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Weather basics and important terms to understand the weather report

16.07.2025

In order to be safe in the mountains, a sound knowledge of the weather and the most common terms in the weather forecast are essential. When planning, before and during an (alpine) tour – whether in winter or summer – you should always check the weather forecast, the actual weather at the start of the tour and how the weather will develop over the course of the day. The basic prerequisite for this is to understand the weather forecast and be able to interpret it correctly.

The weather can be quite exciting, especially when you’re in the mountains. This is because special climatic conditions prevail there: The topography of the mountains with its mountains and valleys has an effect on the weather – which can sometimes be very different from place to place and can change extremely quickly. The Alpine region is an extremely complex but also fascinating world for meteorologists – but also for everyone who spends time there.

The term “weather”

What do we actually mean when we talk about “weather”? The term refers to the current physical state of the lowest layer of the atmosphere at a specific location at a specific time. Weather is made up of factors such as temperature, humidity, precipitation and wind, which are used to describe the state of the atmosphere. However, before describing these weather parameters, the driving force behind the weather – radiation – will be discussed.

01

Radiation

Radiation from the sun is the only significant source of energy for the earth. A large proportion of the incident solar radiation is already reflected back into space in the atmosphere and therefore never reaches the Earth’s surface. How much radiation reaches the Earth’s surface varies due to three processes:

Reflection,
Dispersion and
Absorption

The filter effect of the atmosphere, the angle of incidence of the radiation, the presence of clouds and the intensity of air pollution are decisive factors here.

Radiation angle:

Surfaces that are normal, i.e. at right angles to the light source, absorb a significantly larger proportion of the incident radiation than those that are at a shallower angle to it. This results in the large temperature differences on Earth (which are subsequently the driving force behind our weather). At the equator, solar radiation hits the earth’s surface at a more vertical angle than at the poles. As a result, more energy is absorbed and the ground heats up. At the poles, the angle of incidence is much flatter and most of the radiation is reflected.

© snow institute

Standard measures in planning © snow institute

Radiation during the day:

During the day, solar radiation (short-wave radiation) hits the earth’s surface and heats the ground. Depending on whether the sky is cloudy or clear, the incident radiation from the sun is reduced due to reflection, absorption and scattering by clouds. At the same time, the earth also emits radiation (long-wave radiation). Overall, however, the radiation balance, i.e. the ratio of incident to emitted radiation, is positive during the day. More energy arrives on earth than is emitted, resulting in warming.

Radiation at night:

On a clear night, the incident radiation is almost zero. However, the earth continues to emit long-wave radiation. The energy balance is negative. The energy escapes to the ground and temperatures fall. On cloudy nights, the Earth’s surface loses less energy as the clouds reflect most of the radiation back to the ground. This greatly reduces the drop in temperature at night.

Albedo:

The albedo describes the reflectivity of a body and depends on the type and composition of the body. The brighter and smoother a body, the more radiation is reflected. New fallen snow, for example, reflects 95% of the incident short-wave radiation, compared to a maximum of 12% reflected by coniferous forest.

Radiation on the mountain:

The radiation increases with every meter of altitude, so it is significantly stronger on the mountain. This is because the layers of air above us are thinner at altitude and therefore reflect and absorb less radiation. The danger of getting sunburnt, for example, is considerably higher. In addition, diffuse radiation must be taken into account on the mountain: Clouds (and also fog) and snow scatter the sun’s rays in all directions. In spring, when the sun is higher, you can therefore easily get sunburn due to the diffuse radiation despite the cloudy sky.

EXCURSUS: What does this mean for the snow cover snowpack?

The color, the albedo, the moisture, the insulating capacity and the high radiation emission into the atmosphere give snow special properties that cause the thermal behavior. This differs greatly compared to other floor coverings. Due to its high albedo, the incident sunlight is almost completely reflected back into the atmosphere by the snow, and absorption is correspondingly very low. The snow cover snowpack absorbs little energy, but at the same time it continuously radiates a large amount of energy. The heat balance therefore remains slightly negative, which is why the snow can remain powdery, especially in the height of winter, even when the sun is shining and the air temperature is above 0 °C.

When the sky is clear at night, the snow cover snowpack continues to radiate a lot of energy, but there is hardly any energy input. This causes the snow cover snowpack to cool down. Due to its good insulating properties, the drop in temperature is limited to the layers near the surface. The difference between the surface temperature of snow and the temperature of the deeper layers can often be between 10 °C and 15 °C.

Good firn conditions in spring

A warm day with high energy input and a subsequent clear night, which allows the snow to radiate optimally, are good conditions for an upcoming “firn day”. During the night, the snow surface freezes on the surface. The following day, when the weather is fine, this frozen layer gradually softens again due to sunlight. A thin film of moist snow forms on a hard surface crust, colloquially known as firn. As the day progresses, a progressive softening of the surface crust and soaking of the snow cover snowpack can be observed as the day warms up. At a certain point, the snow cover snowpack loses stability and there is an increased risk of avalanches.

02

Air temperature

The air temperature is a measure of the heat content of the air. The air is not heated directly by solar radiation, but indirectly by the long-wave rays emanating from the ground and by the process of turbulent mixing. This means that when solar radiation hits the ground – grass, forest, rock, etc. – it heats up. The lowest layer of air (a few millimeters) is then heated by thermal conduction. Through small-scale, turbulent eddies, this heated air mixes with the layer of air above. As this process takes time, the highest temperature is not measured at the same time as the highest position of the sun, but with a time delay of two to three hours after midday.

The air temperature is always measured two meters above the ground and protected from radiation (in the shade) and is given in degrees Celsius (° C), Kelvin (K) or Fahrenheit (F). In principle, the higher the air pressure, the higher the temperature – and as the air pressure is highest at sea level and decreases with altitude, the temperature also decreases with altitude (vertical temperature gradient). The decrease in temperature with altitude is usually around 0.65 °C per 100 meters of altitude. In very dry air, it can even be up to 1 °C per 100 meters of altitude.

The distribution of air temperature plays a special role in inversion weather conditions. The higher layers of air are then warmer than the lower ones. This inversion weather situation often occurs in the fall and winter months when cold air collects in valleys and lowlands. It is then cloudy and cool in the valleys, but sunny and mild in the mountains. An inversion weather situation can be very persistent during winter high-pressure conditions.

Note: It’s a good idea to take a look at the webcam to get a better idea of the conditions on the mountain.

03

Air pressure

Air pressure is the weight of air on a certain surface. It is given in hectopascals (hPA) or millibars (mbar) (1 hPa = 1 mbar). Air pressure decreases with altitude. The average air pressure at sea level is 1013 hPa, which corresponds to the pressure of a 10 m high water column. For forecasts and analyses, meteorologists usually use the sea level at which the air pressure is 850 hPa, which is approximately the case at an altitude of 1500 m above sea level. However, air pressure is not only dependent on altitude, but also changes horizontally. The large-scale air pressure distribution determines the current weather situation, which is why weather changes are often associated with changes in air pressure.

Decrease in air pressure with altitude:

At high altitudes, above 2,500 m, the air pressure decreases exponentially. This means that there are fewer air particles in the same amount of air that we breathe in. Our body therefore has to work harder (including breathing faster and deeper) to absorb the required amount of oxygen. The thin air can be unpleasant for people. Altitude sickness with shortness of breath, headaches, nausea, insomnia etc. can be the result from an altitude of around 2,000 to 2,500 meters. Through appropriate acclimatization, the body can slowly adapt to an altitude of up to approx. 5,300 m, above which it is no longer possible to stay permanently. Climbing the highest mountains above 8,000 m is possible without oxygen, but the vast majority of expedition participants use oxygen, which they carry in bottles and breathe through a mask from a certain altitude and depending on the effort.

High pressure areas and low pressure areas

Low pressure areas form when the air is heated by solar radiation. Heated air expands, has a lower density compared to its surroundings and rises. As the air masses heat up, more water vapor can be absorbed and transported upwards. This results in the formation of clouds as the air cools down. This is why low-pressure areas are often associated with cloudy and rainy weather.

High pressure areas form when cold air masses sink due to a higher density. High pressure areas are often associated with fair weather, as the air is dried/heated as it sinks and clouds disperse.

The position of high and low pressure areas determines the weather situation.

04

Wind

On the mountain, wind can very quickly become unpleasant and – especially in winter – sometimes even dangerous. Wind causes the perceived temperature to be colder than the air temperature (see wind chill effect) and the body cools down more without appropriate protection.

Strong winds (from approx. 40 km/h) can set large objects in motion and throw people off balance. If you are in rough, difficult terrain in such wind peaks, it can quickly become dangerous.

Wind also plays a decisive role in avalanche risk; it is even referred to as the “master builder of avalanches”, as it is responsible for the formation of drifting snow that is suitable for snow slabs. Such transported snow is already possible from a wind speed of approx. 15 km/h.

Incidentally, the wind direction always indicates where the wind is coming from. A westerly wind, for example, comes from the west and a mountain wind blows down from the mountain.

If the snow is only transported by the wind close to the ground, it is referred to as drifting snow. If this impairs visibility – i.e. if the swirling snow is at eye level or above – it is referred to as blowing snow.

How is wind created?

A key factor in the formation of wind is the heating of the earth’s surface and the surrounding air by the sun. Heated air rises upwards and cools down. In colder areas, cold, dry air masses sink downwards. This results in a distribution of different air pressures. To equalize these differences in air pressure, the air flows from the areas with high air pressure to those with lower air pressure. This causes the layers of air to move – the result is wind. Colder air flows to where warm air rises.

Influence of mountain surfaces (relief) on the wind

The wind speed increases with altitude due to ground friction, which is why it is generally windier on the mountains than in the valleys. An exception to this are some passes and notches, where the so-called jet effect can ensure that it is windier there than on the peaks. This happens because the wind tries to flow around obstacles such as mountains and seeks lower passages (passes, notches).

Due to their relief and exposure, mountains can have different effects on the wind. For example, the mountains can accelerate and deflect the wind, as well as disrupt its laminar flow, creating turbulence. For this reason, the main wind directions at higher altitudes in the mountains can also differ from the winds predicted in the weather forecast in valley systems.

Valley constrictions lead to a higher velocity of the air flow. If the wind blows through a constriction (e.g. narrow valley section), the flow lines at the entrance are compressed at right angles to the direction of flow, resulting in compression of the air (and thus an increase in air pressure).

At higher altitudes above the ground, the air flow is normally laminar, i.e. uniform. However, the air flow can become turbulent due to the uneven surfaces of mountains. When winds encounter obstacles such as mountains, the wind speed decreases on the downwind side (lee side). At this point, vortices repeatedly form in the air flow. These vortices can manifest themselves in the form of sudden gusts of wind. Although the wind speed is lower on the lee side than on the windward side (windward side), an increased gustiness occurs there.

The wind direction always indicates where the wind is coming from, for example a westerly wind comes from the west and the mountain wind blows down from the mountain.

EXCURSE: Wind signs in the snow

The snow cover snowpack is a true quick-change artist. On one side you sink in waist-deep, on the other you stand on it without collapsing. Impressive snow cornices show how strongly the wind shapes the snow cover snowpack in high alpine regions. The influence of the wind not only changes the amount of snow on a given area, but also the layering and density of the snow layers. In most cases, this constant change remains hidden from the outside. Not only the already deposited snow, but also the falling snow is influenced by the wind. Especially with loose, dry snow, the wind always leads to drifting. Wind caps form upwind (windward side) and snowdrift accumulations snowdrift deposits downwind (side facing away from the wind). The snow is also blown off e.g. crests and broad ridges and then deposited again in gullies and bowls.

© snow institute I LWD Tirol

© snow institute I argonaut.pro

© snow institute I argonaut.pro

© snow institute I argonaut.pro

© snow institute I argonaut.pro

© snow institute I argonaut.pro

© snow institute I argonaut.pro

© snow institute I argonaut.pro

Uphill and downhill wind system/uphill and downhill winds

The mountain and valley wind circulation are thermally induced wind systems that occur in the mountains due to horizontal temperature differences.

After sunrise, the ground heats up considerably, which means that the air close to the slope is warmed more than the air further away, resulting in an upslope wind in the morning.
To replace these air masses, equalizing currents from the foothills of the valleys flow through the valleys – the valley wind.

At sunset (or shortly before), upslope and downslope winds come to a standstill. After a brief standstill, the wind system then reverses.

The mountain slopes radiate a lot of heat and cool down faster than the air above the valley. The cooler and therefore denser air sinks and flows down the slopes – the downslope wind.

The cool air from the surrounding slopes flows together at the bottom of the valley and then gradually (due to the limited valley volume) begins to flow out of the valley – the mountain wind. This in turn comes to a standstill at sunrise.

Glacier wind

The surface temperature of a glacier never exceeds 0°C at any time of the year. In summer, the temperature differences between glacier ice and air are usually greater than in winter. Due to diurnal warming, the air in ice-free areas around a glacier is warmed and rises as a valley wind. As soon as this air comes into contact with the cold surface of the glacier, it cools down. The air becomes heavier and sinks. A glacier wind flows down the mountain. Glacier winds occur particularly at glacier tongues and can reach deep down into the valley. These winds can be felt directly below a glacier front as cool, moist winds. Due to their inertia, the winds can also develop their effect several hundred meters beyond the end of the glacier. At the same time, the daily slope and valley winds flow in the warmer layer of air above this gentle glacier breeze. At higher altitudes, the flow is in turn influenced by the prevailing weather and the general winds in the region.

Wind chill effect

The wind chill factor shows how temperature and wind affect our skin and body. Everyone knows the feeling that the wind cools us down. What can be pleasant on hot days becomes very unpleasant and even dangerous in the cold. The wind draws energy from the skin – if you are not/lightly clothed -, accelerates the exchange with the environment and increases the evaporation of sweat. The process of evaporation draws additional energy from the environment, i.e. if this is intensified by wind, it can lead to faster cooling.

To protect us from the cold in winter and when skiing, we wear warm clothing, i.e. this garment traps air in its fibers – e.g. wool, synthetic fibers or down. This air cushion is warmed by body heat and thus forms an insulating layer. However, if this clothing gets soaked by rain or snow, the wind penetrates the clothing and it loses its insulating effect. You therefore need to choose the right garment depending on the expected weather conditions. Because while the wind makes cold temperatures feel even colder and you therefore want to wear the most insulating clothing, the same clothing prevents the body from regulating its temperature in warm temperatures, when there is no wind and during physical exertion, you sweat more, overheat, etc., which can then become just as critical.

In the following table we can see how the wind speed affects the perceived temperature:

05

Air humidity

Water occurs in the atmosphere in three different forms (aggregate states): gaseous (water vapor), liquid and solid. How much water vapor can be contained in the air is strongly dependent on the temperature – cold air cannot absorb as much water vapor as warm air. How high the air humidity is or whether and when saturation and condensation occur is not uninteresting to know, especially for winter sports enthusiasts. There are two ways in which humidity can be measured.

The relative humidity indicates what percentage of the air’s absorption capacity has just been reached – so if the relative humidity is 100 percent, the air is saturated and can no longer absorb anything. At 50 percent relative humidity, the reservoir is half full, so to speak, and can either be increased by adding water vapor (e.g. through evaporation) or by cooling the air.
The dew point is the temperature to which an air parcel must be cooled for saturation to occur. Each air parcel contains water vapor and can be cooled until this water vapor can no longer be retained or saturation occurs. If an air parcel has already absorbed the maximum possible amount of water vapor and cools further, condensation occurs. This results in dew or, if the air temperature is below 0 °C, frost. However, tiny droplets can also form in the air, which we perceive as fog. The temperature at which condensation occurs is known as the dew point.

If the dew point is low (below -10 °C), i.e. there is little water vapor in the air, the snow cover snowpack can radiate strongly (emits a lot of energy). For winter sports enthusiasts, this can be seen in hard slopes or frozen snow surfaces. A high dew point (higher than -5 °C), which is close to the air temperature, means that the air is (almost) saturated and there may be fog, snow or rain.

06

Precipitation

When we plan a tour, we keep a close eye on the precipitation. While in summer we tend to focus on sunshine and rain is not considered ideal mountain weather, in winter we look forward to precipitation in the form of snow (and then, of course, the sun). The decisive factor for snowfall is the temperature within the cloud and the air temperature. It is decisive for the height to which the precipitation falls in the form of snow crystals, i.e. it snows. The precipitation forecast can either be read from the weather report or can be seen from precipitation animations. The amount of precipitation is usually described or displayed in mm and describes the precipitation in liquid form (rain) – but not the amount of new snow.

So what does it mean when 10 mm of precipitation falls?

10 mm of precipitation means that 10 liters of liquid precipitation (rain) fall per square meter. The decisive factor is the period for which this precipitation is forecast. Will it fall in one hour or spread over four hours? In winter, it makes a big difference whether new snowfall occurs within a short period of time or over a longer period.

Snow has a much lower density than liquid water. Therefore, 10 mm of precipitation at a temperature around freezing point can mean about 10 cm of new fallen snow. If it is very cold and the snow is very powdery (low density), 10 mm of precipitation can also mean 20 cm of snow or even more if the density is very low.

And what does the probability of precipitation say?

The probability of precipitation indicates how likely it is that it will actually rain or snow. For example, if the weather forecast states that the probability of precipitation for the next day is 20 percent, this means that there has been precipitation on 2 out of 10 days with the same weather conditions in the past.

07

Weather forecast / weather services

The use of weather forecasts is an important part of tour planning. Nowadays, it is very easy to access the latest weather reports thanks to weather apps. There are of course differences in quality between the various products and the respective weather forecasts are based on different forecast models.

In principle, however, they are all more or less reliable. Short-term weather forecasts are generally more reliable than long-term forecasts that extend over more than five days. Therefore, a final weather check should always be carried out shortly before the tour. The following list is a selection of weather services and websites that offer a weather forecast:

Austria:

Switzerland:

Italy:

Servizio Meteorologico dell’Aeronautica Militare Italiana: Previsioni Meteo, Osservazioni, Satellite e Allerte | Meteo Aeronautica Militare (meteoam.it)

Avalanche report Tyrol, South Tyrol, Trentino

Germany:

International:

Other providers/apps:

Cover picture: © snow institute | argonaut.pro

Teaching materials on the topic: