This map shows the average fractional morning cloud cover in May along the track of the eclipse, derived from 17 years of satellite observations of the globe. The nominal time of the observations is 7 a.m. Image: J. Anderson. Data: CIMMS/SSEC/University of Wisconsin.

Weather Discussion

Solar system geometry favours Australia once again, as the May annular eclipse revisits Queensland and the Northern Territory, nearly paralleling the path of the total eclipse of November 2012. This time, however, it embarks a little farther west, rising in mid-eclipse over the northern parts of Western Australia. For those wishing to catch this lunar-solar conjunction, Australia promises by far the best weather prospects.

In May, the southern hemisphere summer has given up and the season stands on the threshold of winter. The Earth's semi-permanent high-pressure zones have settled in at Australia's latitude, helping to bring the transition to the dry season. The Intertropical Convergence Zone (ITCZ) and the South Pacific Convergence Zone (SPCZ) have been nudged a little northward beyond the Australian continent. These two cloud and precipitation regions are caused by converging wind flows and are visible in the cloud cover map above. The ITCZ is the band of heavier cloudiness running eastward from the Philippines, while the SPCZ angles southeastward across New Guinea, past Fiji and Tonga. The two join together over Borneo. The upshot of all of this interplay of highs, lows, and convergent wind flows is that the eclipse track enjoys a marvellously sunny climate while over Australia but crosses into a much cloudier regime once it leaves the land and encounters both the SPCZ and the ITCZ.

The transition from sunshine to cloud is abrupt and dramatic. Over much of the outback interior of Australia, the table above shows that cloud amounts range between 25 and 35 percent. This climbs to between 40 and 50 percent along the Queensland coast, and then to between 60 and 70 percent over the Pacific Islands, though there are some parts of Polynesia that are more favourable than others, as can be seen in the satellite map of cloudiness in the table.

Tennant Creek and its environs in the Nothern Territory are the choicest places for the cloud averse, but just about any part of Australia will offer excellent weather prospects. At the challenging western edge of the track, in Western Australia where the Sun will rise in eclipse, average May morning cloud amounts generally range around 30 percent, giving good prospects for a horizon-bordered photo of a ringed, oval Sun.

Once away from the Australian continent, the track touches Papua New Guinea and the Solomon Islands, crossing the very cloudy climate along the SPCZ. The cloud-cover map shows that this cloudiness lets up a bit as the shadow reaches Kiribati, but the relief is short-lived, for the ITCZ lies in waiting as the track stretches to its most northerly point. The one remaining bit of land in the path—Fanning Island—lies on the south edge of the ITCZ, and though the path then moves into sunnier skies, only ship-borne observers will be able to sample the late afternoon antumbral shadow.