In Aconcagua weather changes are so unpredictable that sometimes it is impossible to reach the summit for lack of experience in making a personal forecast. The weather of Aconcagua is always related to the weather in Chile (See weather considerations).
Here are some tips for making an empirical 48-hour weather forecast approximately:
When at sunset appear from the West "wool-like" clouds, this means a storm is to be expected in the next 24 hours.
Fresh and transparent air with breezes coming up from the valleys and a light blue or greyish blue sky at sunset means the weather will be good for the next 24 hours.
High clouds around 10000 meters (cirrus), coming slowly from the South, indicate good weather.
A feeling of psycho-physical sluggishness, followed by hot sunsets and wind in all directions, means upcoming weather changes.
A strong storm and snowfalls are to be expected when cirrus begin to approach quickly from the West accompanied by cirro-strata.
Good weather for the next 24 or 48 hours is announced by cold winds from the South.
At night, the shine of stars may mean different things: if they shine firmly and seem to twinkle still in one place, it means there will be no perturbations; on the contrary, if they seem to "jump" and "dance" frantically, changes are to be expected. The moon and its halos do not indicate bad weather, but if it has a halo and in addition there are cirro-strata, there will likely be precipitation. When the moon has a yellowish disc around, like an aureole, not a halo, that shows there will be precipitation soon, since the layers of the atmosphere are saturated with water. From the altitude camps it is possible to see the valleys. If they are hazy at dawn, this means there will be 24 hours of good weather (the fact that they are not hazy does not mean the opposite). From the camps at 5350 meters or from the shelters on the Normal Route at 5800 meters, it is possible to see the Pacific Ocean. At dawn if the horizon is yellowish orange or light green, the weather that day will be good.
