The dew-point is determined by the wet- and dry-bulb thermometer (or psychrometer). The instrument may be made as follows: For the frame find a board eighteen inches long, two inches wide, and one half inch thick; bore a hole in one end so as to hang the apparatus on a nail when not in use. Get two all-glass thermometers with cylindrical bulbs, and the degrees Fahrenheit engraved on the stem. Cover the bulb of one thermometer with a thin piece of cotton cloth, fastening it securely by a thread. When this cloth covering is wet with water and exposed to evaporation in the air, it constitutes the "wet-bulb thermometer"; the other thermometer has no covering on its bulb, is not wet at any time, and constitutes the "dry-bulb thermometer."

The range of temperature of the open air in the following table is from 36° Fahrenheit to 75° Fahrenheit, and of depression of temperature in the wet bulb, from 1° to 13° Fahrenheit, giving a range in both directions of sufficient scope for the needs of northern farmers during the growing season. The temperature of the dry-bulb (or open-air temperature) is found in the left-hand column of the table; the difference in degrees between the readings of the dry- and wet-bulb is entered in the horizontal line at the top, from 1° to 13°. To find the temperature of dew-point at any observation, find in left-hand column the temperature of dry-bulb, then follow the horizontal line opposite that figure till you reach the perpendicular column under the difference between dry- and wet-bulb readings, and the figures at the meeting of these two columns will give the temperature of dew-point. For example, suppose the dry-bulb stands at 65° and wet-bulb at 55°: the difference is 10°. Pass across the page in the line of 65° till you intersect the vertical column under 10°, and you read 47°, which is dew-point under these conditions. If the dew-point is 10° or more above frost-point (32° Fahrenheit), there is little danger of killing frost; but if the dew-point is less than 10° above 32°, danger may be apprehended. If a line is drawn from the intersection of 43° — 1° and 67° — 13°, of the table, this may be called the danger line, and all dew-point temperatures below this line indicate danger of frost, and are printed in italics. This margin of 10° is taken because the temperature on a still night will often sink several degrees below the first dew-point, and the temperature of the air at five feet above the ground is several degrees above that at ground level. For these reasons combined, a margin of 10° may be safely assumed as the limit of safety.

Table for determining the temperature of dew-point from the readings of the dry-bulb and wet-bulb thermometers (Hazeri)