Waiting on Weather

Waiting on Weather

Fall and winter in the North Pacific is a time for strong winds, big waves and storms.  Low pressure systems originating in the west cross the Pacific and move toward the Bering Sea.  When doing ship-based research, deploying and recovering instruments over the side with a crane is a tricky operation even in calm weather; attempting it in rough weather is simply not worth the risk.  Hence, monitoring the weather is a crucial part of offshore operations. 

National weather services such as Environment Canada have prediction centers whose models generate forecasts and create updated weather maps many times a day.  On the upper map below (Figure 1), the L represents the center of a low pressure system (usually accompanied by bad weather) and the H, a high pressure one.  The contour lines (isobars) indicate regions of equal air pressure; stronger winds occur in areas where the pressure gradient is larger and the isobars are closer together.  On the lower map below (Figure 1), the arrow-like symbols (Figure 2) illustrate wind speed and direction.  The end where the “point” should be shows the direction toward which the wind blows.  The “feathers” indicate its force, a triangle if 50knots, one full line is 10knots and a half line is 5knots.  In a low pressure system, the wind moves counter clock wise (at least in the northern hemisphere); the air rises in the atmosphere where it eventually cools down and its water content condenses forming clouds.  On that same map, the regions of different colors indicate wave height.

Figure 1: Forecasted weather map for Canada for December 1^st 2010 showing the location of low and high pressure systems (top). Forecasted sea state map of the Northeast Pacific for the same date (bottom).  Both maps were created by Environment Canada.

Figure 2: Symbol used to indicate wind speed and direction on a weather map.  In this example, the symbol shows wind blowing at 45knots, roughly to the east.

In the case of operations involving remotely operated vehicles or manned submersibles, when the waves are higher than 3meters or the wind is blowing above 25knots, diving is not possible.  Research cruises usually lose at least a few days due to bad weather and fewer of them are scheduled in the winter months.  The weather in the Northeast Pacific can also change very quickly.  Sometimes a dive can start, but has to end early because the wind is picking up and the swell is expected to follow.  In the case of deep sites like Endeavour, contingency time allowing for the two hour long ascent also needs to be factored in; making the planning of operations with respect to weather somewhat of an art.  Once deployed, instruments connected to cabled observatories will not be affected by rough weather; this continuous presence is one of the main advantages of this sampling tool.  But after the storm the nice weather eventually comes back and we have beautiful days where the ocean looks like a mirror (Figure 3).

Figure 3: The end of a beautiful day aboard the R/V Thompson