The figure above depicts summit weather at Quelccaya prior to and during our 2012 fieldwork. The short synopsis is that during our time at Moraine Camp and the AWS we experienced clear sky, low humidity, and low wind speeds. Just prior was an interval of high humidity, snowfall, moderately-high wind speeds, and lower temperatures.
Here are some details shown in the graph, and implications (click the graphic to see a larger version). Some measurement are presented only through the 11th due to sensor changes and service. We were at the summit from the 9th through the 15th, and near the ice cap 6-16 July:
- Hourly incoming solar radiation is shown in orange. Clear sky continued from the 9th through until the 15th (not shown).
- Blue squares are average daily relative humidity, which increased during our time in Cusco and then dropped abruptly on the day we first reached the summit.
- Snow accumulation & ablation is depicted as the dashed blue line. This is actually a smoothed recording of changes in surface height, where decreasing distance represents snowfall and increasing distance is a combined record of melting, sublimation, wind scour and settling. The measurements are hourly, yet must be smoothed to remove a spurious diurnal cycle due to the impact of air path temperature on the speed of sound - since acoustic sensors are used. Anyway, processes were less continuous than the smoothed plot indicates, suggesting: a) about 1.5 cm of snowfall during the afternoon of July 4th, b) another ~1.5 cm of snowfall late on the 5th, and c) ~1.5 cm of sublimation on the 9th. Note perfect conditions for sublimation on the 9th, including clear sky and high radiation, decreasing humidity, high wind speeds and lowest temperatures of the interval.
- Average wind speed (red diamonds) parallels the humidity pattern, reaching a maximum on the day gear was carried to the summit and then dropping. For several 24-hour periods, it averaged < 3 m/s, and on our 'hottest' day of work at the station the average wind speed was <2 m/s - or <5 miles/hour. Very calm, especially at nearly 6,000 m elevation in the Andes.
- Two different measurements of temperatures are represented on the graph, both as triangles and both as the maximum hourly values each day. The pink values are measurements in a naturally-ventilated, multiplate radiation shield whereas the cyan symbols depict measurements in a sophisticated, fan-aspirated shield compatible with the US CRN system (Climate Reference Network). Note that radiation shield differences can yield huge differences in measured temperature: on 2 days in this interval the difference is >6°C. If data were available from the multiplate shield on the 13th (sensor was being replaced) a 10+ degree difference would not be surprising. Wind speed and solar radiation critically influence temperature measurement!