METEOROLOGY AftD CLIMATOLOGY. 
9 
if the traveller finds snow lying on grass, moss, or other vegetation, he 
is certainly not above the snow-line. It is necessary also to notice that 
glaciers may descend unmelted a long distance below the level of per¬ 
petual snow. While the conditions of snow lying on the ground in the 
Arctic regions and above the snow-line in any part of the world are 
matters pertaining more to geology and mountaineering than to meteor¬ 
ology, the duration of snow-showers, the character of the snow 3 and the 
depth to which it lies on ground below the snow-line are too important 
from their bearing on climatology to be overlooked. 
The character of the snow as it falls varies from the sleety, half-melted 
drops common in warm air to the fine dust of hard, separate ice-crystals 
found in the intense cold of a Polar or Continental winter. The 
feathery appearance of lightly-felted flakes is an intermediate type 
between the two extremes. In measuring the depth of snow as it lies, 
care should be taken to select open ground where there is no drifting, 
and when the snow is not too deep the measurement can usually be best 
made with a walking-stick on which a scale of feet and inches (or of centi¬ 
metres) has been cut. Such a stick is useful for measuring the depth of 
streams, and for many other purposes. The result should be entered as 
“ depth of fallen snow/' so that there may be no risk of confusing the 
figures with the amount of snowfall estimated as rain. Speaking 
roughly, a foot of snow is usually held to represent an inch of rain. A 
violent storm of wind, accompanied with falling snow, is termed a 
blizzard in the western United States, and a bur an in Siberia; but in 
recording such a phenomenon it is better to describe its nature than to 
give it a name which may possibly be misleading. 
Frost .—The appearance of frost in the form of hoar-frost (the way in 
which atmospheric wafer-vapour is deposited in air below the freezing- 
point), or of thin ice formed on exposed water, should always be carefully 
looked for and noted. In hot, dry countries the intense radiation from 
the ground at night often reduces the temperature below the freezing- 
point, although, during the day, the ground may be very hot. The appear¬ 
ance of frost at sunrise is a valuable check on the readings of a minimum 
thermometer, and in most cases is a more trustworthy datum. Similarly 
in cold countries, where snow is lying on the ground or ice covering the 
rivers, the appearance of thaw, especially in cloudy weather, is a delicate 
test of the rise of the air temperature to the freezing-point. The traveller 
