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Ask Eric: It's Raining Out, So Why Doesn't Radar Show It?

All afternoon it has been raining in Brentwood, NH. Why isn't it showing up on radar or registering on weather apps?
- Sarah in Brentwood
A bit of a weather puzzle on Sunday afternoon as many reported rain, yet there was very little showing up on radar. Did doppler take the day off?
The case calls for a little weather detective work. We know that Sunday was a pretty gloomy day, and those clouds were hanging low. We can look back at METAR code to find out exactly where those clouds were. METAR is short for aerodrome routine meteorological report, so you get the impression that it's pretty useful for aviation. Pilots absorb piles of METAR and TAF (terminal aerodrome forecast) reports that are shorthand weather observations helping to guide their way. The benefit is that you can squeeze a ton of info into a small space. For a full tutorial on how METAR works, check out this METAR 101 site from Wunderground.
Below is the METAR report from the Manchester, NH airport on Sunday at 1953 Zulu Time, or 3:53pm EDT. Aside from the clouds, this report shows the date (23rd), time (1953z), wind speed and direction (7 knots from the east), visibility (4 miles), weather conditions (light rain),  temperature (22C), dew point (19C), and altimeter reading (29.99" Hg)

KMHT 231953Z 07007KT 4SM -RA FEW007 BKN010 OVC025 22/19 A2999

As for the cloud translation, this METAR shows that there was a layer of overcast skies at 2,500 feet, with some clouds even lower than that.  All of this info comes from an automated weather station at the airport (there are also human observer METARs, like Mt. Washington). But what does this have to do with radar? This observation tells us that much like an ice berg, a lot of the weather was below the surface.

There are a couple of radar sites in the area - WBZ's own personal radar is in Worcester, and a National Weather Service radar site in Taunton, MA. There's also one somewhat nearby in Gray, ME. Brentwood, NH happens to be a pretty fair distance from all of them. For this example, we'll look at WBZ's radar site and the Taunton site.

SAVE Doppler Explain1

As the crow flies, the Worcester radar is about 63 miles away. The Taunton radar is about 90 miles away. The distance is important, because the lowest radar beam is at an elevation of 0.5º. A radar works by starting at this elevation and sending out short bursts of radio waves. That energy encounters projectiles along the way (not just rain and snow, but birds, bugs, dust, etc) and some of the energy is reflected back to the radar site. Then the doppler determines what exactly it's looking at, how it's moving (toward or away), and what the concentration is. It does this for the lowest slice, with several scans to follow that are at higher tilts (0.9º, 1.5º, 1.8º, and so on). This builds a profile of the lower levels of the atmosphere. We call the final picture a VCP, or volume coverage pattern.

Save Doppler Explain2

Since the initial scan is not flat to the surface, it ends up being higher and higher up into the sky the farther away from the radar site you go. With some math we can determine exactly how high up the beam is at a given point. So in the case of Brentwood, the WBZ radar was seeing action at about 7,700', but no lower. The Taunton radar can't see any lower than 12,400'. When you have shallow precipitation like we did on Saturday, it can hide beneath the beam! We see this a lot during ocean-effect snow. Sometimes the moisture being picked up from the ocean and flung onshore is so low to the surface/shallow that it never shows up on radar, and subsequently we get lots of emails saying 'It's still snowing up here!' when the radar has gone quiet. Another example of why all your reports are so important.

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