Errigal, Co. Donegal: summit wind, exposure, and wind chill

Mount Errigal, at 751 metres, is the highest peak in County Donegal and part of the Derryveagh Mountains. Its distinctive conical shape, particularly prominent from the west, means it stands relatively isolated from surrounding peaks, leading to significant exposure to prevailing westerly and south-westerly winds.

The mountain's steep, often scree-covered slopes offer little shelter, especially at higher elevations. The summit ridge, in particular, is highly exposed. Wind speeds commonly increase with altitude, a phenomenon known as wind shear. On Errigal, this effect is pronounced, meaning wind at the summit can be considerably stronger than forecasts for lower elevations suggest.

The Wind Agent's Shear Glass instrument provides height-matched wind speeds at 10, 80, 120, and 180 metres above ground level, allowing users to estimate conditions closer to the summit than a standard 10 m forecast. While 180 m is below the summit, it provides a more accurate representation of upland conditions than valley-floor data.

The prevailing wind direction in Donegal is from the west to south-west. These directions typically bring the strongest and most persistent winds to Errigal. Winds from the north-west can also be significant, especially after the passage of a cold front.

Specific hazards related to wind direction on Errigal include:

• Summit Gusts: The exposed nature of the summit means that gusts can be significantly higher than the average wind speed, and strong enough to cause loss of balance, particularly on the narrow summit ridge. Commonly cited thresholds for safe walking on exposed ridges range from 40–50 mph (approximately 65–80 km/h) for experienced walkers, with lower thresholds for less experienced individuals or in poor visibility.
• Turbulence over Ridges: Winds crossing sharp ridges or passing over the mountain can create localised turbulence and eddies, making conditions unpredictable even in moderately strong winds.
• Lee-side Shelter and Rotor Effects: While the lee side of the mountain may offer temporary shelter, it can also be subject to rotor effects, where turbulent air descends rapidly, creating sudden, strong gusts. This is particularly relevant for winds from the west or south-west, where the eastern slopes would be in the lee.

Always consider the wind's direction relative to your planned route, particularly on exposed sections or when traversing ridges.

Wind chill is a critical factor on Errigal, even on days with moderate air temperatures. The combined effect of wind and cold air can make the 'feels like' temperature significantly lower than the actual air temperature, increasing the risk of hypothermia.

The Wind Agent's delta-T chart provides a modelled wind chill temperature. This is calculated using a commonly cited formula that combines air temperature and wind speed. For example, an air temperature of 5 °C with a wind speed of 30 km/h can result in an effective temperature of approximately 0 °C. At 0 °C with the same wind, the effective temperature can drop to around -6 °C.

Factors influencing wind chill:

• Wind Speed: The primary driver; even a small increase in wind speed can significantly lower the effective temperature.
• Air Temperature: Lower air temperatures amplify the effect of wind.
• Exposure: Areas of high exposure, such as the summit and ridges, will experience the full effect of wind chill.

Adequate layering of clothing, including windproof outer layers, is essential for mitigating the effects of wind chill.

The meteogram below presents the modelled wind forecast for Errigal over the coming days. It displays wind speed, gust speed, and direction at a standard height of 10 metres, adjusted for the mountain's elevation. This provides an overview of the expected conditions, including any significant changes in wind strength or direction.

Key elements to observe:

• Wind Speed and Gusts: Pay close attention to the difference between average wind speed and gust speed. On a mountain, gusts are often the critical factor for safety.
• Wind Direction: Note any shifts in direction, which can indicate frontal passages or changes in the synoptic pattern. A shift to a more exposed direction (e.g., westerly or north-westerly) often brings stronger winds.
• Trend: Observe the overall trend in wind conditions – whether they are increasing, decreasing, or remaining stable. This helps in planning the timing of your ascent and descent.

Remember that this is a model forecast. Actual conditions on the mountain can vary, especially with localised effects not fully captured by global models. Always combine forecast data with real-time observations if available and your own assessment of conditions on the ground.

Understanding the typical wind patterns for Errigal throughout the year can aid in long-term planning. The wind rose below illustrates the historical distribution of wind speeds and directions at this location, derived from reanalysis data, which provides a smoothed representation of past weather conditions.

Insights from the wind rose:

• Dominant Directions: Identify the most frequent wind directions, which for Errigal are typically from the westerly and south-westerly sectors, reflecting Ireland's Atlantic climate.
• Strongest Winds: Observe which directions are associated with the highest wind speeds (represented by the darker shades on the rose). This helps in understanding which wind directions pose the greatest challenge.
• Seasonal Variation: While the wind rose provides an annual overview, it is important to remember that wind patterns can vary significantly by season. Winter months (October to March) generally experience stronger and more frequent strong winds due to deeper Atlantic depressions.

This climatological data helps set expectations for conditions at different times of the year, informing decisions about when to plan ascents or when to anticipate more challenging weather.

Ascending Errigal requires careful planning, with wind conditions being a primary factor. Mountain rescue organisations consistently advise that walkers should be prepared for rapidly changing weather, especially in exposed upland environments.

Key planning points:

• Personal Limits: Understand your own and your group's tolerance for wind. Factors like experience, fitness, and equipment play a significant role.
• Route Choice: Consider alternative routes or turning points that offer more shelter or an easier descent if conditions deteriorate.
• Equipment: Always carry appropriate gear, including windproof and waterproof outer layers, warm clothing, navigation tools, and emergency supplies. Goggles can be essential in strong winds with driving rain or hail.
• Check Multiple Sources: Consult various weather forecasts and compare them. The Wind Agent's model comparison chart can highlight discrepancies between different forecast models.
• Turn-back Times: Establish clear turn-back times and conditions before starting your ascent. It is commonly cited that turning back is a sign of good judgement, not failure.

Mountain safety guidance, such as that provided by Mountaineering Ireland, consistently emphasises that the decision to proceed or turn back rests solely with the individual or group leader. The Wind Agent provides data to inform that decision, not to make it.