Science Course with Ryan Morse: Vacuum candle and air pressure

Beyond The Forecast

(WFRV) – High and low pressure play a massive role in driving our daily weather. During our full forecasts on TV, you will often see the Hs and Ls on our maps. Pressure is tough to explain to many people, but here is a quick visual you can do to help better understand it. 

Do it yourself experiment

The materials you will need for this experiment are very easy to find. They include birthday candles, an empty clear glass bottle, matches, a plate or bowl, and food-colored water.  


  1. With parental supervision, burn some wax and place the candle in the middle of the plate
  2. Fill the plate with the food-colored water
  3. Light the candle
  4. Immediately cover the candle with a glass bottle. 

What is going on? 

When the flame heats the inside of the bottle, hot air expands. In the beginning, water slowly rises.

As soon as the flames goes out, the water level rises faster as the air turns cooler and contracts. The contraction results in lower pressure inside of the glass bottle compared to outside the bottle.

A weak vacuum then forms inside the bottle because air flows from high to low pressure. This results in the colored water rising in the bottle. When pressure is equalized, the water will stop rising.

Wind directions on actual weather maps don’t not exactly flow directly from high to low pressure because of other forces at play.

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