Measuring the wind: met masts, LiDAR and SoDAR

The amount of energy a wind turbine generates rises steeply with wind speed, so small differences in the wind resource make large differences in output. That makes wind measurement one of the most important investments in early development. A project with poor measurement data carries uncertainty that affects its design, financing and long-term returns.

Why measurement matters

Regional wind maps and modelled datasets are good for identifying promising areas, but they cannot capture everything that shapes the wind at a specific site:

• Local terrain — ridges, valleys and slopes accelerate or slow the wind
• Surface roughness — trees, crops and buildings slow the wind near the ground
• Turbulence — affects both energy capture and turbine loads
• Wind shear — how wind speed changes with height
• Direction — the prevailing directions determine layout and wake effects

On-site measurement captures all of these at the actual location and heights of interest.

Measurement methods

Meteorological masts

A met mast is a tall lattice or tubular tower carrying instruments at several heights:

• Anemometers to measure wind speed
• Wind vanes to measure direction
• Temperature, pressure and humidity sensors to calculate air density
• A data logger recording measurements at regular intervals

Met masts give direct, well-understood measurements and are widely accepted by lenders. Their main limitation is height: modern turbines have hub heights and rotor tips well above what many masts reach, so wind at the top of the rotor must be extrapolated.

LiDAR

LiDAR (Light Detection and Ranging) devices sit on the ground and use laser beams to measure wind speed and direction at many heights above them. Advantages include:

• Measurement up to and beyond modern hub heights
• Fast deployment and relocation
• Smaller land footprint

LiDAR in complex terrain may need correction, and devices must be validated against reference measurements.

SoDAR

SoDAR (Sonic Detection and Ranging) uses sound pulses to similar effect. It is less commonly used than LiDAR today but still appears in some campaigns.

Combining methods

Many campaigns combine a met mast with a remote sensing device. The mast provides a robust reference at its heights, while LiDAR extends the measurement up through the rotor. Together they reduce uncertainty in wind shear.

How long to measure

Wind varies by season, and the Indian monsoon creates strong seasonal patterns. A campaign usually needs at least a full year to capture the annual cycle. Longer campaigns reduce uncertainty further.

But even several years of site data cannot capture the full long-term variability of the wind. That's where long-term correction comes in.

From measurements to a long-term estimate

The process typically follows these steps:

1. Quality control — remove data from iced sensors, faulty instruments or tower shadow effects.
2. Long-term correlation — compare site data with a long-term reference dataset (such as reanalysis data or a long-running nearby station) over the same period, then use that relationship to estimate the site's long-term average.
3. Vertical extrapolation — estimate wind at hub height from measurements at lower heights, using measured wind shear.
4. Horizontal extrapolation — model the wind at each proposed turbine location using flow models that account for terrain and roughness.
5. Energy yield — apply turbine power curves, wake losses and other losses to estimate output. See wake losses and turbine spacing.

Each step adds uncertainty, which is why results are given as P50 and P90 estimates.

Siting the measurement equipment

Equipment should be located where it represents the planned turbine positions — similar elevation, exposure and terrain. In complex terrain, one mast may not represent the whole site, so multiple measurement points may be needed.

Practical considerations

• Permits — met masts may need local approvals and aviation clearance depending on height and location; check requirements with the relevant authorities
• Land agreements — a short-term agreement with the landowner for the mast footprint, guy wires and access
• Security — equipment is valuable and vulnerable to theft or tampering
• Maintenance — regular checks to keep instruments calibrated and data flowing
• Data recovery — gaps in data increase uncertainty; remote monitoring helps catch problems early

What landowners should know

If a developer asks to install a measurement mast on your land:

• It is usually temporary, for one or more years
• It needs a small area plus space for guy wires
• Access for maintenance visits will be needed
• Ask for a written agreement covering duration, compensation, access, removal and restoration

Measurement does not mean a wind farm will definitely be built — it is the first step in finding out.

Common mistakes

• Measuring for less than a full seasonal cycle
• Placing the mast where it does not represent turbine locations
• Neglecting maintenance, leading to long data gaps
• Relying on mast data alone for modern tall turbines without remote sensing
• Skipping long-term correction and treating one year as typical

Using data from operating wind farms

Where nearby turbines are already operating, their production and SCADA data can be a valuable addition. It shows how real machines perform in the local wind regime, though differences in hub height, turbine model and exposure must be accounted for. For repowering projects, years of operating data from the existing turbines are often one of the strongest inputs available.

Where we fit

Through wind project development and site services, we help developers identify measurement locations, secure short-term land agreements with landowners, and coordinate the specialist partners who install equipment and analyse the data.