2015-09-28 17:09:39.000 – Ryan Knapp, Weather Observer/Staff Meteorologist
Welcome back to our weekly #MWOMetMonday.
Our first question is from Tim Myers on Twitter who asks:
While we do not keep records on the length of rime ice, one event for me sticks out as having the longest rime ice formations I’ve personally witnessed and measured. During the end of January into the start of February 2012, a moist flow from the south set up as cold air encroached from the north. The one-two punch of these two allowed rime to build. But let me back up a bit and explain what rime is. Rime ice forms when the summit is in the fog and below freezing. Clouds (or summit fog in our case) are made up of supercooled water droplets. When I say “droplets,” I am not talking about rain or drizzle size droplets; these droplets measure 50 micrometers (0.00197 inches) or less. So in other words – very, very small. Supercooled means that the droplets are below freezing but are not in solid form. There are many reasons that factor into why this occurs, but the primary reason is because these little droplets lack a nuclei. If you want to see a video with a further explanation and how to see it yourself, we found this one:
So, as these supercooled droplets flow over the summit, they eventually hit a surface – a wall, rocks, summit sign, jackets, eyelashes, etc. As soon as they hit a surface, that surface acts like a nuclei and the stability of the droplet becomes compromised and freezes instantly. As more and more droplets smash into a surface, they start to build on each other growing into the direction of the wind. So a wind from the west would cause the rime to grow towards the west. If you have turbulent flow over and around solid objects (rocks, buildings, etc), you can see the turbulent flow as the feathers twist and curve into the flow over and around the object. Over time, the physics of ice formation allows formations anywhere from just a coating (what we usually see on our clothing when we deice the instruments – the “glazed doughnut” look) to large feathers and spikes growing several inches or feet long.
Getting back to the original question, the longest rime ice feathers/spikes I have witnessed were over 10 feet in length. During the event, we were deicing the instrumentation once to twice an hour with occasionally a third deicing trip necessary. After the event, everything was cocooned in rime leaving a twisted dream-like view all around. Here is what our weather tower looked like after that event when we finally cleared for a bit.
Since we only had one question this week, I will instead answer a common question I have seen about our pictures of last night’s lunar eclipse – How did you capture that?
The camera was a Canon 60D. The lens was EF-S 18mm-200mm f/3.5-5.6 IS. The tripod was a heavily weighted mount.
At the start, the moon was bright, so F-stop was f/10, exposure time was 1/250 sec, ISO was 100, with a focal length of 200mm shot in JPEG+RAW format.
At the partial eclipse, the F-stop was f/10, exposure time was 1/500 sec, ISO was 400 to 800 (400 as it started and 800 as it slivered), focal length was 200mm and shot in JPEG+RAW format.
At total eclipse, the F-stop was as wide as my lens allowed at 200mm which is f/5.6, exposure time was 1.6 secs, ISO was 1600, and it was shot in JPEG+RAW format.
Winds were 30 to 45 mph on the summit, so I had to shoot in a sheltered area to minimize shaking from the winds. A remote trigger was used to minimize shaking from hitting the shutter button. Image stabilization and auto-focus were turned off as those can typically blur an image at night; auto-focus is bad as it is not bright enough with enough contrast for it to work effectively and IS tries to compensate but usually overcompensates at night making it blurrier than having it off. The movement of the Earth and the moon is easily picked up when zoomed in, so a fast exposure time is ideal which sounds counter intuitive to those that have shot stars or other night scenes. However, since the moon is so bright (at least for the start and end of the eclipse), a small aperture and fast shutter time could be utilized easily to keep things sharp. However, because of these short times and small apertures, stars were not present in the image as they are not bright enough to be captured. Then the images were edited in Adobe Lightroom from the RAW formats, exported as JPEGs and posted online.
Ellen Estabrook2023-11-08T07:34:12-05:00November 7th, 2023|Comments Off on A Glimpse at METAR Reports
A Glimpse at METAR Reports By Alexis George, Weather Observer & Meteorologist METAR observations are submitted every hour of every day at Mount Washington Observatory. METAR is a format for reporting weather information that gets
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Mount Washington Observatory is a private, nonprofit, member-supported institution with a mission to advance understanding of the natural systems that create Earth’s weather and climate. It serves this mission by maintaining a weather station on the summit of Mount Washington, performing weather and climate research, conducting innovative science education programs, and interpreting the heritage of the Mount Washington region. Our weather station is located on the summit of Mount Washington in New Hampshire, at Mount Washington State Park.