Bioprecipitation, the concept of rain-making bacteria, was proposed by David Sands from Montana State University. Such microbes – called ice nucleators – are found in rain, snow, HP Pavilion dm4-1008tu Battery and hail throughout the world. These bacteria may be part of a constant feedback between terrestrial ecosystems and clouds and may even have evolved the ability to promote rainstorms as a means of dispersal. They may rely on the rainfall to spread to new habitats, much as some plants rely on windblown pollen grainsHP Pavilion dm4-1008tx Battery.
Cooling air to its dew point
Late-summer rainstorm in Denmark. Nearly black color of base indicates main cloud in foreground probably cumulonimbusHP G62-101XX Battery
In general, clouds form in the troposphere when one or more lifting agents causes air containing invisible water vapor to rise and cool to its dew point, the temperature at which the air becomes saturated. The main mechanism behind this process is adiabatic coolingHP G62-103XX Battery. Atmospheric pressure decreases with altitude, so the rising air expands in a process that expends energy and causes the air to cool, which reduces its capacity to hold water vapor. If the air is cooled to its dew point and becomes saturated, it normally sheds vapor it can no longer retain which condenses into cloud.
The altitude at which this begins to happen is called the lifted condensation level, HP G62-104SA Battery which roughly determines the height of the cloud base. Water vapor in saturated air is normally attracted to condensation nuclei such as dust, ice, and salt that are small enough to be held aloft by normal circulation of the air. If the condensation process occurs below the freezing level in the troposphere, the nuclei help transform the vapor into very small water droplets. HP G62-105SA Battery The average size of a newly formed droplet is around .002 mm (.00008 in). High clouds that form above the freezing level are composed mostly of ice crystals. An absence of sufficient condensation particles at and above the condensation level causes the rising air to become supersaturated and the formation of cloud tends to be inhibited.HP G62-106SA Battery
There are three main agents of vertical lift. One comprises two closely related processes working together. Frontal lift and cyclonic lift occur when stable or slightly unstable air, which has been subjected to little or no surface heating, is forced aloft at weather fronts and around centers of low pressure. HP G62-107SA Battery Newly formed cloud droplets that are lifted beyond the condensation level tend to increase in number and coalesce until they grow to a size of up to .04 mm (.002 in). They remain aloft as long as the drag force of the air dominates over the gravitational force for small particles. If the cloud droplets continue to grow past this size, HP G62-110ED Battery they become too heavy to be held aloft as the gravitational force overcomes the atmospheric drag, and they fall from the cloud as rain. When this process takes place just above the freezing level, the vapor tends to condense into supercooled water droplets, which with additional lifting and growth in size, can eventually turn into freezing rain. HP G62-110EE BatteryAt temperatures well below freezing, the vapor desublimates into ice crystals that average about 0.25 mm in length. Continuing lift and desublimation will tend to increase the number of ice crystals which may combine until they are too heavy to be supported by the vertical air currents and fall out as snow. HP G62-110EO Battery