0.1 Meteorological instruments: rain gauges, temperature, relative  (Page 3/4)

Dry vs. wet bulb differences decrease with temperature (i.e., at 50% relative humidity the difference between a dry and wet bulb is greater at 40oC than at 10oC). The difference also is less with increasing elevation because the lower air pressure means the atmosphere can hold less water and the evaporation rate is less. These non-linear controls means that one has to use a chart to determine the relative humidity for a given difference in temperatures at a given elevation. A Colorado chart for 1500 m elevation will not work in Vietnam!

Two main methods are: (1) a sling psychrometer , where the thermometers are rapidly swung in a circle (or a fan is used to blow air by the wet bulb); and (2) Stationary method, where the wet bulb has a continuous water supply. Stationary method may not work as well if no wind, and over time evaporation will lead to mineral deposition on the sock, so it will need to be replaced more often.

Provides instantaneous rather than continuous values.

3.2. Mechanical methods (“hygrograph”) use the principle of expansion and contraction, usually with hairs held at high tension; as the humidity changes the hairs expand and contract, and again a system of levers transforms this into the vertical movement of a pen on a chart. Cost is about $500-$1000. Many instruments combine temperature and relative humidity, so upper part of the chart is temperature and lower part is relative humidity (“hygrothermograph”).

3.3. Electronic instruments now available, and cheaper than a mechanical recorders, but much more than electronic thermometers; electronic hygrographs inherently must have a thermistor to measure temperature since relative humidity is temperature-dependent.

Wind

4.1. Standard method is with a vertical cup anemometer. This uses a mechanical or electronic sensor to record each turn. A standard anemometer records turns per unit time, which yields wind speed; a totalizing anemometer records total number of turns, which is wind run. Wind speed can be converted into wind run by multiplying the average speed for a given period of time, or integrating shorter-term measurements for the time period of interest. Cost about $400-1000;

4.2. More recent is the sonic anemometer , which measures the effect of wind on the speed of sound. This generates three-dimensional wind speeds, which are useful for measuring the upward flow of CO2 and water vapor (“eddy covariance”), while cup anemometers only measure lateral winds (two dimensions, but only one integrated value). Cost is in the thousands of dollars.

4.3. Location. Standard measurement height for wind speed is 2 m, but measurements are often made at 30 cm next to an evaporation pan, at 10 m, or above the vegetation canopy. For accurate wind measurements the anemometer needs to be 10 times the height of the nearest vertical element. So in a forest that is 30 m high, the clearing for an should have a radius of 10 x 30 m, or 300 m, which converts to a diameter of 600 m! In the U.S. a totalizing anemometer is almost always placed at a height of about 30 cm adjacent to an evaporation pan, as the evaporation rate is highly dependent on wind run (or wind speed).