On Saturday 24 October at 9 am, the roasting bulletin showed 7 degrees, an overcast sky and a south-west wind at 24 km/h, the morning after storm Austen, for which the Belgian weather service (IRM) had issued a yellow wind warning on Friday. On Sunday, at the same hour, it was 11 degrees and the pressure had dropped from 1013 to 1001 hPa. Laurent roasted the same six coffees on Golden on both days: Ethiopia, Vietnam, Bon Café, Colombia, Brazil and Decaf. This guide shows where weather and coffee roasting meet in Laurent's decisions, with dated examples, and what you can check at home with the bag on your shelf.
Why we roast outside, in plain sight
Meteo Koffie roasts on Saturdays and Sundays at the Marché de l'Abattoir, Anderlecht, on Golden, a gas roaster that stands in the open air of the market. Anyone buying a bag sees the drum, the flame and the person working at it. Open-air roasted coffee in Brussels means, in practice, a workshop without walls: cold, damp and wind reach the metal.
Laurent reads the air before the first batch, adjusts fire and time on Golden and holds the same taste target for each origin. The weather becomes part of the batch's identity, next to the origin, the day, the place and the hands that roasted it. We call this weatherroir. The person at the flame decides the taste; the weather supplies the working data.
How to reach the stand, the opening hours and the three-bell ritual are in the guide to the open-air roastery. Here we stay with the sky.
Three readings Laurent takes before the first batch
The bulletin on prajire.meteokoffie.com gives, for every roasting day, the temperature at 9 am, 1 pm and 5 pm, the humidity, the wind, the pressure and the rain. Three of them change the work at the flame.
Temperature: what the drum gives away
A hot drum loses heat to the air around it, and the loss grows with the gap between metal and air. At 7 degrees, like last Saturday at 9 am, a larger share of the flame goes into the market air before it reaches the beans than on a 20-degree morning. Laurent makes up for it with fire and time, in small steps.
The temperature also moves during the day. On 24 October it went from 7 degrees at 9 am to 11 degrees at 1 pm; on 25 October, from 11 to 14. Meanwhile Golden's metal stores the heat of the morning batches, so the right setting for the first batch is too much by midday.
Humidity: how long drying takes
The first phase of a roast drives the water out of the green bean. Air humidity can fall a long way in one day: on Saturday 1 August it was 85% at six in the morning and 23% in the afternoon. In damp air Laurent accepts a slightly longer drying phase and watches for two signs: the bean turns from green to yellow, and the smell of grass becomes a smell of bread. Last weekend the bulletin recorded 78% on Saturday and 66% on Sunday.
Wind: where the flame goes
Wind pulls at the flame and cools the drum's metal, so coffee roasted in the wind needs an eye on the flame for the whole batch. On 19 September the bulletin recorded a south-west wind at 22 km/h, with gusts above 40 km/h. On a day like that Laurent puts the windscreen on the windward side, turns the gas tap as soon as the flame spreads out and looks at the flame more often than at the clock. On last weekend's bulletin: south-west 24 km/h on Saturday, west 27 km/h on Sunday.
Pressure also appears on the bulletin and on every batch page. We record it as part of the batch's identity, with the other figures.
Cold, heat, rain: what changes at the flame
Roasting coffee in cold weather
The first batch on a cold morning needs the most heat. The drum starts cold, the air pulls heat from the metal and the gas bottle pushes less hard. Propane pushes at roughly 10 atmospheres around 25 degrees and roughly 5 around 1 degree; after an hour of continuous flame the bottle cools itself, because of the gas evaporating inside. You can see the sign: at the same tap position, the flame shrinks.
In the cold, Laurent lets Golden heat up longer before the first green beans go in, starts the first batch with a full bottle, out of the wind and off the cold concrete, and raises the fire at charge. With restraint: too much fire at charge marks the bean, with almost black tips (tipping) and burnt patches on the flat side (scorching). The damage happens in the first minutes and comes back in the cup as a burnt taste.
Heat and a day that warms up
In warm, dry air, drying goes faster, so Laurent starts with a lower fire at charge. When the air warms through the day, with 17 degrees at 9 am and 23 at 4 pm as at the end of summer, Laurent lets morning drying run 20 to 30 seconds longer and keeps the flame a little higher. In the afternoon Laurent cuts the flame earlier and empties the drum about 15 seconds before the morning time. Fire and time trade places; the target stays the same.
Roasting in the rain
In rain, Laurent mostly changes the order of the batches. Saturday 19 September was dry, from 17 degrees at 9 am to 23 degrees at 5 pm, with 60% humidity on the bulletin. The Ethiopia batch for filter went into the drum mid-morning; Brazil and Colombia for espresso followed after lunch.
On Sunday 20 September, it rained all morning, 6.5 mm on the bulletin, with humidity above 90% until around 10 am. Laurent reversed the order: Brazil and Colombia in the morning, in the rain, and Ethiopia after 2 pm, when the rain had stopped and humidity had come down towards 63%. For filter batches the fire is cut shortly after first crack; a gust that steals heat at that moment gets corrected on the spot, eyes on the flame.
What weather and coffee roasting mean for your cup
On every coffee's page you will find the roast date, the batch code and the air Laurent worked in. An example from the weekend: Colombia, batch MK-261025-CLM-COL, roasted on 25/10/2026, cloudy, 13 degrees, 69% humidity, 999 hPa, medium roast.
Laurent holds the same taste target from one weekend to the next. Two bags of the same origin are still two different batches: a different sack of green coffee, a different day, different air during the work at the flame. The weather line tells you where the bag comes from; the coffee roast date tells you when it is ready to drink.
That is where the weather becomes useful at home. A bag roasted on Saturday is good for filter from Wednesday and for espresso from the following Saturday, according to how long coffee rests after roasting. Every new bag, with a new date and a new weather line, is the moment to check your recipe, before you blame the grinder or the coffee.
The new-bag test, to run at home
The recipe for a new bag of coffee starts from the setting you found for the old one. The steps, in order:
- Write on the old bag with a marker: the roast date, the weather line and your recipe. For espresso: 18 g in the basket, 36 g in the cup and the seconds. For filter: 15 g of coffee to 250 g of water, the grinder setting and the total time.
- Keep the last 30 to 40 g of the old bag until the new one reaches its resting window. A bag from 24 or 25 October is good for filter from the middle of this week.
- On the same morning, brew one cup from the old bag and one from the new, same recipe, same setting.
- Compare the time first, then the taste. Change one thing, usually the grinder, by one step.
- Write the setting you landed on onto the new bag, under the weather line.
What the first cup tells you:
- The time is almost the same (2 to 3 seconds apart for espresso, 15 seconds for filter): keep the recipe. If you taste a new nuance, you have found the difference between batches.
- Espresso much slower, with very thick crema: the new bag is still too fresh. Wait two or three days before touching the grinder.
- It runs faster and tastes sour: one step finer.
- It runs slower and tastes bitter even though the bag has rested: one step coarser. If the bitterness stays, look at the signs of a grind that is too fine.
After three or four bags you have your grinder setting for each origin on paper, with the weather of each batch next to it. Then you can see for yourself how close to target Laurent kept batches from different weekends.
The bulletin, the bag and the Club
Every roasting weekend has its bulletin. The latest is the bulletin for 25 October 2026 (in French), with the hourly temperature, the wind, the pressure and the six batches of the day. Saturday is under 24 October.
If you want to blend two origins at home, start with the blending guide.
For freshly roasted coffee in Anderlecht, one bag a month, without a trip to the market, there is the Meteo Koffie Club: pick it up at the stand at the weekend or get it delivered on Monday within Belgium, with the weather report on every bag, and you can pause it. The Colombia from the example is in stock on its page, so you can run the test with a batch whose air you already know.
Saturday and Sunday, Marché de l'Abattoir, Anderlecht.
Brut. Sauvage. Imprévisible.
