Cloud types

Clouds cover roughly two-thirds of our earth s surface and play an important role in influencing global climate by affecting the radiation budget. Cirrus clouds are one example of a cloud type whose optical properties are not accurately known. Cirrus clouds form in the upper troposphere and are composed almost exclusively of non-spherical ice crystal particles. The impact of cloud coverage on dispersion of pollution in the atmosphere is an area of great concern and intensive study. 

Effect of aerosol particles on cloud drop number concentrations and size distributions Clouds and fogs are characterized by their droplet size distribution as well as their liquid water content. Fog droplets typically have radii in the range from a few /an to 30-40 /an and liquid water contents in the range of 0.05-0.1 g m" Clouds generally have droplet radii from 5 /an up to 100 /im, with typical liquid water contents of 0.05-2.5 gin"5 (e.g., see Stephens, 1978, 1979). For a description of cloud types, mechanisms of formation, and characteristics, see Wallace and Hobbs (1977), Pruppacher (1986), Cotton and Anthes (1989), Heyms-field (1993), and Pruppacher and Klett (1997). 

Stephens, G. L., Optical Properties of Eight Water Cloud Types, CS1RO Aust. Div. Atmos. Phys. Tech. Pap., No. 36, 1-35 (1979). 

The effect of aerosol on the UV flux reaching the earth s surface is reviewed. Under cloudless conditions UV is scattered or absorbed by aerosol. The net effect depends on the optical properties of the aerosol (scattering or absorbing). In cloudy conditions the UV flux attenuation is more pronounced but depends also on the cloud type. In most instances of clear or cloudy skies an attenuation occurs, with a noteworthy departure the case of partly cloudy sky in a rural area. 

Cloud Type Cloud occurrence is seasonally dependent in the inter-... 

Cloud patterns associated with fronts frequently can be seen from the perspective of a satellite, but a person on the ground, who cannot observe a front in its entirety, must infer its presence from the characteristic progressions of cloud types and weather that occur. 

Cloud types often are classified based on altitude. High clouds have their bases above 7 km (23,000 ft) and include the wispy mare s tail clouds known as cirrus the cirrocumulus, known as mackerel sky and the layers of cirro-stratus. Middle clouds have altitudes between 2 and 7 km (6500 to 23,000 ft), and are either the rounded altocumulus or the layered altostratus. Low clouds have bases from near Earth s surface to about 2 km (6500 ft), and include stratocumulus, stratus, and nimbostratus. Nimbostratus clouds usually bring rain or snow. Clouds with vertical development extend from about 2 to 7 km or more, and include cumulonimbus (thunderhead clouds) and cumulus. 

The most important greenhouse gas at present is not carbon dioxide but water vapour, simply because there is so much of it in the atmosphere (Box 6.1). Volcanoes emit large amounts of water vapour (c. 1 Ttyr-1 Skelton et al. 2003), but even so this flux is minor compared to evaporation from the oceans and evapotranspiration from plants (c.0.25% see Fig. 3.12). In a warmer world, such as during the Cretaceous, the atmosphere can hold more water vapour. However, the extent of the warming caused by extra atmospheric water vapour is difficult to predict because clouds also exhibit an albedo effect, and the balance between the greenhouse and albedo effects varies with cloud type and altitude (Lovelock Whitfield 1982). 

Cloud Feedback. Cloud feedback mechanisms are extremely complex and are still poorly understood. Changes in cloud type, amount, altitude, and water content can all affect the extent of the climatic feedback. 

The presence of clouds in the troposphere modifies somewhat the radiation field in the stratosphere (Lacis and Hansen, 1974) by altering the albedo and introducing a highly scattering medium. Because the reflectance properties of clouds vary considerably with cloud type, numerical models dealing with this problem have to define statistical properties of the cloud distribution. 

FIGURE 7.2 Frequency distribution for liquid water content average values for various cloud types over Europe and Asia. 

Warren S. G., Hahn C. I, London J., Chervin R. M., and Jenne R. L. (1986) Global Distribution of Total Cloud Cover and Cloud Type Amounts over Land, NCAR Technical note TN-273 + STR, National Center for Atmospheric Research, Boulder, CO. 

Direct measurements of ambient supersaturations in clouds have been extremely challenging. Not only does one try to measure a small deviation from saturation, but clouds are frequently patchy with supersaturated regions next to subsaturated ones corresponding to dry entrained masses. Previous measurements have indicated that ambient supersaturations are usually less than 1% and almost never exceed 2% (Warner 1968). A median value of 0.1% was reported in these measurements. Most of our knowledge of these supersaturations is based on theoretical calculations using measurements of atmospheric conditions and are rather similar to that presented here as an example. Ranges of supersaturations expected in various cloud types are given in Table 17.3. 

Cloud Type Updraft Velocity, ms-1 Maximum Supersaturation, % Reference... 

Cloud Type Droplet Concentration (cm ) Liquid Water (g m" ) Mean Droplet Size (/im)... 

Minimum droplet diameters are a few micrometers, where large droplets exceed 100 /xm. In general, droplet spectra are wider for orographic clouds, less wide for stratus, and rather narrow for cumulus cloud types. Continental cumuli drop sizes range only from a few micrometers to around 20 /um in diameter. Frequency distributions of the mean cloud droplet size for various cloud types are shown in Figure 15.30. 

FIGURE 1530 Frequency distributions of the mean cloud droplet size for various cloud types. 

Figure 15.31 shows a zonally averaged climatology based on six cloud types high clouds (Ci, Cs), middle clouds (As, Ac), low clouds (St, Sc), cumulus, cumulonimbus, and nimbostratus. The altitude, thickness, and cloud cover are shown with a 10° resolution. Note that the cirrus base heights vary with latitude but their thickness is fixed at 1.7 km due to limitations of the observations. 

FIGURE 15.31 Zonally averaged climatology of cloud type, cover (number over bar indicates percent occurrence), and thickness for both Northern and Southern Hemispheres at 10° latitudinal intervals. Reprinted from K. N. Liou, Radiation and Cloud Processes in the Atmosphere, 1992, by permission of Oxford University Press. 

Liquid water contents are dependent on the cloud type, cloud base temperature, and height above cloud base. In stratiform clouds, the values are comparatively low, usually 0.1 g m . In cumulus clouds, typical peak values are 0.5 g m , which increase with increasing cloud intensity to more than 3 g m for severe thunderstorms. (As the cloud base temperature increases, a cloud of a given type tends to have a higher liquid water content, increasing with height from cloud base to near cloud top and then falling abruptly to zero at cloud top.)... 

There are very large numbers of surface observations for winds, temperature, humidity, precipitation, cloud types and covers, radiation, etc., over the continents and islands. Matty of the surface stations report at least twice a day at 00 and 12 UTC, but some of them report every hour. Other importaut surface observations come from traveling cotttmercial ships that provide atmospheric arrd oceartic data at synoptic times. In addition, 100 or more drifting buoys gather atmospheric and oceanic data. 

In addition to the heat flux sensors located on the meteorological tower up wind of the spill point and near the spill point, two heat flux sensors were located on the mass flux array 25 m downwind from the spill point to measure heat flow between the ground and the dispersing cloud. Type K thermocouples were positioned on the downwind mass flux and dispersion array towers at multiple heights to measure cloud temperature as it moved downwind. 

Weather pertains to atmospheric conditions that constantly change, hourly and daily. In contrast, climate refers to the long-term composite of weather conditions at a particular location, such as a city or a state. Ghmate at a location is based on daily mean conditions that have been aggregated over periods of time that range from months and years to decades and centuries. Both weather and climate involve measurements of the same conditions air temperature, water vapor in the air (humidity), atmospheric pressure, wind direction and speed, cloud types and extent, and the amount and kind of precipitation. 

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