Diurnal Changes of Temperature at Different Altitudes.—The curve representing the diurnal change in the air at some distance above the ground is probably similar to one representing the change near the ground, except that its amplitude is less. If this be true, then the diurnal rate of fall for a given time at any two levels will be proportional to the daily ranges of temperature at the two levels. It is impossible in practice to keep a kite at exactly the same level for twenty-four hours; hence the daily ranges for the different levels must be found by comparing the rates of rise or fall of temperature for given times with the rates found from records near the ground, made simultaneously with those above. In [Plate IX]., [Fig. 1], the results for six stations, i.e. the kite at 1000 and 500 metres, the Eiffel Tower in Paris (300 metres), the summit of Blue Hill, its base, and the valley (200, 50, and 15 metres respectively), are connected, and a smooth curve is drawn through them. The curve passes approximately through every one of the observed and the computed ranges, except the one at the summit of Blue Hill, which is too great. This evidently is because insolation and radiation, acting through the soil of the hill, heat and cool the air to a greater extent than the free air is heated and cooled at the same altitude, and this must be true at every mountain station. The smoothed curve passes also very slightly to the left of the data for the Eiffel Tower, indicating that the range there is about 1° greater than the true range on account of the heating and cooling of the Tower. From this it appears that the diurnal range of temperature diminishes rapidly with increasing altitude in the free air, and almost disappears in the average at a height of 1000 metres.
The records of the anemometer show that, as a rule, the wind increases steadily as the kites rise, but the increase is greatest between Boston and the top of Blue Hill, due probably to the retarding of the lower winds by contact with the ground. The results are plotted in [Plate IX]., [Fig. 3], together with the mean wind velocity on Blue Hill (209 metres), and the velocity on a tower in Boston (60 metres). Single records of the kite-anemometer differ much, for sometimes the wind velocity diminished with altitude, and at other times it increased so rapidly that the kites were unable to rise higher. On several occasions when the kites passed from one current into another, having a different direction and a different temperature, the wind suddenly increased, and was stronger between the two currents than above or below that plane.
Diurnal Changes of Humidity at Different Altitudes.—It is found that as night approaches the humidity at the altitude of 1000 metres diminishes, while at the earth it increases. This agrees with the evidence furnished by the cumulus clouds that form during the day between 1000 and 2000 metres, and disappear at night, thus visibly indicating an increase of humidity by day and a decrease by night. If the trend of the humidity-curve at a height of 1000 metres is assumed to be the reverse of its trend at the ground, then the results from the kite-meteorograph show the minimum humidity to be at the coldest and the maximum humidity at the warmest part of the day. The mean daily ranges for different altitudes are plotted in [Plate IX]., [Fig. 2]. The part of the curve at the left of the zero line shows the range at different altitudes, with the minimum humidity near the warmest time of day, while the part at the right of the zero shows the ranges at different altitudes, with the minimum humidity at the coldest time of day.
Plate X.—Changes with Height recorded by Kites at Blue Hill.
Types of Change of Temperature with Altitude.—When the records of temperature and humidity made aloft by the kite-meteorograph and at the stations near the ground are plotted in relation to altitude, they are found to be easily divisible into a few types. In [Plate X]., Type 1 represents the decrease of temperature on most fair days from the ground to altitudes of a mile or more, when no clouds are met. The continuous line, plotted from the records of the ascent, represents the day conditions, and the broken line, plotted from the records of the descent, represents the night conditions. This curve shows that with increasing altitude the temperature falls uniformly during the day and approximately at the adiabatic rate represented by the dotted lines. The fall of temperature with increasing altitude during the night is slower than during the day, and in fact, from the earth's surface to an altitude of a few hundred metres, there is often a rise of temperature with height, so that the air at altitudes of from 300 to 500 metres may be considerably warmer than it is at the ground. This was shown in the descent on October 8, 1896, and is found in Type 3.
When clouds are traversed during the flight, the temperature curve assumes the form of Type 2. The continuous curve is plotted from the records of an ascent; the broken curve from the records of the descent, both occurring in the day-time. The temperature falls at the adiabatic rate in unsaturated air till the base of the cumulus cloud is reached. It falls at a slower rate in the cloud, the rate probably being that computed by physicists as the adiabatic rate for air in which condensation is taking place. Above the clouds, the fall of temperature appears to be very slow.
Type 3 is a condition which persists throughout the day and night, and it resembles the night form of Type 1. The temperature rises very rapidly for a short distance above the ground and then falls, with increase of height, somewhat slower than the adiabatic rate. The rise of temperature near the ground with increasing height is more marked after sunset than during the day-time.