The amount of water in the atmosphere is clearly important to plants, which will loose more water when there is less in the air, due to a steeper gradient. A common way to get some idea of this amount is to measure the RELATIVE HUMIDITY, but this does not indicate the actual amount of water, only the amount relative to full saturation at that temperature.
However, since what is important to the plant, and therefore the ecologist is the evaporating power of the air, a better technique may be to use an ATMOMETER. Here is description of how to make and use such a piece of equipment from An Introduction to Field Biology by Bennett and Humphries.
"A useful comparative method for measuring the total evaporating power of air is the atmometer which measures the actual evaporation from a moist surface. The apparatus used is similar to a potometer, but has a porous thimble or other evaporating surface replacing the plant shoot. Any kind of potometer may be adapted in this way (a and b below). A simple atmometer may be made from a piece of capillary tube by grinding one end flat on an oil stone and fitting a rubber tap washer to support a disc of filter paper, cut out with a cork borer. This end of the tube should be bent through a right angle so that both the longer part of the tube and the evaporating surface are horizontal. The tube is filled with water and a moist disc of filter paper
applied (c below). As the water evaporates air is drawn in and its speed of movement measured. Because gusts of wind increase evaporation, the average of several readings should be taken after the apparatus has first had time to reach a temperature equilibrium with its surroundings. This apparatus can be used to find out how transpiration rate is modified by environment. Useful comparisons might be made between the inside and outside of a wood, at different heights, in the open and by a hedge or other windbreak, and on north and south sides of hedges on sunny and overcast days. It can prove instructive to follow such comparisons with potometer experiments."