In its original sense the word climate meant the degree of inclination of the sun’s rays at any particular spot at a specified date; but nowadays it is employed to designate the average type of weather experienced in a district. In this latter sense it comprises the results of the combined effects of temperature, atmospheric pressure, the degree of moisture in the air, the direction and force of the wind, and the amount of rainfall; the study of climate constituting the science of meteorology. Although the climate of the British Isles is of an exceedingly changeable type, yet the average of the seasonal changes is far from being the same in all parts of the country. Taking temperature alone, we find, for instance, that while the average for the whole year in Shetland is as low as 43° Fahrenheit, in the Scilly Islands it rises to 53°. It is a very general idea that in our islands the winter temperature of a place depends upon whether it is situated in the north or the south. This, however, is a mistake, the temperature having little relation to latitude, but growing colder as we pass from the west to the eastern side of the country; the south of England, as a whole, being milder than the north, not because it is the south, but because it includes such a large extent of land in the west. The degree of elevation above the sea-level has much to do with temperature and the amount of moisture in the air; high lands being, as a rule, colder and drier in winter than those lying at lower levels. On account of the comparatively high elevation of a considerable proportion of its area and its easterly position, coupled with the prevalence of north-easterly and easterly winds in spring, Hertfordshire ought to have, on the whole, a cold and bracing climate for the greater part of the year; and, as a matter of fact, such is the case. Indeed, it is commonly said that northern Hertfordshire is so cold and bracing, that only strong and robust constitutions can stand it; but that for those blessed with such constitutions it is one of the healthiest counties in the kingdom.

Hertfordshire, however, like England on a small scale, has local climates of its own, dependent upon differences in elevation above the sea-level, in the amount of rainfall, in the nature of the geological formation and soil, and also in aspect, especially as regards protection from the east wind in spring. The slope of a hill facing south or south-west receives for instance far more sun in winter than one which looks in the opposite direction; while it also escapes the full blast of the bitter east wind. Places situated on chalk, and above all on gravel, are drier, and consequently—if on the same level and with a similar aspect—also warmer than those on cold, heavy clay or marl.

As if to confirm and perpetuate the above-mentioned popular error, it happens that the northern and north-western districts of the county, that is to say, those constituting the chalk area, are very much colder and more bracing than those to the south and east, whose substratum is clay. This difference depends, however, not on differences of latitude, but on the lower elevation of the southern as compared with the northern districts, coupled with the protection from cold winds afforded to the low lands by the high ground. To those who are in the habit of travelling by the Midland railway from the north-western corner of the county to London, the differences between the climate of the northern and the southern districts is made self-apparent in early spring by the extraordinary difference in the condition of the hedges and trees on the two sides of the Elstree tunnel. It is true that on this particular route the district south of the tunnel is in Middlesex, but to the eastward much of the lowland is in Herts. On the northern side of the range of chalk-hills pierced by this tunnel the hawthorn hedges may be seen at a certain period of the spring to be absolutely devoid of a sign of green; while on the opposite side they will be in full leaf. There is, in fact, about a fortnight’s difference between the Elstree and the Mill Hill side of this range in regard to the development of spring-vegetation; and while the northern side is exposed to the full force of the east wind, the combes and valleys with a south-westerly aspect near the summit of the opposite flank are so warm and sheltered that hardy species of bamboo and palm will grow in the open air almost as luxuriantly as in similar situations in Surrey or Sussex.

We see, then, as has been well remarked, that the division of the county, along the line indicated in the section on its geology, into two very unequal portions—namely, a large north-western area with a relatively dry soil and atmosphere, and a smaller south-eastern tract with a comparatively moist soil and atmosphere—forms a sufficient approach to an accurate climatic division.

Here we may mention that the daily temperature and the amount of moisture in the air, together with the barometric pressure and a number of other details, are recorded at the Meteorological Office in London from reports received from a host of observing stations (either public or private) scattered at intervals all over the country; and at the end of each year the averages, or “means,” of these observations are worked out for the British Isles and England generally, and likewise for the various counties and other local districts. This enables comparisons to be instituted between the climates of different places with much greater accuracy than would otherwise be possible.

In the year 1905 the mean temperature for the whole of England was 48·7°, while that of Hertfordshire was 48·9°, as deduced from observations taken at four stations, of which Bennington showed the lowest mean of 48·4°, and New Barnet the highest of 50·2°. The aforesaid county mean of 48·9° was, however, 0·6° above the average; the average mean for a series of years thus being 47·8°, or about 1° lower than that for England generally.

If we turn to the map here given we notice that, speaking generally, the rainfall of England decreases steadily as we pass from west to east. The moisture-laden clouds, driven by the prevalent winds across the Atlantic, precipitate their contents on reaching the land, more especially if the land be high, and in consequence the country beyond is less wet. Hertfordshire occupies a middle position between the heavy averages of Wales and S.W. England and the minimum of Essex and the neighbourhood of the Wash, as we should expect. The difference between the average of the various stations in our country is remarkable. In the year 1905, which is taken throughout as the basis of comparison, the highest rainfall in England and Wales occurred at Glas Lyn, near Snowdon, and was no less than 176·6 inches; whereas the lowest was registered at Shoeburyness, in Essex, this being only 14·57 inches; while the average rainfall for Great Britain was 27·17 inches. In Hertfordshire, as we shall see, the average in that year for the whole county was 23·47 inches, but this is 1·5 inches below the general average for a series of years, which is 24·52 inches. In the same year the average for the four chief observing stations in the county was, however, 24·22 inches, with a maximum of 25·88 inches at New Barnet and a minimum of 22·51 inches at Bennington. These extremes were exceeded by a maximum of 28·29 inches at Pendley Manor, Tring, and a minimum of 19·31 inches (or rather more than 5 inches above the Essex minimum) at Hillside, Buntingford.