Measuring Precipitation

• Freezing rain – this involves supercooled rain that falls to the ground, is exposed to the cold ground surface, and freezes. As you know, this is quite dangerous if you're traveling on the road when this happens.

• Sleet – you commonly see sleet when the temperatures are near freezing. Sleet is defined as a mixture of snow and rain that can be caused by partial melting of flakes as they fall.

MEASURING PRECIPITATION

The branch of meteorology associated with measuring precipitation is called watershed hydrology. The goal of these researchers is to find the best ways to measure precipitation so that it is accurate.

With respect to rain, it is often measured with a rain gauge. Most rain gauges have some type of funnel that drops water into some collection chamber. The amount of water in this chamber is measured automatically or periodically and then recorded. You need to know the size of the funnel and the size of the collection chamber to know exactly how much rainfall the rain gauge is recording.

The standard rain gauge or SRG is not your typical small glass tube you put outside your backyard. It is a large metal cylinder with a funnel to gather rain. There is a measuring tool in the middle that can take up a total of 2 inches of rain. Beyond that it must dump the rain into a larger outer cylinder. Figure 21 shows you this rain gauge:

Figure 21.

In the wintertime, the funnel and the small inner tube are removed so that snow can collect in the metal outer tube. The snow must be melted in order to get an accurate water equivalent. The downside of this type of rain gauge is that it must be attended to at all times.

Certainly, a rain gauge that can record precipitation over a period of time and documented would be preferable. Rain gauges that do this are called recording rain gauges. They record all precipitation collected during a 15 minute period of time, weigh it, and determine what depth that weight of water would be. A punch tape records the depth. This must be attended to once a month, where the punch tape is collected and changed out each month. There are specialized ones that are heated in order to measure the amount of snow and ice in a bucket that can then tip out and start over. These tipping bucket rain gauges can record the amount of rain in a variable amount of time, because it logs rainfall or snowfall every time the bucket tips. The downside of these is that they can under determine the amount of precipitation if it is raining quite hard.

In addition to these types of rain gauges, you can get optical sensors as well as impact sensors that can measure the size and total number of drops that strike the surface. Larger drops contain more water, which is used to calculate the total depth of the rainfall. Doppler radar sensors can detect how fast a raindrop is falling to the earth. None of these sensors actually collect the rain that falls on them.

Your rain gauge funnel should be one meter off the ground and should have its opening level and horizontal. Needs to be away from any tall objects, but it should also be away from widely open areas; this is because wide open areas prone to wind, which will affect rain deposition. An open space in a grove of trees would be perfect.

Rain gauges are not perfect. The word undercatch refers to the amount of rain caught compared to the actual rain that fell. Most of the undercatch occurs because of the effect of wind, but some of it occurs because of raindrops sticking to the inside of the rain gauge. This is due to the surface tension of water. Under catching because of raindrops sticking to the rain gauge is more prevalent in light rains with only a little bit of rain collection. This is why the people who make rain gauges try to use substances that water doesn't stick to very well. There are comprehensive calculations that can help determine how much the undercatch is.