What radar can't see
Radar has known blind spots. This lesson gives you the numbers for them, so you can tell when your eyes are the only instrument with the answer.
Beam height and range
A Weather Surveillance Radar, 1988, DopplerThe doppler weather radar the National Weather Service runs, roughly 160 of them across the country. When somebody says "the radar", this is the one they mean.'s lowest tilt leaves the antenna at 0.5°. The beam runs nearly straight while the earth curves away beneath it. Standard atmospheric The bending of the radar beam as it travels through air of changing density. Forecasters assume a standard amount of it when working out how high the beam is at a given range. and the 4/3 earth-radius approximation set how fast the gap opens. At the far edge of useful range, the radar is sampling the middle of the storm, not the bottom.
0.5° tilt, standard refraction, height above antenna. At 150 km the lowest beam passes over most of a A thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds.'s low-level The rotating updraft inside a supercell, typically a few miles across. Radar can see it aloft; whether anything is rotating at the ground is a separate question..
Beam width and small features
The beam is a cone, not a line. The third column above is the same story told sideways. A sample 0.41 km across at 25 km is 3.81 km across at the edge of the range. Whatever the radar reports for a point is an average over that whole volume.
So a small feature loses its signature with distance even when the feature has not weakened. A tornado a few hundred metres wide fills a sliver of a sample volume kilometres across, and it is averaged together with the quieter air around it. The leading edge of cool air rushing out of a storm, ahead of the rain. It is what makes the wind pick up sharply just before a storm arrives. structure goes the same way. Distance raises the beam and widens it.
What the radar cannot confirm at all
- The tornado itself. Radar sees rotation aloft and, after the A radar upgrade that sends pulses both horizontally and vertically, so the returns say something about the shape of what they hit. It is what lets radar recognise lofted debris. upgrade, debris already lofted. It does not see ground contact.
- Hail at the surface. How much of the radar pulse bounced back from whatever the beam was passing through on that tilt. It is the familiar green-to-red precipitation picture, and it describes what is up in the beam, not what is reaching the ground. and Maximum Estimated Size of HailA radar-derived estimate of the largest hail a storm is producing. It describes hail in the cloud, so it can disagree with what actually reaches the ground. describe the column, not what survived the fall.
- Wind damage. The radar product showing how fast the rain, hail and debris in the beam are moving toward or away from the radar. Only motion along the beam is measured, and rotation shows up here as inbound and outbound speeds side by side. measures air, not what the air did to a roof.
- Flooding impacts. Rainfall rate is not water over a road.
Then there is the geometry: the The column of sky directly above a radar that it cannot see into, because the antenna cannot tilt straight up. A storm passing overhead is in the worst possible place to be observed. directly above the site, terrain and Terrain standing in the way of the radar beam, so everything behind that hill or ridge is partly or completely unseen at the lower tilts. across a County Warning AreaThe counties and zones one forecast office is responsible for forecasting and warning. Storms crossing the boundary are handled by two different offices., and the wait between looks. A severe-weather One full pass through every tilt in the pattern. A complete new picture of the storm arrives only when it finishes, which is where the four to six minute gap comes from, though extra low-level sweeps can refresh the bottom of it sooner. takes roughly four to six minutes end to end, about four in Volume Coverage PatternThe sequence of tilts a radar sweeps through to build one complete scan of the atmosphere. Which pattern is running sets how long you wait between updates. 12 and about six in VCP 215, which sweeps fifteen elevations.
Work it out for your own town
This is worth ten minutes with a map, because the answer is specific to where you live and nobody will tell it to you. Find the nearest WSR-88D, measure the distance to your town, and read the height of the lowest tilt off the table above.
Almost everywhere in the country has somewhere within its county that comes out badly, and large parts of the interior West and the northern Plains are poorly covered at low levels by any radar at all. If your figure is a few thousand feet, a low-level circulation over your town would not appear on the product the warning meteorologist is watching. The storm would look ordinary. That gap is the reason this course exists, and knowing the size of your own is worth more than knowing the average.
A supercell sits 150 km from the nearest WSR-88D. Of the following, which one can the lowest tilt actually confirm?
Pick one. Think about what the radar can see at the bare minimum.At 150 km the beam center is about 8,600 ft AGL, so the radar samples rotation well above the surface. Ground contact, surface hail size and damage all need a human underneath the storm.