In the last few weeks we’ve enjoyed (or suffered, depending on your point of view) some colder interludes where the temperature has fallen below freezing. As a result, ice in its various forms has been a conspicuous companion on many of my bike rides and walks this past month.
It mightn’t have the hypnotic beauty of falling snow, but there’s something equally beguiling and enchanting about the secretive way ice appears on the ground from out of nowhere.
There are too many varieties of ice to do justice to here, so I’m taking a look at a select few of the ones I’ve encountered since winter returned.
FROST
Frost is the first kind of ice that most of us will see as we move into autumn and winter, but despite its sparkling familiarity I doubt it’s something we spend much time thinking about. After all, what is there to know? If it gets cold and the temperature outside reaches freezing, then we’ll wake up to a frost, right?
Erm…..yes. And no. It’s a bit confusing.
You might have heard the terms ‘ground frost’ and ‘air frost’. The Met Office defines the first as when ice forms on the ground after the surface temperature (measured about 5cm above the ground) falls to 0C (32F). They define the latter as when the air temperature falls to 0C at a height of at least 1m above the ground. The difference is stated because you can get a ground frost without an air frost, without the temperature of the air outside falling to freezing.
During the night, all surfaces lose some of the heat they’ve absorbed during the day. On cold, still nights with little or no insulating cloud cover and no wind to blow warmer air to the surface, that heat easily radiates upwards. Colder air stays next to the ground, and surfaces can therefore cool faster than the air above them.
Surfaces that readily lose heat in this manner, such as grass, logs and car roofs, can easily reach freezing long before the air or other surfaces do, forming a frost. This is why you sometimes see frost on only a few ground surfaces in the morning.
A thick coating of hoar frost at Glenmore Forest Park. Right: Frozen dew at Markinch station
But what exactly is frost? Where does the ice come from?
Well, the picture postcard white frost we all know and love isn’t just water that was already on the ground and then froze. Nor is it frozen dew, although the process by which dew and frost form are very similar.
Water exists in three different states on our planet: ice (solid), water (liquid) and vapour (gas). We can’t see the vapour but it is all around us in the air and, when the air becomes saturated to the point it can’t hold any more moisture, the vapour can condense into liquid water.
One of the ways the saturation point is reached is by cooling the air, and the temperature at which its vapour condenses is called the dew point. Unlike the freezing point of water, the dew point is not a fixed temperature. It changes from day to day depending on weather conditions, but it is always lower than or equal to the air temperature.
When air meets a relatively cool surface whose temperature is at or below the dew point, and whose surface is still above freezing, the vapour condenses into liquid water on that surface. Into dew. If the surface is below freezing, the vapour skips the liquid stage altogether and instantly crystalises into ice. Into frost. This process is called ‘sublimation’ and produces the white, needle-like crystal covering of ‘hoar frost’ we wake up to in the mornings.
If dew forms, only to freeze later in the night, you still get icy surfaces in the morning but it’s not strictly a hoar frost because it has been through the liquid stage. Some people call this a ‘silver’ or a ‘white’ frost, although others use the latter of those two to describe a hoar frost….which in this country we tend to just refer to as ‘frost’.
Like I said, it’s confusing!
SURFACE HOAR
If there is snow on the ground and we get a spell of prolonged cold, calm and clear conditions, we might get to see one of the most beautiful ice formations.
When there is plenty of water vapour in the air, either from the air itself or more likely from snow evaporating from deep inside the snow pack, large feathery ice crystals can form on the surface of very cold snow. Each can be more than a centimetre in length, roughly resembling reptile scales or guitar plectrums in shape.
These wafer thin crystals reflect light off their large surfaces, and on a sunny day the whole snowy landscape sparkles with thousands of bright pinpricks of light. Because they are not well bonded to the surface they are also easily dislodged, and the thick accumulations scatter into the air as you plough through the snow, creating a sparkling cloud around your feet.
Top: surface hoar in Glen Affric during the cold winter of 2008/09. Bottom: sparkling surface hoar in the Cairngorms – one of the most beautiful sights in winter.
In this country, where we commonly use the word ‘frost’ to describe any frosty white coating we find outside of a morning, the term ‘hoar frost’ is often reserved for when folk encounter this particular phenomenon. That’s technically correct as it IS a hoar frost, formed by sublimation, but to avalanche forecasters and backcountry skiers / boarders etc, whose interest lies in the stability of the snowpack, its special characteristics warrant its own frosty classification. Surface hoar.
Surface hoar can be one of the most beautiful sights of the Scottish winter, but given our predominantly wild and mild weather it’s sadly quite rare here. I saw some modest growth of it on Ben Venue last month, but nothing approaching the beautiful scenes many of us experienced during those cold winters between 2008 and 2010.
On the other hand, it is perhaps fortunate that surface hoar is relatively uncommon, because its benign beauty can be potentially lethal if it is buried under new snow.
Though delicate in appearance, the crystals are surprisingly resilient if they get covered before they can melt or be blown away. Standing upright beneath the new snow, they do not bond well with either the old or the new snow in the snowpack and therefore create a very thin layer in between the two. A slippy layer that is filled with air pockets and is susceptible to collapse.
These instabilities can endure for weeks after surface hoar is buried, and are the cause of many avalanches around the world.
RIME
While it might be frosty at the car park or in the glen as you start your walk, once you get higher up into the rocky landscapes of the high hills, rime is probably the icy phenomenon you’re most likely to encounter in Scotland.
It is one of the defining features of the Scottish winter, not least for climbers, and
is most commonly seen plastering the sides of vertical surfaces such as rocks, crags, fences, aerials, cairns and, perhaps most conspicuously of all for hillwalkers, trig points.
Clockwise from top left: rime on a boulder on Ben Cruachan; a rime-encrusted wire fence on Culter Fell; Ben Klibreck’s trig point doubling in size; rime in the Trossachs after freezing fog.
Rime can resemble frost or snow depending on its extent and thickness, but it is neither. Unlike frost, which is formed when water vapour strikes a freezing surface, rime forms when very cold ‘supercooled’ water droplets, already condensed in the air as cloud, strike a freezing surface.
‘Supercooled’ means the tiny droplets are below freezing but still in liquid form. Incredibly, water can exist as a liquid down to around -40C provided there are no impurities in it. But once a supercooled water droplet comes into contact with an impurity such as dust or an ice particle, or indeed a surface, it freezes instantly. The next droplet hits that frozen droplet and freezes to it. As does the next, and the next.
Because it is windy at altitude and the hills are often cloaked in freezing cloud for days on end, rime accumulates very quickly on surfaces that point INTO the wind. The result is a landscape transformed.
In its most modest forms it has the appearance of small lumps or feathery spikes, or peculiar accumulations of ice on just one side of a stem of grass. But in its most wonderful excesses when it can grow inches or even feet in thickness, especially if the conditions persist and the wind direction remains constant, it looks like huge solid fingers of ice protruding from pretty much everything around you.
Rime isn’t a phenomenon confined to the high hills, however, as it can form wherever you get supercooled water droplets striking cold objects. At lower altitudes freezing fog is a source of rime, but because freezing fog forms in quite calm conditions, the coating of rime tends to be thinner and more evenly distributed than up in the hills, and is therefore often mistaken for frost.
HAIR ICE
I’ve saved what I think is the strangest and most exquisite ice formation until last, and it’s something I saw twice last month during a run of subzero days.
The first encounter was in Beecraigs Country Park in West Lothian when it caught my eye at the side of a path. A few days I later saw it again in the Trossachs, just off the woodland path I was using to descend from Ben Venue.
At a glance it looks like soggy white tissue or perhaps a fungus hanging off a branch, so you could be forgiven for ignoring it. But it’s not until you’re inches away from it that you see the white mass is made from countless tiny hairs protruding from the wood’s surface. And it isn’t until you touch it with a warm un-gloved finger, and it instantly melts, that you realise it is ice.
Hair ice, found on dead wood in West Lothian. From a distance it looks like tissue or fungus and you could easily miss it.
This is ‘hair ice’, a beautiful and exceedingly delicate structure that forms when water within the dead wood is forced out of tiny pores in its surface. When the water reaches the freezing cold exterior it freezes instantly. The next droplet to be forced out freezes to the first and pushes the one before it outwards. As does the next, and the next, slowly forming a hair of ice that grows outwards from the wood.
Intriguingly, the consensus in scientific circles appears to be that fungi are responsible for this lovely phenomenon. Fungi living inside dead wood work at digesting it, causing it to rot and decompose. The decomposition process produces carbon dioxide as a waste product, which increases pressure inside the wood and pushes moisture out of the tiny pores.
It’s a different process to that of ‘frost flowers’, which can sometimes appear on the stems of plants. In those instances, freezing temperatures cause sap inside the plant to expand and split the stem along its length. If water is still being drawn up through the stem it reaches the split, oozes out and freezes on contact with the exterior. The results are petal or ribbon-shaped protrusions of ice that resemble flowers, hence the name.
Both hair ice and frost flowers are rare, delicate and short-lived sights, and encountering one is a real treat on a cold winter’s day.