Fog, rain and humidity

Most consumer drones have no water-resistance rating, so rain, drizzle, wet fog and condensation can all reach the electronics. Fog and mist also cut visibility quickly. The dew point is the best early hint: when the gap between temperature and dew point is small and shrinking, fog or low cloud is likely on the way.

Moisture and drone electronics

DJI's own FAQ is clear for its consumer drones. Asked whether the Mini 4 Pro, Air 3S or Mavic 4 Pro is waterproof, the answer is no: avoid contact with water, return and land as soon as possible if it starts to rain, and let the drone dry completely, inside and out, before the next flight. DJI's printed guidelines for the Mini 4 Pro also list rain, snow and fog among the conditions not to fly in, and warn against leaving batteries wet or out in the rain.

Why it matters:

• Open motors and vents. Drones need airflow for cooling, so the body is not sealed.
• Electronics and connectors. Water can short-circuit boards and corrode contacts over time.
• Camera and sensors. Droplets on the lens or the obstacle sensors blur the image and can confuse the drone's vision systems.

IP ratings explained

An IP (ingress protection) rating, defined in the IEC 60529 standard, has two digits:

• First digit, solids (0–6): 5 means dust-protected and 6 dust-tight.
• Second digit, water (0–9):

An "X" means that part was not tested, so IPX4 is rated for splashing water and has no dust rating.

In practice, IP ratings are rare outside enterprise drones. DJI rates the Matrice 350 RTK and Matrice 30 at IP55: dust-protected and resistant to water jets. The Matrice 350 RTK page adds that the rating is not permanent and may drop with wear and tear. The Mini 4 Pro, Air 3S and Mavic 4 Pro spec pages list no IP rating (checked October 2026).

In Altiwind, the rain-chance check for a drone without an IP rating uses these defaults: under 20% is within your limits, under 50% is marginal, and 50% or more is outside your limits. Your drone profile records whether your model has a rating.

Fog, mist and visibility

The Met Office defines fog as visibility below 1,000 m. Mist is the same suspended droplets at lower density, with visibility above 1,000 m. In METARs you will see FG for fog and BR for mist; our METAR guide decodes them.

Altiwind's visibility check:

So any fog is outside your limits, and thick mist is marginal. Watch the cloud base too: a METAR showing BKN004 means broken cloud at 400 ft, about 120 m, so a drone climbing to that height would fly into it.

The main fog types, per the Met Office:

• Radiation fog: forms on clear, calm nights as the ground cools the air above it. It usually occurs in winter and tends to clear as the sun warms the ground.
• Advection fog: forms when warm, moist air flows over a colder surface, such as coastal sea fog.
• Freezing fog: fog below 0 °C, whose droplets freeze onto surfaces as rime. See flying drones in cold weather for icing.

Dew point spread as a fog hint

The dew point is the temperature the air must cool to, at constant pressure, to reach 100% relative humidity. The spread is the temperature minus the dew point.

• When the spread shrinks to about 2–3 °C (5 °F) or less, pilot training material from AOPA says to expect fog. When the two numbers meet, fog is almost certain.
• The trend matters more than a single value. A spread that shrinks through a clear, calm evening points to radiation fog in the coldest hours before dawn.
• After sunrise, warming widens the spread again and the fog usually lifts.

Example: at 21:00 the temperature is 9 °C and the dew point 7 °C, the sky is clear and the wind light. Overnight the temperature falls towards 7 °C, and fog by dawn is likely. A METAR showing 04/04 means the air is already saturated.

Dew point is more useful than relative humidity here. Relative humidity rises and falls with temperature through the day, while the dew point tells you directly how far the air is from saturation.

Condensation between warm and cold

Condensation forms on any surface colder than the dew point of the air around it. The classic case is bringing a cold drone indoors.

Room air at 20 °C and 50% relative humidity has a dew point of about 9 °C. A drone carried in at 0 °C sits below that, so moisture condenses on the lens, the gimbal and inside the body until the drone warms past 9 °C.

To avoid it:

• Put the drone and batteries in a closed case or bag before you go inside, and let them warm slowly to room temperature before opening it.
• Do not charge a cold battery straight away. DJI gives a charging range of 5 to 40 °C for the Mini 4 Pro.
• If the lens fogs in the field, give it time to reach the outside temperature before you take off.

Sources

• DJI FAQ pages for the Mini 4 Pro, Air 3S and Mavic 4 Pro, and the Matrice 350 RTK and Matrice 30 spec pages, checked October 2026
• DJI Mini 4 Pro printed guidelines, via the DJI downloads page
• IEC, IP ratings (IEC 60529)
• Met Office, Fog
• NOAA/NWS, Why dewpoint vs humidity?
• AOPA, Weather: fog facts