Part 5

Several thousand observations with various agencies, such as kites and pilot balloons and more especially measurements made with theodolites and nephoscopes, show that there is a definite twist to the right with elevation. The amount of the deflection is shown in Figure 13. Turning of the Wind with Altitude. Here the average yearly values are given for directions and velocities. Thus if the mean wind direction at Blue Hill is from a point a little to the north of west, 306 grads or 275 degrees, and the mean velocity 7 metres per second; the clouds at 1000 metres elevation will move from 312 or 280 degrees and at a speed of approximately 11 metres per second (24 miles an hour).

These however, are average values. In individual cases the difference between surface winds and stratus clouds may be considerably greater. It may be as much as 180 degrees; that is, the cloud may move directly opposite to the wind. In general there will be a difference of 10 to 20 degrees.

WIND AND RAIN

The law of wind direction, approximate cooling and rain.

When the lower clouds are moving from the north or northwest, without sharply defined edges, the LOW is east or northeast of the observer; and rain or snow is not likely unless there is a rapidly falling temperature.

FIG. 13. TURNING OF WIND WITH ALTITUDE]

When a stream of warm air with a high absolute humidity flows north on the east side of a LOW, and a cold northwest wind follows quickly after the LOW, rain or snow may be expected.

Any rapid chilling of warm, moist air produces cloudiness and rain or snow; but a cold stream blowing into a warm area will not produce as much rain as a warm stream blowing into a cold area.

DURATION OF WIND

The average duration of wind from various directions is as follows:

From the north about 16 hours each week; from the northeast, the same; from the east, 11 hours; from the southeast, 10 hours; from the south, 24 hours; from the southwest, 27 hours; from the west, 33 hours; and from the northwest 31 hours.

During an individual disturbance lasting about 36 hours, we may have 8 hours of southwest wind; 4 hours of west wind, backing during the next 4 hours to south; 2 hours of south wind; 2 hours of southeast wind; 2 hours of east wind; 8 hours northeast wind and 4 hours north wind, 2 hours northwest, when it may be considered that a new pressure distribution prevails.

The above values hold only for a storm moving with normal velocity. LOWS are often blocked by slow moving HIGHS in advance. In such cases the duration of east winds is greater.

THE WINDS OF A YEAR

The following table shows the marked increase in the prevalence of northwest and west winds during winter months, the decrease in north winds during July, the increase in northeast winds in May, also in east winds; the increase of south and southwest winds in July; and the falling off of southeast winds in December. See Table, page 72.

In cities near the Atlantic Coast, a continuance of northeast wind, especially in the fall and winter months, results in frequent altho not necessarily heavy rains. On the other hand a period of continued northwest and west wind is a dry period.

In summer, southeast and east winds bring fog and cooler weather; while southwest winds are favorable for the development of thunderstorms.

WINDS OF A YEAR

TABLE I.--Number of Hours the Wind Blows from Different Directions.

-------------------------------------------------------------------- Jan. Mar. May July Sept. Nov. Year Feb. Apr. June Aug. Oct. Dec.

Boreas (N) 98 74 71 70 60 40 59 59 67 80 82 96 850 Kaikias (NE) 41 46 65 94 101 55 79 79 77 91 48 30 819 Apheliotes (E) 34 37 52 58 63 48 51 51 52 58 34 31 576 Euros (SE) 37 37 45 41 54 45 62 62 52 45 39 34 534 Notus (S) 82 66 95 99 143 155 128 128 118 93 81 65 1245 Lips (SW) 112 77 81 79 118 170 135 135 133 108 119 131 1402 Zephyros (W) 180 177 155 125 107 137 125 125 108 131 169 194 1732 Skiron (NW) 160 162 183 154 98 94 105 105 113 138 148 163 1607 --------------------------------------------------------------------

THE SEA BREEZE

When the weather has been clear and moderately warm for two or more days, and the winds are light and variable, there may occur on the third day a moderate wind from the east, known as the sea-breeze. This occurs during anticyclonic conditions. Preceding the sea-breeze, the winds are very light, there are no clouds, and the temperature rises rapidly during the forenoon. This heating is due to a slow dynamic compression as the air slowly descends and the surface air does not flow away. There is no cooling because there is no evaporation due to air movement. The absolute humidity is low, often less than ten grams per cubic metre. Cumulus clouds do not form because there is no uplift of the lower air and consequently no chance for condensation of whatever water vapor may be present. No thunder-heads form notwithstanding the heat. The heat, while dry, is nevertheless extremely trying to men and animals. Relief comes in the early hours of the afternoon by the arrival of the sea-breeze.

The usual explanation of the origin of the sea-breeze is that the land being excessively warm, the air over a relatively cool ocean moves in to take the place of the warm and therefore lighter air, which it is assumed has risen. Unfortunately for this explanation, the air over the land has not risen; but on the contrary is falling slowly. Again the sea-breeze does not begin at the place where the temperature contrast is greatest, namely, just inside the shore line; but comes in from the sea. Nor does the flow extend far inland, which would be the case if there were up-rising currents. The sea-breeze is very shallow, generally not extending upward more than 200 metres, and often not above 100 metres. It does not penetrate far inland, as a rule not more than 15 kilometres, 9 miles.

The sea-breeze is probably caused by a slow descent of dry, warm air, on an incline sloping from northeast to southwest. As it reaches the surface it is twisted more to the right; that is, becomes an east wind. It carries inland with it some of the air over the ocean which is much cooler and heavily saturated.

MUGGY DAYS

There are certain days, more noticeable in summer than at other times, when the air is heavily laden with water vapor; and there is little or no cooling of the body due to evaporation. We perspire freely but as the sweat does not evaporate, there is a constantly increasing amount of water on the skin.

It is not altogether a question of temperature, for another day may have as high or even higher temperature. It is essentially a matter of ventilation. On muggy days we are somewhat in the condition of the unfortunate prisoners in the Black Hole at Calcutta. They did not die by poisoning, as has generally been accepted, that is, lack of sufficient oxygen and an excess of carbon dioxide; but because they were unable to keep the skin sufficiently cool. There was no ventilation; no movement of the air and the body became over-heated and exhaustion followed. No matter how much water there may be on the skin if the surrounding space is saturated, one feels oppressed. A vigorous fanning of the air helps evaporation and cools us. That is why a brisk northwest wind routs a muggy condition.