This is part 3 of a 5 part series. Be sure to read the earlier parts first if you haven’t already.
- Getting the images on your computer
- Visualizing the data
- Examples: Lake Winnipesaukee; Lake Sunapee (this article)
- Getting the images while on the ice
- Additional information
Let’s continue with Winnipesaukee for a moment, but zoom out one notch. And let’s start by looking at what’s changed in the past few weeks. First at 1/17, in infrared false color.
Then 1/27 (1/22 was cloud covered), in infrared false color.
Then 2/1, in infrared false color:
You can see ice coverage growing, shrinking in places, and changing character, all of which makes sense given the weather we’ve had.
Zoom back in to the area just north of Leavitt Beach and look at the 2/1 image.
I’ve circled in yellow a lead that was at least partially covered in black ice on Monday (2 days before this image was taken). It was thin but some skaters crossed it, very carefully.
You can see how different newly forming primary ice looks from the thick snow ice that these plates are composed of, but it’s hard to tell the difference between black ice and open water (which the area to the north definitely was).
Switch to NDWI (water index) and the difference between black ice and open water jumps out.
Switch to NDSI (snow index) and the distinction between black ice and open water is even clearer.
Switching between visualizations like this is very powerful. No single visualization can tell you everything you’d like to know, but by comparing multiple different kinds you can get a clearer picture of what’s going on.
Jamie’s post to the NH Nordic Skating list on 1/27 (the archives are only viewable by group members) about satellite images and how to interpret them has more detail and is a better example than any I’ve got here. Go read that.
In that email, he was describing the images from 1/17 and how they compared to what he saw on the lake that day. On 1/29 he and a group skated down the entire eastern shore of the lake, and found decent conditions. On 1/31 I tried to skate on the lake, following their route, and was stymied by a wide variety of bad conditions. Let’s look at what happened, from the satellite perspective.
1/17, pretty much all black ice. Promising! But possibly not quite ready for skating. Hard to say without getting out there and ground truthing.
1/27, following Jamie’s guidance, looks like gray ice! Much more definitive. But is it good ice?
And are those clouds covering the eastern shore? Hard to tell. Switching to NDSI (snow index) makes it clear: yes, there’s a little bit of cloud coverage, but no, the clouds aren’t hiding anything important.
But how much variation between ice and snow covered ice is there? It’s a little hard to tell using the infrared image, so let’s look at NDWI (water index).
Much clearer. Easy to see now how the group was able to skate the eastern shore, and why that specifically was relatively good quality.
2/1, the day after I found much worse conditions (back to infrared false color).
The eastern shore looks whiter than on 1/27, but it’s still hard to tell exactly what’s going on. Take a look at the NDWI (water index) image and comparing it to the NDWI image from 1/27 makes it clear: what was ice with areas of snow is now much more evenly covered with snow.
And here’s a side-by-side split-screen comparison of the NDWI visualizations from 2/1 (on the left) and 1/27 (on the right), to make the difference even clearer. (I’ll explain how to use the comparison tool in another article).
Again, impossible to say for sure what’s going on without getting out there. But by comparing multiple visualizations, you can get a better sense of what’s going on and see how conditions are changing over time.
Now that you’ve seen some examples of how to analyze the images to find skateable ice, continue on to part 4, getting the images on the go.
— Christopher Boone, some rights reserved (CC BY-NC-SA 4.0)