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I am residing in New Delhi (pardon me for any irrelevant data I might include), and I noticed a really interesting thing. It's an average $30^{\circ}\mathrm{C}$ out here, and yet, most/some weather forecasting media are reporting the conditions as 'foggy'.

Is there a way to describe why this is happening? Yes it's overcast all the time, with some rain , but then it shouldn't amount to any hindered visibility , via fog, or mist for that case. And we can safely rule out smog, for two reasons : That smog is purely a local phenomenon to a specific region, and weather forecasts are meant to cover a wider area... And, that the rain ought to be enough to make the particulate matter settle down. Also, I'm skeptical about the forecasts, because the heat ought to be enough(peak temperatures reach $35^{\circ}$- $40 ^{\circ}~\mathrm{C}$), to completely evaporate the rain water, rather than it getting condensed to fog.

I'll be doing my reading on mist and fog formation, in case I might have missed anything crucial. But, an explanation would be just as helpful. Thanks! Edits are welcome.

These were some of the forecasts I noticed enter image description here enter image description here

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    $\begingroup$ Was it actually foggy? $\endgroup$ – tfb Jul 26 '16 at 17:19
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    $\begingroup$ I'm voting to close this question as off-topic because it's about meteorology not physics $\endgroup$ – John Rennie Jul 26 '16 at 17:20
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    $\begingroup$ @JohnRennie rather than closing the question, could you possibly direct the question to the correct place. I'm curious about this. $\endgroup$ – Abhinav Jul 26 '16 at 17:43
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    $\begingroup$ @Abhinav You might be quibbling re the definition of "fog" ... I say this, because, in the USA, Californians refer to thick and low clouds as "fog" --- while folks on the east-coast do not $\endgroup$ – PERFESSER CREEK-WATER Jul 26 '16 at 18:32
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    $\begingroup$ Probably Earth Science SE is the most appropriate place for this question. $\endgroup$ – sammy gerbil Jul 27 '16 at 1:40
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You get fog when the relative humidity at ground level goes above 100%. This is easily achieved by a drop in temperature without a lot of airflow.

Looking at the saturated vapor pressure in the range of 25 - 35 C gives us a sense of what temperature drop you would need to go from 80% RH (muggy but not unusual) to "fog". This plot (raw data from http://www.novaweather.net/vapor.jpg) demonstrates that if you start with 80% relative humidity, a 4 degree drop in temperature will cause fog:

enter image description here

Just to clarify: the blue line is the saturated vapor pressure of water (from above source); the red line is 80% of the blue line, and the green dashed line is the red line shifted left by 4 °C.

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Water boils at $100 ^\circ~\mathrm C$. $30$-$45^\circ~ \mathrm{C}$ is not nearly enough to boil water, but the constant heat of the sun is enough to evaporate water over time. It takes about $4.314~\mathrm{J/K}$ to evaporate 1 kg of water (under normal atmospheric pressure). The can slowly evaporate the water on the ground, but $30$-$45 ^\circ~\mathrm{C}$ is not nearly hot enough to prevent it from raining, also keeping in mind that the temperature is much colder the higher up you are in the atmosphere. As a result, rain droplets form and fall as rain.

Fog or mist is essentially low clouds. Sometimes when the ground is very hot, incident rain water can evaporate little by little, causing steam to rise up into a fog.

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