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Do atmospheric conditions affect extraction time in your espresso?

Espresso blend beans
Have you noticed how changes of atmospheric conditions along the day affect extraction time in your espresso? April and May are months typically chilly in early mornings but after middays temperatures become warmer...
In our experience at markets and at our roastery the difference in extraction time could change in between 2-4 seconds faster, or the opposite if we go from warm to colder temperatures. So, we have to adjust the grinder to produce a finer ground in order to respect the espresso golden rule (18g x 36ml x 30sec). Although at markets we are exposed directly to sudden changes of humidity, this also apply to your grinder at home, although in less harsh variations.
If you have excellent coffee beans (Coffee Gems beans, of course), an excellent machine, and you are a skilled barista, but do not have the coffee grinder adjusted for the day's conditions, the drinks produced can be of poor quality: either sourish, or bitter and harsh. In fact, most of the time that you get an espresso that is of poor quality, the reason is that the grind was wrong for the humidity conditions at the time. No amount of tasty syrup or fancy latte art can hide the poor quality of espresso that was badly extracted because the grind was off.

Part of the daily routine before brewing the first shot should be to check the calibration of the grinder. Unfortunately in our experience, many espresso coffee lovers don't bother to do it. All you need is a kitchen timer and a scale.

Optimal Extraction Times

What Type of Coffee Is Best for Espresso?

The optimal double espresso is 30 seconds for 36ml espresso by using double portafilter. Since this point you should play around with 1-2 seconds above or below this mark to find your perfect shot. 

If the extraction takes longer than this, coffee will be under extracted and therefore it will have not enough flavour and your espresso will be sourish. This is because the grind is too coarse, the grounds will not pack tightly together, and the water will pass through them too quickly. The extraction will be too fast, and espressos will be weak. You must make the grind finer.

If the extractions take longer than 30 seconds, they will be bitter. When the grind is too fine, the grounds will pack too tightly together, and it will be harder for the water to penetrate the coffee puck. This will result in an extraction that is too slow, hence bitter drinks. You must make the grind coarser.

Adjusting for Weather

What Type of Coffee Is Best for Espresso?

Coffee is a very sensitive and delicate product so any variation in atmospheric conditions will impact its density. This is one of the reasons that you should empty the chamber of your grinder every night, and start with fresh coffee beans in the morning. On a humid day, or on a day that becomes humid, the coffee beans will absorb water from the air more quickly than on a dry day.

If you do an extraction with coffee that had been exposed to humidity, the water from the espresso machine will not penetrate it as quickly. This will produce a slower extraction than normal. Essentially, as a day grows humid, the effect on extraction is the same as if the grind got finer. Humidity will slow the extraction down. You have to compensate by making the grind a little coarser.

Equally, on warmer days (or changes from chilly to hot days as in Spring or Summer), the espresso will tend to extract slightly faster. The effect of dry air is the same as if your grind were coarser. You will need to compensate for dry air by making your grind a little finer.

You should calibrate the grind first thing in the morning and if the weather changes during the day, or if you think that conditions have changed, you should time a shot to see whether you have to adjust the grind. Experienced espresso makers adjust the grinder very quickly to  put it in the right timing...