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

4.4. Wind direction is recorded on a 360 degree circle using a wind vane. There are mechanical wind vanes that can record on a chart, but most people now use a wind vane combined with a data logger. The reality is that most people use wind speed, and ignore wind direction.

Evaporation

5.1. Standard method is an evaporation pan. This is a metal pan that holds water, and one carefully measures the change in water level, usually on a daily basis and sometimes twice a day. U.S. Weather Bureau Class A pans are 120 cm in diameter, 25 cm deep, and filled to 20 cm. Most evaporation pans are above the ground and therefore tend to overestimate evaporation because the meal sides absorb solar radiation. There also is a transfer of sensible heat from the air to the pan because evaporation cools the water in the pan and thus creates a temperature gradient from the air to the pan. To avoid this problem evaporation pans should be floated in a large lake, but this is usually impractical. Sunken pans are more accurate, but are rarely used because they accumulate debris and it is hard to detect a leak should it occur.

Since the pans overestimate evaporation, the pan evaporation value needs to be multiplied by an empirical pan coefficient of about 0.7 to 0.8 to get potential evapotranspiration. The problem is that this coefficient varies seasonally and with location, so there is no single value!

One also has to keep the water level relatively constant, as the evaporation rate will increase as the water becomes shallower due to the proportionally greater radiation load and greater advective heat input per unit of water depth.

An adjacent rain gage is needed to subtract any rainfall from the change in water level. Errors can occur due to birds drinking or bathing in the pan, and they also need to be kept clear of algae and debris

Under night-time freezing conditions the data are not reliable, as some energy needed to melt the ice before evaporation can occur. If the pan freezes solid, the welds may break and the pan may leak; at about $3000 for the U.S. Class A pan, this is expensive!

Daily evaporation rates are relatively low (usually 0-10 mm), so the depth measurements need to be very accurate. In the U.S. people raise and lower a point gauge, which has a Vernier scale to the nearest 0.001 inch or 0.001 cm. In Vietnam water is added from a graduated cylinder until the water level reaches a similar point as the U.S. point gauge, and the volume added is divided by the relatively large pan area to get an accurate evaporation rate. Pressure transducers aren’t used because they generally do not have a sufficient degree of precision and accuracy.

5.2. Atmometers or evaporimeters use a volumetric change in a narrow glass tube with a saturated cloth or paper at the bottom. The change in water level is recorded daily. Different values will be recorded from an evaporimeter in a shelter versus one in the open because of the much lower wind speeds in the shelter. Big advantage is that these are much cheaper, as they can range from about $20 to perhaps $300. Not as widely used as they should be!

Radiation

6.1. Two main types of radiation are short-wave (“solar”) and long-wave (infrared or thermal). Measurements are generally made by measuring the temperature increase due to radiation hitting a strip of metal that is painted black to absorb radiation. A dome can be used to restrict the type of radiation that is allowed to hit the metal strip.

Measurements of solar radiation can be divided into: (1) direct , which would be measured using a long tube pointed directly at the sun (“ pyrheliometer ”); (2) diffuse , which comes in at all angles, and (3) reflected from the ground. Albedo is the ratio of reflected short-wave from the ground divided by the incoming short-wave.

Long-wave by definition is diffuse, and it is measured as incoming (from the sky) or outgoing (from the ground).

6.2. Pyranometer : measures direct and diffuse short-wave from the sky; glass dome transmits 0.34-2.8 µm, while quartz domes transmit 0.25-4.0 µm;

6.3. Radiometer measures short and long wave;

6.4. Net radiometer measures the difference between incoming and outgoing. Values are positive during the day and negative at night.

6.5. Pyrgeometer: measures long-wave.

6.6. Duration of direct solar radiation. Another, cruder method of measuring solar radiation is with a spherical ball. This concentrates the solar radiation onto a strip of special paper, and the presence of direct solar radiation is indicated by burning a narrow strip as a result of the concentrated solar radiation. The thickness of the burned strip indicates the magnitude of solar radiation, but it is difficult to convert this into an amount. Hence this is most useful for indicating the duration of sunshine (direct solar radiation) rather than the amount.