In the United States, spring is a season that many people associate with rains (or as the old proverb states "April Showers Bring May Flowers"). After a long winter of mainly frozen precipitation (ice and snow), the floral scents and warm days of the springtime bloom heighten our awareness of rain, particularly when heavy showers fall from the first thunderstorm.

Did you ever stop to think how rain is measured? There are many ways of doing this: Rain gauges, weather radars, satellites in orbit around the Earth, and even special underwater microphones that listen to the impact of raindrops on the ocean's surface.

Of all these methods, which one do you think does the "best" job? Be careful, though...measuring rainfall is a surprisingly tricky business, and the answer might surprise you!

In the question, I mentioned three principal ways that we measure rainfall. These are: 1) rain gauges; 2) ground-based weather radars; and 3) satellites. Why of these does the best in terms of accuracy and global coverage?

As it turns out, none of them do, if they are considered individually.

Let's look at the simplest method: Put out a bucket and measure the water depth after a shower with a ruler. The problem with this is that the heaviest rains often fall from thunderstorms, which contain strong, gusty winds. The winds blow a lot of the rain sideways through the air, right over the top of the bucket, so less falls in. Also, because thunderstorms are often only a few miles in diameter, rain gauges (mainly located in major cities spread several tens of miles apart) often miss these storms, or are located on the edge. The result is that a significant amount of rain goes undetected. And how do we solve the problem of putting rain gauges across the vast oceans, particularly the tropical oceans, where two thirds of the Earth's rain falls?

Now on to weather radars. These are familiar to all of us who watch the nightly news or the Weather Channel. There are dozens of radars evenly spaced across countries like the United States. Radar beams sweep out horizontal circles about 300 miles in diameter. In this manner, we get much more complete coverage than that provided by rain gauges. But radars do not measure actual rain…they detect the amount of microwave energy scattered by water drops and ice particles moving through the air. Statistical methods are required to correlate the amount of energy received with what is measured on the ground using rain gauges. These relationships are not perfect, and they vary a lot from location to location, and even within then same storm system. Also, the curvature of the Earth limits how far the radar beam (which projects outward from the radar transmitter as a straight line) can detect rainfall, which is usually heaviest in the lower regions of clouds. At large distances from the radar, the radar beam sweeps through the tops of the clouds and misses a lot of the rain. Studies have shown that radars and rain gauges often don't agree to within better than a factor of two. Not all countries can afford to maintain a network of sensitive radars, and we have the same problem as before…how do we place radars across the world's oceans?

Finally, we have the satellites. These provide the most comprehensive picture of all…snapshots of the clouds taken as frequently as every 30 minutes, and the coverage is truly global. At last, we can detect rain over the tropical oceans. There are many satellite techniques in use. Some measure the infrared energy emitted from cloud tops, and then infer the rain rate based on how deep the clouds are. Some measure microwave radiation upwelling from the atmosphere. Rain drops absorb some of the microwave, and ice particles in cloud tops scatter the rest of the microwave energy. The absorption and scattering can be physically related to the amount of rainwater contained in a cloud. Finally, at least one satellite, NASA's Tropical Rainfall Measurement Mission (TRMM) behaves just like a ground weather radar, but aims its beam straight down through the clouds from an altitude of 400 km instead of sweeping out a circle on the ground. But many of these methods of measuring rain from space are relatively new, and they are far from being perfect. It's difficult to measure a quantity such as infrared or microwave energy and use this information to understand water falling through the sky.

So there are advantages to all three techniques, and disadvantages as well. It turns out that the best way to measure rain is by combining all of these techniques together, and several research groups are now doing this very thing. The results are encouraging.

Now who would have ever thought that counting rain drops falling from the sky could be so difficult?

This week's question comes from Dr. Jeffrey Halverson. Dr. Halverson investigates severe storms at the NASA Goddard Space Flight Center, serves as the Education and Outreach Scientist for NASA's Tropical Rainfall Measurement Mission satellite, and teaches courses on meteorology at the University of Maryland Baltimore County. He holds a PhD in Environmental Sciences and writes a column on interesting weather phenomena in the bi-monthly publication Weatherwise. Dr. Halverson is also an avid amateur astronomer and enjoys hiking throughout the Mid Atlantic to better understand the region's complex geology.