clouds also accompany midlatitude disturbances but are typically covered, as are altostratus, by higher clouds. The altocumulus clouds are often found in association with altostratus decks and can be distinguished by a lumpier appearance.
The lowest level clouds also contain cumuloform and stratiform variants. Fair weather cumulus, the "popcorn" clouds seen on fair days, are often below the resolution of regional satellite images. When the cumulus clouds grow into towering cumulus or thunderstorms (cumulonimbus), their high tops and isolated rounded shape are easily identifiable. Cumulonimbus often form along the leading edge of the cold fronts that are associated with cyclones. Stratocumulus forms by the spreading out of cumulus clouds or breaking up of stratus decks. Large decks of straocumulus are often found off the West Coast of the United States. Stratocumulus cloud lines often form off the East Coast of the United States after the passage of a cold front. Stratus clouds are low-based clouds with uniform features and are difficult to distinguish in the visible from altostratus.
Fog, the lowest of all clouds, can often be observed from satellites. On visible images, fog is relatively featureless and difficult to distinguish from higher stratus clouds. If the fog is located over land, either along the coast or in mountain valleys, it can sometimes be detected by the manner in which it follows ground contours. For example, the fog bank may follow the contours of a bay or harbor, or branch into mountain valleys. The branching effect is a good way to distinguish mountain fog from snow cover. Fog can be difficult to observe in infrared images because its temperature is often very close to ground temperature. It can, at times, be even warmer!
Reference: "Earth's Mysterious Atmosphere - ATLAS 1 Teacher's Guide with Activities." (For use with Middle-School students.) NASA EP-282/11-91, pp. 44-54.