Stability: why day and night winds differ

Atmospheric stability refers to the tendency of an air parcel to resist or enhance vertical motion. It is primarily determined by the vertical temperature gradient, or lapse rate, compared to the adiabatic lapse rate.

• Unstable atmosphere: If a parcel of air is warmer than its surroundings when lifted, it will continue to rise. This occurs when the environmental lapse rate (how temperature decreases with height) is steeper than the dry adiabatic lapse rate (approximately 9.8 °C per 1,000 metres). Unstable conditions promote vigorous vertical mixing.
• Neutral atmosphere: A lifted air parcel cools at the same rate as its surroundings, so it neither accelerates upwards nor sinks back down. This state is common during strong winds or overcast conditions, where mechanical turbulence or cloud cover dampens surface heating effects. The environmental lapse rate is close to the adiabatic lapse rate.
• Stable atmosphere: If a parcel of air is cooler than its surroundings when lifted, it will sink back to its original position. This happens when the environmental lapse rate is less steep than the adiabatic lapse rate, or when temperature increases with height (an inversion). Stable conditions suppress vertical motion, leading to layered air and reduced mixing.

The stability of the boundary layer significantly influences how momentum is transferred between the free atmosphere and the surface, directly affecting observed wind speeds and gust characteristics.

During the day, solar radiation heats the ground, which in turn warms the air directly above it. This heating creates an unstable layer near the surface, as warm air is less dense and tends to rise. This process is known as convective mixing or thermal turbulence.

Convective mixing effectively transfers momentum from higher altitudes, where wind speeds are typically greater, down to the surface. This means that on a sunny, warm day, surface winds can be stronger and gustier than they would be under stable conditions, even with the same pressure gradient. The vertical motion acts to homogenise the wind speed profile, reducing the shear between the surface and higher levels.

For example, if the mean wind at 100 metres is 15 m/s and the 10-metre wind is 5 m/s under stable conditions, strong daytime mixing might increase the 10-metre wind to 10 m/s by bringing down faster air, while slightly reducing the wind at 100 metres as momentum is distributed downwards. This effect is particularly noticeable on days with light to moderate winds, where thermal effects can dominate over mechanical turbulence.

After sunset, the ground cools rapidly by radiating heat into space. This cools the air directly above the surface, creating a stable layer or even a surface temperature inversion where temperature increases with height. This stable layer suppresses vertical mixing.

With vertical mixing inhibited, the surface layer becomes decoupled from the faster-moving air aloft. Friction from the ground slows the air near the surface, and without the downward transfer of momentum from higher levels, surface wind speeds often decrease significantly, leading to night-time lulls.

This decoupling means that wind shear can become very pronounced. While the 10-metre wind may drop to near calm, significant wind speeds can persist just tens of metres above the surface. This is a critical factor for operations such as drone flights or crane work, where conditions at 10 metres may not represent conditions at working height. For instance, if the 10-metre wind drops from 5 m/s to 1 m/s at night, the wind at 80 metres might only decrease from 12 m/s to 10 m/s, resulting in a much stronger shear.

A strong surface inversion can lead to the formation of a low-level jet (LLJ). This phenomenon occurs when the stable layer near the ground effectively 'caps' the atmosphere, preventing vertical mixing. Above this stable layer, the wind can accelerate as it is no longer slowed by surface friction, and the Coriolis force can act more efficiently.

Low-level jets are typically found at heights between 100 and 600 metres above the ground, often reaching their maximum strength in the early morning hours just before sunrise, when the surface inversion is most pronounced. Speeds within an LLJ can be significantly higher than the geostrophic wind, sometimes exceeding 20 m/s (approximately 40 knots) even when surface winds are light.

For example, if the 10-metre wind is 2 m/s, an LLJ at 200 metres could exhibit speeds of 15 m/s. This represents a substantial increase in wind speed over a relatively small vertical distance. LLJs are particularly important for wind energy, aviation, and for operations involving tall structures, as they can introduce unexpected high winds at operational heights.

A temperature inversion is a layer where temperature increases with height, rather than decreasing. Surface inversions, formed by nocturnal cooling, are the most common type and lead to the stable conditions discussed. However, inversions can also occur aloft, often associated with high-pressure systems or frontal boundaries.

When an inversion layer exists, it acts as a lid, trapping air below it. This can lead to a variety of effects:

• Pollution trapping: Pollutants emitted near the surface are confined beneath the inversion, leading to poor air quality.
• Fog and low cloud: Moisture can be trapped, leading to the formation of fog or stratiform clouds.
• Trapped wind: If there is a pressure gradient below the inversion, the wind can be confined to this layer. This can result in persistent, moderate winds near the surface that do not mix with the air above, or conversely, very light and variable winds if the pressure gradient is weak.

In Ireland, inversions are common during calm, clear nights, particularly in inland areas. They can significantly alter local wind patterns, creating microclimates where wind conditions differ markedly from regional forecasts.

Over land, stability is strongly influenced by diurnal heating and cooling. Over the sea, the large thermal inertia of water means that sea surface temperatures change much more slowly. Therefore, stability over marine environments is primarily determined by the difference between the sea surface temperature (SST) and the air temperature.

• Unstable conditions: Occur when cold air moves over warmer water. The warmer water heats the air from below, creating instability and promoting vertical mixing. This is common in winter when cold continental air masses move over the relatively warmer Atlantic. It leads to gustier conditions and reduced shear.
• Stable conditions: Occur when warm air moves over colder water. The colder water cools the air from below, creating a stable layer. This is common in spring and early summer when warm air flows over still-cold coastal waters. It leads to smoother, less gusty winds and increased shear near the surface, often accompanied by sea fog.

For example, if the air temperature is 15 °C and the SST is 10 °C, the air is cooled from below, creating a stable marine boundary layer. This would typically result in a smoother wind profile with less gustiness compared to a situation where 5 °C air moves over 10 °C water, which would be unstable and gusty. The Wind Agent's marine mode considers these factors.

Atmospheric stability has a profound impact on both wind shear (the change in wind speed with height) and gust behaviour.

• Unstable conditions (daytime, cold air over warm water): Vertical mixing is strong, which tends to reduce wind shear. Momentum is transferred downwards, increasing surface wind speeds and gustiness. The wind profile becomes more uniform with height. The gust factor (ratio of gust to mean speed) tends to be higher due to the vigorous convective eddies.
• Stable conditions (night-time, warm air over cold water): Vertical mixing is suppressed, leading to increased wind shear. Surface winds are decoupled from faster winds aloft, resulting in lower surface speeds and often a very smooth flow. The gust factor tends to be lower, as there is less turbulence to generate strong gusts. However, if a low-level jet forms, it can introduce high shear at its base and top.

Consider a mean 10 m wind of 5 m/s. In unstable conditions, the 80 m wind might be 8 m/s, with gusts of 8 m/s at 10 m (gust factor 1.6). In stable conditions, the 80 m wind could be 15 m/s, but the 10 m wind might be 2 m/s with gusts of 3 m/s (gust factor 1.5). The Shear Glass is critical for understanding this vertical variation.

While stability is not directly plotted on a standard meteogram, its effects can be inferred from several parameters:

• Temperature and Dew Point: A large spread between temperature and dew point at the surface, combined with strong daytime heating, suggests an unstable atmosphere. Conversely, a small spread and decreasing temperature with height (or an inversion) indicates stability.
• Wind Speed and Gusts: Strong diurnal variations in surface wind speed (calm nights, gusty days) are characteristic of stability changes. A meteogram showing a marked drop in wind speed and gust activity after sunset, followed by an increase after sunrise, strongly suggests a stable nocturnal boundary layer and an unstable daytime one.
• Cloud Cover: Clear skies, especially at night, promote strong radiative cooling and thus stable conditions. Overcast skies tend to dampen diurnal temperature swings and keep the boundary layer more neutral.
• Delta-T (Air-Sea Temperature Difference): For marine locations, a positive delta-T (air warmer than sea) suggests stability, while a negative delta-T (air colder than sea) suggests instability and potential for sea fog.

The Wind Agent's meteogram, combined with the Shear Glass, provides the necessary information to interpret stability effects on your specific operational height.