TIPPING-BUCKET RAIN GAUGE
Of devices for obtaining an automatic record of rainfall, the tipping bucket (or, as the British call it, the “tilting bucket”) is probably the most serviceable, and it is the one most widely used in this country. This instrument is as simple as it is ingenious. The “bucket” is a little metal trough, pivoted in the middle, so that it can tilt back and forth, seesaw-fashion. It is divided into two compartments by a central partition. Rain falling into the funnel-shaped receiver at the top of the gauge flows into whichever compartment of the bucket is uppermost, until the weight of the water causes the bucket to tip, thus emptying one compartment and presenting the other to the incoming stream. When the second compartment is filled, the bucket tips in the opposite direction. The parts of the gauge are of such dimensions that each tip of the bucket corresponds to 0.01 inch of rainfall. The gauge is connected electrically with registering apparatus indoors, so that every tip of the bucket is recorded. The registration sheet shows the time of occurrence as well as the amount of rainfall.
The two most important things about the wind that are observed and recorded by meteorologists are its direction and its force. It is the universal custom to regard as “the direction of the wind” the direction from which, rather than toward which, it blows. Moreover, it is only the horizontal direction of the wind that is ordinarily observed, though many winds have a considerable upward or downward slant, and, locally, a wind may even blow straight up or straight down. The direction of the wind may be observed in several makeshift ways, such as by watching the drift of smoke from chimneys, or, as sailors do, holding up a wet finger to the breeze. Instrumentally and scientifically it is observed with a special type of vane, much more accurate in its indications than the weather vanes and weather cocks of ornamental and symbolical architecture. The nonscientific vane, once set in motion, is likely to be carried too far by its own momentum, and may even spin completely around under a sudden impulse. In the scientific vane this tendency is restrained by means of a spread tail; the pressure of the wind on the diverging blades serving to hold the vane in the correct position. The vane, like most other meteorological instruments, is self-recording at all important meteorological stations. The type used by the Weather Bureau registers the direction of the wind every minute.
The force of the wind is obtained from an anemometer. Most anemometers do not, however, show this directly, but are designed to measure the speed or so-called “velocity” of the wind, from which its force may be computed. The speed is observed in miles per hour or meters per second. In considering some of the possible effects of wind it is well to bear in mind that its force increases as the square of the velocity. This means, for example, that a wind of 20 miles an hour is four times as strong, and one of 30 miles an hour nine times as strong as a wind of 10 miles an hour.
One of the external features of a weather station that invariably attracts the attention of the passer-by is an instrument consisting of four hemispherical cups revolving horizontally in the wind. This scientific whirligig is the Robinson cup anemometer, which, in spite of its shortcomings, is the most widely used instrument of its class throughout the world. As generally constructed, the cups are supposed to turn 500 times for a mile of wind movement. Actually the relation between the speed of the cups and the speed of the wind is somewhat variable, and at high velocities the indications of the instrument are seriously erroneous. The Robinson anemometer has a dial from which direct readings can be made, but at large stations it is connected electrically with a registering device in the observer’s office, which makes a mark for each mile of wind and shows how the speed of the wind varies through the day.
There are many other types of anemometer, and some of them tell a much more detailed story of the wind’s variations than does the Robinson instrument. On the other hand, thousands of weather observers dispense with anemometers altogether and merely estimate the strength of the wind from its effects. This applies to nearly all observers at sea, and, in Europe, to the vast majority of observers on land. Such estimates are recorded on a scale ranging from zero, for a calm, generally up to ten or twelve for the strongest winds ever experienced. Several different scales are in use. The best known is the Beaufort Scale, devised by Admiral Sir F. Beaufort, in 1805. The following table of the Beaufort Scale, as adapted for use on land, is from the “Observer’s Handbook” of the British Meteorological Office:
| Beaufort number | Explanatory titles | Specification of Beaufort Scale for use on land based on observations made at land stations | Equivalent speed in miles per hour at 33 feet |
|---|---|---|---|
| 0 | Calm | Calm; smoke rises vertically | 0 |
| 1 | Light air | Direction of wind shown by smoke drift, but not by wind vanes | 2 |
| 2 | Slight breeze | Wind felt on face; leaves rustle; ordinary vane moved by wind | 5 |
| 3 | Gentle breeze | Leaves and small twigs in constant motion wind extends light flag | 10 |
| 4 | Moderate breeze | Raises dust and loose paper; small branches are moved | 15 |
| 5 | Fresh breeze | Small trees in leaf begin to sway; crested wavelets form on inland waters | 21 |
| 6 | Strong breeze | Large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty | 27 |
| 7 | High wind | Whole trees in motion; inconvenience felt when walking against wind | 35 |
| 8 | Gale | Breaks twigs off trees; generally impedes progress | 42 |
| 9 | Strong gale | Slight structural damage occurs (chimney pots and slates removed) | 50 |
| 10 | Whole gale | Seldom experienced inland; trees uprooted; considerable structural damage occurs | 59 |
| 11 | Storm | Very rarely experienced; accompanied by widespread damage | 68 |
| 12 | Hurricane | Above 75 |
The clouds receive more attention at some weather stations than at others. A routine observation consists of noting the kinds of clouds visible, the direction or directions from which they are moving, and the degree of cloudiness—i. e., the extent to which the sky is clouded, stated in tenths, from O = cloudless, to 10 = completely overcast. At many of the more important stations the movements of clouds are observed with a nephoscope. The reflecting nephoscope, used in this country, consists of a black mirror in which the image of the moving cloud is watched, the direction of its motion being read off from the graduated circular frame of the mirror. There is also a device for measuring the apparent speed of the cloud. From this the actual speed can be calculated if the height of the cloud is known. There are other nephoscopes, such as Besson’s in which the cloud’s movements are watched directly, and not by reflection.
BESSON’S COMB NEPHOSCOPE