When the temperature and dew point spread is 4°F (2°C) or less, what weather condition becomes likely?

Why →The dew point is the temperature at which air becomes saturated (100% relative humidity) and condensation begins. When the temperature-dew point spread narrows to 4°F (2°C) or less, the air is near saturation. Further cooling from overnight radiation, advection, or uplift will cause condensation as fog, low clouds, or precipitation.

The trap →A small spread means nearly saturated air, the opposite of dry. High winds and turbulence come from instability, not saturation.

Field note →METARs always include temperature/dew point (e.g., "18/16" = 18°C temp, 16°C dew point, only 2°C spread). This close spread means fog or low ceilings are likely if temperatures drop overnight. One of the fastest weather reads you can do from a METAR.