METEOROLOGY OE 1 897. 
17 
§27. The daily mean dew point and daily mean rela- 
tive humidity are the means of the corresponding dew 
points and relative humidities from observations at 7 a. m. 
and 7 p. m. 
§28. The maximum and minimum temperatures are 
obtained by self-registering thermometers, which record 
respectively the highest and lowest temperatures that take 
place between the times of observation. The mean tem- 
perature for the day is the mean of these two readings. 
In general, this is higher than would be obtained from 
the mean of the air temperatures at 7 a. m. and 7 p. m. 
§29. As great variation in temperature is characteristic 
of the climate of the arid regions, a column showing the 
daily range is therefore given. It will be noticed that the 
average daily range for the year is nearly 30°, and that the 
range of certain days may exceed 50°, an amount which is 
unknown in the regions east of the Mississippi. It is in con- 
sequence of such variation, that while the temperature 
seems moderate, if not warm, it is not uncommon on days 
when the temperature has been below zero in the morning, 
to find an overcoat unnecessary at noon. The same valu- 
able features are characteristic of the summer climate, for 
while the temperature of the day may rise to 90° or over, 
the range gives cool nights, and the conditions for physical 
rest. 
§ 30. The barometer readings are those of the mercurial 
barometer, read twice daily. In addition to this a baro- 
graph is also used. It will be noticed that the barometer 
varies from day to day and from month to month. 
§31. Terrestrial radiation serves to indicate the effect of 
radiation on the temperature of the air next to the earth’s 
surface. The reading of the instrument gives the tempera- 
ture indicated by the minimum thermometer placed with its 
bulb a few inches above the grass surface. This is nearly 
the temperature of the air at that point. This instrument 
is placed about six feet lower than the instruments in the 
shelter close by. The amount that the radiation thermom- 
eter is lower than the minimum thermometer in the shelter 
is placed in the column of radiation. 
§32. The important effect of radiation is that it may 
cause frost to occur, even when the lowest air temperature 
is above freezing. It will be noticed, that during the months 
of April, May and June, the temperature near the ground 
becomes 4° or 5° lower than the temperature of the air six 
feet above the ground, and that at times it may descend 10° 
