What radar can't see
- View
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.
In this lesson you will
- Explain why beam height increases with range, and estimate the height at a given range.
- Explain beam broadening, and why small features are undersampled at range.
- List the hazards radar cannot observe at ground level: the tornado itself, surface hail, wind damage, flooding impacts.
- Describe the cone of silence, terrain and beam blockage, and volume-scan timing gaps.
Beam height and range
A WSR-88D's lowest tilt leaves the antenna at 0.5°. The beam runs nearly straight while the earth curves away beneath it. Standard atmospheric refraction 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.
| Range | Beam center AGL | Beam width |
|---|---|---|
| 25 km (13 nm) | 836 ft | 0.41 km |
| 50 km (27 nm) | 1,914 ft | 0.83 km |
| 100 km (54 nm) | 4,794 ft | 1.66 km |
| 150 km (81 nm) | 8,638 ft | 2.49 km |
| 200 km (108 nm) | 13,447 ft | 3.32 km |
| 230 km (124 nm) | 16,796 ft | 3.81 km |
0.5° tilt, standard refraction, height above antenna. At 150 km the lowest beam passes over most of a supercell's low-level mesocyclone.
Only a spotter can confirm what is happening at the surface. Everything below the beam is inference. Your report replaces the inference with an observation.
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 meters wide fills a sliver of a sample volume kilometers across, and it is averaged together with the quieter air around it. Gust-front 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 dual-pol upgrade, debris already lofted. It does not see ground contact.
- Hail at the surface. Reflectivity and MESH describe the column, not what survived the fall.
- Wind damage. Velocity 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 cone of silence directly above the site, terrain and beam blockage across a CWA, and the wait between looks. A severe-weather volume scan takes roughly four to six minutes end to end, about four in VCP 12 and about six in VCP 215, which sweeps fifteen elevations.
SAILS, and MESO-SAILS doing it more than once, insert extra sweeps of the lowest tilt into the middle of a volume scan. They do not make the volume scan shorter. What they shorten is the wait between looks at the bottom of the storm, which is the part you are standing under, so it is the right thing to have been optimized. Between those looks, nothing is watching the ground but you.
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.
Check yourself
A supercell sits 150 km from the nearest WSR-88D. Of the following, which one can the lowest tilt actually confirm?
- A tornado in contact with the ground
- Rotation in the cloud layer above the surface
- Hail size once it reaches the ground
- Wind damage to a structure
Hint
Pick one. Think about what the radar can see at the bare minimum.
Answer
Rotation in the cloud layer above the surface
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.