Guide

Look closerWeather

Ripples on icicles

December to March

Icicles grow on cold days when sun or a warm roof melts snow and the water freezes again as it drips. Look closely and many icicles have ripples running around them, spaced about a centimeter apart.

Why this time of year

December through March brings snow plus days that stay below freezing. A sunny cold day after a snowfall, or a roof warmed from inside, gives icicles the steady drip they need to grow.

How to notice it

  1. After a snowfall, on a day below freezing, look along roof edges, gutters, fences, and railings for icicles that are still dripping.
  2. Pick a small icicle hanging low, away from any roof edge, and look along its length for evenly spaced bumps or rings.
  3. If you find ripples on both a thin icicle and a fat one, compare them: researchers found the spacing stays close to 1 centimeter whatever the growing conditions.

Try this with kids: Find a small icicle hanging low on a fence, railing, or bush, away from any roof edge. Without touching it, count how many ripples you can see from top to bottom.

Word to know: “Ripples”: small rings of bumps around an icicle, about a centimeter apart.

Safety

  • Do not stand or walk under icicles hanging from a roof or gutter, and do not knock them down; falling icicles can cause serious injury, and kids tempted to knock them off are the ones most prone to injury.
  • Look from a safe distance, away from the building edge. Never climb a ladder or roof to reach icicles.
  • A row of icicles along the eaves can be a warning sign of an ice dam, where melted snow refreezes at the roof edge and can back up under shingles. Snow and ice removal on or below a roof is dangerous and is best left to professionals.

Sources

  • ar5iv.arxiv.orgChen and Morris, U Toronto Physics, New J Phys 2013: 'In all cases, the ripple wavelength is close to 1 cm'; 'the ripple wavelength is remarkably universal, independent of the growing conditions'.
  • arxiv.orgAbstract: 'Natural icicles often exhibit ripples about their circumference'; pure-water icicles do not ripple; dissolved ionic impurities trigger ripples.
  • utoronto.caU Toronto release Dec 1 2022: ripples depend on something foreign in the water, whatever the impurity.
  • wxguys.ssec.wisc.eduUW-Madison Weather Guys: icicles form on days below freezing when sunshine melts snow or ice; need constant flowing water layer.
  • umass.eduUMass BCT: ice dams form when melted snow refreezes at roof edges; escaping house heat warms the upper roof.
  • extension.missouri.eduMissouri Extension: icicles on the overhang are a warning sign of ice dams; roof snow removal dangerous.
  • newswise.comLoyola Medicine: avoid walking under hanging ice; kids tempted to knock off spikes are most prone to injury.
  • ncei.noaa.govNOAA NCEI Boston Logan normal snowfall: Nov 0.7, Dec 9.0, Jan 14.3, Feb 14.4, Mar 9.0, Apr 1.6 in.
  • ncei.noaa.govNOAA NCEI Boston Logan: days with low at or below 32 F: Dec 19.8, Jan 24.8, Feb 21.7, Mar 16.2, Nov 6.8.

Checked against these sources on November 3, 2026.

In the guide for December, January, February, March

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