Introduction:Not sure whether to bring an umbrella because the weather forecast is ambiguous? Try this little experiment originating from the UK: brew a cup of hot Americano, drop in a sugar cube without stirring, and observe where the bubbles gather — you can preliminarily determine whether the weather will clear up or rain within the next hour. This method involves the principles of coffee crema’s surface tension and atmospheric pressure changes. It’s simple to perform and offers some reference value, making it a great daily observation for pour over coffee enthusiasts.
As we enter late March, Guangdong experiences frequent overcast and rainy weather, especially around the Qingming Festival when continuous rainfall is common. Although phone weather forecasts may show rain all day, many people find that sometimes the sky clears up when they step outside, leaving them wondering whether to bring an umbrella. Such mismatches between forecasts and actual conditions often leave people feeling at a loss.
There’s an interesting and simple method circulating among the public: just brew a fresh cup of hot Americano, drop in a sugar cube but don’t stir it, and observe where the bubbles form from the dissolving cube — you can roughly predict whether the weather will clear up or continue raining. This little experiment was first documented in British writer Matthew Cole’s book "A Cup of Coffee Knows the Weather," and its principle is closely related to the layer of coffee crema on the surface and changes in atmospheric pressure.
To successfully conduct this observation, it is recommended to use freshly ground and brewed hot Americano rather than instant coffee or pour over coffee. Only freshly ground coffee forms enough crema on the surface to help bubbles accumulate and reflect air pressure differences. In addition, the choice of sugar matters — it’s recommended to use sugar cubes instead of regular granulated sugar. Due to their manufacturing process, sugar cubes retain slight gaps between crystals, which makes it easier to produce uniform, tiny bubbles as they dissolve.
The procedure is actually very simple: first, brew a cup of hot Americano. It’s recommended to use medium-fine grind with water temperature around 90–93°C, and control the brewing time to about 2–3 minutes to extract approximately 240 ml of coffee. Once the coffee cools slightly to around 70–80°C, drop in a standard sugar cube without stirring, letting it naturally sink to the bottom and gradually dissolve.
The key observation is: the bubbles released from the dissolving sugar cube will naturally gather in a certain area on the coffee’s surface. If the bubbles concentrate in the center of the cup, it usually indicates higher atmospheric pressure, suggesting a possible clearing trend within the next hour; conversely, if the bubbles distinctly gather near the edges or sides of the cup, it means the current pressure is relatively low, and rainfall or overcast weather is more likely to follow.
This phenomenon isn’t without scientific basis. The coffee’s crema forms a thin film with surface tension, and the bubbles released by the dissolving sugar float on this liquid surface. Under high-pressure conditions, air flows from the center outward, causing the air above the high-pressure zone to sink, making it easier for bubbles to gather in the central area. Under low-pressure conditions, airflow tends to concentrate in the center while the edges have even lower pressure, leading bubbles to accumulate near the cup’s rim.
In short, high pressure generally suppresses cloud formation, resulting in more stable weather and clear skies; whereas low pressure facilitates cloud and rain development, often bringing overcast or snowy conditions. Therefore, by observing the distribution of these tiny bubbles, one can preliminarily infer short-term weather change trends.
However, this method should only be used as a simple reference before heading out and cannot fully replace official weather forecasts. Especially during seasons with rapidly changing climates, such as spring-to-summer transitions, when weather systems move quickly and localized rainfall is frequent, it is advisable to use multiple prediction tools for a comprehensive judgment. Moreover, if using instant coffee or pour over coffee that hasn’t formed sufficient crema, it’s difficult to observe obvious bubble aggregation, so it’s not recommended for this experiment.
In summary, this little trick of using sugar cubes and coffee crema to observe bubble patterns not only adds fun to daily life, but also allows you to observe subtle signals of natural changes while enjoying your coffee. For pour over coffee enthusiasts, why not try this method next time before heading out? It might offer some interesting scientific insight into whether you should bring an umbrella.
The above content is compiled by CoffeeHunters, a coffee news website.
