10.1 Wind, Sea Breezes and Local Effects

How Aviation Wind Is Reported

For a CASA Remote Pilot Licence (RePL) holder, wind is the weather element that most often decides whether a flight goes ahead. In aviation, wind is reported as the direction the wind is coming from, in degrees true (not magnetic), and in knots (kt). A METAR (Meteorological Aerodrome Report) wind group such as 18010G18KT means the wind is blowing from 180° true at 10 kt, gusting to 18 kt.

Two details matter for every remote pilot:

• True, not magnetic. Runway numbers are magnetic, and you will sometimes need to convert wind direction to magnetic for crosswind calculations on a runway. For off-runway RPA work, treat the reported wind as true and compare it directly with your planned track (also plotted true on a VNC or VTC).
• From, not toward. A wind from 180° is blowing toward 360°. If you launch an RPA to the south into a southerly wind, the aircraft is flying into a headwind on the outbound leg and a tailwind on the return; the tailwind leg is where groundspeed and therefore landing distance (or RPA recovery distance) will be greatest.

Surface Wind vs Low-Level Wind

A remote pilot cares about two wind regimes that are reported differently:

• Surface wind — the wind at approximately 10 m above ground level (AGL), as reported in a METAR or Automatic Weather Station (AWS). This is the wind that affects launch, recovery, and the low-level transit of most RPA operations under the standard 120 m (400 ft) AGL limit.
• Low-level wind — the wind through the depth of the operating area, typically from the surface up to a few thousand feet above mean sea level (AMSL). Wind speed and direction can change markedly with height because of the weakening of surface friction.

Surface friction slows the wind near the ground and turns it slightly inward toward low pressure; above the friction layer the wind aligns more closely with the isobars and is usually stronger. For an RPA operating at 120 m AGL this means the wind at the aircraft can be noticeably stronger and from a different direction than the wind you feel on the ground at launch.

Gusts and Crosswind

Two derived quantities decide most go/no-go calls:

• Gusts — short-period increases above the mean wind. A report of 18010G18KT means the mean wind is 10 kt but peaks reach 18 kt. For RPA work the gust value, not the mean, is the number to compare against the aircraft's published wind limit.
• Crosswind component — the component of the wind perpendicular to your takeoff/landing track or direction of travel. A wind from 090° at 12 kt against a north-south takeoff track gives an all-crosswind situation: the full 12 kt is crosswind.

Key habit: always check the gust against the aircraft wind limit, and the crosswind component against the manufacturer's crosswind limit (if published) or your own conservative personal minimum.

The Sea Breeze

Along the Australian coast — where most RPA operations occur — the sea breeze is the dominant local wind feature during the day. It is a thermally driven circulation:

1. Daytime heating: the land surface warms faster than the adjacent sea. The air over the land becomes warmer and less dense and begins to rise.
2. Onshore replacement: cooler, denser air over the sea flows inland to replace the rising warm air. This is the sea breeze — an onshore wind that typically starts mid-morning and strengthens through the afternoon.
3. Return flow aloft: the rising air over the land flows seaward at a few hundred to a few thousand feet and subsides over the sea, closing the circulation.
4. Night reversal: after sunset the land cools faster than the sea. The air over the land becomes cooler and denser than the air over the water and drains offshore as a land breeze, reversing the daytime circulation.

A mature sea breeze can reach 20-30 kt on the coast even when the synoptic (large-scale) gradient wind is light. It also shifts the wind direction — a morning light northerly over a Sydney beach can become a strong afternoon southerly as the sea breeze sets in from the east/south-east. The leading edge of the sea breeze is sometimes marked by a line of small cumulus cloud and a visible wind shift.

Local Effects: Channelling and Funnelling

Wind is rarely uniform across an operating area. Terrain and built structures reshape it:

• Terrain channelling: valleys and gaps between ridges constrain the flow and align it with the gap, so the wind direction in the operating area may differ by 30-60° from the reported AWS wind a few kilometres away.
• Funnelling: a gap between ridges, or between rows of buildings, accelerates the flow (Venturi effect). Wind speed at the rotor can be well above the area-averaged value.
• Obstacle-induced turbulence: buildings, trees, and ridges shed mechanical turbulence downwind; this is covered in section 7.2.

Worked Example: A Coastal Afternoon Operation

You are briefed to fly a survey at a coastal sports field at 1300 local time. The 0900 AWS observation reports 18008KT, a light southerly. The forecast suggests a sea breeze developing in the afternoon.

• At 0900 on site the wind feels light from the south; you confirm 8 kt with a hand-held anemometer at launch height, well below the RPA's 15 kt wind limit.
• By 1230 the wind has veered to 15015G22KT as the sea breeze establishes. The gust value (22 kt) now exceeds the aircraft limit, even though the mean wind (15 kt) does not.
• You stop the operation, land, and re-schedule for the early morning when the sea breeze has not yet developed.

The lesson: a coastal site that is flyable at 0900 can be unflyable by 1300 purely from the sea breeze. Always check the Bureau of Meteorology (BoM) aviation forecast for the expected sea-breeze timing and strength, and confirm on site with an anemometer before each flight.