Tag Archives: meteorology

Snow, then sun, then heavy fog, then rain. WHY?

What happened?

Recently at the Campus of university of Notre Dame, IN, US, a strange (at least to me) weather phenomenon appeared that caught my interest. So here is what happens.

It started off with a snow storm, which resulted into more than 40 cm of snow pack on the ground.

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The temperature rebounded at a rapid rate to more than 10 degree celsius. The sky was cleared and we enjoyed a beautiful day. Just one day.

Processed with VSCO with a5 preset
Processed with VSCO with a5 preset

Same day, at night, fog started to form. The fog persists to the next day until afternoon when the rain started, and temperature fell back to zero degree celsius. A strange thing to me is that, the fog in the second day has a steep density gradient, dense near the ground and started to diminish above tens of meters.

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The rain started in the afternoon. It was pretty heavy.

The next day, which is today, it started to feel cold again.

So my initial conjecture is like this.

  1. Starting with the heavy snow, which is like to be resulted from the instability from jet stream, the thick snow pack is formed.
  2. Temperature started rising due to the periodic instability warm air from equator is carried over. Also, the temperature rises because the formation of the snow releases sensible heat that negatively feedbacks on the cold environment.
  3. Then, the fog forms due to the fact that the difference between temperature and dew point has reached beyond a certain limit (~2.5 degrees). The decreasing temperature is caused by the melting of the snow, also could be due to the cold instability is coming back. Also, at the same time, humidity rises due to the melting of snow (which could be limited as melting leads to water..)
    1. The large gradient of the fog, then, is caused by the temperature gradient, low near ground due to melting.
  4. The reason for the rain afterwards is then due to the geostropic flows caused from instability that creates the updraft, which leads to rain cloud formation and heavy rain quickly afterwards.

What my advisor informed me

So, after this initial thought, I went to consult with my advisor, who is a world-leading expert in meteorology. He suggested and corrected several parts of my reasoning, some due to conceptual error, some due to lack of knowledge of real information.

  • The temperature rises is most probably due to the advection from the Michigan Lake, which carries warm air and large amount of moisture over. This can also explains the increase in moisture.
  • He corrected that the radiation from sun will not lead to significant snow melting as the albedo (level of absorption of radiation, 0 -> all radiation reflected, 1 -> all radiation absorbed) is low for snow, hence most radiation from the sun is reflected away. This then implies the reason for increased temperature is due to instability, advection from lake, or both.

Conclusion

So in general my hypothesis is mostly in the right track. Fog dynamics is interesting.

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