Roasting in changing weather conditions

It is well known that the weather makes a difference to the roasting process. The change of seasons. A clear sunny day versus rainy weather.


At first you should think that the temperature was the only factor. But  more factors play a role – like humidity.

And each factor may influence different parts of the roasting proces. So it’s complex. And it easily gets confusing. It also depends on the type of roaster and ventilation.

I will not claim that I got hold of all factors affecting your roasting. But here I can mention some of the factors:


At first you should think that the temperature was the only factor. If the surroundings are colder and the air sucked into the roaster during it takes more energy to heat up your beans

Also, what temperature do your beans have before roasting ? Does your bean storage change in temperature ?

Bean storage temperature
If your beans are 25°C and you heat them up to 200°C in 8-9 minutes, that is an average of 20°C per minute. If your beans instead is only 5°C, then there is a 20°C more “road” to heat up till 200°C – so that will take an extra minute to reach.

But temperature is not the only factor – the humidity also affects the roast.



The roaster sucks in air from the outside during the roast. Even though you are roasting indoors the humidity / H2O content in the room is significantly affected by the weather outside.

The level of humidity in the air can work in two ways:

(1) If the air is already heated humid air will transfer heat quicker than dry air. Water molecules transfer heat more efficiently than other air molecules, which are mainly Nitrogen and Oxygen.

So, if you have any airflow in your roaster: Air with higher water content will transfer heat quicker.

Have you tried being in a sauna compared with a steam bath (also called Turkish Hamam) ? The steam bath is about 40°C, but the sauna is like 80-90°C. That’s because the heat in humid steam bath air will hit you much more efficiently than the dry sauna air.

(2) The roaster sucks in air from the outside during the roast. If the air is not heated up before entering the roasting chamber the humidity will compete with the beans for the heat. So; dry air will make a quicker roast than humid air.

It might also be a factor that drier air gives quicker drying of the beans because it removes water quicker from the beans.


Relative and Absolute humidity

But the air on a rainy day in summer doesn’t have the same water content as a rainy day in the winter.

Lets compare a humid day at 1°C and at 25°C.

Moisture meter shThe humidity is measured with a moisture meter also called a hygrometer. But it will only give the relative humidity.

Relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature.


As you see on the graph below; warmer air can hold more water than colder. The x-axis is the temperature in Celsius. The y-axis on the left is grams of water per cubic meters (m3). The lines are levels of relative humidity.

Rel vs abs humindity

When outside air is sucked into the roaster we need to know the absolute amount of water per volume air.

Lets look at high humidity, 100% (rainy weather):
At 25°C the air holds 23 grams water per m3
At 1°C the air only holds 5 grams per m3

So if its 25°C outside and 100% humidity, there will be 4,5 times more water in the air sucked into the roaster – compared to 100% at 1°C.

As you can see on the graph, the difference in water content is much bigger at 25°C than around freezing point. So, in warm weather changes in humidity makes a bigger difference: At 25°C a dry day with 30% humidity has 7 grams water … compared to a humid day at 90% has 21 grams of water.

You can see this on a cold glass of water. On a warm humid day quickly water will condense on the outside of the glass. Thats because the warm air contains a lot of water. But right by the cold glass, it gets cooled down and the reach “dew point” – and condense from water vapor into fluid water.

Dry or humid climate
From the graph you can also see that this has biggest impact at higher humidity. Here in Denmark we got high humidity (often above 80%). But if you live in a low humidity climate (like in the desert), look at the 10% curve on graph: the water content doesn’t change much at different temperatures.


Calculate your absolute humidity
If you want to observe this in relation with your roasting – you can either have a moisture meter in the room, close together with a thermometer … Or get data from an official weather station nearby.

And then calculate the absolute humidity – like with this one
It also takes in Barometric pressure, but it doesn’t have a big influence.

Note: If you use your own moisture meter – good calibration is needed. Moisture meters can easily be 15% wrong.


PRactical experience

How big the influence is depend on how much airflow you use. And remember the coffee beans also loose water during roasting; going from a water content of around 10% to around 2 %.

Experiencing HIGHER humidity making heating during the roasting process more efficient:

Roaster Morten Riiskjær roast 150 tons of coffee a year, both on a 12 kilo Probat roaster using gas – and on a 60 kilo automatic roaster (listen to him roasting by smell in the podcast from december 2016).
He says: “When the weather is both warm and humid – you really have to watch out because the roast quickly runs out of hand”. So he uses lower charge temperature and lower heat around First Crack.

Experiencing LOWER humidity making heating during the roasting process more efficient:

Bob Werby had been roasting a lot on his 1 kilo Bullet R1 for a year – when he changed the humidity levels in his house. Going from 60-65% rel humidity to under 50% using dehumidification Equipment.

This made the roasts quicker. With same settings and batch size he reached first crack at around 7:15 minutes at 47% humidity – compared to an average of 8:50 min to first crack with humidity levels above 60%.


If you would like to explore this in details join the humidity project

Atmospheric Pressure

A roaster told me about this impact on the roasting process:

A fan typically controls the airflow in a coffee roaster. The fan has speed settings. However, the amount of air that the fan moves does not only depend on the fan speed setting, but also depend on the backpressure from the rest of the flow path. So if you have a chaff filter or collector depleted with chaffs it gives more resistance to the fan and the airflow is lower.

And … if the outside air is denser (higher barometric pressure, higher water content) it gives more resistance to the fan and the airflow is lower.

I suppose how much the weather affects the airflow in your roaster  depends on extent of airflow: If you roast with a low airflow it wont affect as much.


More factors

In general, lower water content in the air gives quicker drying … This should give a faster roast … But the question is if it has any significant importance ?

And roasters living in high altitudes say that affects the roasting too.

Somebody said that the gas burner is affected by weather conditions.

…. So many things influence the roasting process