‘Any way the wind blows’ – if only that was true in Copenhagen

DMI’s finest sets the record straight on why cyclists always seem to be struggling into a force 12 hurricane at this time of year

As far as complaints go in Copenhagen, it’s right up there with “You’re killing the hygge”.

Why, oh why, oh why does it feel like we’re cycling into the wind on our way to work, and then cycling into the wind on her way back?

Blowing in the wind
Well, we’ve had enough at the wind-swept CPH POST offices, which are located just around the corner from the always blustery Copenhagen Lakes.

Given that September and October are the windiest months in Denmark – although the storm season tends to come a little later in the year – we thought there was no time like the present to consult an expert on all matters wind.

DMI’s finest
Henrik Vedel is a senior scientist at the Danmarks Meteorologiske Institute (DMI), the national weather forecaster, who describes himself as “maybe not the best wind expert, but the most fanatical biker at DMI”.

He commutes by bike every day, travelling over 20 km, and he even has a small computer attached to the bike for scientific purposes.

He has to be the man with the answer to that eternal question.

Henrik Vedel

Are we always cycling into the wind, or is it just a false impression?
No. We do not always cycle into the wind. But it’s not totally a false impression. It is fairly easy to feel like that because most people will have ‘something’ against them. It’s because we bike faster than the typical inland wind speed in Denmark.

Commuters bike on average at 20 km/h and the wind blows at 18km/h, 1.5 metres above the ground. Therefore, more often than not you face a headwind rather than a tailwind, resulting in wind resistance instead of a push. So if you bike at 25km/h and the relative wind is 4m/s (14.4 km/h), you are faster, so the wind is against you in both directions.

Is the wind resistance always dominant?
There are different factors that interrupt biking, depending on your speed. If the bike speed is fast enough, the bike is mainly affected by the wind resistance, but if the bike goes slow, there would be other resistances, for example, the rolling resistance of the wheels. In fact, the wind resistance increases in correspondence to the square of the along-track relative wind speed. So if you double the wind speed, the resistance becomes four times bigger. This means that even small changes may cause a considerable impact on the power needed to maintain a certain bike speed. For instance, a very modest change from 20 to 22 km/h leads to 21 percent more wind resistance – but of course this doesn’t necessarily mean you need to make 21 percent more effort.

Does the wind direction change twice a day exactly when people leave home and get back?
The answer is again: No. As we move faster than the wind, we may feel it’s coming into our face, but that can happen no matter which direction the relative wind is blowing – except the crosswind that comes from the side. In fact, the crosswind does not cause wind resistance nor slow you down. But it can be hard for you to keep your balance.

If not twice a day, how often does the wind direction change?
The wind direction usually changes gradually, and the same direction may remain for several days in a row. Although moderate changes are often seen, large changes from east to west or north to south in just one day are uncommon.

It often makes me sweat a lot to bike in wintertime, so I often wear a baggy sweater to both keep the heat and ventilate. What outfit would you recommend?
It’s always recommended to be as slim and smooth as possible. The smaller your area against the wind the less resistance is. The clothes should be tight as well, since loose outfits can easily increase the resistance. It’s better to take off your jacket than having it open. Big sweaters that let in air are the worst choice!

Does that mean that it is easier to cycle if you are small, and that bigger people have more difficulties?
In a controlled condition, the answer is yes, because smaller people experience less wind resistance. But in general they are also less strong. In typical flat-land situations, larger and stronger bikers will have a benefit against the wind.

I commute to Central Copenhagen by bike and it’s less windy as I get surrounded by the buildings. Do buildings in Copenhagen somehow lessen the impact of the wind?
In general, obstacles reduce wind speed. For example, it’s lower in the forest than in a field, and more in the sea than on land. This is because of the so-called ‘roughness length’. In general, buildings also reduce wind speed. However, depending on the wind direction, large buildings can locally redirect the wind, either creating a lee, or speeding up the wind.

If obstacles reduce the wind speed, does that explain why we feel more wind in this flat country?
Yes. Since Denmark has a lot of open fields and coasts, we feel the effect of the wind more. We have many fields with relatively low vegetation like wheat. We also have a lot of coasts: if you are at a coast where you have the wind coming in from the sea you will always have more wind. In western Jutland this effect is very strong because the westerly wind is dominant.

I find it easier to bike in the city. Why is that?
It may be because the other commuters protect you from the strong wind resistance. When the relative wind speed is fast enough that the wind resistance is dominant, having other commuters as a lee helps you a lot with the amplified resistance. This is why racing bikes always go in line. The faster the relative wind is, the better the effect is – hide behind the other commuters when the wind is too strong!

Why is the Danish wind so gusty?
It’s quite different from day to day. Some days can be gustier than other days, depending on the weather fronts. On the days when the warm front passes, the wind is relatively stable. And the cold front comes afterwards, when you typically have showers and the wind becomes gustier. It’s more irritating to bike because you need to adjust to the force. And if it’s in town, gusts in different directions can make it more difficult for you to keep your balance. When the wind passes fast over a building it creates turbulence, and sometimes this turbulence can bring down air parcels from aloft where the wind speed is higher, resulting in a strong wind gust.

On a windy day, but not during a storm, our editor’s cargo bike got caught by a gust right next to Copenhagen’s Lakes and ended up being flipped over.
What do you think happened?
It could be an example of turbulence bringing down a very fast-moving air parcel from aloft – more quickly than he could react to. Or possibly also a redirection of the wind by a building.

Sometimes I’m scared to bike if it’s too windy. Has the wind ever caused any serious accidents in Denmark?
In the old days, there were of course some accidents like deaths from floods or blackouts due to fallen trees bringing down power lines. Nowadays, storms and hurricanes in general rarely cause casualties. Also, the authorities take care of it. An important part of the work of the Danish preparedness authority, Beredskabet, is to warn you and provide evacuation, if necessary, should the sea level rise above what the dikes can withstand. However, using hand-held smartphones while cycling is more dangerous than the wind. In contrast to the wind, we can easily do something about it. Please do so!

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