When condensation takes place in a warm stratum it will be in the form of minute massive spherical particles or spherules. If these spherules are then whirled aloft by ascending currents it is possible for them to be cooled to far below the freezing point without turning to ice; they will, however, congeal instantly when they touch one another or are jostled by touching any solid or liquid surface. They may give a coating of ice to the limbs of trees and the coating may increase until the limbs break, and the surface of the earth thus may be covered with thin ice called _sleet_. =Hail.= There is a difference of opinion among meteorologists as whether the thunderstorm whirls about a vertical axis, like the tornado and the hurricane, or whether it rotates about a horizontal axis. One may well account for the formation of the hailstone by assuming that its alternating layers of snow and ice are caused by the horizontal roll of a thunderstorm, the under part of which has a temperature at or above freezing and the upper half much below freezing. A raindrop is formed in the lower part, frozen in its course through the upper part, receives a fresh coating of water or snow with each revolution and a freezing before its circuit is completed. It thus gains in size until it becomes too heavy to be sustained by the whirling storm-cloud, when it falls to earth. Hail usually has the size of small peas, but occasionally it falls in chunks sufficiently large to kill cattle in the fields. On August 15, 1883, a hailstone weighing eighty pounds is said to have fallen in Kansas. =Frost.= Frost is composed of beautiful crystallizations, similar to snow. Chapter VII describes the process of formation in detail. =Cloud.= Cloud is formed by the cooling by expansion as currents of air are carried aloft. Clouds are composed of minute watery droplets or of ice spiculæ, depending on their temperature, and the latter largely is determined by elevation. A cloud differs from mist or rain in the size and number of its particles, and from fog in its position and the method of its formation. There are three fundamental formations, the cirrus, cumulus, and stratus. The others are combinations of these. The cirrus are thin, high, veil-like clouds, always composed of ice spiculæ; the cumulus look like great banks of snow with bulging, oval tops in which thunder heads may form; the stratus spread out like a great blanket. The cirrus usually fly at the top of the storm stratum, some five to seven miles high; the other clouds at some lower level. When rain is falling from a cloud, it is called nimbus. =Fog Is Cloud at a Low Level.= It is formed by warm water vapor rising from lakes or rivers into the cool night air at the bottom of valleys, or by the cold waters of oceans being forced up over a bar, where the coldness that they impart to the adjacent air condenses some of its vapor.