CHAPTER XXXIV.

CLIMATE AND WEATHER.

=The Climate= of a country has an important influence on the health and character of its inhabitants. The character of a climate depends on four main conditions:—

1. The distance from the equator. 2. The height above the sea. 3. The distance from the sea. 4. The prevailing winds.

There are other conditions which are of subsidiary importance, but which have great influence in modifying the climate of any given locality. Thus:—

5. The nature of a surface—its aspect, shelter, slope; the colour of the soil or rock, the reflection from rocks or sheets of water, and the influence of vegetation.

6. The cultivation of the soil.

7. The drainage of marshes and damp soils.

8. The planting and clearing away of forests.

=The Distance from the Equator= is the most important factor in relation to climate. The sun’s rays become less powerful as they fall more obliquely, in travelling from the equator. This primary factor in producing climate is largely modified, however, by the relative distribution of land and water, and by the character of the prevailing winds of a district.

=The Elevation= of a locality affects the temperature and the barometric pressure, both falling as the height is increased. The amount of fall varies with the latitude of the place, with its situation in regard to surrounding districts, the degree of moisture of the air, the presence of winds, the hour of day, and the season of the year. It is usual to allow 1° Fahr. for every 300 feet of ascent above the level of the sea, and one-thousandth part of an inch depression of the barometer for every increase of one foot in height.

=Hills, Plain and Valley.=—The law of decrease of temperature with increase of altitude, is liable to great modifications, and even subversions, from various causes. The chief cause producing such modification of the law is the _elevation in relation to the surrounding district_. Thus, in the case of rising ground, the higher parts become rapidly cooled by radiation. The air here is likewise cooled by contact, and becoming heavier in consequence, flows down to low-lying ground. Hence places on rising ground are not so fully exposed to the intensity of frosts at night as places in the valley.

Valleys surrounded by hills and high grounds, not only retain their own cold and heavy air, but serve as reservoirs for the cold air falling from neighbouring heights. One finds, in consequence, mists in low situations, while adjoining eminences are quite clear.

The =air of mountains= is (1) cooler than that of lower districts with the exception already named. (2) It is less dense in proportion to the altitude; its pressure at the height of 16,000 feet being only half that at the sea level. (3) Its absolute humidity is decidedly diminished; there is some difference of opinion as to the relative humidity. (4) The air is as a rule purer. It is generally free from dust, and to a large extent aseptic (that is, free from microbes). (5) The amount of ozone is commonly greater than in lower regions. In addition to these characters, (6) the light is intense, and (7) the direct heat of the sun is greater, and the difference between sun and shade greater than in lower regions.

Owing to these peculiarities of mountain air, it is of great value as a restorative. The circulation of blood is increased, nutrition is improved, the chest expands, and the increase in its size may be permanent.

The presence of =forests and sheets of water= counteracts the effects of radiation from the earth. Thus if a deep lake fills the basin of a valley, the cold air descending from higher levels cools the surface water, which sinks and is replaced by warmer water from below. In this way deep lakes are sources of heat in winter, and places on their shores are free from the severe frosts which are peculiar to other low-lying situations.

If the slopes of a hill are covered with trees the temperature of its sides and base are considerably increased, as the trees obstruct the descending currents of cold air. The frosts of winter are felt most severely in localities where the slopes above them are destitute of vegetation, and especially of trees. It follows that in any given locality, the best protection against the winter cold is ensured by a dwelling situated on a slope a little above the plain or valley from which it rises, with a southern exposure, and sheltered by trees planted above it. Such local conditions should always be carefully enquired into, when a choice of site is possible, as the temperature of one part of a neighbourhood may differ by several degrees from that of another part near at hand. This is particularly important in the case of invalids.

=Forests= tend to modify a climate, and mitigate its extremes, whether situated on the slopes of mountains or on plains. In America, as elsewhere, the effect of destruction of forests has been to produce greater variation in the annual rainfall, to lengthen periods of drought, and to increase the power of floods and cloud bursts. Trees are heated and cooled by radiation like other bodies, but from their slow conducting power, the periods of their maximum and minimum temperatures are not reached for some hours after the same phases of the temperature of the air, and the effects of radiation are not confined to a small surface on the soil, but distributed to the level of the tree-tops. For these reasons, trees make night warmer and day cooler, thus giving to forest districts something of the character of an island climate. Evaporation occurs slowly from the damp soil beneath trees, as it is screened from the sun, and the trees prevent a free circulation of wind. Hence the relative humidity and rainfall are increased. At the same time forests mitigate the disintegrating effect of the rainfall on the soil.

=Ground covered with Vegetation= has a more uniform temperature than bare soil, the effect being much the same as that of forests, though on a smaller scale.

All growing vegetation evaporates a large quantity of water. A plant evaporates 200 pounds of water while it forms one pound of woody fibre; the effect of a forest must, therefore, be enormous. At the same time, vegetation, and especially trees, retain moisture in the soil. The water-supply of barren regions may be greatly increased by planting trees.

_The absence of vegetation_ leads to extreme fluctuations of temperature. An extent of sand, for instance, raises the temperature of the air greatly during the day, as it is a bad conductor; but at night, radiation is very great, and the temperature falls accordingly.

=Relation of Sea to Climate.=—Water has the greatest specific heat of any known substance, being four times greater than that of the earth’s crust. On this account it takes longer to heat and to cool than the earth. Unlike the earth, likewise, it allows free penetration of the sun’s rays,—in clear water probably to a depth of at least 600 feet; consequently, the surface of the water becomes less rapidly heated. The freezing point of fresh water is 32°, while that of sea-water is 27·5°-28·4°. Thus, the sea remains open at a temperature at which inland lakes freeze, and has, therefore, a greater influence in moderating winter cold and summer heat. Another factor rendering it more competent to mitigate extremes of temperature than lakes, is the presence of currents, causing admixture of the water of different climates. Of these currents the most important for this country is the Gulf Stream, an immense stream of water which, when it leaves the Gulf of Mexico, is travelling at the rate of four to five miles an hour, and has a surface temperature of 88° F.

It is important to distinguish between the _surface_ temperature and the _deep-sea_ temperature, the latter being fairly constant. The whole of the depths of the sea is filled with water at or near 32° Fahr., which in the tropics is 40°-50° below the temperature of the surface-water.

The influence of seas on climate is so great as to lead to a classification of climates into oceanic, insular, and continental.

An =oceanic climate= is least liable to violent changes of temperature. It can only be obtained by a sea-voyage.

An =insular climate= presents smaller differences between the temperature of summer and winter than the interior of great continents, especially when the island is small and in the midst of the ocean. In the British Islands, the prevailing winds being westerly, places on the east coast are less truly insular than similarly situated ones on the west coast; and their climate approaches more nearly that of inland countries.

A =continental climate= is drier and more subject to extreme alternations of temperature than insular and oceanic climates.

=Isothermal lines= (lines of equal mean temperature) around the world bend up and down, the bendings being determined by the relative position of continents and oceans. New York has the same mean temperature as London, though New York is as far south of London as Madrid. This fact illustrates the fallacy in judging of the climate of a locality by the annual mean temperature. Means, it has been well said, are general truths but particular fallacies. One should know the extremes of temperature, and the extremes for each month of the year, as well as the amount and distribution of the rainfall, and the amount of sunshine, before judging of a local climate.

=Winds= are due to differences in atmospheric pressure caused by changes in temperature and moisture. Inasmuch as the temperature and degree of moisture of air vary with the prevailing winds, their consideration becomes very important. Winds bring with them the temperature of the air they have traversed: thus, in England, south winds are warm, while north winds are cold. Winds coming over an ocean cause less variation in temperature than those which have passed over an extensive tract of country. Thus, moist ocean winds are accompanied by a mild winter and cool summer, while dry continental winds cause the reverse conditions. The amount of moisture capable of being carried by a current of air increases with its temperature; therefore, equatorial winds become moister as they proceed, while north winds become drier. The south-west winds, in the British Isles, being both oceanic and equatorial, are very moist, while the north-east winds, being both northerly and continental, are peculiarly dry and parching.

Owing to the atmospheric pressure diminishing from the south of Europe northwards to Iceland, south-west winds are the most prevalent in Great Britain; and as this diminution of atmospheric pressure is greatest in the winter months, south-west winds are most common at this season. The result is that the temperature of these islands is higher than that due to mere latitude, and the temperature on the west coast is fairly uniform from Shetland to Wales.

_Mountain ranges_ have an important bearing in determining the character of the prevailing winds. If the range is perpendicular to the direction of the winds, the latter lose the greater part of their moisture, and the places to leeward being exposed more completely to solar and terrestrial radiation (from comparative absence of aqueous vapour), winter becomes colder and summer hotter. The difference between the climates of the west and east parts of Great Britain is largely due to this cause. In Ireland, the mountains are not grouped in ranges running north and south, but in isolated masses, and the difference in climate between the east and west coasts is consequently less marked.

The =prevailing winds= have a great =influence on the rainfall=. (1) Thus if the wind has traversed a considerable extent of ocean, the rainfall is moderately large. (2) If a wind reaches into a colder region, its saturation point is lowered, and the rainfall is greatly increased; and if a range of mountains lies across its path, the rainfall on the side facing the wind is greatly increased, but diminished on the opposite side of the range. (3) If a wind after reaching land proceeds into lower latitudes or warmer regions, the rainfall is small, or absent. This accounts for the rainless summers of California, North Africa, and South Europe.

The =Barometric Pressure= varies daily, being at its maximum at about 9 a.m. and 9 p.m. The average range in the tropics amounts to 0·1 inch, but in this country does not usually exceed 0·02 inches. During the year the minimum barometric pressure usually occurs about the end of October, while the maximum is usually at the end of May or early in June. The ordinary variations in barometric pressure with changes of weather have little apparent effect on health; but more extreme changes produce marked effect. In mountain-climbing faintness and nausea may be caused at great altitudes. At the opposite extreme, in pier-driving and laying the foundations of bridges, men have to work in air-chambers at a pressure of from three to four atmospheres. Then what is known as “caisson disease” may be produced. The usual symptoms are discomfort or pain in the ears, giddiness, bleeding at the nose, vomiting, or even temporary paralysis. In such occupations it is most important that on leaving the air-chambers the atmospheric pressure should be gradually lowered.

The use of the barometer as a weather indicator is based on the fact that moist air is lighter than dry air. Hence, if the air is moist and rain imminent, the barometer falls rapidly. The maximum daily range in this country is rarely greater than 3 inches. Weather observations can be based on records kept at one spot. Their value is greatly enhanced, when such observations are compared with others distributed over a wide area. The wider the area from which such observations are collated, the more accurate the deductions that can be secured. If observations of places at which the barometrical pressure is identical be recorded on a map, we have a _synoptic map_, and the lines of equal barometrical pressure connecting these points are called =isobars=. The modern development of meteorology, enabling forecasts of weather to be made with approximate accuracy, is based chiefly on telegraphic communication of information, enabling isobars to be constructed.

It is found that =isobars= arrange themselves into seven chief forms (1) Cyclones. (2) Secondary cyclones. (3) V-shaped depressions. (4) Anti-cyclones. (5) Wedge-shaped isobars. (6) Cols. (7) Straight isobars.

Each of these varieties is shown in Fig. 43, which embraces the conditions in Europe, the eastern part of the United States, and over the North Atlantic on a certain day.

The closeness of the isobars, _i.e._ the rapidity of changes in atmospheric pressure determines the _barometric gradient_. The steeper this gradient, the greater the velocity of the wind in any given place. The distance between two isobars is equal to a change of a tenth of an inch in the mercury in the barometer. The direction of the wind in a given place is from the higher to the lower isobars. This is expressed in _Buys Ballot’s law_, which states that in the northern hemisphere, if you stand with your back to the wind, the lowest pressure is to your left and in front.

=Cyclones= or depressions are areas of low barometric pressure. A cyclonic system (Fig. 43) is formed by circles of concentric isobars. The differences between cyclones and anti-cyclones are as follows:—

_Cyclones._ │ _Anti-cyclones._ │ Wind moves in the opposite │ Wind moves in same direction as direction to the hands of a watch.│ the hands of a watch. Barometer is lowest in the centre. │ Barometer is highest in the │ centre. Area comparatively small. │ Area comparatively large. Gradient from centre to │ Gradient not steep. circumference steep. │ Short duration. │ Long duration. Velocity of wind great. │ Air comparatively quiet. Weather bad; much rainfall. │ Weather fine. Cool in summer; warm in winter. │ Hot in summer; cold and frosty │ in winter.

Cyclones usually travel from west to east, and are always associated with bad weather. The essential point in determining the character of the weather, both in cyclones and anti-cyclones, is the barometric gradient. Thus, according to the gradient, a cyclone may mean mild wet weather, a gale, or a hurricane. The turning point of a cyclone, just before the barometer begins to rise again, is called the _trough_. Cyclones are usually oval in shape, except in the tropics, where they are smaller and circular. The ordinary course of events in a cyclone is shown in Fig. 44, reading it from left to right.

In =Secondary Cyclones=, bad weather is usually associated with a stationary barometer and no wind. They are incompletely circular looped concentric isobars, with the lowest pressure in the centre. They frequently follow primary cyclones.

=V-shaped Depressions= are angular areas, with the lowest pressure in the centre, frequently forming between adjoining anti-cyclones (Fig. 43). In the northern hemisphere the tip usually points south. They usually move with great rapidity from east to west, and are always associated with squalls or thunderstorms. Their movement is very uncertain, and their forecast therefore more difficult than that of cyclones and anti-cyclones.

=Anti-cyclones= are associated with calm and cold in the centre, while on the borders the wind blows around the centre, spirally outwards in the direction of the hands of a clock. An anti-cyclone is usually accompanied by a blue sky, dry cold air, a hot sun, a hazy horizon, and little or no wind.

=Wedge-shaped Isobars=, unlike V’s, usually point north. They are areas of high pressure moving along between two cyclones, being really projecting parts of an anti-cyclone. The fine weather accompanying them is only temporary, because they are never stationary, and are generally followed by cyclonic disturbances. At the narrow end of the wedge thunderstorms or showers often occur, and at the wide end fog is common.

=Cols= or necks of relatively low barometric pressure occur between two anticyclonic areas. Like straight isobars they are intermediate systems. Over cols the weather is dull and gloomy; in summer they may be associated with thunderstorms.

=Straight Isobars= obviously do not enclose any area of high or low pressure. They form an intermediate condition, preceding the formation of a cyclone; and are usually associated with a blustering wind and hard sky.

=Weather forecasting= is necessarily somewhat difficult and uncertain. If one is dependent on _observations at a single point_ the following rules are useful:—

(_a_) If the barometer falls slowly and steadily bad weather will follow.

(_b_) The barometer falls for rain with S.W., S.E., and W. winds.

(_c_) When the barometer falls rapidly, heavy storms may be expected.

(_d_) The barometer rises rapidly for unsettled weather.

(_e_) The barometer rises gradually for fine, settled weather.

The _Thermometer_ also is of great value as a weather indicator, especially if one knows what is the average temperature at the place of observation for each day of the year. Thus:—

(_a_) A temperature continued for some time above or below the average, indicates a probable change.

(_b_) Electric storms follow unusual warmth in summer.

(_c_) A low thermometer and almost steady barometer are succeeded in winter by gales from N.N.W. or N.E.

The veering of the wind in England is also useful as an indicator. Thus:—

(_a_) When the wind, in shifting, goes round in the same direction as the hands of a clock—_i.e._, from N. by E. to S., or from S. by W. to N.,—favourable changes of weather may be looked for.

(_b_) When the wind _backs_—that is, veers round in the opposite direction—bad weather generally follows.

The direction of the wind is an important factor. Thus:

(_a_) Settled N.W. winds bring cold and fine weather.

(_b_) Continued W. and S.W. winds are followed by rain.

Clouds give useful indications. Thus:—

A _mackerel sky_, that is, one covered with lines of cirrus clouds, causing halos around the sun and moon, presages rain in summer and thaw in winter. By degrees the light clouds descend and pass into either masses of cumulus, or into dense, horizontal stratus, which form at sunset and disappear at sunrise. Both these kinds pass into the grey, shapeless nimbus, which soon covers the entire sky and is followed by rain.

When numerous observations can be _synoptically studied_, forecasting becomes much more nearly certain. For this purpose telegraphic communications are indispensable. The continent of Europe is better placed than England for accurate forecasting. Areas of high pressure coincide usually with large areas of land, of low pressure with large surfaces of water. Thus England is placed near the boundary of the usual anticyclonic and cyclonic systems, and its chief disturbances come from the Atlantic from which early communication is impracticable. Furthermore cyclonic disturbances may be diverted from their course by a coastline or mountains or by the formation of an anticyclonic area. In view of these uncertainties, the large proportion of correct forecasts is somewhat surprising.

The =Moisture of the Air= depends upon the amount of vapour present in it, and the ratio of this to the amount which would saturate the air at the actual temperature. The former is called the _absolute humidity_, the latter the _relative humidity_. The _dew point_ is the point at which condensation of some of the vapour in the atmosphere occurs, either as dew, rain, snow, or hoar-frost. The amount of moisture which the atmosphere can retain before such condensation occurs, varies with the temperature (see page 101). Thus the air is drier at noon than at midnight, though the amount of vapour present in the two cases be the same; and it is for the most part drier in summer than in winter. This refers to the relative humidity, which is highest in cold weather. The absolute humidity is higher in summer than in winter; it varies more in continental than in maritime and insular climates; and there are daily variations according to the state of the sky, the movements of air, etc. The relative humidity is expressed as a percentage of what would be required to produce saturation at the given temperature. The usual relative humidity is 50 to 75 per cent. A moist air prevents excessive changes of temperature due to radiation. It protects the earth from too great intensity of the solar rays by day and from too rapid loss of heat by radiation at night. The inhalation of a dry air plays an important part in the cure of consumption. When the air is almost saturated with moisture, evaporation from the skin and lungs is diminished, and there is a feeling of oppression and disinclination to work caused by the interference with the tissue changes of the system.

=Rainfall= is caused by over-saturation of a column of moist air. This may be due to the contact of the air with a cold surface, as the ridge of a mountain or a large surface of water, or to the impact of a colder wind.

The amount of rainfall varies greatly. In some parts there is no rain, as in the desert of Sahara; while on the south-east slopes of the Himalayas, which are exposed to winds laden with moisture, it may be several hundred inches.

The _latitude_ of a place has a great influence. As a rule the rainfall decreases with increasing distance from the equator; but local conditions may produce great modification, or even alterations of this law.

The _elevation above the sea-level_ has a varying influence. In the Swiss Alps it is said that the rainfall increases with the elevation; but this rule does not hold good in America.

The _nearness of large surfaces of water_ in summer tends to increase the rainfall, when water is colder than its surroundings, while in winter it has the opposite effect. The neighbourhood of the sea is for the west of England and islands adjacent, a cause of increased rainfall.

The _influence of winds_ on the rainfall has been already considered. In Great Britain south-west winds more especially increase the rainfall. In their course they have travelled over the Gulf Stream and the general equatorial current, and have thus received warmth and moisture. The condensation of their moisture liberates a large amount of latent heat, thus raising the temperature of this country. In summer, however, south-west winds are _cool_ and moist, as the Atlantic is not so hot as the continents of Asia and Europe over which other winds have travelled.

In England the average rainfall is about 33 inches, in Scotland 46, and in Ireland 38 inches. In the east of Great Britain, the rainfall is from twenty to twenty-eight inches. On the west coasts of Scotland and Ireland it is from 60 to 80 inches; and in some parts of Cumberland may be about 150 inches per annum. The annual rainfall varies greatly from the average for a number of years. In this country it has been estimated that the maximum annual rainfall exceeds by one-third, and the minimum annual rainfall is less by one-third than the average rainfall of a series of years.

The _number of rainy days_ by no means corresponds with the amount of rainfall. There are fewest rainy days at the equator, where the rainfall is greatest. The rain diminishes the relative humidity of the air, and purifies it from dust.