Today's stories: · Weather Prediction · Shopping in Cuetzalan Pete offers a description of the automatic weather information system: This video requires the QuickTime Plugin to be viewed.

Expedition Weather Prediction

The Situation
Caving when the weather is bad is a calculated risk. Caves by their very nature are natural conduits for rainwater, the amount of water taken by a cave depending on the size of its catchment area. If this area is large, even a small amount of rainfall can be focused and channelled into a small cave, flooding it to the roof for an extended period of time. The consequences for cavers trapped behind the water can be dire.

In Cuetzalan, it is evident that the majority of water falling on the surrounding area drops into the active cave systems that we are currently exploring. To visualise this, you could imagine the cave systems as the fingers on a hand with Alpazat at the wrist, where all the caves meet.

Weather Prediction
The expedition relies on outside agencies to keep it informed about the weather. We have direct assistance from the Meteorological Office in the UK, which provides long term weather pictures tailored to our needs. However, here in the hills of Mexico, the weather systems are localized and difficult to predict accurately. The fluctuating pressure and humidity sometimes lead to snap storms with little or no warning. For this reason, we decided to bring along some weather measuring equipment (kindly loaned to us by Campbell Scientific, UK).

The equipment is set up at base camp and comprises three sensors (pressure, temperature and relative humidity) connected to a self-powered datalogger. I have set it up to collect information from each of these probes every fifteen minutes, and store the information away in its memory. Each morning (or more often if the situation warrants) the information is downloaded to a laptop computer and a graph is generated.

We are on the lookout for a sharp drop in temperature and/or increase in humidity, which would suggest that a weather front is moving in. This will then give us a brief time in which to alert the people down the caves.

Bad Weather Warning
The main thrust of the expedition is currently down the resurgence at la Cueva de Alpazat which is also the main resurgence of water from the Cuetzalan area into the Tozan River. If the rain hits, we would have very little time before Alpazat responds by flooding. For this reason, we always have someone on standby in the base camp ready to speed down to Alpazat to sound the alarm.

When the runner arrives at the cave entrance, he alerts the cavers underground by attaching a telephone to the cable running from the mouth of the cave all the way through to the underground camp. A constant tone on the line is the signal for the speedy evacuation of the cave.

Pete Simpson