Ice clouds

The temperature dependence of the equilibrium between water vapor and ice can be described by the Clausius-Clapeyron equation. Following (17.7), one finds that 

Note that (17.94) is similar to the Clausius-Clapeyron equation for water vapor-water equilibrium (17.8), with the enthalpy of sublimation replacing the enthalpy of evaporation. 

FIGURE 17.26 Average frequencies of appearance of supercooled water, mixed phase, and ice clouds as a function of temperature in layer clouds over Russia (Boronikov et al. 1963). 

FIGURE 17.27 Pressure-temperature phase diagram for water. The dashed line corresponds to supercooled water and its metastable equilibrium with water vapor. 

Finally, if the Clausius-Clapeyron equation is applied to the equilibrium between ice and water, we get 

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The stratosphere is very dry clouds do not form at lower latitudes because the temperature is not low enough. However, the stratosphere over Antarctica is distinctive the temperature can drop to below -90 Celsius during the winter and spring months, leading to the condensation of water vapor and nitric acid vapor, that is, to the formation of ice clouds (polar stratospheric clouds or PSCs). 

Since feedbacks may have a large potential for control of albedo and therefore temperature, it seems necessary to highlight them as targets for study and research. Besides the simple example above of cloud area or cloud extent, there are others that can be identified. High-altitude ice clouds, for example, (cirrus) have both an albedo effect and a greenhouse effect. Their occurrence is very sensitive to the amount of water vapor in the upper troposphere and to the thermal structure of the atmosphere. There may also be missing feedbacks. 

Collection of supercooled liquid water in clouds is simple, using only a plate or screen exposed to RAM air the water is later melted and stored prior to analysis (6 ). Collection of frozen cloud particles is a little more problematical since the liquid water content can be low, and individual particles are more subject to bounce-off during impactive collection. Collection of snow particles aboard the aircraft is most difficult of all due to the low aerodynamic diameter exhibited by these particles in RAM air streams. Successful methods for the collection of snow and ice clouds are still in an active stage of development. 

Caltech unified GCM (Global) Bulk liquid and ice in both stratiform and subgrid convective clouds Diagnosed from predicted cloud water content single size distribution constant cloud droplet number based on observations None None Simulated based on MIE theory with different parametrizations for liquid and ice clouds... 

Neptune s atmosphere has distinctive visible strac-tures, including cloud bands, spots (similar to spots that are storms on Jupiter and Saturn), and methane ice clouds. Cloud bands circle the planet at given latitudes. Spots are stormy areas that have connter-clockwise rotation as a result of shear between clond bands. The Great Dark Spot occnrs at about 20 degrees south latitude and the Lesser Dark Spot at abont 50 degrees south. Methane ice clouds are white and appear approximately 31 mi (50 km) above dark spots, but do not rotate with the spots. 

The surface of Mars is covered by meteorite craters, some up to 200 km in diameter. The question as to whether water exists on Mars has been the subject of scientific controversy for many years (see Chap. 11). Costly Mars missions with the goal of mapping the surface have afforded important results on now dry river valleys. The weather on Mars is characterized by ground-level fog, thin ice clouds and (often very violent) dust storms, which vary not only seasonally but also daily. The question as to whether our neighbour planet harbours life (of any kind), or if it ever did so, gave rise to a media sensation at a NASA press conference on August 7, 1996. The researchers, who had been studying the 1.9 kg Mars meteorite ALH 84001, came to the conclusion that it bore clear evidence of previous life forms ... 

Mixed-phase cloud processes A variety of mixed-phase and ice cloud models exist, describing the homogeneous and heterogeneous formation of water droplets and ice crystals. The implications of aerosol particles on mixed-phase clouds may be evaluated if their ice nucleating properties are known. 

Partly because of limited analysis of Antarctic stratospheric temperatures, early studies that followed the discovery of the ozone hole were not specific about the type of particles of which the observed PSCs were composed. It was generally assumed that the particles were mainly water ice (Steele et al., 1983). Stratospheric ice clouds are frequently optically thick and brilliant in color. Such clouds form when temperatures drop below the temperature at which ice forms (frost point), and are now referred to as Type 2 PSCs. However, sensitive satellite measurements (McCormick et al, 1982) suggested that optically thinner PSCs were also present at warmer temperatures. 

The ozone hole today Scientists have also learned that the ozone hole forms each year over Antarctica during the spring. Stratospheric ice clouds form over Antarctica when temperatures there drop below —78°C. These clouds produce changes that promote the production of chemically active chlorine and bromine. When temperatures begin to warm in the spring, this chemically active chlorine and bromine react with ozone, causing ozone depletion. This ozone depletion causes the ozone hole to form over Antarctica. Some ozone depletion also occurs over the Arctic, but temperatures do not remain low for as long, which means less ozone depletion in the Arctic. 

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