10.3 Thunderstorms, Fronts and Fog

Thunderstorms

A thunderstorm is a convective cloud system that has grown into a cumulonimbus (CB) — a towering cloud with strong updrafts on the building side and strong downdrafts and precipitation on the mature side. A mature CB contains almost every hazard an aircraft can encounter, concentrated in one cell.

Hazards Inside and Near a CB

• Severe turbulence — updrafts can exceed 3,000 ft/min; downdrafts can exceed 2,000 ft/min. Any of these exceeds a small RPA's capability by an order of magnitude.
• Microbursts — intense, localised downdrafts (1-2 km across) that hit the surface and spread outward as a radial outflow. A microburst can produce a 30-50 kt wind shear within seconds and is one of the most lethal hazards to any aircraft near the ground.
• Lightning — a direct strike can damage or destroy a small RPA's electronics; a near-strike can induce currents in the airframe. Lightning also signals that the cell is mature and violent.
• Hail — can damage rotors, gimbal, and airframe.
• Heavy rain — reduces visibility and can load the airframe with water, increasing mass.
• Icing — supercooled water in the upper parts of the CB can coat the airframe; less relevant for most low-level RPA work but possible in mature cells.

The Never-Near Rule

For a remote pilot the guidance is absolute: never operate near, beneath, or within a cumulonimbus. A practical separation minimum used by many RPA operators is 10 nautical miles (nm), about 20 km, from any visible CB cell, and more if the cell is moving toward the operating area. If a CB is visible and building within that distance, land and secure the aircraft.

A CB does not need to be directly overhead to be dangerous. A mature cell 5 km away can drop a microburst onto an otherwise clear operating site with no warning, and the outflow from a dying cell can produce a wall of gusty wind that reaches the site well before any rain does.

Fronts

A front is the boundary between two air masses of different temperature and humidity. The two types a remote pilot most often encounters are:

Cold Front

A cold front is the leading edge of a colder, denser air mass that pushes under a warmer air mass and lifts it sharply. The lift is intense and concentrated, so a cold front is marked by:

• A squall line — a line of thunderstorms or heavy showers just ahead of or along the front.
• A sharp wind shift — typically a veering (clockwise) change of 30-60° as the front passes, often gusty.
• Brief, intense weather — heavy rain, reduced visibility, turbulence, then rapid clearing behind the front.
• A temperature drop behind the front.

For RPA operations a cold front is a do-not-fly event within roughly an hour either side of the passage. The squall line can arrive before the surface front; the wind shift can flip a light headwind into a strong crosswind in minutes.

Warm Front

A warm front is the leading edge of a warmer air mass that slides over a retreating colder one. The lift is gentler and spread over a wider area, so a warm front brings:

• Gradual deterioration — lowering cloud base, thickening cloud, slowly reducing visibility, and eventually rain or drizzle over a wide area ahead of the front.
• A wind shift that is smaller and slower than with a cold front.
• A temperature rise after passage.

Warm fronts are less violent than cold fronts but can still produce low cloud and poor visibility that make a visual RPA operation unsafe. The lesson is the same: avoid operations near or behind a front until the air mass has settled.

Using the Forecast

The Bureau of Meteorology (BoM) aviation forecasts (TAF, area forecast) indicate expected frontal passage times. A remote pilot should check the forecast for the planned operating window and treat a forecast frontal passage within the window as a no-go.

Fog

Fog is cloud at ground level — suspended water droplets that reduce visibility below 1,000 m (and often to a few hundred metres). For a visual-line-of-sight RPA operation fog is an immediate no-go: it removes both visibility and the ability to maintain visual contact with the aircraft.

How Fog Forms

Fog forms when air cools to its dew point — the temperature at which the air becomes saturated and water vapour begins to condense. The gap between the air temperature and the dew point is the temperature-dew-point spread:

• When the spread is large (e.g., T 25°C, DP 10°C), the air is dry and fog is unlikely.
• When the spread is small and falling (e.g., T 16°C, DP 14°C, with evening cooling forecast), fog risk rises.
• When the spread reaches about 0°C, fog is likely if there is enough condensation nucleus and light wind.

Two Fog Types

• Radiation fog — forms on a clear night with light wind (2-5 kt). The ground radiates heat to space and cools; the air in contact with the ground cools to its dew point. Calm conditions let the fog stay shallow; light wind mixes the cooled air through a deeper layer and produces a thicker fog. Common inland and in valleys; typically burns off after sunrise.
• Advection fog — forms when warm, moist air moves over a cooler surface (e.g., moist tropical air flowing over cooler southern waters in winter, or moist air flowing over cold ground). Can persist day and night and is common along the southern Australian coast.

Reading the Spread as a Fog Predictor

A METAR reports temperature and dew point, for example T 16/14. The spread here is 2°C. If sunset is approaching, the wind is light, and the sky is clear, the spread will narrow further as the ground cools; fog is a real risk within a few hours. The decision is to finish the operation well before dusk and not plan a return at first light without re-checking the temperature-dew-point spread.

Key habit: before any early-morning or late-evening operation, check the temperature-dew-point spread in the latest METAR and the BoM forecast. A small, falling spread is a fog warning.