Laserfiche WebLink
<br />Clouds can be made up of unfrozen water droplets, ice crystals or a combination of them. <br />Within a convective cloud having a portion of it colder than freezing, some of the sub-freezing water <br />droplets remain in a liquid state, and termed "supercooled". Convective clouds often create a condition <br />in which both unfrozen water droplets and ice crystals co-exist simultaneously. It is the supercooled <br />cloud volume that is critical to the formation of rain and hail. Supercooled water can remain unfrozen <br />down to as low as -40 C (-40 F) before spontaneously changing to ice. When spontaneous freezing <br />occurs, it is termed homogeneous nucleation. <br /> <br />Supercooled water droplets containing ice nuclei freeze first. The speed at which supercooled <br />water droplets convert into ice crystals increases as cloud temperature decreases, that is, as clouds <br />grow in height above the freezing level. The process of vapor deposition starts to have a significant <br />effect within clouds when ice crystals and supercooled water exist in the same medium. Surface <br />pressures over ice crystals are lower than those over water droplets which creates a pressure gradient <br />between them. This gradient causes liquid to flow from the droplets to the ice crystals, thereby <br />growing at the expense of the droplets. Once ice crystals develop, they continue growing rapidly by <br />using up surrounding water vapor and cloud water from nearby water droplets. Continuous unequal <br />movements of water droplets and ice particles inside convective clouds ensure random collisions of <br />ice and water droplets. The collisions promote the processes of coalescence, accretion and <br />aggregation. Coalescence is a process in which' the urifrozen water droplets collect other water <br />droplets by impact, the freezing occurring after the impact. Accretion, or riming, occurs when droplets <br />freeze upon impact with cloud ice particles. Aggregation is the process in which ice particles collect <br />or attach to other ice particles. In advanced stages of cloud growth, ice particles will shatter, coalesce, <br />grow larger and repetitively collide in a complex manner through the processes just mentioned. When <br />the various sizes of ice particles eventually fall out of the cloud and drop below the freezing level, they <br />begin melting. If melting is not complete, then hail, graupel or snow arrives at the ground as <br />precipitation instead of rainfall. <br /> <br />The sizes and concentrations of all nuclei present in the atmosphere as well as their chemical <br />and electrical properties all combine in important ways to determine how efficiently a cloud system <br />can produce precipitation. Although there are massive amounts of water vapor in the atmosphere at <br />any time, precipitation won't occur if certain conditions required for the formation of precipitation are <br />absent. <br /> <br />Two cloud types produce all precipitation: "warm clouds" and "cold clouds". A "warm" cloud, <br />is one in which its temperature is not below freezing and does not produce ice crystals in its cloud <br />volume. The warm cloud is generally characterized by a relatively slow growth. Cloud water droplets <br />eventually may grow tp sufficient size and weight to fall from the cloud if given enough time. While <br />falling, cloud droplets collect other cloud droplets by scavenging them along their downward paths. <br />Although this type of cloud occasionally appears in Western Kansas, it doesn't playa dominant role <br />in producing precipitation here. However, large size warm-rain drops can be important embryo sources <br />in the production of hail when they merge into sub-freezing clouds that are not of the warm-rairi <br />variety type and become carried aloft rapidly by updrafts where they freeze and grow into large hail. <br /> <br />4 <br />