These climates are in the polar front region in winter, and thus have moderate temperatures and changeable, rainy weather. Originally, h signified low latitude climate (average annual temperature above 18°C) while k signified middle latitude climate (average annual temperature below 18°C/64°F), but the more common practice today (especially in the United States) is to use h to mean that the coldest month has an average temperature that is above 0°C (32°F), with k denoting that at least one month averages below 0°C.iv. If the annual precipitation is less than half the threshold for Group B, it is classified as BW (desert climate); if it is less than the threshold but more than half the threshold, it is classified as BS (steppe climate). It corresponds more or less to the coldest tundra climates and to the Holdrige reasoned that plants net primary productivity ceases with plants becoming dormant at temperatures below 0 °C (32 °F) and above 30 °C (86 °F).The temperature profile of the atmosphere is a result of an interaction between However, when air is hot, it tends to expand, which lowers its density. In the Southern Hemisphere, Group D climates are extremely rare due to the smaller land masses in the middle latitudes and the almost complete absence of land south of 40° South latitude, existing only in some highland locations.The second and third letters are used as for Group C climates, while a third letter of d indicates 3 or fewer months with mean temperatures above 10°C and a coldest month temperature below “38°C (“36.4°F).d. In the tropics the rainy season is provoked by the tropical air masses and the dry winters by subtropical high pressure.
To qualify, the wettest month must have at least 60 mm (2.4 in) of average precipitation if all twelve months are above 18°C (64°F), or 30 mm (1.2 in) if not; plus at least 70% of the total precipitation must be in the same half of the year as the wettest month — but the letter used indicates when the dry season occurs, not the “wet” one.This would result in Khartoum, Sudan being reckoned as BWhw, Niamey, Niger as BShw, El Arish, Egypt as BWhs, Asbi’ah, Libya as BShs, Ömnögovi Province, Mongolia as BWkw, and Xining, China as BSkw (BWks and BSks do not exist if 0°C in the coldest month is recognized as the h/k boundary).
For tropical oceanic locations, such as the summit of For the climate of the mountains named the "Alps", see Many of these areas would be Oceanic (Ca. a) the alpine climate proper which occurs when the mean b) the alvar climate, the coldest mountain climate since the biotemperature is between 0°C and 1.5°C (biotemperature can never be below 0°C).
The growing season (for plants) is about 180 days. If alpine climate is defined by the tree line, then it occurs as low as 650 metres (2,130 ft) at 68°N in Sweden,The variability of the alpine climate throughout the year depends on the latitude of the location. On occasion, a fourth letter is added to indicate if either the winter or summer is “wetter” than the other half of the year.