Southwest winds brought unseasonably warm temperatures to the Marquette area on April 17th, with high temperatures rising into the lower 70s. At the Marquette Coast Guard weather station (MCGM4), early afternoon temperatures were hovering right around 70 degrees through 2 PM under the offshore winds.
A lake breeze developed and moved onshore shortly after 2 PM, which caused the temperature at the Marquette Coast Guard to drop 20 degrees in 12 minutes. The temperature went from 69 degrees at 2:00 PM to 49 degrees at 2:12 PM.
During the same time period, winds shifted from the west (off the land) to the northeast (off Lake Superior). With the lake breeze already past the weather station, temperatures continued to hover around 50 degrees for the rest of the afternoon.
A similar situation occurred the previous day (4/16) at MCGM4. The temperature dropped from 54 degrees at 11:30 AM to 37 degrees at 12:06 PM (a 17 degree drop), as the lake breeze shifted winds to the east.
What is a Lake Breeze?
A lake breeze is a thermally produced wind blowing during the day from the surface of a large lake to the shore. It is caused by the difference in the rates of heating of the surfaces of the lake and of the land. In this case, temperatures over the land rose in the lower 70s, while Lake Superior water surface temperatures were in the middle 30s. This can be seen in the image below from April 16th.
For current Lake Superior water temperatures, visit the Michigan Sea Grant Coastwatch website.