Page:The Journal of Indian Botany.djvu/345

THE ECOLOGY OF THE UPPER GANGETIC PLAIN 299 heat, what in the form of visible light, and what ultra-violet. Table II shows the observed records of mean cloudiness, based on hourly observations extending over a number of years ; these same records are plotted in graphical form in Fig. 3.

Mean monthly cloudiness on the scale of 0—10, for Allahabad.

Clouds become common in June, perhaps two weeks before the rains actually begin. June to September are the cloudy months. In June there are densely cloudy days interspersed with days of burning sunshine. During July and August the clouds are denser and more persistent, but even then there are breaks when the sun shines from a nearly cloudless sky with the intensity of the dry season. From September on the number of sunny days gradually increases, till in November the sky is cloudless or overspread with only thin hazy clouds for weeks at a time. This is broken at some period during December, January or early February by the " winter rains", when for a short time monsoon rains are repeated on a miniature scale. During most of the time from the end of the rains to the beginning of the next monsoon the sun shines down with unbroken violence, accentuated during March, April, and May by low humidity.

Temperature. The climate of the Upper Gangetic Plain is distinctly continental. The nearest large body of water is the Bay of Bengal, more than 400 miles to the eastward. The temperature accord- ingly exhibits a large range between winter and summer, and between day and night, despite the fact that Allahabad is barely outside the tropics. The lowest temperature occurs in December, when the mean is 59.0° F. ; January is 0.3° warmer. From then on there is rapid rise to a maximum of 91.6° F. in May and June. The high point that should be reached in June and July is prevented by the increasing cloudiness of June, and the following monsoon. In July the mean temperature is 84.4°, falling to 83.0° in September. Here what would be the normal descending curve is met again, and the fall is rapid to the 59.0° of December. Table III shows the temperature in degrees Fahrenheit by months for the year (see Figs. 1 and 3).