Southeast Alaska’s summer has been an unusual one from several angles (read a summer review here). The latest thing to get people’s attention, at least in Haines where I live, was a striking wall of fog that moved in on the 5th (of Sep 2025). I received more than a few comments (“cool” “simply amazing” “eerie”), questions, and photos about it, so I took that as a cue to blog about it. I love hearing people’s weather experiences and questions, and, of course, seeing their photos. (I use a lot of photos in my Alaska Weather Calendar and actively encourage photo submissions from anyone.)

Here are three by locals who had their cameras at the ready:

visit Joe Ordonez’s website

visit Kathleen Menke’s website

What’s going on here?

The science of weather is a complicated one in that there are many variations, exceptions, approximations, howevers, etc to what you might have thought, or heard, or seen in a book or a website — things that have been simplified, understandably so, to make it quicker or easier to learn. But taking it a little deeper can reward you with the new insights. Nonetheless, if you are in a hurry, let me see how I do with a once-sentence explanation, for starters.

The cool-looking fog that people were talking about came off the open ocean, pushed into the inner channels by the same high pressure area responsible for creating it.

Go deeper? Let’s revisit high and low pressure systems. On the simplest level you may have heard that low pressure systems bring clouds, rain or snow — generally “bad” weather. High pressure systems bring clearing, dry weather… “good” weather. Well yes, but… The fog that packed into much of the inside passage recently was due to a high pressure system over the ocean, and if you were stuck at an airport waiting for it to clear, or out fishing wondering if you might be in the path of an unseen cruise ship you might not have put it in the “good” weather column.

Let’s dig deeper about high pressure systems, or “highs” for short. We’re talking about an area at the surface of the earth (ground or ocean level) where the air pressure is higher than surrounding areas. Analogous to a hill in the topographic sense. It’s true, the most noted effect of a high is to clear clouds. This is because in a 3 dimensional view there is going to be sinking air above the surface high. Sinking air warms as it is subject to the increases in pressure of lower altitudes. This warming lowers the relative humidity, causing clouds to evaporate. (a “low” works in the opposite: rising, cooling air causing condensation.)

Here’s a surface map showing the huge high pressure area covering much of the NE Pacific. There’s a healthy low in the Bering Sea but it’s not going to be able to progress much farther east with the high in the way. The map is valid 4 am on the 6th.

Here’s the all-important “however:” the sinking air can’t sink through solid ground or ocean. Instead, it spreads out horizontally, ie., radially away from the center of highest pressure. This produces the usually gentle winds of a high, blowing from higher to lower pressure as one would expect. It also means a shallow layer of air from the surface to a few hundreds to a thousand feet or so up is not part of the sinking effect, so is no longer warming. This relatively thin layer of atmosphere is now most strongly effected by the ground or water it is in contact with. I’m going focus on this layer’s behavior over the ocean in this article.

Water has a much, much greater heat capacity than air, so when the two are in contact for any amount of time the air is going to respond most dramatically and cool quite quickly to very near the water’s temperature, called the sea surface temperature (SST) in the lingo. This has several consequences: 1) cooling the air very often condenses out water vapor into fog or stratus (a cloud type that’s basically the same as fog, only lifted a short distance off the surface). 2) The cooling increases the density of the air making it more stable, which discourages it from mixing with the air above. Because it is so stable this weather feature — called a marine layer — is very persistent. It’s basically not going away until the high pressure is disturbed by an outside force such as a low.

As the winds blow out from the center of the high on all sides, the fog and/or stratus moves with it. Sometimes it dissipates as it strays too far from the source, but sometimes it invades adjacent land areas. How often a given piece of land gets this fog invasion — no big secret here — has to do with how exposed to the open ocean it is. I’ll dig a little deeper into this toward the end of the post.

The recent fog

Here are two NOAA polar orbiting satellite images from the general time period, showing the coast from Yakutat in the upper left corner through Haida Gwaii archipelago in British Columbia in the lower right:

2025-09-03 1209 pm ADT

Above, on the 3rd, dense stratus/fog had been packed into the southern coastal areas, covering the water and adjacent lower terrain, but the northern 2/3rds of SE was basking in rare sunshine… even the outer coast. Off the northern coast the cloud coverage at that time looks to be more of a stratocumulus, with lots of small cells and weak areas or breaks. Below, the next day, the 4th, the cloud mass had progressed north and deepened. Also, the northern part looked much more solid and had bumped up against the outer coast, covering Sitka, and had started to push into Cross Sound, the only large ocean entrance to the northern panhandle. Most folks in the northern inner channels were still enjoying the ignorant bliss of high pressure’s good persona. (click for larger versions). But that was about to change…

2025-09-04 1241 pm ADT

Here’s a 4-frame animation showing this marine push (as it’s sometimes called in weather geek speak) moving further north over a 10-hour period during the wee hours of September 5th. This is from the GOES satellite, showing the GeoColor product in which, at night, fog/low clouds look blue.

Digging yet deeper

Looking at the maps and satellite images above it should be pretty intuitive why places on, or closer to, the outer coast are more exposed to this marine layer and its typical weather than those on the inner channels, despite both being “on the ocean.” Besides stratus and fog which I’ve gone on about, that would also includes a more moderate temperatures range between summer and winter (seasonal) and between day and night (diurnal). For a more objective measure try this: Take a map and draw a circle around the piece of land in question, radius, say, 30 miles. The percentage of the circled area that is water vs land tells you how “marine” the spot is to first approximation, vs the opposite, continental. Then refine that by looking at any mountains or ridges positioned to block the marine influence. And, yes, how far from the outer coast, the true open ocean, the location is is very important too, but not the only factor. When it comes to other kinds of fog, there are more factors yet. For further reading I’d recommend feature articles I’ve written for the Alaska Weather Calendar on fog, in the 2025 issue and on this concept of continentality, in the 2026 edition.

Looking at some examples from Southeast Alaska, consider Yakutat, Sitka, Ketchikan, Gustavus, Juneau, Haines & Skagway mapped below. This list is in order from most to least marine. They are circled below. (click for larger versions of all the maps)

Below are zoomed in sections. I think these illustrate the point quite well.

Thanks for reading and I’d love to hear if you have any comments or questions, ideas for future topics, or photos to share.