Wind shear and the Shear Glass

Wind shear describes the variation of wind velocity over a distance. In meteorology, this typically refers to the change in wind speed or direction with increasing height above the surface. This phenomenon is a direct consequence of surface friction, which slows the air closest to the ground, allowing the wind higher up to move more freely.

The rate at which wind speed increases with height is often described by a power law or logarithmic profile. While simplified, these models illustrate the principle:

S(h) = S(z_ref) * (h / z_ref)^α

Where:

• S(h) is the wind speed at height h.
• S(z_ref) is the wind speed at a reference height z_ref (commonly 10 metres).
• α (alpha) is the shear exponent, which varies with terrain roughness and atmospheric stability.

Typical α values range from 0.1 over very smooth surfaces like open water to 0.4 or more over rough, urban terrain or during stable atmospheric conditions. For instance, if the wind speed at 10 m is 10 m/s and α is 0.14 (typical for open grassland), the speed at 80 m would be approximately:

S(80) = 10 m/s * (80 / 10)^0.14 = 10 m/s * (8)^0.14 ≈ 10 m/s * 1.30 ≈ 13.0 m/s

This 30% increase in speed over 70 metres highlights why understanding shear is crucial for operations at height. The Wind Agent uses direct model outputs at multiple levels rather than a single exponent, providing a more nuanced representation.

Wind shear encompasses two primary components:

1. Speed Shear: This is the change in wind speed with height. As discussed, it typically increases with altitude due to reduced surface friction. This is the most commonly considered aspect of shear for many operations, as higher speeds exert greater forces.

1. Directional Shear: This is the change in wind direction with height. Near the surface, friction not only slows the wind but also causes it to turn towards lower pressure. As height increases and the influence of friction lessens, the Coriolis effect becomes more dominant, causing the wind to turn clockwise (veer) in the Northern Hemisphere. This means the wind at 100 m might be from a slightly different direction than the wind at 10 m.

For example, over land in the Northern Hemisphere, the 10-metre wind commonly backs (turns anticlockwise) by 10° to 30° relative to the geostrophic wind aloft. Consequently, as you ascend from the surface, the wind direction will typically veer (turn clockwise). A hodograph chart visually represents this turning with height.

Both speed and directional shear are important. For lifting operations, a significant change in direction can cause a suspended load to swing unexpectedly. For drones, directional shear can introduce unexpected drift and control challenges. The Shear Glass presents both components through its display of speed and direction at discrete height levels.

The Wind Agent's Shear Glass is an instrument designed to visualise wind conditions at multiple standard heights simultaneously. It presents the modelled wind speed and direction at 10, 80, 120, and 180 metres above ground level (AGL). These levels correspond to common operational heights for various industries, such as crane jibs, drone flight ceilings, and wind turbine hub heights.

Each height level on the Shear Glass displays:

• Mean Wind Speed: The 10-minute average wind speed, typically in metres per second (m/s) or kilometres per hour (km/h).
• Gust Speed: The maximum 3-second average wind speed within the 10-minute period, also at the respective height level.
• Wind Direction: The direction the wind is blowing from, in degrees true (0-360°).

By presenting these values side-by-side, the Shear Glass allows users to quickly assess the presence and magnitude of both speed and directional shear. For instance, a rapid increase in speed from 10 m to 80 m indicates strong speed shear, while a noticeable difference in direction arrows between levels points to directional shear.

This direct display removes the need for manual calculations or estimations based on single-level forecasts, providing a more comprehensive and height-specific understanding of the wind environment. It is a critical tool for planning and executing operations where wind conditions vary significantly with altitude.

For operators of cranes, drones, or other equipment working at height, the Shear Glass provides specific, actionable insights:

For Crane Operations:

• Identify the relevant working height (e.g., jib height or load height). If your jib is at 100 m, you would interpolate between the 80 m and 120 m readings. For example, if 80 m shows 15 m/s and 120 m shows 18 m/s, a linear interpolation suggests approximately 16.5 m/s at 100 m.
• Compare the wind speed at your working height against your operational limits. The Wind Agent's exceedance fan can then show the probability of exceeding this limit.
• Observe directional shear. If the direction at the jib height is significantly different from the ground, it indicates potential for load swing, especially for large or sail-like loads.

For Drone Operations:

• Assess the wind speed at your planned flight altitude. A drone flying at 50 m needs to contend with different wind conditions than one at 10 m. The Shear Glass provides speeds at 10 m and 80 m, allowing for interpolation for intermediate heights.
• Consider the gust speeds at your operational height. Gusts are critical for drone stability and battery life.
• Directional shear can cause unexpected drift, requiring more active pilot input and potentially reducing flight efficiency. A drone launched into a southerly at 10 m might encounter a south-westerly at 50 m.

Common Misreading: A frequent error is to assume the 10 m wind speed is representative of all heights. This can lead to underestimating wind forces at altitude, potentially compromising safety or operational efficiency. Always refer to the height-matched wind on the Shear Glass for your specific working height.

Wind shear is not constant; it varies significantly throughout the day and night, largely influenced by atmospheric stability. One of the most pronounced variations is the occurrence of nocturnal shear peaks.

During the day, solar heating warms the ground, which in turn heats the air above it. This creates convective turbulence, mixing the air vertically and distributing momentum. This mixing tends to reduce the vertical shear, making the wind profile more uniform with height.

After sunset, the ground cools rapidly, often becoming cooler than the air above it. This leads to a stable atmospheric boundary layer, where vertical mixing is suppressed. Without the daytime mixing, the air near the surface remains slowed by friction, while the air aloft, decoupled from the surface, can accelerate. This can lead to a significant increase in wind speed with height, creating very strong speed shear, often peaking in the early hours before sunrise.

These nocturnal shear peaks are particularly relevant for operations that occur overnight or in the early morning. A light surface wind might mask strong winds and significant shear at operational heights of 50 metres or more. The Shear Glass and the shear heatmap are crucial for identifying these periods of elevated shear.

The shear heatmap provides a visual representation of how wind shear evolves over time and across different heights. It typically displays wind speed or the rate of shear (e.g., ΔV/Δh) across a grid, with hours on the horizontal axis and height levels on the vertical axis. The intensity of colour in each cell indicates the magnitude of the wind speed or shear at that specific height and time.

How to interpret the heatmap:

• Vertical Gradients: Look for strong colour changes as you move up a column (for a given hour). A rapid transition from lighter to darker colours indicates significant speed shear.
• Temporal Patterns: Observe how these vertical gradients change over the 24-hour cycle. You will often see stronger shear (darker colours at higher altitudes while lower altitudes remain lighter) during night-time hours, reflecting the nocturnal shear peaks discussed previously.
• Operational Planning: Use the heatmap to identify periods when shear might be a concern. For example, if your operation is sensitive to shear above 100 metres, you can scan the rows corresponding to those heights for high-shear conditions.

This chart complements the instantaneous view of the Shear Glass by providing a temporal context, allowing for proactive planning to avoid periods of high shear. It helps to understand not just 'what is the shear now?' but also 'how will shear develop over my operational window?'

It is crucial to understand that the values presented by The Wind Agent's Shear Glass and related charts are derived from numerical weather prediction (NWP) models, not direct measurements at those specific heights. While these models are highly sophisticated and constantly validated against observations, they are still representations of reality.

Key considerations:

• Spatial Resolution: NWP models have a grid resolution (e.g., 2.5 km for the ECMWF IFS). The wind values are representative of the average conditions within that grid cell, not a precise point measurement at your exact location.
• Vertical Resolution: Models have discrete vertical levels. The Wind Agent presents data at standard levels (10, 80, 120, 180 m) that align with key operational heights. For intermediate heights, values are interpolated.
• Terrain Effects: While models incorporate terrain, local micro-scale effects (e.g., flow around a specific building or hill) might not be fully resolved. This is particularly relevant for very complex terrain.

Therefore, while the Shear Glass provides an excellent forecast and situational awareness tool, it should always be used in conjunction with local knowledge and, where available, on-site measurements. Your own operational documents and safety procedures govern your limits, and model outputs are there to inform your decision-making process, not dictate it.

The Shear Glass is designed to help you make informed decisions by visualising wind conditions at relevant heights. To effectively use it for limit decisions, follow these steps:

1. Identify Your Critical Height: Determine the highest point of your operation (e.g., crane jib, drone ceiling, top of a structure). This is your primary height of concern.
2. Set Your Limits: Refer to your operational manual or risk assessment for the maximum allowable mean wind speed and gust speed at your critical height. These are your limits, not those provided by The Wind Agent.
3. Consult the Shear Glass: Locate the closest height levels on the Shear Glass to your critical height. Interpolate if necessary. For instance, if your limit is 100 m, examine the 80 m and 120 m values.
4. Assess the Forecast: Observe the forecasted mean and gust speeds at your critical height for your operational window. Pay attention to both speed and direction.
5. Use the Exceedance Fan: The Wind Agent's exceedance fan allows you to input your specific height and limit. It then calculates the probability of exceeding that limit based on ensemble forecasts, providing a probabilistic assessment of risk.
6. Review the Agreement Spine: For longer-term planning, the Agreement Spine shows the consistency of different model runs, indicating the confidence in the forecast for your specific height and time.

By systematically comparing the height-matched forecast from the Shear Glass against your defined operational limits, and using the probabilistic tools, you can make a well-grounded decision regarding the feasibility and safety of your operation.