The weather in Mendoza now

The weather and Forecast in Punta de Vacas ( 60 Kms. of Base Camp )

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The weather and forecast in Aconcagua´s base Camp Plaza de Mulas

The Weather and forecast in Aconcagua´s camp 3 (5.894 mts.)

The Weather and forecast in Aconcagua´summit ( 6.952 mts.)

In the mountain the exposure to the whims of the atmosphere is maximized. Besides pathological problems derived from altitude and the cold, weather is usually the key factor to achieve or not the expedition's objectives. Although mountain meteorology is very complicated and difficult to forecast , we both carry recent reports and try to obtain our own data, that correctly interpreted, will allow us to forecast the weather and to design our tactics. For that reason, any serious expedition (and even more the commercial ones!) must have both a clear and scientific weather forecast technique and a flexible, easy to modify itinerary depending on forecast, without affecting high priority acclimatization tactics.
Forecast procedure should not be excessively complicated, but limited to recognize and evaluate the indications of the local phenomena, that in the Aconcagua are reduced almost exclusively to storms coming from the Pacific cyclonic systems.


Storm in the Aconcagua

Amongst the different types of storms studied by meteorology, typical storms in the Aconcagua are generated by the arrival of low pressure systems called "vaguadas" that move eastwards f rom the Pacific Ocean. Meteorology usually predicts these systems by the passage of a warm front (valuable for forecast) that preceeds a cold one (the actual storm occurrence). But in the Aconcagua, given the high altitude and the predominantly radiative character of the temperature, the warm front passage is difficult to detect unless standardized meteorological procedures are performed, and these are far for the mountaineers to achieve. And in addition, the Andes mountain range dividing western marine and eastern continental climate, generates an orographic Fohen type (same as the Chinook) effect known regionally as Zonda.
This process produces three events with forecasting utility: a "vaguada" that moves to the East and can be detected in remote stations of the Pacific Ocean, precipitations in Chile windwards of the Andes, and local alterations of barometric pressure and cloudiness.

Regional Forecasting


Weather forecast is classically based on four phenomena: cloudiness, barometric pressure, intensity of wind and wind direction. Wind data will be very affected in the Aconcagua by orography and differential exposure of the camp sites; our forecast will be based on both regional official information and local variations of pressure and their interpretation under observation of cloudiness.
Regional official informations that usually preceeds a bad weather situation are, in occurrence order: a low pressure in the Chilean oceanic islands (Easter, 3500 km -2000 nmi- West of the coast and 800 km -450 nmi- North; and Juan Fernández or Robinson Crusoe, 700 km at West of the coast -400 nmi- and 100 km -60 nmi- South), and rain in Santiago. Unfortunately Chilean radio broadcast does not report technical data like pressure values, but present weather at the cities (good, sunny, storm, cloudy, rainy, etc.) and a brief forecast for the rest of the day or the next day. In the Aconcagua only Chilean broadcasts are listened to , mainly above the 5000 m -16,500'-. We must pay attention and register the following events for Santiago and the V Region:

Clear sky: low possibilities of bad weather in the Aconcagua.
Dimmed sky: possibilities of bad weather in the Aconcagua.
Raining: definitely bad weather in the Aconcagua, with snowfalls and winds.
If a warm front arrival or proximity is announced, attention must be paid to a later cold front announcement.
If a cold front from the Pacific arrival or proximity is announced, a high probability of storm in the Aconcagua is to be expected.
A low pressure system formation or approach in Robinson Crusoe island (archipelago Juan Fernandez) announcement would be of great value to anticipate the development of the above mentioned phenomena, but this kind of data is not usually reported by broadcasting radio.
Our expeditions obtain this information from our base in Mendoza through our friends of Robinson Crusoe island (where we take trekking groups every year) who give us updated reports on low pressure systems. Forecast for 3 days in Easter, Juan Fernández and Santiago is also obtained daily from the Internet through
http://www.usatoday.com/weather/
basemaps/foreign/samerica/wfchile.htm selecting Easter Island Chile, Island Robinson Crusoe Chile and Santiago Chile, whose data is informed by phone to the Plaza de Mulas hotel.
This is the tool we have to know whether a great scale regional phenomenon affecting the Aconcagua zone may happen.


Local Forecasting


Regional information might be insufficient since the mountain is able to generate its own climatic condition, with local phenomena that may not be forecasted at a greater scale.
First of all, we can empirically observe typical cloudy phenomena, well known by guides and rangers.
Cloudiness usually begins with small cumuli at the south (watching from Plaza de Mulas downwards the Horcones valley, on Plomo and Juncal range), or at the northwest (looking from Plaza de Mulas to the left of Cuerno), that grow in size every hour. Under a process of worsening weather, these clouds begin to appear between 2 and 4 pm, it is completely cloudy in the evening and usually it may become clear after sunset. First clouds appear earlier each day, and bad weather is installed when clouds appear between 10 am and 12.
Lensed stratocumuli and hooked cirrus (claws) are another kind of clouds that foretells bad weather, but they generally indicate a Zonda phenomenon, normally forecasted through Santiago or Juan Fernandez info. A big lensed stratocumulus placed as a hat or "mushroom " on the summit is usually the confirmation that bad weather has arrived, although if that cloud appears at dusk and if there are no other indicators for bad weather, it might probably vanish at dawn.
If a remarkable, growing accumulation of very sharpened or globulose white alto-cumuli exists, usually coming from the West, from the North and the Northeast on the zone of Tambillo mount, they can be seen from altitude camps or on summit ascent. Should this happen before 5 pm, it can cause local bad weather.

Registering our own data

Finally, you can give your expedition a professional touch by carefully processing and analyzing meteorological variables. In "Towards Horizon" expeditions barometric pressure and temperature figures are daily downloaded into a worksheet in our PC and then data versus time are traced to see the evolution in course during many days. After the first three days of registry, we will be able to apply Gachon's rules to interpret these data:

Pressure reduces:
Slowly (1 to 2 hPa/day): bad weather coming, confirmed if, in addition, temperature rises slowly (1 to 2ºC/day, 2 to 4ºF/day) and/or sheepped sky (cirrocumulus) is observed.
Abruptly (3 to 4 hPa/day): immediate and intense bad weather, confirmed if, in addition, temperature descends abruptly (3 to 4ºC/day, 5 to 7ºF/day).
Oscillating: long bad weather.

Pressure grows:
Slowly: good weather, confirmed if in addition temperature descends slowly and sky is clear or with medium and isolated cumulus. If increase starts from a normal pressure (between 599 and 601 hPa for Plaza de Mulas), good weather will last a period of time equivalent to the time pressure took in reaching its maximum value.
Abruptly: temporary improvement, confirmed if in addition temperature descends abruptly; or unstable weather.
Oscillating: improvement in the weather will be delayed.

Pressure remains stable:
In a low value (below 599 hPa in Plaza de Mulas): unstable good weather.
In an average or high value (600 hPa or more in Plaza de Mulas): weather will remain unchanged, confirmed if temperature is also stable.
Pressure units equivalences arise from the definition of 1 normal atmosphere:
1013,23 hPa or mbar = 760 mmHg = 29,92 "Hg
Since pressure and temperature vary throughout the day, registered values must be comparable. Normally pressure increases slowly from sunrise to midnight (about 2 or 3 hPa throughout 18 hours) and diminishes again during the night (in 6 hours). Pressure is recorded every even hour and draw up two curves: one with daily maximum and another with daily minimum pressure . We have found that our forecasts become more reliable (no matter being favorable or unfavorable) the more uniform the daily pattern of variation is, when the maxima pressure registers are at 0 or 2 am and minima ones are at 6 or 8 am, and without other peaks during the day or the night. For the temperature we calculate the average between 6 pm of previous day and 8 am, since in high altitude temperature of the rest of the day is affected by cloudiness.
Then data is processed during the morning, forecast is analyzed and tactical decisions are taken.
Should you wish to calibrate the altimeter in the airport, runway level is 705 m asl (2,313 ft). In Mendoza city you can do it in the NE corner of Plaza San Martín, where a board in the floor indicates 748 m (2453 ft). In Puente del Inca, the platform of the old railway station is at 2719 m (8920 ft). For Plaza de Mulas we always use a constant value of 4260 m (13,976 ft) that we have determined by doing fast flights in helicopter with stable weather from Puente del Inca. Altimeter calibration is only a temporary and ephemeral change of scale, and it does not have any relation with the calibration of the barometer.
We continue with the forecasting process from altitude camps using a conversion program that allows to translate pressure data P measured in altitude to Plaza de Mulas values P0, and then continuing the registry by using calculated P0 values. The formula that allows to make this conversion is the following:
P0 = P [(a - Z0)/(a - Z)]b
where a = 44 365.5723 m, b = 5.256, Z the altitude of the point of measurement in meters, Z0 that of reference level in meters, P and P0 the corresponding pressures in any unit (both the same). In order to use altitudes in feet a = 145 556.3396 ft. Replacing values for the altitudes of the habitual camp places, we can express P0 = P k, where the values of factor k are the following: