Rainfall

RAIN (since 0000 GMT)

The total amount of rainfall measured since 0000 GMT [6 PM CST or 7 PM CDT] just above the ground; it is measured in discrete tips of the bucket (approximately 0.01 inch per tip, or 0.254 mm).

The Micronets use an unheated tipping-bucket rain gauge with a 30 cm diameter opening 0.6 m above the ground. The gauge works by funneling rainfall into one of two small buckets mounted on either side of a balance pivot. As each bucket fills, tips, and brings the other bucket beneath the funneled rain, the tip is counted (one tip is 0.254 mm or 0.01" of rain). However, each observation contains a running accumulation of rainfall since either 6 PM CST or 7 PM CDT. As each new evening begins, the accumulated rainfall is reset to zero.

If the rainfall is very light, the bucket may only tip once every half hour or so. Thus, even if the rain is continuous, rainfall might be recorded during every other 15-minute interval. If the rainfall is very heavy, there is a strong likelihood that rain has splashed out of the bucket. Thus, measurements during heavy rainfall periods generally underestimate the total amount of rainfall.

Unheated gauges were chosen because of constraints on cost and power; the drawback is that unheated gauges underestimate snow or freezing rain totals. In addition, wet snow can clog the bucket while dry snow often blows away. Therefore, do not rely on the gauge observations during frozen precipitation events. Moreover, once air temperatures rise above freezing, the water equivalent of the melting precipitation is finally measured. The result is a delayed measurement of winter precipitation.

Sensor Used:

Met One Tipping-Bucket Rain Gauge

Standard Units:

millimeters of water per day

Accuracy:

± 5%

RELH

The average relative humidity at a height of 1.5 meters above the ground during the last 5 minutes. [Note: The relative humidity sensors were removed from the micronets during December 2008 through March 2009. Archived relative humidity data from the micronets can be downloaded on the Data Request page.

Relative humidity changes when either the moisture content of the air or the air temperature changes.

Sensor Used:

Vaisala HMP45C

Standard Units:

Described as a percentage

Accuracy:

± 3% in the range 10% - 98%