Weather Whys: How does the green screen work?

Weather Whys

The green screen is a tool that meteorologists use daily to show you the forecast at home, it is also used throughout the TV, movie and video production industry. Although it is widely used, many people don’t understand how it works. One of the questions I get most often while shooting a Facebook live or sharing photos in-studio is “why isn’t the wall changing behind you?” or “why can’t I see the forecast on the wall?” — basically some variation of “how does the green screen work?” So I thought, why not explain it?

Let’s start with the basics. The green screen is exactly that — a green screen of fabric or wall that has been painted green. Depending on the studio this can also extend onto the floor but at FOX21 ours is just the wall. Here’s a picture of what it looks like all the time, even when I am using it for the forecast. The wall is not the technical part of the operation. It is just a wall, nothing fancy. The technical term for it is a “chroma key.”

The green screen in the FOX21 studio.

The directors in our control room use a program to select the green color of the chroma key and replace that with our weather graphics or any other picture or video they want. We call this process “keying out.” The green screen can actually be ANY color but green is most commonly used, blue is sometimes used too. Bright green is a popular choice because it isn’t a color you would usually find on the body or in a wardrobe. Why is that important? It is important because if someone in front of the green screen wears green they will key out too! Check out the GIF below for a look at how that works. Do you see the shamrocks showing up on the cloth? The cloth is the same color as the wall so it “keys out” too and shows the same picture. This is why you’ll never see any of us wearing green at the wall. Sometimes colors or shiny fabrics interact with the chroma key and you’ll notice a small amount of “keying out” on our clothes. We do our best, but it is sometimes unavoidable.

A look at how anything that matches the chroma key will also key out. Don’t match the green screen or you will disappear into the graphics.

While I, and the rest of the FOX21 Storm Team, am doing weather at the wall I use the monitors on either side to see what YOU see at home. Since the wall never physically changes, it is important that I have these monitors as a reference so I know what I am pointing at and what graphics I am showing. The camera that films us at the wall also changes from a teleprompter that the anchors read to the same picture as the two monitors so we have three places to look in the studio (this also means that we are not reading anything).

I look at the monitors on either side of the wall as well as the camera in front of me to see what YOU see at home. It is how I know where I am pointing and what I am pointing at.
A closer look at the monitor at the side of the green screen.

So there you have it! The green screen is not anything fancy, it is just a wall. All the magic happens through computers behind the scenes.

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