What is 'hoarfrost' and why did it affect Edinburgh trams today?

‘Hoarfrost’ is a type of very thin ice that forms when condensation is exposed to freezing temperatures and last night Edinburgh experienced the perfect conditions for it to develop.

Edinburgh Trams were suspended due to 'hoarfrost' this morning

As a result, this morning, Edinburgh Trams saw significant disruption, with all services being suspended while they assessed why the trams were not able to draw power from the lines.

After investigating, Edinburgh Trams announced that ‘hoarfrost’ had formed on the overhead line and that was what was preventing the trams from drawing power as normal.

In response, they had “no choice” but to suspend the services as the trams could not operate safely.

‘Hoarfrost’ is a deposit of icy crystals on objects that are exposed to the free air, usually things such as leaves or blades of grass, but in this case, the trams power lines were impacted.

It forms in a similar way to dew, the difference being at which temperature each occurs.

‘Hoarfrost’ forms in damp conditions when water vapour in the air comes into contact with surfaces like that of the power lines which are already below freezing point.

The ice crystals form immediately and then the ice continues to spread as more condensation freezes.

Last night, conditions were perfect for ‘hoarfrost’ to develop.

Usually in winter, Edinburgh Trams deploy what they refer to as ‘icebreaking’ trams which remove the ice on the overhead line ready for normal service to start, but that wasn’t possible this morning due to the ‘hoarfrost’ preventing the trams from accessing the power.

The company made an agreement with Lothian Buses so that customers who had already purchased tram tickets could use them to ride the bus instead.

As day broke and temperatures began to rise, an Edinburgh Trams spokesperson said: “Weather overnight created the perfect conditions for a particular type of ice called ‘hoarfrost’ to form on the overhead line.

"This ‘hoarfrost’ prevented the trams from drawing power as normal and caused the trams to fail.

"Therefore, we had no choice but to suspend service. However, with perseverance and the help of rising temperatures, full route services are now resuming.

"We’d like to thank our customers for their patience as we know how important our services are for those who need to make essential journeys.”

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