by Glaciers are often imagined as frozen landscapes where precipitation arrives as snow and temperatures remain safely below freezing. But that picture is changing.
In a warming climate, rain is increasingly reaching places that once received mostly snow. In some regions, rain is also falling directly onto snow-covered glaciers and ice sheets. These rain-on-snow events can alter the structure of snow, increase surface melting, darken reflective surfaces, and send additional water into the glacier system.
Rain may seem harmless compared with a heat wave or a major summer melt event. Yet its effects on snow and ice can be surprisingly complicated.
A rainfall event can deliver heat directly to a snowpack. It can cause snow grains to become wetter, reduce the surface’s ability to reflect sunlight, create an ice crust when temperatures fall again, and increase the amount of liquid water moving through the snow. On glaciers and ice sheets, this water can sometimes reach deeper layers and influence the movement and stability of ice.
Recent research shows that rainfall over Greenland has increased substantially over recent decades, while scientists are also observing broader changes in the balance between snow and rain across the Arctic.
Understanding rain on snow is therefore becoming increasingly important for understanding how glaciers respond to climate change.
What Is Rain on Snow?
A rain-on-snow event occurs when liquid precipitation falls onto an existing layer of snow.
The rain does not necessarily melt the entire snowpack immediately. Instead, it interacts with the snow in several ways.
Rainwater can:
- Add heat to the snowpack
- Increase the liquid-water content of snow
- Cause snow to become denser
- Create ice layers or crusts after refreezing
- Accelerate snowmelt
- Carry heat downward through the snow
- Change the surface’s reflectivity
- Increase runoff
- Alter the physical structure of the glacier surface
The National Snow and Ice Data Center’s Arctic Rain on Snow Study (AROSS) specifically investigates these events because their effects extend beyond glaciers to ecosystems, wildlife, infrastructure, and Arctic communities.
For glaciers, the most important question is what happens after the rain arrives.
Why Rain Is Different From Snow
Snow and rain deliver precipitation in very different forms.
Snow generally accumulates on a glacier and can contribute to its mass. Fresh snow is also highly reflective, meaning it can send a large proportion of incoming sunlight back into the atmosphere.
Rain behaves differently.
Instead of adding frozen material to the glacier, rain arrives as liquid water. If temperatures are near or above freezing, the rain can contribute to surface melt rather than building the snowpack.
This distinction is important because glacier mass balance depends on the difference between what a glacier gains and what it loses.
The U.S. Geological Survey explains that glaciers gain mass primarily through snowfall and lose mass through processes such as snow and ice melting. When losses exceed gains, a glacier has a negative mass balance.
As temperatures rise, a greater share of precipitation can shift from snow to rain, changing this balance.
Rain Carries Heat Onto Snow and Ice
One of the simplest ways rain can influence a glacier is through heat.
Rain develops within relatively warm air masses. When liquid precipitation reaches a cold snowpack, the water can release heat as it cools or freezes.
This is one reason rain-on-snow events can cause unusually rapid changes in snow conditions.
The National Oceanic and Atmospheric Administration’s rain-on-snow modeling work shows that snowmelt during rainfall depends on several interacting energy exchanges, including precipitation, radiation, sensible and latent heat, wind, and the condition of the snowpack.
In other words, rainfall is not simply “water landing on snow.”
It is part of a larger energy exchange between the atmosphere and the frozen surface.
When the atmosphere is already warm, rainfall can become an additional source of energy that pushes a snowpack closer to melting.
Rain Can Transform the Snowpack
Snow is not a solid block of ice.
It is a complex collection of ice crystals, air spaces, and layers that change continuously.
When rain enters a snowpack, it can move through the spaces between snow grains.
This can change:
- Snow density
- Grain structure
- Liquid-water content
- Refreezing patterns
- Ice-layer formation
- Drainage pathways
If temperatures fall after the event, some of the water can freeze again.
The result may be a hard ice layer within or on top of the snow.
The NSIDC notes that rain falling on snow can refreeze and form an ice crust.
For glaciers, these structural changes matter because the snowpack is not merely a temporary cover. It can influence how energy and water move through the upper layers of ice.
Rain Can Remove the Protective Snow Cover
Fresh snow is generally bright and reflective.
This property is known as albedo.
A bright snow surface reflects much of the incoming sunlight, helping limit the amount of solar energy absorbed by the surface.
When snow disappears and darker glacier ice becomes exposed, more sunlight can be absorbed.
NOAA’s Greenland Ice Sheet monitoring explains that fresh snow increases albedo and reduces melt potential, while exposed bare ice has a lower albedo and can absorb more solar energy.
Rain can contribute to this transition.
If rainfall causes existing snow to melt or become heavily saturated, the protective snow layer may disappear sooner. Once darker ice is exposed, solar heating can increase.
This creates a potentially important feedback:
Rain → snow loss → darker surface → greater solar absorption → more melting.
Rain is therefore capable of influencing glacier melt indirectly as well as directly.
The Albedo Effect Makes the Problem More Complicated
The relationship between rain and melting becomes especially important because of albedo.
Imagine two glacier surfaces receiving the same amount of sunlight.
One is covered with fresh, bright snow.
The other consists of older, darker ice.
The darker surface generally absorbs more solar energy.
That means the condition of the snow cover can influence how much additional energy reaches the ice.
Research on glacier surfaces in the Tibetan Plateau and surrounding regions has found that declining snow and ice albedo can substantially affect glacier melt.
Rain can contribute to this process by accelerating the transition from snow-covered surfaces to exposed ice.
Greenland Is an Important Example
Greenland provides one of the clearest examples of why scientists are paying closer attention to rainfall.
The Greenland Ice Sheet is enormous, and its surface receives both snow and rain depending on location, season, and atmospheric conditions.
A 2021 study using a high-resolution climate model found that rainfall over Greenland increased between 1980 and 2019, with particularly strong increases in northwestern Greenland. The study also found substantial increases in September rainfall and rainfall intensity.
More recent research has reinforced concerns about changing precipitation phase.
A 2025 observational study using satellite radar data found that rain occurrence over Greenland is strongly sensitive to warming. The researchers estimated that rain could double with approximately 2.3°C of local near-surface warming under their analysis.
These findings matter because precipitation that once arrived as snow can increasingly arrive as rain as temperatures rise.
Rainfall Can Increase Meltwater Runoff
When rain falls on snow or ice during temperatures favorable for melting, some of that water can become part of the glacier’s runoff.
This can increase the amount of water leaving the glacier surface.
NASA documented this process during a major Greenland melt event in August 2021, when rainfall intensified an already active period of surface melting. Meltwater can also enter crevasses and vertical drainage channels called moulins, potentially transporting water deeper into the ice sheet.
Runoff is important because it represents a direct loss of water from the glacier or ice sheet.
If precipitation falls as snow and remains frozen, it can contribute to the glacier’s accumulation.
If the same precipitation arrives as rain and runs away from the surface, it does not provide the same long-term mass benefit.
Rain Can Reach Deep Into the Glacier
A glacier’s surface is not completely isolated from its interior.
When meltwater enters cracks, crevasses, and drainage channels, it can travel downward.
This creates a connection between atmospheric conditions and processes occurring deeper within the ice.
In Greenland, scientists have studied how surface meltwater can reach the base of the ice sheet and influence ice movement. Research has shown that the relationship between surface water and ice flow is complex and varies across the ice sheet.
This means that rainfall can have effects beyond surface melting.
However, it is important not to assume that every rainfall event automatically causes a large acceleration of glacier flow. The response depends on factors such as drainage systems, temperature, glacier geometry, bed conditions, and the timing and quantity of water.
Rain-on-Snow Can Create Ice Layers
Not all rain simply disappears into meltwater.
If temperatures fall after a rain event, water within the snowpack can freeze.
This can create a dense layer of ice.
Repeated cycles of rain, melting, and freezing can produce complicated layers within the snowpack.
The Arctic Rain on Snow Study has documented how these events can produce ice crusts with significant consequences for Arctic ecosystems and communities.
On a glacier, such layers can alter the physical characteristics of the snowpack.
Scientists are interested in these changes because the structure of snow affects how water, energy, and air move through the upper surface.
Rain Can Change the Timing of Melt
The timing of rainfall can be just as important as its total amount.
A rain event occurring early in the melt season may initiate processes that would otherwise happen later.
For example, rainfall can:
- Warm the snowpack.
- Increase its liquid-water content.
- Encourage melting.
- Reduce or alter surface snow.
- Expose darker ice.
- Increase solar absorption.
- Extend the period during which meltwater is produced.
This means a relatively short rainfall event can potentially influence the trajectory of melting beyond the hours during which the rain actually falls.
Rain Is Becoming More Important as Temperatures Rise
Climate warming does not simply mean “more rain everywhere.”
The relationship between precipitation and warming varies by region, altitude, season, and atmospheric circulation.
In some very cold areas, additional precipitation can still fall as snow.
But in locations close to the freezing threshold, relatively small temperature changes can strongly affect whether precipitation falls as snow or rain.
This is particularly important for glaciers located near the boundary between solid and liquid precipitation.
A 2026 study of Greenland precipitation projects continued changes in precipitation and a nonlinear increase in the fraction falling as rain, particularly along lower-elevation and southern parts of the ice sheet.
What Happens at High Elevations?
Mountain glaciers provide another important example.
At very high elevations, temperatures can remain well below freezing, meaning precipitation is still likely to fall as snow.
But lower portions of a glacier can be much warmer.
The result is a transition zone.
Near the upper glacier, precipitation may arrive as snow.
Farther down, the same storm can produce rain.
As the freezing level rises, the boundary between snow and rain can move upward.
That matters because the area receiving rain instead of snow can expand.
Over time, this can reduce the amount of fresh snow available to protect lower glacier surfaces.
Rainfall and the Glacier Mass Balance
To understand the long-term impact, it helps to return to the concept of mass balance.
A glacier gains mass primarily through snowfall and loses mass through melting, sublimation, ice discharge, and other processes.
A healthy glacier over a particular period does not necessarily need to grow, but its long-term mass balance determines whether it is gaining or losing ice.
USGS describes mass balance as the difference between accumulation and ablation.
Changing precipitation from snow to rain can influence both sides of this equation.
Less snow means less accumulation.
More rain can mean more melt and runoff.
Reduced snow cover can lower albedo.
Together, these processes can push the balance toward greater mass loss.
Rain Is Not the Only Cause of Glacier Melt
It is important to place rainfall in context.
Rain is one factor among many.
Glacier change is influenced by:
- Air temperature
- Snowfall
- Rainfall
- Solar radiation
- Cloud cover
- Wind
- Humidity
- Surface albedo
- Dust and black carbon
- Glacier geometry
- Ice thickness
- Ocean temperature for marine-terminating glaciers
- Changes in atmospheric circulation
The current loss of glaciers worldwide cannot be attributed to rainfall alone.
Rather, increasing rainfall is one part of a larger climate system in which warming temperatures are changing both precipitation patterns and the energy balance of glaciers.
Rain and Ice-Darkening Feedbacks
Another important interaction involves the darkening of snow and ice.
Snow and ice can contain or receive dust, soot, organic material, and other particles.
These materials can reduce surface reflectivity.
NOAA research notes that light-absorbing impurities deposited on snow and ice can increase absorption of solar radiation and accelerate melting.
If rain removes some snow and exposes darker surfaces, these impurities can become even more important.
The resulting sequence can look like this:
Warm weather → rainfall → snow loss → darker surface → greater solar absorption → additional melt.
This is one reason scientists study rainfall together with albedo rather than treating precipitation and sunlight as completely separate processes.
Rain-on-Snow Beyond Greenland
Although Greenland receives significant scientific attention, rain-on-snow events are not limited to one region.
They occur across the Arctic and in mountain environments around the world.
The NSIDC’s AROSS project maintains data and research on rain-on-snow events across the Arctic, including observations from northern Europe, Siberia, and North America.
Mountain glaciers in regions such as the Himalayas, the Andes, the Alps, Alaska, and western North America can also experience shifts between snow and rain depending on elevation and weather conditions.
The precise response varies greatly from one glacier to another.
The Himalayas and Rainfall
The Himalayan region demonstrates why precipitation can have a complicated relationship with glacier mass balance.
Heavy precipitation can sometimes temporarily reduce glacier mass loss when it arrives as snow, because snowfall adds mass and increases surface reflectivity.
A 2026 study of glaciers in the western Himalaya found that some extreme precipitation events were associated with less negative or even positive annual mass balance, largely because of increased snowfall.
This is an important reminder:
More precipitation does not automatically mean more glacier melting.
What matters is the form of that precipitation.
Snow can add mass.
Rain can increase melt and runoff.
At temperatures near freezing, a small shift in conditions can therefore produce very different glacier responses.
What Does This Mean for the Future?
As the atmosphere warms, the boundary between snow and rain is expected to shift in many cold regions.
This does not mean every glacier will suddenly experience frequent rain.
Some high-elevation and polar environments will remain cold enough for snowfall to dominate.
But glaciers near the melting threshold may experience increasingly frequent rainfall events.
That could change:
- Snow accumulation
- Melt-season timing
- Surface albedo
- Runoff
- Snowpack structure
- Ice drainage
- Glacier mass balance
The significance of these changes will vary from region to region.
How Scientists Study Rain on Glaciers
Studying rainfall over glaciers is difficult because many glaciers are remote and weather conditions can be extreme.
Scientists therefore combine several methods.
Weather Stations
Weather stations measure air temperature, precipitation, humidity, wind, and radiation.
Snow Pits
Researchers can examine snow layers directly to determine how rainfall and freezing events have changed the structure of the snowpack.
Satellites
Satellites can detect changes in snow cover, surface melt, albedo, and precipitation over enormous areas.
Radar
Radar observations can help distinguish precipitation types and identify changes in rain and snow over ice sheets.
Climate Models
Models allow scientists to examine how precipitation patterns may change under different warming scenarios.
Combining these approaches provides a much clearer picture than any individual measurement.
Why Rain on Snow Matters for Glacier-Fed Water Supplies
Glaciers act as long-term stores of frozen water.
In many mountain regions, meltwater contributes to rivers and downstream water systems.
Changes in rainfall and snowpack behavior can affect the timing of water release.
A rain-on-snow event can produce a rapid pulse of water rather than storing that precipitation as snow for later release.
This can alter seasonal runoff patterns.
The consequences vary greatly depending on the region, glacier size, reservoir systems, and local climate.
A Changing Glacier Landscape
If rainfall becomes more common on glaciers, visitors may not immediately notice.
A glacier does not transform overnight simply because rain falls on it.
The important changes often accumulate over many years.
Snow may disappear earlier.
Bare ice may remain exposed longer.
Meltwater channels may develop.
Ice layers may form inside the snowpack.
The glacier may gradually lose mass.
Over decades, these repeated processes can contribute to visible retreat and thinning.
Final Thoughts
Rain on snow and ice is becoming an increasingly important part of the story of changing glaciers.
Rain can bring heat to a snowpack, increase liquid water, accelerate melting, create ice layers, alter drainage, and contribute to runoff. When rainfall removes bright snow and exposes darker ice, the resulting decrease in albedo can allow the surface to absorb more solar energy.
Greenland provides particularly strong evidence that rainfall patterns are changing. Research has documented increasing rainfall over the ice sheet, while newer studies indicate that continued warming could produce further shifts from snow toward rain.
At the same time, rain should not be viewed as the single cause of glacier loss. Temperature, snowfall, sunlight, wind, impurities, ocean conditions, and glacier dynamics all interact.
The key change is that rain is entering environments where snow once dominated.
That seemingly simple shift can influence the entire surface system of a glacier.
A glacier’s future depends not only on how much precipitation it receives, but also on what form that precipitation takes, when it arrives, and what happens to it afterward.
As the climate continues to warm, understanding these interactions will help scientists explain why some glaciers are losing mass faster, how water moves through ice, and how frozen landscapes around the world are being transformed.
