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Deana

Why is fog white and does the sun melt it?

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I really need help with how fog is formed, all of the websites online are quite complex. Firstly, is fog white because light from the sun is trapped in its suspended water droplets? If the heat from sun melts these suspended water droplets, is the light released and thus...the fog burned off? This is my understanding of fog, is this correct?  Please explain your answers very simply as I am struggling to piece it all together...

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An explanation of why clouds/fog are white can be found here:

https://www.metoffice.gov.uk/learning/clouds/why-are-clouds-white

Quote

Clouds are white because light from the sun is white. As light passes through a cloud, it interacts with the water droplets, which are much bigger than the atmospheric particles that exist in the sky.

When sunlight reaches an atmospheric particle in the sky, blue light is scattered away more strongly than other colours, giving the impression that the sky is blue.

But in a cloud, sunlight is scattered by much larger water droplets. These scatter all colours almost equally meaning that the sunlight continues to remain white and so making the clouds appear white against the background of the blue sky.

In terms of fog itself...Air has both a temperature and a dew point temperature, the latter is the measure of moisture. The dew point temperature is the point at which the air must be cooled for the moisture in the air to be condensed out. For example, if you have air with a temperature of 5c and a dew point of 2c, you would need to cool the air down to 2c for the moisture in it to condense into clouds, or if at the surface, fog. 

Fog tends to form on calm, still nights, where the surface can cool down quickly to this dew point temperature without the air being mixed up. If there is a breeze then this cooling process doesn't occur anywhere near as efficiently, and therefore fog tends not to form. 

If fog forms, then the next day to clear the fog, we either need the surface air to warm up (so that the temperature raises above the dew point) or for drier air to move in at the surface (so that the dew point falls, a lower dew point indicates drier air and vice versa). The term "fog burning off" simply refers to the process of the sun heating the surface, promoting both mixing of the air above and surface warming, which eventually causes the fog to disperse.

 

Note: This is the main kind of fog we see in the UK during the cooler months, known as "radiation" fog. There are other ways that fog is formed, but this is your typical stuff that will burn off the following morning after a previously clear, calm night.

Edited by Nick L
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8 minutes ago, Deana said:

I really need help with how fog is formed, all of the websites online are quite complex. Firstly, is fog white because light from the sun is trapped in its suspended water droplets? If the heat from sun melts these suspended water droplets, is the light released and thus...the fog burned off? This is my understanding of fog, is this correct?  Please explain your answers very simply as I am struggling to piece it all together...

Melt is the wrong word. Melting is the process of going from the solid to liquid phase. 

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5 minutes ago, Weather-history said:

Melt is the wrong word. Melting is the process of going from the solid to liquid phase. 

Aye WH, and things are further complicated when bona fide forecasters inform us all that 'any early fog should soon burn off'?:cc_confused:

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