Page:Handbook of Meteorology.djvu/87

factors, appearance and altitude, aid the observer in determining the name and character of a cloud. For all practical purposes the physical form and appearance must always be the chief feature in cloud determination; experience will teach the observer to determine whether the cloud in question is to be classed as “upper,” “intermediate,” or “lower”; a distinction which is sometimes essential. The following is the classification elaborated by Abercromby and Hildebrandsson and adopted by the International Meteorological Congress:

(a) Detached clouds with rounded upper outlines.
(b) Clouds of great horizontal extent suggesting a layer or sheet.
The first (a) are most frequent in fair weather; the second (b) are wet-weather clouds.

Upper clouds, 30,000 feet (9000 meters)			$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	a. 1. Cirrus[1]
				b. 2. Cirro-stratus
Intermediate clouds, 10,000 to 23,000 feet (3000 to 7000 meters)			$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	a. 3. Cirro-cumulus
				a. 4. Alto-cumulus
				b. 5. Alto-stratus
Lower clouds, less than 6500 feet (2000 meters)			$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	a. 6. Strato-cumulus
				b. 7. Nimbus
Clouds of diurnal ascending currents	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	top 6000 feet (1800 meters); base 4500 feet (1400 meters)	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	a. 8. Cumulus
	$\scriptstyle {\left\{{\begin{matrix}\ \\\ \end{matrix}}\right.}$	top 10,000 to 260,00 feet (3000 to 8000 meters); base 4500 feet (1400 meters)		b. 9. Cumulo-nimbus
High fogs, less than 3500 feet (1000 meters)				10. Stratus

1. Cirrus (Ci).^[1] —Detached clouds of delicate and fibrous appearance, often showing a featherlike structure, generally of a whitish color. Cirrus clouds take the most varied shapes, such as isolated tufts, thin filaments on a blue sky, threads spreading out in the form of feathers, curved filaments ending in tufts, sometimes called cirrus uncinus, etc.; they are sometimes arranged in parallel belts which cross a portion of the sky in a great circle and, by an effect of perspective, appear to converge toward a point on the horizon, or, if sufficiently extended, towards the opposite point also (Ci-St and Cu-Ci, etc., are also sometimes arranged in similar bands).

Cirrus clouds moving from the southwest indicate falling temperature; moving from the northwest they indicate the probability of rising temperature. They are the mares’ tails and cattails of sailors’ cant. Near the horizon, cirrus clouds may have a stratiform appearance.

1. For the sake of uniformity of definition and description, the following paragraphs are taken from the report of the Committee.