Two high-pressure systems straddle the North American continent, one northeast of Hawaii in the Pacific, the other between Bermuda and the Azores in the Atlantic. T he chart of average April sea-level pressure shown in Figure 2 sets the stage for this discussion of eclipse weather. Somewhat surprisingly, it is the presence of water-the Great Lakes and the Gulf of St Lawrence-that has the biggest impact on cloudiness. Unfortunately, the spring season and the location of the storm tracks through this area superimposes the benefits given by the terrain onto a very cloudy climatology, giving limited relief to eclipse travellers. Only after reaching the Middle Atlantic States south of Lake Erie does the Moon’s path cross the rugged topography of the northern Appalachian Mountains where some advantage might be gained by hiding behind a topographic barrier. From Texas to the Great Lakes, the track moves across the low topography of the Great Plains and the Mississippi Valley where topographic and cloud-cover variations are muted. In Mexico, where the Sierra Madre Oriental Mountains reach heights above 2600 metres (Figure 1), the spring season is already so dry that the cloud amounts barely respond to the presence of the mountains, on both the windward and leeward slopes. Locations in the lee of high terrain tend to be sunnier-sometimes considerably sunnier-than those on the windward side. In the 2017 eclipse, the rugged terrain of the western mountains had a large influence on the formation of clouds (though the day itself was sunny and needed no help from the terrain). Figure 1: Topographic map showing the track of the eclipse across North America. In Maritime Canada, the last of the winter snow has yet to melt and fresh snowfalls are a threat with every weather system. Over the United States, southern parts of the track are already well into the thunderstorm season, while to the north, spring storms and occasional snowfalls still hint of the departing winter. In Mexico, the winter dry season is in its last month before the summer rains begin. April is a month of transition across the continent, with winter storms gradually giving way to the convective buildups of spring and summer. North America’s second total eclipse of the past seven years comes at a very different season than that in 2017. Animation of the passage of the lunar shadow created in WinEclipse, a program developed by the late Heinz Scsibrany.
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