Understanding Weather Radar

Figure 1. A Flashlight Beam

We always want to point the center, most intense part of the beam at the most intense part of the cell. The intense part of the beam is called the weather beamwidth and is defined by the angle where the energy drops in half. The image below shows a side and end view of the radar beam for a 12-inch antenna. The weather beamwidth is 8-degrees wide. An 18-inch antenna has a 5.6-degree weather beamwidth. Notice that there is still energy out to 14-degrees. We’ll address that in just a minute, but before we do let’s get an idea of the size of the beam.

Figure 2. Side and End View of Radar Beam

Back when we learned to fly, we were taught the one-in-60 rule. If you were 60nm from a VOR and 1-degree off track you were 1nm off track. For radar there is a more useful form of the rule. If you want to know how many feet 1-degree is at any range just add two zeros to the range. For example, 1-degree at 40nm is 40+00, or 4,000 feet. Armed with our new rule let’s visualize how large the beam is. Looking at the blue lines in the image below at 40nm, 1-degree is 4,000 feet. So, our 8-degree weather beamwidth is eight times that or 32,000 feet tall and wide. At 80nm 1-degree is 8,000 feet, so our beam is 64,000 feet or +/-32,000 feet around the center of the beam. Our little flashlight beam grew in diameter and reduced in intensity just by stepping back a few feet. At 80nm our little pulse of radar energy gets spread out over 64,000 feet, or over 10nm in diameter.

Figure 3. Visualizing the Size of the Radar Beam

Let’s go back to our dark room and pick up our flashlight again, but this time we’ll be pointing it lower towards the floor. Place a mirror on the carpet in the middle of the room and another mirror leaning against the wall. When we shine the light on a surface it reflects at the same angle of incidence. The light bounces off the mirror on the floor and doesn’t reflect back. This is what happens when the radar energy hits a calm lake, calm seas, or crops. However, when our light hits a rough surface like the carpet the light scatters and a lot reflects back to us. This is what happens when the radar energy strikes a rough surface like the canopy of trees or rough ground. When our light hits the mirror leaning against the wall all the light is returned. Mountains, buildings, bridges, and roads reflect radar energy extremely well and these stronger returns can be seen well outside of our weather beamwidth. This is called the ground beamwidth and is the 14-degrees we discussed earlier for the 12-inch antenna, and 10 degrees for an 18-inch antenna. So, as you’re setting tilt, aiming the center, most intense part of the beam at the center of the weather (blue lines) you’ll pick up the much stronger ground returns in the ground beamwidth (brown lines). Setting tilt is a compromise between detecting all the weather while minimizing the ground returns.

Figure 4. Ground return Characteristics

But why do we want to see the most intense part of the cell, and where is that? For that matter, why do we look at the colors at all? Based on studies we know that with higher reflectivity comes higher probabilities of turbulence and hail size. Both good reasons to detect the maximum reflectivity and look at the colors. If we enter the turbulence probability chart below at the beginning of the red color, we could say that we have a 55% chance of light turbulence, a 40% chance of moderate turbulence, and a 5% chance of severe turbulence. Another way to say it would be that we have a 45% chance of experiencing less than light turbulence, a 60% chance of encountering less than moderate turbulence, and a 95% chance of experiencing less than severe turbulence. These are probabilities however, and unfortunately the relationship between reflectivity and turbulence is not as direct as we would like and so we must use other methods to assess the threat. The chart also only applies to convective weather, so we must determine if we are dealing with convective or stratiform weather first. Without going into cloud physics, the simple answer is that we need to look for convective activity and how much reflectivity is carried aloft. Think of a small garden fountain and a much larger fountain at a nearby park. The lifting force required for the larger fountain is like the vertical development in a strong cell. These stronger updrafts also cause strong downdrafts and turbulence. So, looking at reflectivity is important, but looking at how much reflectivity is carried aloft is even more important. So, let’s look at how we can detect cells and analyze them for avoidance. 

Figure 5. Reflectivity vs Turbulence and Hail Size

At cruise altitude we recommend the cruise ground park technique (Figure 6). Start with the tilt all the way down (-15 degrees) and slowly raise the beam until you see ground returns at the outer edge of the display. There are three main benefits to this technique. First, by starting with the beam very close to the aircraft you won’t over scan any cells close to the aircraft. Second, any storm cells will walk out of the ground returns making them easy to identify. And, if you can still see ground returns behind the cell then you know your energy hasn’t been attenuated (radar shadow) and that it is an accurate representation of the cell. If you can’t see ground returns behind the cell it may be stronger or extend further than it appears. The cruise ground park technique will not show the maximum reflectivity because the ground returns are being picked up from the ground beamwidth and not the weather beamwidth. If a cell is in your path and needs to be analyzed, you should tilt up and down to find the maximum reflectivity. For analysis we want to look at how much reflectivity is carried aloft, and we’ve already given you the tools to do that. Assume we’ve detected a cell about 40nm ahead of the aircraft that we need to analyze. The bright band or area of maximum reflectivity exists around the freezing level. This is a very dynamic area in the cell where water changes state from liquid to solid creating very high reflectivity and the release of latent heat (energy) that causes cells to build. This area also contains icing, lightning, hail, and turbulence. Going back to our water fountain example this area is the pump for our fountain. We need to determine if this cell is like our little garden fountain or the more powerful fountain in the park. For our analysis we want to look at several things. We’ve already observed the maximum reflectivity so now we want to look at how much reflectivity there is above the freezing level. We’re looking at two items. First, where the higher reflectivity ends (yellow & red) and where the wet top ends (green). The higher these areas extend above the freezing level the more dangerous they are. Our tools are our outside air temperature gauge, so we know the approximate freezing level (-2⁰ C/1,000’), and our rule of thumb that 1-degree at 40nm = 4,000 feet.

Figure 6. Cruise Ground Park Technique

For this exercise we’ll use a 12-inch, 8-degree beamwidth antenna. The procedure works the same for all antennas, but we need to keep the beamwidth difference in mind at the beginning. In this example the aircraft is in level cruise flight at 38,000 feet. The first step is to bring the tilt up one-half of the beamwidth, or 4-degrees for the 12-inch antenna. This puts the bottom of the beam parallel to the aircraft’s flight path. For an 18-inch antenna we would raise the beam about 3-degrees. 

Figure 7. Antenna Raised by ½ Beamwidth

Next, we’re going to raise the beam until the wet tops disappear. Here our flashlight analogy breaks down some. Radar is designed to reflect off water droplets of sufficient size and quantity. Wet hail and wet snow reflect energy extremely well because they look like large water droplets to the radar. Frozen particles like ice crystals, dry hail and dry snow reflect poorly. Dry hail for example, only reflects about 3% of the energy that a raindrop does. The wet tops may appear as light green because of the very low reflectivity. The good news is that if you are flying at 38,000 feet and raise the beam 4-degrees and there are still returns present your analysis is over. The radar is telling you that this cell had enough energy to pump moisture over 38,000 feet into the atmosphere and is dangerous.

Figure 8. Measuring the Wet Tops of Cells

We’ll continue our example by raising the beam until the wet top disappears. At +6-degrees the returns just disappear. We started our analysis at +4-degrees and the returns disappeared at +6-degrees. So, we raised the beam a total of 2 degrees which at 40nm is 8,000 feet. Since we’re flying at 38,000 feet the wet top of this cell extends to 46,000 feet and is dangerous. This analysis was very quantitative, but the simple version is that if you tilt up and something is there then the cell is above you. If you operate a radar that can increase gain you would use it to make the less reflective ice crystals more visible and get a better measurement. The term wet top refers to the reflective top that the radar can see. Keep in mind there is a dry top consisting of all ice crystals that the radar can’t see that extends higher and contains updrafts.

Sometimes looking at radar returns on the display can be confusing but here is where radar shadows (attenuation) and our tilt control can help. As mentioned earlier mountains, buildings and cities all reflect a lot of energy and it can be hard to tell these returns from weather returns. The image below shows a mountain in the lower right-hand part of the display with very strong returns and a radar shadow behind the mountain. On the left side of the display are returns from rough terrain and cities. Notice that the returns have very sharp edges but no shadows. Weather normally has more rounded edges. But it doesn’t matter if the shadow is from mountains or a storm cell – if you can’t see through it, don’t try to fly through it. An easy way to tell that these returns are not from weather is by raising the tilt. Storms are much taller than most mountains, terrain, and cities. So, if you raise the tilt a little and they disappear they are likely not a storm cell.  

Figure 9. Ground Returns

The image below shows approximate antenna sizes with their weather and ground beamwidths. These vary slightly among manufacturers because of the type of antenna and the fact that they are not always perfectly round to fit the swept volume of the radome. The chart also shows the size of the beam at different ranges. I often hear pilots say I was flying the XXXX, and its radar isn’t as good as the one on the YYYY. If the same radar is paired with different size antennas it significantly impacts performance. Think of a 45,000-foot storm cell. If the same radar is installed on three different aircraft with different antenna sizes think of what this does to our little pulse of energy. For a 30” antenna the beam is 45,000 feet tall at 140nm, so all our energy gets reflected back to the radar. For an 18-inch antenna the beam is about 45,000 feet at 80nm. Beyond that the energy is going around and above the cell. Our 10-inch antenna is 40,000 feet at only 40nm and at 320nm our little pulse of energy is spread over an area 320,000 feet tall and wide. 

Figure 10. Approximate Antenna Sizes

Hopefully this has helped you understand the radar’s beamwidth and reflectivity. A wonderful, free tool that can help you visualize the weather beamwidth, ground beamwidth, and the area of maximum reflectivity can be found at https://pohperformance.com/radar.html. It’s available as a web-based tool and as iOS and Android apps so you can take it with you in the cockpit. In the image shown we’re at 38,000 feet with a 12-inch antenna and 40nm range selection. With the tilt set at -7.25 degrees the vertical view shows where the center of the beam intercepts the bright band. You can also see where the weather beamwidth (dark gray) and ground beamwidth (light gray) intercept the ground. The tool also accounts for the earth’s curvature which is more noticeable on longer range selections. The bottom plan view shows where the beam intersects the ground. As discussed previously, the amount of ground return that you see on your display will depend on the reflectivity characteristics of the surface. 

Figure 11. Radar Visualization Tool

Next time we’ll look at how some of the features and functions of the radar can aid you in detection and analysis. 

Test Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached via email at stephen.hammack@Honeywell.com.