What every Volcanic Cloud Researcher should know about Hydrometeors and Meteorological Clouds

Water on Earth exists in 3 states: solid (snow, ice), liquid (water), and gaseous (water vapor). It frequently changes its physical state (freezing-melting, condensation-evaporation, deposition-sublimation).

Each change of state involves a transfer of heat between latent and sensible forms.

Solid <=> liquid: 80 cal/g

When ice or snow melts, it takes 80 calories to melt 1 gram. This heat is drawn from the environment (atmosphere), where it is held as sensible heat, and stored as latent heat. Thus, sensible heat is expended (the environment becomes cooler) and latent heat is accumulated.

When freezing occurs, latent heat is reduced by 80 cal/g. This heat is released as sensible heat into the environment and causes it to warm up.

Liquid <=> gas: 600 cal/g

Evaporation: latent heat accumulated, sensible heat expended by 600 cal/g

Condensation: the reverse happens

The terminology for water vapor in the atmosphere is explained below:

Water Vapor Terminology

Water vapor is water in the gaseous phase. Meteorologists have defined several different terms to express the amount of water vapor in air. Some refer to the actual amount, or concentration, of water vapor in the air, and some relate the actual amount to the amount that would saturate the air. Air is said to be saturated when it contains the maximum possible amount of water vapor without bringing on condensation. At that point, the rate at which water molecules enter the air by evaporation exactly balances the rate at which they leave by condensation.

Specific humidity is the ratio of the mass of water vapor in a sample to the total mass of the moist air, including both the dry air and the water vapor. The mixing ratio is the ratio of the mass of water vapor to the mass of only the dry air in the sample. As ratios of masses, both specific humidity and mixing ratio are dimensionless numbers. However, because atmospheric concentrations of water vapor tend to be at most only a few percent of the amount of air (and usually much lower), they are both often expressed in units of grams of water vapor per kilogram of (moist or dry) air.

The partial pressure of a given sample of moist air that is attributable to the water vapor is called the vapor pressure. The vapor pressure necessary to saturate the air is the saturation vapor pressure. Its value depends only on the temperature of the air. (The Clausius-Clapeyron equation gives the saturation vapor pressure over a flat surface of pure water as a function of temperature.) Saturation vapor pressure increases rapidly with temperature: the value at 90°F (32°C) is about double the value at 70°F (21°C). The saturation vapor pressure over a curved surface, such as a cloud droplet, is greater than that over a flat surface, and the saturation vapor pressure over pure water is greater than that over water with a dissolved solute.

Relative humidity is the ratio of the actual vapor pressure to the saturation vapor pressure at the air temperature, expressed as a percentage. Because of the temperature dependence of the saturation vapor pressure, for a given value of relative humidity, warm air has more water vapor than cooler air. The dew point temperature is the temperature the air would have if it were cooled, at constant pressure and water vapor content, until saturation (or condensation) occurred. The difference between the actual temperature and the dew point is called the dew point depression.

The wet-bulb temperature is the temperature an air parcel would have if it were cooled to saturation at constant pressure by evaporating water into the parcel. (The term comes from the operation of a psychrometer, a widely used instrument for measuring humidity, in which a pair of thermometers, one of which has a wetted piece of cotton on the bulb, is ventilated. The difference between the temperatures of the two thermometers is a measure of the humidity.) The wet-bulb temperature is the lowest air temperature that can be achieved by evaporation. At saturation, the wet-bulb, dew point, and air temperatures are all equal; otherwise the dew point temperature is less than the wet-bulb temperature, which is less than the air temperature.

Nucleation and CCN (based on Encyclopaedia Brittanica materials)

The formation of cloud droplets and cloud ice crystals occurs associated with suspended aerosols of natural and anthropogenic origin, which are ubiquitous in the Earth's atmosphere. In the absence of such aerosols, relative humidities much greater than 100 percent with respect to a flat surface are required for water vapour to condense spontaneously into liquid water or to deposit into ice. The development of clouds without aerosols, which occurs only in a controlled laboratory environment, is referred to as homogeneous nucleation. In the atmosphere, aerosols serve as initiation sites for the condensation or deposition of water vapour. By having a discrete size, they reduce the amount of supersaturation required for water vapour to change its phase. (Air containing water vapour with a relative humidity greater than 100 percent with respect to a flat surface is said to be supersaturated.) Aerosols that are effective as embryonic sites for the conversion of water vapour to liquid water are called cloud condensation nuclei. The larger the aerosol and the greater its solubility, the lower is the supersaturation required for the aerosol to serve as a cloud condensation nuclei. Cloud condensation nuclei in the atmosphere become effective at supersaturations of about 0.1 to 1 percent. The concentration of such nuclei in the lower troposphere at a supersaturation of 1 percent range from roughly 100 per cubic centimetre in oceanic air to 500 per cubic centimetre in a continental atmosphere.

Aerosols that are effective for the conversion of water vapour to ice crystals are called ice nuclei. In contrast to cloud condensation nuclei, the most effective ice nuclei are hydrophobic having molecular spacings and a crystallographic structure close to that of ice. The measurement of hydrophobic/hydrophillic characteristics for various materials involves the measurement of contact angles. This kind of work for volcanic ash is lacking.

While cloud condensation nuclei are always readily available in the atmosphere, ice nuclei are often deficient. (Examples of condensation nuclei include particles of sodium chloride [salt] and ammonium sulfate, while the clay mineral kaolinite serves as ice nuclei.) Consequently, liquid water that is lifted and cooled below 0 C can often remain liquid at subfreezing temperatures because of the absence of effective ice nuclei. Except for ice crystals that are effective at 0 C, all other ice nuclei become effective only at temperatures lower than freezing. Liquid water at temperatures less than 0 C is referred to as supercooled water. In the absence of any ice nuclei, freezing of supercooled water droplets that measure a few micrometres in radius requires temperatures at or lower than -39 C (a process called homogeneous ice nucleation). When ice nuclei are present, heterogeneous ice nucleation can occur at warmer temperatures.

Hydrometeors, Clouds and Precipitation (based on Encyclopaedia Brittanica materials)

Hydrometeors are the particles involved in clouds and precipitation. They are mostly made up of solid and liquid H2O. Rain, Hail, Sleet, Graupel and Snow are several kinds of hydrometeors. Hydrometeors can form in an air parcel when environmental conditions change. For example if air rises, it may cool adiabatically as pressure derived from the decreasing weight of air above the parcel falls. If the water vapor content or specific humidity is the same in the cooled air, its specific humidity will rise. Eventually, if this continues, humidity will reach 100% and a cloud can arise when either liquid H2O or ice forms. A variety of processes, including convection, convergenge and topography can help to form clouds.

Clouds have smaller particles and precipitation has larger ones. Because the precipitation particles are larger, they fall, but cloud particles often do not. The relationship between size of the particle and its fall velocity in the atmosphere is controlled by gravity and drag forces. After a few seconds, any particle in the atmosphere will reach its terminal velocity. Rain is a hydrometeor consisting of liquid water. It deforms during fall and may grow or shrink depending on humidity. Large raindrops break up. The motion of a particle in a fluid is controlled by a balance of gravity and drag forces. Motion may involve smooth streamline laminar flow or turbulent flow if the particle is falling more rapidly. The Reynolds Number is a dimensionless value used to characterize the flow of falling particles. The accumulation or aggregation of hydrometeors is part of the norm of clouds. Snow is an accumulation of ice particles. Large raindrops result when small raindrops accumulate even smaller ones during fall. Many hydrometeors don't reach the ground because they evaporate or sublime. Virga is the visual clue of such a process.

There are many types of clouds. The variety reflects the types of hydrometeors they contain, the optical depth of the cloud and the mode of formation. Optical Depth is a logarithmic measure of cloud turbidity (usually between 0 and 4) and is related to the size and number of particles in the cloud.

The evolution of clouds after cloud droplets or ice crystals have formed depends on which phase of water occurs. A cloud in which only liquid water occurs (even at temperatures less than 0 C) is called a warm cloud, and precipitation emanating from such a cloud is said to be the result of warm-cloud processes. In such a cloud, the growth of liquid water from a cloud droplet to a raindrop occurs first as continued condensational growth due to additional water vapour condensing in a supersaturated atmosphere. This process is effective, however, only until the droplet attains a radius of about 10 micrometres. Above this size, further increases in its radius by condensational growth are very slow, since the mass of the droplet increases as the cube of its radius. Subsequent growth thus occurs only when the cloud droplets develop at slightly different rates due to spatial variations in the initial aerosol sizes and solubilities and to magnitudes of supersaturation. Cloud droplets of different sizes fall at different velocities such that cloud droplets of different radii collide. If the collision is strong enough to overcome the surface tension between the two colliding droplets, coalescence occurs with a new, larger single droplet resulting. This process of cloud droplet growth is referred to as collision-coalescence. Warm-cloud rain results when the droplets attain a sufficient size to fall to the ground. Such a raindrop (perhaps about one millimetre in radius) contains on the order of 1,000,000 cloud droplets of 10-micrometre radius. This type of precipitation is common from shallow cumulus clouds over tropical oceans where the concentration of cloud condensation nuclei is small enough thatthere is only limited competition for the available water vapour.

A cloud that contains ice crystals is termed a cold cloud, and precipitation resulting from such a cloud is said to be due to cold-cloud processes. In this type of cloud, ice crystals can grow either by deposition directly from water vapour that is supersaturated with respect to ice or as a result of the evaporation of supercooled water and subsequent deposition onto the ice crystal. Whether ice forms is not only a function of temperature, however, because the activation energy barriers of vapor to liquid and vapor to solid reactions are significant. Because of these differences, many clouds can contain particles of supercooled liquid water, and some may contain both ice and water and the Bergeron Process. Because the saturation vapour pressure of liquid water is greater than or equal to the saturation vapour pressure of ice, ice crystals grow at the expense of the liquid water. For example, air that is saturated with respect to liquid water is supersaturated with respect to ice by 10 percent at -10 C and by 21 percent at -20 C. This results in a rapid conversion of liquid water to ice. In a cloud with numerous supercooled cloud droplets, such a substantial and rapid change of phase permits large ice crystals of snowflake size to develop quickly from tiny crystals by depositional growth alone. Clouds that are converted only to ice crystals are known as glaciated clouds. Ice crystals that grow by deposition have much lower densities than solid ice because of the air pockets occurring within the volume of the crystal.

Ice crystals also can grow to precipitation size through aggregation or by riming. Aggregation occurs when the arms of the ice crystals interlock, resulting in a clump. This collection of intermingled ice crystals occasionally can attain sizes of several centimetres in diameter. In riming, supercooled water freezes directly onto ice crystals, causing them to grow; the accumulation of dense ice on the crystals increases their fall velocity. When the riming is substantial, the crystal form of the snowflake is lost and a more or less spherical precipitation-sized particle called graupel results. In cumulonimbus clouds wherein the graupel is repeatedly wetted and then injected back toward high altitudes as a result of strong updrafts, very large graupel, or hail, results.

Frozen precipitation falling to levels much warmer than 0 C reaches the surface as rain. Such cold-cloud rain at the ground is distinguished from warm-cloud rain by its larger size. Melted hailstones,in particular, make a large radius impact when they strike the ground. Cold-cloud rain may refreeze if a layer of subfreezing air exists near the surface. If the freezing occurs in the free atmosphere, sleet or ice pellets is produced. When the freezing occurs only at impact on the ground, freezing rain results.