A small temperature and dew point spread means fog or low clouds are more likely: 14 °C and a dew point of 13 °C is a 1 °C spread, and the air is close to saturation. Fog is a cloud at the surface. A fast-moving cold front brings gusty winds and brief, intense showers or thunderstorms, then clearing and cooler air.
When the spread gets small
Temperature is how warm the air is. Dew point is the temperature at which that air would become saturated if it cooled at constant pressure. The spread is the gap between them. When the gap is large, the air has room to cool before water condenses. When the gap is 1 °C, as in 14 °C over 13 °C, saturation is close. The remote pilot should anticipate fog or low clouds, and visibility that may drop below the minimums.
Section 107.51(c) already states the visibility floor for the flight: at least 3 statute miles, observed from the control station. The word "fog" does not add a second visibility number on top of that rule. The practical point of the 1 °C spread is that the observation may deteriorate until the flight no longer meets the 3 statute mile minimum. Clear skies and unlimited visibility are the wrong expectation. Severe turbulence at altitude is also the wrong expectation for this temperature group. The group is a saturation clue, not a turbulence clue.
Fog forms with that small spread and with light wind. It is a cloud on the surface, or very near it. It is not something that shows up only over mountains, and it is not always tied to high winds. High wind is a different picture. Light wind lets the saturated air stay in place long enough for the cloud to sit on the ground. How a METAR prints the temperature and dew point group is on the METAR page. What counts as a ceiling once those clouds form is on the ceiling page.
The front that matches the forecast
A fast-moving cold front is the brief, violent one. Expect gusty winds and short, intense showers or thunderstorms. After the front passes, the weather clears and the air is cooler. That sequence is the answer when the question asks what a fast-moving cold front brings.
The long, wet pattern belongs to a warm front: widespread low clouds and steady rain that can last for many hours. Persistent fog with no precipitation is its own problem, and it is not the description of the fast cold front. If you assign the hours of steady rain to the cold front, you have the two fronts swapped. Stable air and the layered clouds that go with a warm front are discussed on the stability page. The cold front's showers and thunderstorms are the unstable, showery end of that contrast.
Using the two clues before launch
Section 107.49 requires the remote pilot in command to assess local weather before flight. Read the temperature and dew point. A 1 °C spread is a reason to expect fog or a low cloud base and to watch whether visibility from the control station is still at least 3 statute miles. Read the front. If a fast-moving cold front is the feature in the forecast, plan for gusty wind and a short period of intense showers or thunderstorms, then cooler air and clearing, rather than a long gray rain.
A worked morning looks like this. The nearest METAR shows 14 °C and 13 °C, with visibility still above the minimum and a few clouds. You do not treat that as a guarantee of a clear afternoon. The spread says saturation is close, so fog or low cloud is the development to brief. Separately, the forecast discussion says a fast cold front crosses the site near midday. You brief gusty wind and a short burst of heavy showers or a thunderstorm, and you plan the launch window around that passage instead of expecting steady rain all day.
The trap
The spread question offers turbulence and "unlimited visibility" as decoys for a 1 °C gap. The front question offers the warm-front rain and a fog-with-no-precip picture as decoys for a fast cold front. The fog question offers high wind and "mountains only." Keep the three answers in their own boxes: small spread means fog or low cloud, a fast cold front means brief intense weather then clearing and cooler air, and fog is a surface cloud that likes a small spread and light wind.
Weather items are a small slice of the scored test, and they are concrete. Run them on the free practice test. The weather lessons are in the course.