46 THE OBSERVATION AND FORECASTING OF WIND

176. WIND OBSERVATIONS

The best indication of the true wind direction and force is given by the direction and state of the sea-surface waves (not swell), provided that the wind has not changed direction in the previous half-hour (the time it takes for waves to re-orientate themselves to the new direction). Wind direction is named according to the point of the compass from which it blows, e.g. W, NE, SSW, etc., and wind force is numbered according to the Beaufort Scale (a copy of which is in chapters of Basic Meteorology).

Accurate estimates of the Beaufort wind force are only acquired by learning the scale thoroughly and by experience; there is a tendency for inexperienced observers to overestimate the force of light winds and underestimate strong ones, and to make insufficient allowance for the wind created by his own vessel’s movement (the “way wind”), particularly in fast powerboats. In calm conditions, for instance, a powerboat making eight knots creates a way wind of force 3 in the opposite direction to her course (fig. 42-12(a)). If the true wind is dead astern force 3, this will balance the way wind and there will appear to be no wind at all at the boat (fig. 42-12(b)). If the true wind is right ahead, the apparent wind,(i.e. the wind experienced at the boat) is of a strength the aggregate of both the true wind and the way wind, i.e. force 5-6 (fig. 42-12(c)). If the true wind is N force 3 and the boat is heading East, the apparent wind will be from a direction half-way between the true wind and the way wind (i.e. North-East) and slightly greater in strength than either of them (fig. 42-12)d)). In fact, the apparent wind at any vessel is the resultant of the parallelogram of forces between the true wind and the way wind. In theory, the direction and strength of the apparent wind can be found by drawing a parallelogram, as in fig. 42-12(e). Draw a line representing the way wind in the reverse direction to the vessel’s course and of a length to represent the boat’s speed in knots (in this case, 8 parts). Draw a line to represent the true wind’s direction and force on the same scale. Complete the parallelogram formed by the first two lines, by drawing lines parallel and equal in length to them. The diagonal line towards the boat’s position then gives the direction and strength of the apparent wind at the boat, in this case NNE, 12.4 knots. In practice, of course, there is no need to go into detail like this; the theory has been mentioned here only to illustrate that the apparent wind at any craft in motion does not represent the direction and force of the true wind. For this reason, wind speed and direction indicators are of no meteorological value on vessels at sea.

On small craft, some form of anemometer can be useful, in order to gain an idea of wind speed and for tactical sailing when racing. The type generally found on yachts is the cup anemometer, either as a hand-held instrument or, in more sophisticated form, where the ‘cups’ are mounted at the masthead with an electrical connection to a graduated dial in the cockpit or chartroom. Simple and robust f1ow-meter types of anemometer are very much cheaper; these consist of a glass cylinder with a tube at the bottom which is pointed into the wind, which streams past a sliding disc in the cylinder, which rises to indicate the wind strength against a graduated scale. Masthead wind indicators record both speed and direction in the cockpit, and fall into two categories, both in price and complexity. The cheaper varieties indicate over octants of the compass card, so that wind variations over 45° may take place without being registered as a change in direction. The more elaborate and expensive devices follow the masthead wind-vane, so that they may be used for tactical sailing and not just as indicators of wind-veers or backs of wind.

177. FORECASTING WIND

Some useful rules have been evolved for the lone forecaster in his observation of wind. Perhaps the greatest lesson that modern weather forecasting has taught is that if an earth-bound man wants to forecast tomorrow’s weather, then he must look aloft to where the signs of that weather are being born. Winds show considerable variation between the upper and lower atmosphere. In the lowest layers of the atmosphere, up to about 600 metres (2,000ft.) above the surface, the wind speed usually increases upwards. The reason is that near the surface, the air flow is slowed by friction. But this effect depends on the lapse rate (§2 of this Study); if the air is unstable, fast-moving air from aloft moves freely down to the surface, increasing the wind there, particularly on sunny and showery days. If the air is stable, the fast-moving air remains aloft and the surface wind is quite light, as in fog, or at night if the sky is clear of clouds.

At levels above about 600 metres, the wind usually increases upwards to near the tropopause. The most prominent features of high-level winds are the jet streams, ribbons or cores of very fast-moving air, in which speeds of over 100 knots, occasionally 200 knots may be reached. The main jet streams occur in temperate latitudes, pursuing a wavy course around the globe in each hemisphere, with the air flow from west to east and changing their positions from day to day.

Starting to forecast surface weather with winds which are 3 to 7 miles aloft may seem like putting the cart before the horse but, in fact, the converse is the case. The jet stream controls the movement of the smaller depressions and anti-cyclones and influences the direction of motion and development of the larger ones. It is the handmaiden of ‘mares‘ tails and explains why such clouds presage bad weather. It may also help in forecasting several days of bad weather and, therefore, it is useful to be able to recognise it. It is now evident that the old weather lore about ‘mares‘ tails is effective because the teased-out cirrus clouds stream in the high-speed upper winds and reveal the presence of the latter.

The Wind Orientation Rules (W.O. Rules) for the Northern Hemisphere are:- 

Stand with your back to the lower wind and if high clouds advance from the left, then the weather will deteriorate;

Similarly, if high clouds advance from the right, then the weather will improve;

Lower and upper clouds are moving on parallel or anti-parallel courses, then the weather will probably remain much the same.

The Wind Orientation Rules for the Southern Hemisphere are the same as above, except that in (a) and (b), the observer should stand facing the lower wind.

In applying the W.O. Rules, the lower winds must be truly due to the circulation about Lows and Highs, and not local ones due to sea or land breezes, mountain or valley winds. The best indication of lower wind direction is the motion of the low clouds; usually, the base of the clouds is well above the land or sea-surface and higher than the normal depth of the sea or land breezes. The rules pre-suppose that upper cloud can be seen, and this is not always the case. Finding the direction of the upper wind from the motion of clouds aloft is not easy and reliance must be placed on the lines of high cloud which normally lie along the direction of the upper winds. Truly prognostic cirrus nearly always does this, but other forms of cirro or alto clouds may sometimes lie in lines across the wind, as well as lying along the wind direction.