When you're checking radar before your flight, you always see two radar options: composite reflectivity, and base reflectivity. When you look at both, they sometimes look the same, and other times they look very different.

So which one should you use? Should you pick the one that looks more favorable for your flight and just go for it? That's probably not the best way to make your decision (we wouldn't recommend it). Here's how both of them work, and how you can use them for your next flight.

The Radar Basics

As NEXRAD radar sweeps around 360 degrees looking for the next storm you need to avoid, it sends out beams in multiple elevations. The lowest elevation is typically 0.5 degrees, and it goes up from there.

How Base Reflectivity Works

When you look at a base reflectivity radar image, you're looking at the radar echoes from only one of the radar beams, most often the 0.5 degree beam (lowest tilt).

Why the 0.5 degree beam? Because it's the best representation of what's falling out of a cloud and onto the ground. So if you're walking across the ramp to preflight your plane, the base reflectivity image is going to tell you if you're going to get wet (or covered in snow) while you're doing it.

But there are some downsides to base reflectivity. While you know what's coming out of the bottom of a cloud, it doesn't necessarily tell you what's happening higher up in the atmosphere. And since storms reach well into the flight levels, there can be a lot going on aloft that you're not seeing. Like this:

That means that if you're looking at the base reflectivity to plan your flight, climbing up to 10,000 feet could give you a nasty surprise of what's happening aloft.

There's another downside. In mountainous areas, the lowest radar beams can be blocked by terrain. When that happens, there can be weather beyond the next ridge that radar doesn't show you.

And since weather can be radically different on one side of a mountain range vs. the other, it opens up the possibility of you not knowing about precipitation along your route.

How Composite Reflectivity Works

In a composite radar image, the strongest radar echo return for each elevation level is combined into one image. The image gives you more information of what's happening at all elevations of the storm.

Storms, especially ones that are still forming, have a lot of precipitation held up in the air from updrafts. And while that precip may not be falling to the ground yet, it doesn't mean that it's going to be suspended in the air forever. Remember, what comes up, must come down.

Look at the two radar images below. While the base reflectivity image shows the core of the storm, it doesn't show the surrounding area of precipitation compared to the composite image. That can mean a few things.

1) precipitation high in the atmosphere could be evaporating long before it hits the ground, which would make the composite radar image look worse than it really is. 2) it could mean there are strong updrafts holding the precipitation aloft, and that's something you probably want to steer clear of.

Which One Should You Be Using?

So which radar image should you be using to plan your next flight? There isn't a definite 'right' answer, but knowing the difference between base and composite images is the key.

Base reflectivity maps will give you the best picture of what's falling out of the bottom of a cloud, but can also make a storm look smaller than it really is. And if you're flying below the clouds, the base reflectivity will be your best picture of where the precip is falling.

Composite reflectivity is the 'worst case scenario' radar image, and it can show precipitation that's nowhere near your altitude, as well as precipitation that might evaporate and never hit the ground. But at the same time, it can show you areas of strong updrafts and precipitation that hasn't hit the ground...yet.

Steering well clear of the weather is your best option with either radar image. Compare the two, chart a wide course around the precip, and you'll keep yourself out of weather you don't want to be in.