0.1 Meteorological instruments: rain gauges, temperature, relative

Rain gauges

1.1. Principles: Rain gauges should have a sharp edge so and near vertical sides on the inside to minimize rainsplash (drops are either in or out); should have a funnel with a small hole to minimize evaporation losses; sides and funnel should be smooth, hydrophobic material to minimize loss due to wetting of the surfaces conducting water. Standard height in the U.S. is about 75 cm (30 inches). In winter the funnel is typically removed so snow doesn’t accumulate on the funnel. Some gauges are heated in winter to melt and measure snow, but this causes some evaporation losses.

Biggest issue is wind, as the turbulence causes undercatch; much more severe for snow. Most common shields to minimize this problem are an Alter (strips of metal hanging around the perimeter) shield and a Nipher shield (like the opening of a tuba); latter will cause overcatch in rain and hail.

Ideal location is in a forest opening to minimize wind effects. Gauge should be located at a distance that is twice the height of the nearest object, which is an angle of 30 degrees from the top of the object (i.e., 60 m from a 30-m tall tree). If necessary, an angle of 45 degrees is acceptable.

Wind effects can be minimized by installing the opening of the gauge at or near ground level, but this makes it harder to service and more susceptible to materials falling into the gauge and possible clogging it, such as leaves.

1.2. Types: standard rain gage gives total. To get the desired accuracy, the water is typically funneled into a smaller tube inside the gauge that is only 10% of the area of the rain gauge. This 10-fold increase allows precipitation to be measured to the nearest 0.1 mm or 0.01 inches.

1.3. Recording rain gauges provide data on rainfall intensity, and these include weighing buckets, tipping buckets, a siphon gauge recording on a chart, or a storage gauge with a pressure transducer.

Temperature

2.1. Principles. Measurements need to be made in a location that is representative. In most cases this means in a shaded area that is not subject to excessive heating or reflected radiation, such as a grassy area. The thermometer needs to be in a shelter that is painted white and well ventilated so it does not heat up as a result of solar radiation. Traditional wooden shelters have a double roof to minimize heating, and they always open to the north in the northern hemisphere to avoid the sun shining on the thermometers at any time when observations are being made. Electronic thermometers are typically put into a stacked-plate type shelter. All shelters can result in temperatures that are too high under conditions of high radiation loads and no wind; biggest problem is after a fresh snow because of the very high albedo.

2.2. Mechanical measurements all based on principle that higher temperature leads to expansion.

2.2.1. Traditionally liquid-in-glass (=thermometers); usually mercury or alcohol;

Bulb or reservoir and then a narrow tube, so small change in volume leads to a a large change in distance; read directly off the scale for regular thermometer;