Q&A: How Weather Balloons Work

Question: OK, so with all these “weather balloons” in the news, I should probably know they work. I mean one of those things flew over my state. I played with helium balloons as a kid and thought they were pretty cool, but I never did understand why they floated. Keep it simple. It’s been a long time since I did any science or math. Thanks! — CL, Dearborn, MI

Answer: You’re going to need to do some arithmetic to follow my explanation, but that’s what the calculator app is for. 🙂 The first thing to know is that objects can float in air for the same reason objects float in water: Archimedes’ Principle: An object immersed in a fluid (liquid or gas) will feel an upward buoyant force (FB) equal to the weight of the fluid it displaces. Let’s apply that idea to the weather balloon in the main graphic.

First let’s clarify what “displaces” means. For a boat floating in water, part of that boat is underwater. The volume of that part (in cubic meters) equals the volume water that has been “displaced” or “pushed out of the way”. The weight of that water will equal the buoyant force.

In the case of a balloon floating in air, it’s entire volume equals the volume of air displaced since it is totally immersed in the fluid. So now we can do the balloon calculations …

If the balloon is 1.0 m in diameter (typical weather balloons are 1–2 m) at ground level) then it could contain V = 4πR3/3 = 0.52 m3 of gas. Let’s use the lightest possible gas hydrogen. Hydrogen has a density 0.08375 kg/m3 so 0.52 m3 of it will have a mass of 0.044 kg. In Earth’s gravity that much mass will weigh 0.43 N (about 1.5 oz). We’ll ignore the weight of the payload and balloon itself for now.

But that balloon has displaced 0.52 m3 of air. Air at sea level has a density of 1.29 kg/m3 so 0.52 m3 of it would have a mass of 0.67 kg and a weight of 6.6 N (about 1.5 lb). That 1.5 lb is the buoyant force acting to push the balloon upward.

With FB = 1.5 lb, and FG = 1.5 oz, this balloon is will float upward in the direction of the net force. In fact, it could carry a payload weighing close to 1 lb (depending on the weight of the balloon skin). That’s the basic science of balloons — all you need is a large enough FB to float the total weight.

For the greatest lift hydrogen is the best choice, but it comes with the hazard of explosive flammability. When needed for safety the inert gas helium can be used, but it’s in short supply, and costs 2.5X more than hydrogen. Plus you need more of it for a given payload since its density is 0.17 kg/m3 — twice that of hydrogen.

The only practical alternative to hydrogen and helium for balloons is hot air, as in recreational ballooning. The temperature of the heated air is typically 90–100°C above ambient temperature. So on the cool days favored by balloonists, that would be around around 105°C yielding a density of 0.93 kg/m3. That’s considerably higher than hydrogen and helium, and the reason why hot air balloons need to be so large.

Weather balloons have been in use since the late 19th century. But their first wide-spread coordinated use was driven by military demand in WWI. The photo below shows a meteorology student in the Army Signal Corps preparing to launch an early model balloon at Texas A&M in 1918:

Nowadays weather balloons are used around the globe. Just within the continental US, some 75,000 are sent up each year. Multiple launches daily are the norm for some locations. The balloons rise to around 100,000 ft (30 km), measuring atmospheric data and transmitting it back to the station every 1–2 seconds.

As they ascend they expand by a factor of 3–4 because of the decreasing atmospheric pressure. Eventually the balloon bursts and the payload (called a radiosonde) returns to Earth via parachute. NWS says around 20% of the 75,000 launched are found and returned. Here’s a photo of a recent launch at the NWS station in Gaylord, Michigan:

What’s in the payload and how it acquires data is a subject for another post. But if you’re curious, NWS has a great explanation with photos here:

https://www.weather.gov/bmx/kidscorner_weatherballoons

Suffice it to say, weather balloons provide data for our atmosphere from ground level to 100,000 feet. And they do it from 900 locations across the globe. This is the data that feeds our forecast models and helps us predict future weather. So if you ever find a radiosonde, take the time to return it. Each contains a postage-paid return bag with instructions.

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