Picture the Thames in winter with a roast beef booth, a music booth, a pub and a printer, all set up on the ice, while a coach and horses crosses the river. That is not a film set. It is how the Met Office describes a Thames frost fair in 1683, and the last one took place in 1814. This guide covers how the fairs came about, why the river no longer freezes like that, and what real cold does to outdoor cooling plant.

The Little Ice Age and the fairs on the ice

The Met Office's National Meteorological Library and Archive covers frost fairs in its White Christmas factsheet. It puts them in the Little Ice Age, a period of cooling particularly pronounced in the North Atlantic region, conventionally defined as running from the 16th to the 19th centuries. Winters were generally more severe, and the Thames "froze on a number of occasions, sometimes for more than two months".

On five or six occasions between 1607 and 1814, the factsheet says, "the ice was thick enough to enable an entire fair to be held on the river". These frost fairs had stalls selling food and drink and a wide range of games. The sellers even kept warm by lighting fires inside their tents, all of which were pitched directly onto the ice.

If that sentence made you look twice, it is meant to. Fires, tents and ice sit together about as comfortably as a freezer and a sauna, and yet the account is clear that this is what the vendors did.

The factsheet also describes a Frost Fair image from 1683. Along with the roast beef booth, the music booth, at least one pub and a printers, activities included nine pin bowling, skating, chair sliding, sledges, and both horse and human drawn boats. Several coaches and horses can be seen crossing the ice.

The Little Ice Age gets credit for a few other things too. The factsheet mentions the science historian James Burke, who has suggested the period inspired changes in fashion, with the development of buttons and button holes to keep clothing tightly fastened. It also notes it is thought that the colder climate may have made the wood used by Antonio Stradivarius for his violins denser than wood grown in warmer periods. Both are presented as suggestions rather than settled fact.

Why the Thames has not frozen like that since 1814

The factsheet is blunt about it: "the Thames has not frozen over sufficiently to allow a Frost Fair since 1814". That is despite extremely cold winters in 1881, 1895 and 1962-3, when the upper Thames froze over.

It puts this down largely to changes in the river and its built environment. The Thames of the past was wider and shallower, without the modern stone embankments built in the Victorian era, which made it much easier for the water to freeze. Old London Bridge was demolished in 1832. The original medieval wooden bridge was supported by 19 wooden piers with relatively narrow arches between them, which slowed the flow of the water and trapped floating debris, slowing it further and allowing it to freeze. The new stone bridge has much wider arches, so the river flows more freely, and salty water, which has a lower freezing temperature, can penetrate further upstream.

So the answer to "will the Thames freeze" is not just about how cold it gets. The river itself has changed.

Cold snaps this winter, and what they do to outdoor plant

Nobody is predicting a frost fair. But the Met Office has said that, as the record El Niño approaches its winter peak, later in the season it is associated with an increased risk of cold snaps. We cover the wider picture in our piece on the El Niño winter and the 551-mile wall of snow. A cold snap on any day of the year is a different matter for equipment than a mild wet week, and these are the effects we would watch for.

Outdoor AC and heat pump units

  • Frost and defrost. When a unit heats a building, its outdoor coil runs colder than the air around it. In cold, damp weather frost builds on that coil, and the unit runs a defrost cycle to clear it. Seeing a unit steam or pause during a defrost is normal. A unit that ices up and does not clear is not.
  • Frozen condensate. The meltwater from defrosting, and condensate from indoor units, has to drain away. If it runs onto the ground under the unit or along an exposed pipe, it can refreeze and build up ice. A blocked condensate line can cause a fault or a leak indoors.
  • Low ambient limits. Not every unit is designed to run in cooling mode when it is very cold outside. Server rooms and equipment spaces that need cooling all year are the usual example. Check the manufacturer's stated operating range for the unit you have.
  • Keep airflow clear. Snow, leaves and stacked items round an outdoor unit restrict airflow. Clear snow carefully by hand, and do not chip ice off a coil with a tool.

Our sister-site page on using AC in winter goes through running the system in cold weather in more detail.

Cellar and cold room plant

  • Condensing units in the cold. Outdoor condensing units for cellars and cold rooms can struggle with very low ambient temperatures, because the system relies on a stable pressure to feed the evaporator correctly. Systems usually have controls to manage this, and a fault in them often shows up first in the coldest week.
  • Defrost drains. Evaporators defrost and the water has to leave the room. Drain lines that run through cold areas can freeze, so a working drain heater or insulation is worth checking before winter, not after the first ice.
  • Cold rooms in cold weather. It can seem odd to service cooling kit as winter arrives, but a cold room still has a heat load from stock, doors and lights. It still needs the plant to work.

What to do now

  1. Walk round outdoor units this week and clear leaves, debris and anything stored against them.
  2. Check where condensate and defrost water goes, and whether that route could freeze.
  3. Ask for drain heaters, controls and insulation to be checked as part of your autumn visit. Commercial refrigeration maintenance is the natural moment.
  4. Write down who to ring if a unit ices up, and what you would do with stock if a cold room went off.
  5. Keep a note of the manufacturer's operating range for each outdoor unit, so you know what it was designed for.