_Col._--The neck of low pressure between two anticyclones; also called a _saddle_. _Cold Wave._--A rapid and marked fall of temperature during the cold season of the year. The United States Weather Bureau applies this term to a fall of temperature in 24 hours equaling or exceeding a specified number of degrees and reaching a specified temperature or lower; the specifications varying for different parts of the country and for different periods of the year. _Collector._--A device used in measurements of atmospheric electricity for determining the potential gradient. _Continental Climate._--The type of climate characteristic of the interior of a continent. As compared with a marine climate, a continental climate has a large annual and daily range of temperature. _Corona._--1. A colored luminous circular area formed, by diffraction; around the sun, moon, or other source of light seen through clouds or dust haze. Coronas invariably show a brownish-red inner ring, which, together with the bluish-white inner field between the ring and the luminary, forms the so-called _aureole_. Most frequently the aureole alone is visible. Well developed coronas show one or more series of spectral colors outside the aureole. 2. A luminous circle formed by the apparent convergence of auroral beams about the place in the sky toward which the dipping-needle points. _Corposant._--(See _St. Elmo’s fire_.) _Countertrades._--(See _antitrades_.) _Crepuscular Rays._--Beams of light radiating from the sun, seen both before and after sunrise and sunset. The beams are made visible by the presence of water-drops or dust in the atmosphere, and the intervening dark spaces are the shadows of clouds. The beams are actually parallel; their apparent divergence is the result of perspective. _Critical Period._--A period in the growth of a plant when it is especially susceptible to the effects of atmospheric conditions. _Cumulo-nimbus_; _Cumulus_.--Forms of cloud. (See Chapter VI.) _Cyclone._--An area of low barometric pressure with its attendant system of winds. The cyclones of the region within the tropics (_tropical cyclones_) are violent storms; those of higher latitudes (_extra-tropical cyclones_) may be stormy or otherwise. Tropical cyclones are also called _hurricanes_, _typhoons_ or _baguios_. Extra-tropical cyclones are commonly known as _lows_ or _barometric depressions_. _Cyclonopathy._--The abnormal sensitiveness of certain persons to the weather changes attending the passage of barometric depressions. _Cyclonoscope._--A pasteboard dial formerly used in the West Indies for locating cyclones. _Dark Segment._--The shadow of the earth which, in clear weather, rises from the eastern horizon at sunset and sinks below the western horizon at sunrise.