Winter Weather: From Water Vapour to a Unique Snowflake

[Jari Sochorová]

In January, winter climatically peaks in the Northern Hemisphere. Around 22–24 January, the Northern Hemisphere mean daily air temperature (1991–2020 climatological normal) falls to about 8.6 °C, the lowest daily average of the year.

Daily surface air temperature in the Northern Hemisphere; Climate Reanalyzer

Snow is an inseparable part of winter weather. In the following sections, we take a closer look at how a snowflake forms and its “journey” through the atmosphere. To do so, we briefly explore the basics of cloud and precipitation microphysics.

Modelled snow depth, 16 January 2026, 08 UTC (ECMWF); WIndy.com

Cloud Particle Formation

Our story begins in a cloud. For a cloud to form, that is, for tiny cloud droplets or ice crystals to begin forming, the air must first cool, most commonly through ascent.

Schematic diagram of a rising and sinking air parcel; Climate Water Project

For simplicity, let us imagine an air parcel, a small volume of air that we follow as it rises. As it ascends into lower-pressure levels, it expands and therefore cools. The amount of water vapor changes little initially, so the water vapor mixing ratio remains nearly constant, whereas relative humidity increases. This is because the saturation vapor pressure decreases rapidly with temperature (as given by the Clausius–Clapeyron relation), so colder air reaches saturation with less water vapor. At a certain altitude, the air cools sufficiently to reach saturation (100%), and water vapor can begin to condense or deposit to form cloud particles. In the real atmosphere, this process is strongly facilitated by tiny aerosol particles.

Warmer air reaches saturation with more water vapor, so it can hold more water vapor than colder air.

Homogeneous nucleation, the formation of cloud droplets and ice particles directly from water vapor without aerosol particles, is extremely rare in the real atmosphere. It is an energetically demanding and statistically unlikely process that would require very high supersaturation (on the order of hundreds of percent).

The relative size of water molecules to condensation nuclei; NOAA

In practice, therefore, condensation and, in colder clouds, ice formation occur mainly on aerosol particles, which are abundant in the atmosphere and provide surfaces where cloud particles can readily form. For droplet formation, we refer to cloud condensation nuclei (CCN); for ice crystal formation, we use the term ice-nucleating particles (INP). These processes are known as heterogeneous nucleation.

A single aerosol particle can act as both a condensation nucleus and an ice nucleus. The most effective ice nuclei are typically mineral dust particles (e.g., clays and desert dust), soil particles, volcanic ash, and certain biological particles (e.g., pollen, spores, and specific bacteria). By contrast, other aerosols, such as sea salt and many organic particles, often serve as effective condensation nuclei for droplets but are less effective at initiating ice formation.

Schematic illustration of ice-nucleating particle formation from mineral dust aerosols; EGU

Formation of Ice Crystals in Clouds

Because low temperatures alone do not automatically induce the freezing of water droplets, and suitable ice nuclei are often absent, cloud droplets at temperatures below 0 °C (32 °F) are frequently supercooled and remain liquid. As long as ice nuclei or other freezing triggers are not present, such as direct contact with ice, they can persist in the liquid phase well below the freezing point. Particularly small and very clean droplets can remain liquid even at temperatures tens of degrees below freezing. Only at very low temperatures, typically below about −38 °C (−36 °F), do supercooled droplets freeze spontaneously even without ice nuclei (homogeneous ice nucleation).

Occurrence of supercooled liquid water droplets and ice crystals in clouds as a function of air temperature; COMET via NOAA

Current meteorological understanding suggests that, in real clouds, ice particles form primarily by freezing supercooled droplets. The most common pathways are immersion freezing and condensation freezing. In immersion freezing, an ice-nucleating particle is immersed inside a supercooled droplet; once the temperature is low enough, ice forms on the particle and the droplet freezes. In condensation freezing, an aerosol particle first acts as a cloud condensation nucleus on which a droplet forms, and then serves as an ice nucleus that triggers freezing.

Schematic illustration of how MPs (microplastics) can promote atmospheric ice nucleation (a).Experimental images of a gradually cooled MP particle, showing the immersion-freezing process (b). The scale bar in (b) is 20 µm; Philip Brahana 2024

An ice crystal can sometimes grow directly from water vapour without an intermediate liquid-water stage; this is known as deposition nucleation. However, it is relatively uncommon because it typically requires supersaturation with respect to ice and highly effective ice-nucleating particles.

Another possibility is contact freezing, in which a supercooled droplet freezes upon contact with a suitable particle; its importance depends on the frequency of such contacts within the cloud.

Finally, there is homogeneous freezing, which we described above. This process typically occurs in very cold clouds.

Ice nucleation occurs through four mechanisms responsible for forming primary ice crystals in the atmosphere; Wikipedia

From Cloud Particles to Precipitation

For cloud particles to become precipitation particles, i.e., droplets or ice crystals that fall out of a cloud, they must first grow sufficiently. Only when their fall speed, determined by the balance between gravity and air resistance, exceeds the speed of the updrafts do they begin to precipitate. Precipitation particles are typically considered to have radii greater than about 0.1 mm.

However, the available supply of water vapor is not sufficient for all cloud particles to grow to precipitation size, because a liter of cloudy air can contain hundreds of thousands to millions of droplets and ice crystals. Only a fraction of them grow large enough to become precipitating particles, often at the expense of the others.

The primary mechanism by which small cloud particles grow is the diffusion of water vapor. In supersaturated air, water vapor deposits on their surfaces, causing the particles to grow. By contrast, in an unsaturated environment, water evaporates from their surfaces (or sublimates from ice), and the particles shrink and may even disappear entirely.

An illustrative example of the Bergeron–Findeisen process is a fallstreak hole (cavum). It forms when ice crystals develop locally within a thin cloud layer composed of supercooled water droplets, for example, after an aircraft passes through the cloud; NOAA

In clouds where ice crystals and supercooled droplets coexist, the Bergeron–Findeisen process takes place (a diffusion-driven growth process). Because the saturation vapor pressure over ice is lower than over liquid water at the same temperature, the air can be supersaturated with respect to ice while remaining unsaturated with respect to liquid water. As a result, ice crystals grow, while the supercooled droplets gradually evaporate.

Once ice crystals grow large enough for their fall speed to exceed the speed of the updrafts, they begin to fall out of the cloud. During their descent, they can clump together (aggregation) and collide with supercooled droplets, which freeze onto them almost instantly. The crystals then become coated with a layer of frozen droplets, a process known as riming, which can obscure their original shape. Some ice particles can also fragment, increasing their number within the cloud.

Sounding examples: rain, a sufficiently deep warm layer near the surface (left), and snowfall, with temperatures below freezing throughout the profile (right); Windy.com

If the air remains below the freezing point throughout its fall, the precipitation reaches the ground as snow; if it passes through a warmer layer above 0 °C (32 °F), it begins to melt and may fall as rain.

The Remarkable Variety of Snowflakes

A snowflake can consist of a single ice crystal or, more often, an aggregate of ice crystals that forms within a cloud and falls to the ground, where it accumulates as snow. Although snowflakes form through broadly similar processes, which we have largely described above, each one is unique—no two snowflakes are exactly alike.

Because the crystal lattice of ice is hexagonal, ice crystals naturally grow with sixfold symmetry, typically as plates or columns. In plates, growth tends to be strongest at the six corners, while in columns it is strongest along the six edges.

The morphology diagram shows how snow-crystal growth depends on temperature and water-vapour supersaturation; the water saturation line indicates liquid-water supersaturation relative to ice (a). Classic thin, flat stellar-dendrite snowflakes are typically photographed within a narrow range around −15 °C, while slender columns and needles form mainly near −5 °C (b); Kenneth G. Libbrecht 2017

What an ice crystal looks like depends mainly on the temperature and humidity in the cloud: sometimes delicate, highly branched dendrites dominate, while at other times plates, columns, needles, or grains are more common.

As a crystal grows, water vapor deposits onto it, and the crystal retains its hexagonal structure. During its fall, other ice crystals often attach to it, or supercooled droplets freeze onto it. Once a crystal or aggregate becomes visible to the naked eye, it is called a snowflake and can continue to grow. The average diameter of snowflakes is around 5 mm (≈0.2 in), and the largest documented size is reported to be 38 cm (≈15 in).

Close-up of a real snowflake; Photo: Jaroslav Fous

Cloud Types That Most Often Produce Snow

In temperate and subpolar latitudes (and, more rarely, even in the subtropics), snowfall most often comes from mixed-phase clouds containing both supercooled droplets and ice crystals, where snow formation is highly efficient due to the Bergeron–Findeisen process and subsequent aggregation. This is particularly true in the dendritic growth zone, roughly between −12 and −18 °C (about 10 to 0 °F), where crystals grow rapidly and readily aggregate into large flakes.

Cloud type: Significant snowfall can be produced by Ns, Cu con, Cb, and occasionally Sc; Valentin de Bruyn via Wikimedia Commons

Snow most often falls from the following cloud types:

• Nimbostratus (Ns) most commonly produces long-lasting, widespread snowfall. It occurs mainly along warm and occluded fronts and within the broad precipitation area of mid-latitude lows, including the wrap-around region (the “comma head”).

• Stratus (St) and stratocumulus (Sc) usually bring only light snow or brief snow showers, often referred to as low-cloud snowfall.

• Altostratus (As) is often a sign of an approaching warm front. Altostratus itself may produce light snow, but it often gradually transitions into nimbostratus, with precipitation intensifying and typically lasting longer.

• Cumulus congestus produces snow showers. It occurs mainly along cold fronts or in the cold air behind them. During lake-effect snow, it can form long bands (“cloud streets”), often together with stratocumulus (Sc).

• Cumulonimbus (Cb) is associated with very intense snow showers and, in rare cases, thundersnow, which can occur along strong cold fronts.

A striking lunar halo formed by diamond-dust ice crystals; Photo: Jaroslav Fous

Snow can also fall from ice clouds, but less often and usually less intensely than from mixed-phase clouds. These are mainly high-level clouds composed of ice crystals, especially cirrus (Ci), cirrostratus (Cs), and cirrocumulus (Cc). From these clouds, precipitation is typically limited to very light snow or isolated ice crystals, often as virga or diamond dust during dry, very cold weather. Heavy snowfall is rare because ice clouds typically have low ice-water content, the air beneath them is often dry, and they lack the efficient growth and aggregation processes typical of mixed-phase clouds.

Note: In one of our upcoming articles, we will focus on snowfall-favourable situations, covering both synoptic setups and locally driven mechanisms (e.g., orographic snowfall, lake-effect snow, and industrial/anthropogenic snow).

Snowfall and Snow Cover on Windy.com

On Windy.com, you can get a quick overview of current conditions and the expected evolution of snowfall in just a few clicks, or explore a more detailed analysis.

A meteorological radar is available, clearly showing not only precipitation coverage and intensity, but also, more recently, an estimated precipitation type.

Lake-effect snow over the Great Lakes as seen on Windy.com radar (10 November 2025); Windy.com

In addition, you can browse forecasts from numerical weather models. For selected models, you can display layers such as Snow depth and New snow, which represent accumulation over a time period of your choice.

From the meteogram for a specific location, you can track how forecast snowfall accumulation evolves over time, either at an hourly resolution for premium users or in three-hour intervals.

If you want to dive deeper into snowfall forecasting, Windy also offers tools for assessing the atmospheric temperature profile. You can use both measured soundings and model-based (forecast) soundings.

Freezing altitude layer; WIndy.com

A practical feature is the display of the freezing level (0 °C isotherm), either as a standalone layer or as isolines. Because the freezing level often determines whether precipitation falls as snow or rain, it provides a quick way to estimate the snowline. If multiple freezing levels appear in the profile, Windy displays the topmost one.