HIGH-ALTITUDE BALLOONING 101
STEP 4 – MONITORING THE FORECAST
WEATHER FORECAST
Of all potential headaches associated with balloon launches, weather concerns rank right at the top, followed closely by payload recovery challenges (i.e., land access issues) and helium supply shortages. The primary weather concerns for balloon launches are wind, precipitation, and lightning.
An ideal launch day would feature relatively calm winds (a light breeze is fine), mostly clear skies, comfortable temperatures, and no precipitation. However, the weather in Wyoming often does not cooperate with our launch plans, especially during the windy season from November through April. Even just moderately strong winds by Wyoming standards (25 mph) can make it nearly impossible to safely inflate and launch a balloon. Rainfall during launch can cause payload items to become waterlogged, potentially creating problems with predicting the balloon’s ascent rate. Snowfall and snow cover can be problematic for travel safety and road access during payload recovery. Of course, the lightning hazard is obvious.
We begin monitoring the weather forecast for launch day at least a week prior to the event. Weather forecasts are typically not very reliable beyond about 7 days into the future. Within 3 days the forecast usually becomes quite a bit more accurate. We usually consult multiple weather sources, including the National Weather Service (NWS). In fact, the NWS produces location-specific hourly forecast graphs that are often very helpful (click here to see the one for Laramie). Sometimes we may even look directly at the actual forecast maps produced by the computer models. One important factor we consider is forecast consistency and trends. Generally, the more consistent the forecast is or stronger the trend during the week leading up to the launch, the more confidence we can have in it. As they say in the forecasting business, the trend is your friend.
When looking at forecasts for locations in Wyoming, we also consider the following:
- Very broadly speaking, high pressure systems promote fair weather and tend to produce more favorable launch conditions than low pressure systems.
- Upper-level winds over Wyoming are typically stronger in winter/spring (when the jet stream sags south over the continental U.S.) and weaker in summer/fall (when the jet stream migrates north into Canada).
- On most days, the best time to launch is in the morning when the wind near the ground is typically weakest.
- Wind speed and direction can sometimes change drastically in the first few hundred feet above the ground.
- Thunderstorms are most common in the afternoon.
FLIGHT & LANDING PREDICTION
Prior to launch, there are several online tools that can be used to generate a high-altitude balloon flight prediction. We use the SondeHub Balloon Landing Predictor that was originally developed at Cambridge University in the UK. It uses a numerical weather model to obtain a 3D wind forecast for the specific time and location of the launch. Based on this, and assuming a certain ascent rate and burst altitude, it then calculates a 3D flight prediction for the payload (both ascent and descent). This not only gives us an idea of where the payload train will land, but also helps us decide if it’s worthwhile to launch on that particular day.
The Balloon Landing Predictor updates four times each day, in sync with the latest update of the Global Forecasting System (GFS) weather forecast model every 6 hours. This predictor will only calculate flight predictions for launch times less than 180 hours (~1 week) into the future. It is relatively easy to use, requiring several user inputs such as launch latitude/longitude, launch altitude, and launch date/time, as well as the expected ascent rate, burst altitude, and descent rate. We typically start by assuming a 5 m/s ascent rate, 30,000 m burst altitude, and 7 m/s descent rate. The prediction then shows the expected flight path and landing spot on a map based on the forementioned parameters.
If the flight prediction for launch day appears unfavorable for a payload recovery, we might consider using a different ascent rate (to alter the flight distance) or a different balloon (to alter the burst altitude). In some circumstances, it may be wise to postpone the launch until a later date. Overall, we have found that the landing predictions from this website (obtained the night before the launch) are accurate to within 5 miles about 35% of the time, within 10 miles about 70% of the time, and within 15 miles more than 90% of the time.
WHAT EDUCATORS NEED TO KNOW
As we get closer to the launch, we will begin discussing the weather forecast with the teacher/educator. One week before the launch, we will set up the flight prediction simulation and the link to the teacher/educator. The simulation will update every 6 hours, so the page may need to be refreshed to see the latest prediction. The teacher/educator is more than welcome to monitor both the forecast and flight prediction with their students as well.