{"id":1313,"date":"2023-02-19T03:35:03","date_gmt":"2023-02-19T03:35:03","guid":{"rendered":"https:\/\/tomeri.org\/journal\/?p=1313"},"modified":"2024-08-10T21:42:01","modified_gmt":"2024-08-10T21:42:01","slug":"ice-crystals-on-minihaha","status":"publish","type":"post","link":"https:\/\/tomeri.org\/journal\/2023\/02\/19\/ice-crystals-on-minihaha\/","title":{"rendered":"Ice Crystals on Minihaha Creek"},"content":{"rendered":"\n

Saturday, 18 February 2023<\/em><\/p>\n\n\n\n

The winter here has been weird \u2013 a couple of sub-zero weeks, a couple of warmish (32ish) weeks, repeat. I run several days a week, mostly along Minihaha Creek, which winds through south Minneapolis. With the variations in temperature there is a lot of thawing and refreezing, and that, combined with changing water levels in the creek, results in marvelous ice crystals. Mostly they are two dimension intergrowths of needle-like crystals, almost fabric-like in their structure. <\/p>\n\n\n\n\n\n\n\n

Below you see the general context. You can see the frozen creek, and more interestingly you can see differing textures of ice that silhouette the stream’s channel. I presume that the depth (which means the water cools more slowly), and degree of movement of the water (ditto), effects the rate at which crystals form. In geology the more slowly something cools, the bigger its crystals. (Note that ice meets all the geological criteria for something to be considered a mineral and thus a rock, and so I enjoy envisioning that I am running through a (cyro-)volcanic landscape with barely cooled rock.)<\/p>\n\n\n\n

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Minihaha Creek, frozen: Note how the texture of ice reveals the main channel.<\/figcaption><\/figure>\n\n\n\n

Below is another picture — this is perhaps 2 x 3 feet — that shows a region of ice with an acicular (needle-like) fabric of crystals. Water takes on an amazing variety of crystaline forms as it freezes, depending on the temperature and humidity. Acicular forms are common at close to 32\u00b0 F; however, if the temperature drops rapidly, the water can stay liquid well below the freezing temperature (this is called super cooling), until something, usually a bit of dust or other material, creates a nucleus for ice crystals to form around.<\/p>\n\n\n\n

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A ‘fabric’ of intergrown acicular crystals in the middle of the stream (where the water is deeper and freezing proceeds more slowly, resulting in larger crystals)<\/figcaption><\/figure>\n\n\n\n

The above picture shows a homogenous area in mid-stream, but if we focus more towards the edges, there is a clear progression from small (fast-forming, because the water is shallow) crystals to larger ones in the more slowly cooling deeper water. <\/p>\n\n\n\n

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Crystallization, beginning from a surface of pre-existing ice, grows out into gradually deepening water, with crystal size proportional to depth, because deeper water cools more slowly (and probably flow, also proportional to depth, effects it as well). <\/figcaption><\/figure>\n\n\n\n

One day, during a period of intense cold, I noticed an unusual pattern on the ice on part of the stream. It was as though there were tiny pyramids or piles of snow \u2013 about a square centimeter in area \u2013 separated by an equal amount of space. <\/p>\n\n\n\n

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Dark clear ice on Minihaha Creek covered with small discrete mounds of snow. On closer inspection, the mounds prove to be pyramidal structures of ice crystals. <\/figcaption><\/figure>\n\n\n\n

On closer inspection, the \u2018piles\u2019 were not miniature mounds of snow, but rather pyramidal crystalline structures. Perhaps open water had frozen over while there was still a lot of (super cooled?) water vapor in the air, and the vapor then crystallized onto the surface of the ice? I have no idea, really, but that at least helps describe what I saw.<\/p>\n\n\n\n

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Pyramidal mounds of ice crystals on Minhaha Creek formed during a period of intense cold.<\/figcaption><\/figure>\n\n\n\n


If you look closely at the fine structure you can see a lot of different forms of crystal growth like dendrites and hexagonal growth patterns.<\/p>\n\n\n\n

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Fine-structure of ice crystals — note the delicate dendritic structures that divide and then divide again<\/figcaption><\/figure>\n\n\n\n


Stepping back, another thing that\u2019s interesting is that you can see changes in the freezing regime. In the picture below there is an abrupt change from isolated \u2018pyramids\u2019 surrounded by clear ice to pyramids surrounded by speckled ice, the speckles being smaller ice crystals. In geological terms this indicates increased “nucleation,” which most likely indicates a rapid temperature drop in air temperature as the surface ice was growing out over deeper water. <\/p>\n\n\n\n

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A change in the freezing regime as seen in the appearance of many small speckles (tiny ice crystal nuclei)<\/figcaption><\/figure>\n\n\n\n

Another thing I like about ice \u2013 at least ice on the surface of streams \u2013 is its color. Early in the season it can be nearly transparent. Also, if the weather cooperates and there is a very, very light snow, the snow can frost the surface of the ice in such a way that it reveals the temperature of the water beneath the ice. That is, the light dusting of snow persists where the sub-surface water is cold, but melts or sublimates on areas where the water below the ice is warmer. This can reveal the deeper (warmer) primary channels of the stream,and, more interestingly, small springs at the bottom of the channel — usually appearing as circles or ellipses — as in the rightmost picture below. <\/p>\n\n\n\n

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Once the winter has progressed, and the ice has thickened, these features vanish. Instead, the transparence of the ice shifts: early on it goes from clear to a translucent gray and \u2013 supplemented by snow, slush, and simply many freeze-thaw cycles, an opaque matte white. After the solstice, areas of the stream that receive increased light develop a greenish tinge: my reading suggests that this is due to algae, either in the ice, or just below.<\/p>\n\n\n\n

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Later in the season ice can take on a greenish tinge, due to algae growing in or just below the ice. <\/figcaption><\/figure>\n\n\n\n


That’s it!<\/p>\n\n\n\n

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Views: 24<\/p>","protected":false},"excerpt":{"rendered":"

Saturday, 18 February 2023 The winter here has been weird \u2013 a couple of sub-zero weeks, a couple of warmish (32ish) weeks, repeat. I run several days a week, mostly along Minihaha Creek, which winds through south Minneapolis. With the variations in temperature there is a lot of thawing and refreezing, and that, combined with … Continue reading Ice Crystals on Minihaha Creek<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":1340,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"activitypub_content_warning":"","activitypub_content_visibility":"","activitypub_max_image_attachments":4,"activitypub_interaction_policy_quote":"anyone","activitypub_status":"federate","footnotes":""},"categories":[1,51,61,34,72,68],"tags":[],"class_list":["post-1313","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-daily_entry","category-essay-stub","category-geology","category-has-pictures","category-ice-crystals-etc","category-nature"],"_links":{"self":[{"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/posts\/1313","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/comments?post=1313"}],"version-history":[{"count":8,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/posts\/1313\/revisions"}],"predecessor-version":[{"id":1351,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/posts\/1313\/revisions\/1351"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/media\/1340"}],"wp:attachment":[{"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/media?parent=1313"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/categories?post=1313"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/tomeri.org\/journal\/wp-json\/wp\/v2\/tags?post=1313"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}