The atmospheric pressure diminishes with the altitude following an exponential relationship. At sea level the pressure corresponds to 1 atmosphere or 760 mm of Hg. Its value is half of this at 5500 meters and up at the 7000 meters of the peak of Aconcagua, it is at 308 mm of Hg. The humidity of the air diminishes with the altitude but with a higher slope than the atmospheric pressure. From the norm of 4000 meters, when the atmospheric pressure is two thirds that of sea level, the humidity only represents a quarter of its value at sea level:
-0°C with wind of 40 km/h = -15°C. + 7.5°C with wind of 64 km/h = -7.5°C
-25°C with wind of 64 km/h = -60°C, danger of exposed areas freezing in 30 minutes
The temperature also notably influences the relative ambient humidity.
+20°C in 1 kg of air contains 150 grams of water, while at -20°C there is only 7.8 grams.
The freezing affect of the wind is known and terrible, it is enough to remember the following values of the wind chill factor
7.5°C with wind of 64 km/h = -30°C, danger of exposed areas freezing in one hour
The humidity multiples the effect of the cold 13 times.
The peak of Aconcagua can be a sweet and complacent woman if the goddess of the sun (Febo) shines and the god of the wind (Eolo) sleeps, but when the furies of the gods of the wind are unleashed and the dragons of the clouds devour the sky, the peak turns into a terrifying and cold witch. Only when you have seen the two completely different sides of this mountain and its special climate, can you really love and understand it completely.
The Central Argentine Andes and Mt. Aconcagua in particular receive the humidity of the winds that come from the anticyclonic centers of the Pacific Ocean. The climatic phenomenon known as the Zonda wind has an explanation similar to the Fohn of Europe
The anticyclones of the Pacific emit humid winds that blow towards the East, that is to say, winds of the West. These winds rise, and as they crash agains the mountain range mass, get cold and precipitate their humidity in the form of snow in the high peaks. Later, the dried air masses descend over the eastern flank of the mountain range, heating progressively (adiabatically).
This movement of air coming from the Pacific is the principal cause of the snow and wind storms that are unleashed on Aconcagua. But electric storms also exist, and form due to a micro climate of the zone, and are particularly terrible. These electric storms brew exclusively during the summer and are not to be underestimated. All of the North West flank of the mountain, as well as the peak and the crests, are exposed if lightning bolts are produced. In this mountain, due to its great size, another special phenomenon takes place . Strong winds from the West sometimes blow in the high altitudes, affecting the high part of the mountain (5500 meters and above), forming a big mushroom or lentil shaped cloud. Even in good weather, Plaza de Mulas, the presence of the mushroom is an unmistakable sign of strong wind and precipitation in the altitude. Getting inside this mushroom can have a fatal result. When the next formation is observed it is necessary to abandon the upper part of the mountain.
As far as the temperature registered in summer is concerned , you have to consider that during the night (even in good weather) above 5000 meters, negative 20 degrees Celsius is common. At the summit it is frequent to register negative 30 degrees Celsius. During bad weather or in the presence of air masses coming from the South, in Mule Plaza (Plaza de Mulas) it is frequent to see temperatures of mimus 18 degrees Celsius, while in the upper part of the mountain negative 25 Celsius is not too strange. During the majority of the days and nights the temperatures aren’t as rigorous. It is possible to wear a bathing suit around noontime at the base camp (4200 meters) or to stay at the peak at the same time of day, with only a thick shirt or a light anorak.
These great temperature variations oblige the mountaineer to always be prepared. During the winter the zone is heavily covered with snow, and the winds are always violent and constant. The temperature rarely exceeds 0 degrees Celsius. In the shadowy places it is always well below zero. Climbing Aconcagua in the winter season is a great test of psycho-physical performance, and much experience and high quality equipment are a necessity.