#101 Weather

The comfort of our modern lives lets us live sheltered from whatever hostility Nature can muster. We spend the whole year at comfortable temperatures, protected from bad weather and sleeping without fear of our surroundings. Going to the mountains means experiencing Nature in a more primal state, far from civilization. This remoteness exposes us to a necessary return to basics, where the complex environment of the mountains creates weather phenomena that are just as complex and hard to predict. All of it far from huts and other forms of shelter. Despite meteorologists' progress in predicting how the weather will evolve, the mountains remain wild territory. Their layout and their exposure most often generate microclimates that are hard for non-locals to anticipate. The combination of dangerous weather and little shelter is the perfect recipe for disaster. The best defense is to develop an eye for reading the sky and anticipating problems. That is what we are going to try to cover here.

Some meteorology basics

The atmosphere

The Earth is a massive concentration of matter, and like any concentration of mass, it creates its own gravitational field. This gravity has two qualities that interest us here. First, it acts on solids as well as on gases. Second, it decreases with altitude. The Earth thus finds itself surrounded by a layer of gas held in place by gravity, whose thickness is defined by its intensity. This gaseous envelope is what we call the atmosphere. Although it is several hundred kilometers thick, gravity means that 99% of the atmosphere's mass is contained within the first 30 kilometers. Within this zone, meteorologists define two layers:

The gases of the troposphere form what we commonly call air. Air is largely composed of nitrogen (78%), oxygen (21%) and other gases. Among them, the one we will mention most in what follows: water vapor. Its proportion varies from 0.1 to 5%. The air around us is always humid, which is a condition for our survival. Small as these values are, they are enough to build clouds and precipitation.

Atmospheric pressure

The gases making up the atmosphere are themselves composed of molecules in motion. These molecules have mass and exert a weight. Atmospheric pressure is the sum of all these weights. Put another way, pressure is the weight of the air pressing on the Earth's surface. Since gravity decreases with altitude, there are more molecules near the ground than near space. Atmospheric pressure therefore decreases with altitude. This decrease is well known to those who go into the high mountains (altitude \> 4000 meters) since it is responsible for mountain sickness. Atmospheric pressure also varies across the surface of the globe. Not as much as with altitude, but enough to change the weather. Meteorologists then use terms familiar to us: low-pressure areas are called depressions, and high-pressure areas anticyclones. Atmospheric pressure is measured with a barometer, giving a value either in hectopascals or in bars (or millibars, 1 bar = 1000 millibars). Most barometers within our reach are electronic. They use Vidie capsules: the pressure squeezes the capsule, and that compression then changes the electrical voltage across the capsule's terminals. A quick aside. If you want to quantify the drop in pressure with altitude, know that on average you lose 1 hPa every 8 meters of altitude. Which amounts to saying there is as much air between 0 and 5000 meters of altitude as between 5000 meters and space. Barometric watches estimate the altitude we are at using this type of relationship. They are sometimes able to tell the difference from a pressure variation caused by the weather. In that case, the pressure drop is checked against the type of physical activity we are currently doing.

Water vapor

The water vapor in the air is the gaseous form of water (remember that it can also be solid and liquid). This gas is transparent and invisible. On the other hand, when the vapor changes state to become liquid (condensation), it turns into visible droplets. That is how we end up with clouds, fog or dew.

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Meteorologists use relative humidity to quantify the water vapor in the air. This value is a percentage of absolute humidity, which is the maximum amount of water vapor under ideal conditions. When relative humidity is at 100%, the air is saturated with vapor, leading to the creation of droplets and therefore clouds, rain, etc. At 0%, the air contains no trace of vapor, but this value cannot be reached on Earth. Relative humidity varies with temperature. The warmer the air, the more water it can hold in vapor form. Conversely, when the air cools, the vapor condenses and forms droplets. Without the absolute humidity being changed. So during the day, when the thermometer climbs, relative humidity drops, even though the mass of water vapor is unchanged. In the evening, when the thermometer falls again, relative humidity rises and can reach 100% (dew). Put another way, the warmer it is, the more water vapor the air can store. And this water vapor is the essential source of precipitation (snow, rain, hail). This explains why the air is drier in cold regions with sub-zero temperatures than in tropical regions. Incidentally, the higher the relative humidity, the less room there is to store other vapor molecules. If the air is hot and humid, our body struggles to cool itself since sweat can no longer evaporate.

The forces behind the weather

The weather, or rather weather phenomena, are the result of exposing the water vapor (contained in the atmosphere) to conditions of pressure and temperature. These pressures and temperatures are not constant, either over time or across the Earth's surface. The causes of these variations are the Sun, the Earth's rotation and the changing relief of the Earth's crust.

The Sun

The Sun is the star of the solar system around which planets revolve, including the Earth. This star is the seat of countless nuclear reactions, releasing a monstrous amount of energy that spreads through the solar system. When a planet like the Earth is exposed to the Sun's rays, it receives solar energy in a luminous form (it is daytime) and in a thermal form (it is warm). If this planet were too close, the level of exposure would be too intense for life as we conceive it. If it were too far, it would be too cold. The Earth, for its part, is ideally placed. But you still have to be able to capture this energy and transform it. The Earth's surface absorbs part of the solar energy, the rest being sent back into the atmosphere. The gases composing it create the greenhouse effect, returning part of the energy coming from the ground to sender. This back-and-forth system makes it possible to keep nearly 70% of the solar energy and to maintain an average temperature of 15 °C, ideal for our survival and that of the other species around us. Nevertheless, this solar energy is not distributed uniformly across the Earth. Since our planet revolves around the Sun and spins on an off-center axis, the energy at ground level shows strong disparities.

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These disparities create temperature differences across the planet. The equator receives more energy than the poles: it is warmer than the poles. These temperature gaps encourage air movements in the atmosphere. Indeed, warm air is less dense, atmospheric pressure drops and the air tends to rise. The opposite goes for cold air. The air then moves from high-pressure areas toward low-pressure ones. A rebalancing then takes place between the two regions, stirring the air between the equator and the poles.

The Earth's rotation

Taking the solar system as the reference point, the Earth's movement breaks down into two motions. First of all, the Earth revolves around the Sun. Its revolution, the time needed to complete one orbit, defines a year. Then, the Earth spins on its own axis (the Earth's rotation). The time for one complete rotation defines, in turn, the length of a day. The combination of these two motions changes the exposure of the Earth's surface throughout the day and throughout the year. That is how we get cool mornings, hot afternoons and dark nights. This combination is also the origin of the four climatic seasons we know: winter (cold period), spring (transition from cold to hot), summer (hot period) and autumn (transition from hot to cold). The seasons differ because the Earth's movement around the Sun follows an elliptical trajectory, moving the two celestial bodies closer together or farther apart. Finally, the tilt of the Earth's axis of rotation is the reason the seasons are opposite in its two hemispheres.

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The Earth's rotation is also the origin of the Coriolis force. Without going into the details of where it comes from, this force deflects straight-line movements into curved ones. Air coming from high-pressure areas then no longer heads straight for low-pressure areas, but turns in spirals. Together with the Coriolis force, the Earth's rotation creates the jet streams. These currents (courants-jets in French) are born from the temperature imbalance between the equator and the poles and from the resulting air movement deflected by the Coriolis force. The jet streams give birth to the depressions and anticyclones passing over our heads.

The relief of the Earth's crust

The Sun and the Earth's rotation shape the planet's global climate. Since the Earth's surface is not uniform, regions located at the same latitude experience very different local climates. This is due to the nature and the changing shape of the Earth's crust. Let's come back to the greenhouse effect. Two thirds of the solar energy is absorbed by the atmosphere, the ground and the ocean (the remaining third returning to space via the clouds). The ground absorbs part of this energy and sends it back to the atmosphere, whose greenhouse gases in turn send part of it back to the ground. The type of terrain defines the amount of radiation sent back to the clouds and gases. For example, on ground covered with snow and ice, the energy absorbed is lower and the radiation reduced (compared with dry ground). Vegetation cover stores more or less heat, water and carbon dioxide. Mountain ranges, for their part, scatter the solar energy, creating local disparities in radiation (with faces that are more or less cold). Nevertheless, it is the oceans that have the greatest impact on the climate, covering more than two thirds of the globe's surface. On the one hand, they are the foundation of the water cycle: water evaporates from the oceans into the atmosphere before condensing over land and running off until it returns to the oceans. Ocean currents also help redistribute the excess heat received in the tropics toward higher latitudes. The winds, finally, transfer part of their energy to the sea, creating certain currents, including the Gulf Stream.

Clouds

Clouds are a major component of weather phenomena. Located in the troposphere, they are the assembly of a multitude of water droplets and ice crystals in suspension. Learning to recognize them and understanding their dynamics are the keys to reading the sky once you are out in the field.

Cloud formation

A quick refresher: warm air is less dense than cold air. It is also richer in water vapor. When warm air gains altitude, it cools down (remember, in the troposphere, temperature decreases with altitude). The mass of water vapor then condenses to form droplets, which in turn form clouds. To make clouds, warm air ultimately has to rise in the troposphere. This can happen:

It is also possible for warm air to flow over a cold surface, for example over the plains. This warm air then cools without gaining altitude; we speak of cooling from the base. This type of cooling gives rise to low clouds and fog.

Cloud types

The formation mechanism and the pressure and temperature conditions of the troposphere define how the droplets and ice crystals group together. Clouds can then be classified according to their shape and altitude. Clouds made up of several layers are stratiform. Clouds that are a heap of puffballs are cumuliform. A prefix is then added according to their position in the troposphere: stratus for those near the ground, alto for the not-so-near, and cirro for the farthest away. The cloud types are thus:

Some clouds can have heights spanning several levels: nimbostratus, cumulus and cumulonimbus.

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To this list, we can add three other classes of clouds:

Clouds and precipitation

Inside a cloud, droplets and crystals are constantly in motion. These particles merge, melt, evaporate, sublimate, only to condense or freeze all over again. Depending on the pressure and temperature conditions, it is entirely possible to find vapor rubbing shoulders with liquid water or ice crystals. A simple cumulus has a mass of more than a million tonnes, with 500 tonnes of liquid water or ice and 10,000 tonnes of water vapor (the rest being dry air). Not all clouds produce precipitation, but all precipitation comes from clouds. Clouds far from the ground, in the highest level of the troposphere, only very rarely generate precipitation. Nothing to fear, then, from cirrus, cirrocumulus and cirrostratus. Likewise for altocumulus. In general. It is possible to anticipate the type of precipitation from the ambient temperature and the type of cloud. Precipitation will tend to be solid (snow, ice pellets) if it is cold, and liquid if it is warm. As for cloud types, you can rely on the following list:

halos: chance of rain or snow within 24 hours.

Weather phenomena

The way water vapor is knocked about by low- and high-pressure areas produces several weather phenomena. These phenomena are the wind, the various forms of bad weather (rain, snow, hail), thunderstorms, heat waves and cold snaps. We will also cover fronts and UV rays to complete the picture.

Fronts

Fronts are the boundaries between two systems, one of low and one of high pressure. These two systems each try to gain the upper hand over the other, and this confrontation defines the weather above our heads. When a depression (low-pressure system) pushes an anticyclone (high-pressure system) back, the front is called warm. A cold front indicates that the anticyclone is gaining the advantage over the depression. Remember that warm air is less dense than cold air. Depressions contain warm air rich in vapor. Anticyclones, for their part, contain cold air poor in vapor. This difference makes it possible to determine the nature of the front (warm or cold) and its speed of advance by studying the formation of the clouds found there. A cold front is a source of stratocumulus, altocumulus, cumulus and cumulonimbus. A warm front is a source of halos, lenticulars, stratus, cirrocumulus, cirrostratus, altostratus and nimbostratus. When the depression stops advancing (it settles in), the front no longer moves in a straight line but turns on itself, forming a spiral. Once the cold front has passed, the clouds remaining in the sky and gradually dispersing form what is called a post-frontal sky (ciel de traîne). Studying clouds in the distance thus makes it possible to estimate the weather to come. By looking at the type of clouds, we can determine whether the weather is going to deteriorate (arrival of a depression) or improve (arrival of an anticyclone). By watching the direction and speed of the clouds, it is also possible to estimate how long before bad weather is likely, and take shelter if needed.

Wind

Wind is a consequence of the movement of air masses as they pass from high-pressure systems to low-pressure ones. The greater the pressure difference, the stronger the wind. The direction and speed of the wind are mostly imposed by the anticyclones and depressions, but the relief can channel the movement of air masses just like water in a riverbed. Wind is characterized by its direction and its speed, generally expressed in km/h for us mountain folk. Since the measurement time is arbitrary, we speak of instantaneous wind for a 3-second measurement and of average wind over a 10-minute period. Gusts, for their part, are sudden increases in the instantaneous wind, with speed differences of almost 20 km/h compared with the average wind. They can have a direction at an angle to the average wind. Gusts are all the stronger when the air is unstable or when the airflow is disturbed by the relief. Winds can also appear through gravity. This is the case of the katabatic wind, a wind caused by the descent of a cold air mass rushing down a slope. Various weather conditions are needed here: a temperature inversion at altitude and a small local pressure difference. Once under way, the cold air mass descends the mountain, accelerating until it creates a wind whose speed can exceed 200 km/h (generally at the poles). In the mountains, we also find the Foehn effect: at equal altitude, the temperature on the windward side is lower than on the leeward side. The foehn appears when the cloud has lost part of its water vapor as precipitation on the way up.

Rain

Clouds are made up of fine droplets produced by the cooling of water vapor. These droplets are always looking for a surface to settle on. They then cluster together, grow bigger, get heavier and fall to the ground. When the temperature is low enough, the droplets can form ice crystals, which also fall to the ground by gravity. As they fall, they warm up until they become raindrops. Rain is measured in millimeters: this measurement reflects the depth of water falling per square meter. 1 millimeter thus corresponds to 1 liter of water per square meter. Precipitation occurs:

At the end of the winter season, cold air persists at altitude, while the lower layers of the atmosphere gradually warm up. When the temperature gap between these two layers is large enough, powerful updrafts form unstable clouds that will give rise to giboulées — sudden squally showers. A giboulée is a very short form of shower, often accompanied by wind, falling at the end of winter and the beginning of spring. It can bring rain, but also hail, ice pellets or snow.

Snow

Snow follows the same formation principle as rain. The difference is that the crystals form at sub-zero temperatures. The shape of these crystals (the future snowflakes) depends on the air temperature, which defines the amount of water that can cluster together. Snow can fall continuously or in showers. Temperature is key for snow to fall: the flakes reach the ground when the air temperature is below or close to 0 °C. But the quality of the snow also depends on relative humidity and wind. The different combinations create three types of snow:

When the crystals melt on their way down, we get rain. If this form of rain meets a layer of cold air or cold ground, we speak of freezing rain: the drops form ice crystals all over again. If this rain is heavy and long-lasting, the flakes can cool the ambient air all the way down to the ground. We then speak of snow through isothermy.

Hail

Hail is once again the result of water vapor turning into ice crystals. However, a hailstone is the result of crystals accumulating on solid particles, such as dust. Hail forms inside cumulonimbus clouds, with the hailstones forming at the top of the cloud, where the temperature is lowest. Gaining mass through the accumulation of crystals, the hailstone descends within the cloud, and droplets in turn stick and freeze onto its surface. When at last the hailstone is too heavy to stay in the cloud, it falls to the ground. The difference between hail and ice pellets comes from the size of the stone. If the diameter does not exceed 5 millimeters, the stone can bounce off the ground without breaking: we are dealing with ice pellets. Hail, for its part, is made up of hailstones larger than 5 millimeters. Hail and ice pellets always fall in the form of showers. If hail manages to reach the ground, its destructive power can be immense; a tent would not last long. So watch out for cumulonimbus, without fearing hail from every one of these clouds. Fewer than 10% of cumulonimbus produce hail. But these clouds remain ones to fear, because they can produce thunderstorms.

Thunderstorms

Thunderstorms form when the atmosphere is unstable, with warm air near the ground and cold air aloft. For a storm to break out, the warm air must be forced to gain altitude, forming a cumulonimbus. This mechanism occurs when the wind pushes the air onto high ground, or when the ground is very hot (in summer in particular). The cumulonimbus feeds on warm, humid air, generating violent churning of the air within it. The water vapor condenses as it gains altitude, then gets knocked about by the air movements. Crystals and droplets collide at high speed, electrifying the cloud. Using the image of a battery, the violent winds also drive the positive pole away from the negative pole. An electric arc can appear to cancel out the electrification. But if the two poles are too far apart within the cloud, the ground can play the role of the negative pole. The arc then runs from the cloud to the ground: that is lightning. This electric arc also heats the air. In a few thousandths of a second, the air reaches a temperature of 30,000 °C and undergoes an alternation of very strong compressions and expansions. These sudden, successive movements generate the sound waves behind thunder, the sound of the arc. It is easy to see that thunderstorms are violent phenomena, often accompanied by strong winds and intense precipitation. But they are short-lived phenomena, lasting from a few minutes to a few hours. However, if the temperature difference between the air layers remains large, the storm can be followed some time later by a circular storm arc centered on the area where the initial storm occurred.

Heat waves

A heat wave is a weather accident. It is defined by temperatures above seasonal averages and reduced temperature differences between day and night. A heat wave is generally long, lasting from several days to a few weeks. It occurs when an anticyclone remains stable with air coming from a hot source (in France, the air comes from the Sahara). Heat then accumulates faster than it is removed by the wind or into space. A heat wave promotes air pollution in the layer near the ground, since the heavy gases fall by gravity, the air surrounding them being warm and therefore very light.

Cold snaps

A cold snap is the opposite of a heat wave. Also a long-lasting phenomenon, it is defined by temperatures below seasonal averages. For it to appear, an anticyclone must be positioned over a polar region, and a depression must be positioned such that the region hit by the cold snap lies in the currents formed along the front line. With both systems stable, the currents spiral and drive the cold air down from the poles. A cold snap is sometimes accompanied by snow and black ice.

UV rays

A quick reminder: we saw that the Sun emits solar energy through radiation. This radiation contains several electromagnetic waves: visible light, which lights our way, infrared rays, etc. Among the waves emitted, we also find ultraviolet rays (UV), the amount of which reaching the ground depends on the Sun's position in the sky and the composition of the atmosphere. UV rays are filtered in large part by the ozone layer, whose thickness determines the amount of UV reaching the ground. In the end, two types of ultraviolet reach the ground:

Ultraviolet rays quickly become dangerous for our health (see below). You therefore need to watch your exposure to the Sun according to the amount reaching the ground. Precisely to get an idea of the UV level, weather services use a UV index. This index, rated from 1 to 11+ (11 and above), reflects the intensity of the ultraviolet radiation and its health impact on the skin. In general, the information communicated is the maximum value of the index over the day, reached within a window of 2 to 4 hours around solar noon (the point where the rays are perpendicular to the ground).

Weather forecasts

As we explain in our article "Preparing your outing", knowing the weather ahead of your mountain outing is fundamental for your safety and your preparation. For your safety, because the forecast can tell us that conditions will be dangerous. For your preparation, so you pack the right gear to cope. Few countries have their own weather service, as France does with Météo France. For the others, there are several global forecast models. It must be said that designing a forecast model and the computing power required quickly drive up the costs of such a structure. Not to mention that these models must be fed with data describing the current weather. And this data comes from weather satellites and from measurements taken in the field and in the atmosphere.

Forecast models

A forecast model is a modeling of the atmosphere, over a defined region or over the entire globe. The quality of a model depends on its grid cell size. This cell is the indivisible volume within which everything is assumed to be identical. Forecasts will not be the same depending on whether the cell is the size of a building or the size of a country. The most widely used model in the world is the American GFS model. Its widespread use is mainly due to the fact that it is free. The problem here is that the cells are quite large (22 km), making forecasts hit-or-miss in mountain areas. A more precise model is the ECMWF, designed by the Europeans. The grid is finer here, with 9 km cells. It is not as popular as the GFS because its use comes at a cost. An increasingly popular model comes from the Swiss outfit Météo Blue. The cell size here is variable, ranging from 3 km in densely populated areas up to 30 km. The forecasts are the result of a compilation of several models over which Météo Blue runs a layer of artificial intelligence. This model is particularly effective at predicting mountain weather. The latest model in vogue is the ICON model from our German friends. Here too the grid varies from 22 down to 5 km. This model is one of the most accurate to date if you are looking for the weather at any point on Earth. Finally, the leading countries in meteorology have their own models. Météo-France's Arome model is dedicated to the weather in metropolitan France, with very precise results. The cells here are 1.3 km and the model takes the nature of the ground into account in its calculations.

Websites for checking the weather

Our little favorite is Windy.com, which even in its free version lets you compare the different models against each other.

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The Meteo Blue site is very interesting because it gives access to the weather history of a given town. This history comes from both forecasts and measurements taken as close by as possible. For access to weather data linked to summits, the reference remains Mountain-Forecast.com. This site uses the results of the GFS model, on top of which it adds modifications to account for the characteristics of the summit in question. This is where you can pick up information on the freezing level, the temperatures at different altitudes, as well as the broad trends for the days ahead. In any case, the ideal is to be able to compare several forecasts (coming from several models) to get an idea of the weather to come. If the models do not agree with each other, there is a good chance the weather will be unstable. Which already gives you an idea of the weather ahead.

Other useful information

The Internet is full of forums and sites for sharing mountain outings. By digging around a little, it is possible to learn the recent conditions of the terrain and the weather-related difficulties encountered by fellow mountain-goers. The easiest information to find concerns the state of the ground. But once again, you need to sort and cross-check what is written to get quality information. If you now own a barometer, you can make your own forecasts. It remains rough, but it is always better than nothing. When the pressure is low, the weather will most often turn to rain. When atmospheric pressure drops quickly, it can announce wind, and bad weather threatens. Conversely, high atmospheric pressure means calm weather, but not necessarily fine weather. Thus, in summer, high pressure and good weather go hand in hand, but in winter, you can expect fog and low clouds that can last all day.

Recommendations in the field

Let's now look at how, in practice, to adapt to weather phenomena in the mountains.

Facing the wind

Wind is ever-present in the mountains. If it is light, it can help limit condensation in a tent. If it is stronger, the danger rises fast. The first risk of a strong wind is making us lose our balance, with all the consequences that implies. If it is strong enough, it can stop us from moving forward unless we lean into it, or push us in a bad direction. Protecting yourself means either leaving the spot, or using suitable clothing. Reducing lift is also key: minimize the surface exposed to the wind, either by making yourself small, crouched down, pack set on the ground, or by avoiding handling fabrics (tent-type) that can act like a boat's sail. The wind can also lift dust off the ground, considerably reducing visibility. In such cases, wraparound sunglasses or wearing ski goggles help considerably to preserve visibility. Another effect quickly felt is wind chill. Exposure to the wind cools the skin, removing heat by convection: the air warmed by the skin is immediately replaced by cold air. This windchill effect is all the greater when the air is cold to begin with. https://fr.wikipedia.org/wiki/Refroidissement_%C3%A9olien A mountain particularity, the Foehn is a warm wind descending the mountain. This wind can melt snow and destabilize a snowpack. So watch out for snow bridges and for rising river levels downstream. Also pay attention, when pitching your tent, to the wind direction and to the orientation relative to the slope if there are katabatic winds. Finally, remember that the relief deflects the flow of air. Beware of passes or gullies that can locally accelerate the wind.

Facing the rain

Protecting yourself from the rain demands discipline. Protection means wearing a hooded jacket, overtrousers and shoes, all of them waterproof. This waterproofing is not infallible and your protective layers can end up soaked by sweat struggling to get through the waterproof membrane. It is then important to dry your gear as thoroughly as possible. Another solution, if temperatures allow, is to wear a minimum of clothing (t-shirt and shorts) and accept being wet. This only works if it is possible to dry off later. The risk of rain, or of being wet, is getting cold. The moisture pumps away the heat given off by the body, playing the same role as sweat. If we are not careful, the risk is falling into hypothermia. This cooling always comes faster than you expect, and it is harder to warm up again than to protect yourself. Another risk is trench foot. This condition follows prolonged exposure of the foot inside a wet shoe. The foot then stews in its own juices and initially behaves as if it were frozen, even though the temperature is above zero. The best way to avoid this is not to sleep with those shoes on and to let your feet breathe in the open air as much as possible. Rain, finally, can degrade the terrain, especially when it is intense. Floods and landslides can appear after a few hours of intense rain on flat ground or ground without vegetation cover. The greatest risk remains at night, when you are in your tent and without visibility. To protect yourself, place the tent so as to be on raised ground and far from areas likely to slide.

Facing snow

We will come back to snow in general in more detail in the Montagneur 103 section. For now, remember that snow covers the ground, making a trail invisible and complicating navigation. Snow is also dangerous when you are in the tent. Too much accumulation can break the poles, or block the flow of air, asphyxiating the tent's occupants.

Facing thunderstorms

The thunderstorm is the mountain-goer's dread. This phenomenon is violent, sudden and unpredictable, making it very dangerous.

Before the storm

The best protection is, once again, anticipation. For this, it is important to watch the sky and the presence of cumulus and cumulonimbus. In summer, storms tend to arrive in the afternoon. A good trick to avoid them is to set out and return early. Now, that is not always possible, especially when sleeping in a tent. In that case, you will need to find an unexposed area, as low as possible, to camp. But not at the bottom of a basin, because storms often come with intense rain. If you observe a storm in the distance, you can monitor its speed of advance. Since the speed of light differs from that of sound, you can estimate its position by counting the number of seconds between the image and the sound. Sound moves at a speed of 340 m/s through the air, so each second of delay adds 340 m to the distance between the storm and you.

In the storm

If you are now inside the storm, let's not kid ourselves, you're in deep trouble. A few recommendations for coping:

Facing heat

Heat, when it is intense, puts great strain on our bodies. Their only defense is to use sweating to cool down. The principle is to deposit liquid water on the skin. The skin warms this water to turn it into water vapor. This evaporation needs energy, allowing the body to cool itself. For sweating to work, several conditions must be met. First, the body must be hydrated enough to sweat. Second, the relative humidity of the air must be below 100%, otherwise there is no room left to store the vapor coming from sweat. Finally, you must not exceed the maximum amount of heat the body can shed. If these conditions are not met, then the body no longer controls its temperature and heatstroke arrives. Protecting yourself from the heat therefore means preserving these three conditions:

  1. The easiest: simply drink enough water. Nothing beats training for testing different hydration strategies and learning your body's water needs depending on the level of heat. If your urine is yellow, you are not drinking enough. Watch your blood sodium level if you drink several liters a day.
  2. The trickiest. Avoid exerting yourself when the relative humidity is high and the air is hot. If that is not possible, douse yourself generously with water.
  3. Heat can come from an excess of body temperature (it is hot out) and from physical effort (working muscles produce heat). The defense here consists of moderating physical activity when it is hot and taking shelter during the hottest hours of the day.

Facing the cold

Cold is a vicious phenomenon. It makes us sluggish, drains all our motivation and pushes us toward immobility. Hypothermia lies in wait, and its warning signals are shivering, goosebumps and numbness in the extremities. These signals are not to be taken lightly, especially when staying outside for a long time. It is always easier to stay warm than to warm up again. The first defense against the cold is checking the weather forecast. These forecasts will provide the minimum and maximum temperature values, as well as the value of the freezing level, the altitude at which the air temperature turns negative. All this information lets you pack the right gear to cope. Indeed, to face the cold, gear is the best tool at our disposal. It must be backed up by adequate nutrition so that our metabolism can maintain its temperature.

Hydration also needs watching. Cold reduces thirst, and water freezes in bottles and drinking tubes quite quickly. It is up to you to test your gear to find the solution that suits you. Thermoses are widely used when temperatures are well below zero.

Facing UV rays

As we wrote earlier, the ultraviolet rays emitted by the Sun are partly filtered by the ozone layer. Only UV-A and UV-B can reach our skin. UV-A and UV-B, even if their modes of action differ, are both genotoxic forms of radiation. UV exposure can damage the DNA of exposed cells, a major factor in the appearance of skin cancers. Limiting your UV exposure is not just about protecting yourself from the sun's rays. The ground gives off part of the rays, and you can get sunburned under a parasol. This matters all the more when you are walking on snow. Grass, earth and water reflect less than 10% of UV radiation, while this proportion can reach 80% for fresh snow. The means of protection are sunglasses (category 3 or 4), sunscreens and clothing. Clothing is more effective than sunscreen and far less polluting for the environment. Its drawback is that it degrades the effectiveness of sweating. Sunscreens used at altitude must have a UPF protection rating of over 50.