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What should i expect during my flight planning when i see both temperature and dew point are the same? I understand there will be fog but what else? could it mean icing? any difference between 20/20 and 04/04 for example to expect?

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    $\begingroup$ You think you’ll get icing at 20° OAT? $\endgroup$ – Arkhem Dec 18 '20 at 21:32
  • $\begingroup$ @Arkhem - Possibly once you are nearing 10,000 feet AGL. Remember that the temperature on METARs and TAFs is in Celsius. $\endgroup$ – Dean F. Dec 18 '20 at 21:42
  • $\begingroup$ @DeanF. 20° at 10k still wouldn’t be freezing. Unless you’re doing 2° per thousand feet at which case it wouldn’t be 20°... $\endgroup$ – Arkhem Dec 18 '20 at 21:50
  • $\begingroup$ @Arkhem - Since a METAR or TAF would be giving the temperature at the surface, a standard lapse rate of 2°C per thousand feet would put you at “freezing“ at 10k’ AGL. The OP did not specify that he was getting his temperature and dew point from a Winds/temps aloft chart or a skew-t/log-p diagram. $\endgroup$ – Dean F. Dec 18 '20 at 21:57
  • $\begingroup$ So we’ve established that when it’s freezing outside you’ll get icing... how does this relate to it being OAT 20 and dew point 20 on the ground and it not being in icing? Your comment seems irrelevant $\endgroup$ – Arkhem Dec 18 '20 at 22:00
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Both 20/20 and 4/4 will probably result in fog or thick haze/mist.

4/4 is a problem because it tells you that besides the airport being likely fogged in, the freezing level is only a couple thousand feet above the airport and there is very likely icing at that level if there is any cloud up there. Only airplanes capable of flight in icing would be coming and going, if at all, depending on the actual runway visibility.

20/20 is also likely fogged in, but the freezing level is up at 10000 ft so that's not a problem. 15-20C is a sweet spot for carb icing, so saturated air at that temperature is a major concern.

The other thing to take into consideration is temperature tendency. Generally, a zero spread like that in the morning means you can expect to wait a couple hours and the fog will lift. On the other hand, a spread approaching zero in late afternoon is a big red flag.

When I was bush flying on floats in Northern Ontario, the dew point spread was the most important bit of data when checking weather. A spread of one degree in the morning when departing on a trip was no worries because it would only improve (absent frontal conditions etc) as the day warmed up. On the other hand, a tight spread in the afternoon might mean making a customer wait at the lodge until the next morning rather than risk getting caught out scud running home at 500 ft, with clouds forming all around as the spread became zero, necessitating a diversion to the nearest lake on the route.

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Close temperature and dew points will mean the possibility of visible moisture. Fog is one aspect of that. At 20°C, structural icing is going to be rare below 8,000 feet AGL. On the other hand, Induction (carburetor) icing on low power settings with rich mixture settings is very possible. At 4°C, beware of structural icing as well as carb ice.

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As a very practical rough guide with a stable air mass at sea level, for every degree the dew point is different from the air temperature (in degrees C), the cloud base will be approximately 400feet. Eg 7/5 = 800ft in the lower levels.

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