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