Reading the monthly climatology

The monthly climatology chart presents a statistical summary of wind speed for each calendar month, typically based on a 30-year historical period (e.g., 1991–2020) from reanalysis datasets like ERA5 or local meteorological station records. For each month, the chart displays several key statistics:

• Median (P50): This is the middle value of all wind speeds recorded for that month over the historical period. Half of the recorded speeds were above this value, and half were below. It represents the 'typical' wind speed for the month, less sensitive to extreme outliers than the mean.
• Interquartile Range (IQR): This is the range between the 25th percentile (P25) and the 75th percentile (P75). It encompasses the middle 50% of all wind speeds. A wider IQR indicates greater variability in wind speed during that month.
• 10th and 90th Percentiles (P10, P90): These show the range within which 80% of all wind speeds fall. The P10 indicates that 10% of speeds were below this value, and the P90 indicates that 10% of speeds were above it. The total spread from P10 to P90 gives a good indication of the expected range of conditions, excluding the most extreme 10% at either end.

For example, if a chart shows a median of 6 m/s for January, with P25 at 4 m/s and P75 at 8 m/s, it means that in half of all Januaries, the wind speed was between 4 m/s and 8 m/s, and the most typical speed was 6 m/s. This allows for a more nuanced understanding than a simple average, highlighting the expected range of conditions.

In Ireland, the months with higher wind speeds typically occur during the winter half of the year, from October through to March. This pattern is driven by the more frequent and intense passage of Atlantic depressions across or near the island during this period. These low-pressure systems bring stronger pressure gradients and thus higher wind speeds.

Conversely, the summer months, particularly June, July, and August, tend to be the calmest. During this time, the Azores High often extends its influence further north, leading to more settled weather and weaker pressure gradients. The median wind speeds are noticeably lower, and the overall spread of speeds (P10 to P90) also tends to be narrower, indicating less variability.

For instance, climatological data for a coastal site in County Kerry might show:

• January: Median wind speed of approximately 8 m/s, with the P90 around 13 m/s. This indicates that 10% of January days historically experienced wind speeds exceeding 13 m/s.
• July: Median wind speed of approximately 4 m/s, with the P90 around 7 m/s. This suggests that only 10% of July days saw wind speeds above 7 m/s.

This seasonal cycle is a fundamental characteristic of Ireland's temperate maritime climate, directly influencing operational planning for activities sensitive to wind, from marine operations to construction and agriculture.

The Wind Agent's monthly climatology charts are specific to the chosen location, drawing on high-resolution reanalysis data or nearby observations. This allows for a direct comparison of a particular site's wind regime against broader regional or national patterns.

Coastal locations in Ireland, especially those on the west and south-west, typically exhibit higher median wind speeds and a greater overall spread compared to inland sites. This is due to reduced surface friction over the ocean and direct exposure to Atlantic weather systems. For example, a site near the Donegal coast might show a January median of 9 m/s, while an inland site in County Laois might show 6 m/s for the same month.

Worked Example: Consider two hypothetical sites:

1. Coastal Site (e.g., Loop Head, Co. Clare): January median 9 m/s, P90 15 m/s.
2. Inland Site (e.g., Tullamore, Co. Offaly): January median 6 m/s, P90 10 m/s.

If your operation has a wind speed limit of 12 m/s, the climatology indicates:

• At Loop Head, historical data suggests that wind speeds exceeded 12 m/s in more than 10% of January days (since P90 is 15 m/s). The probability of exceeding 12 m/s would be higher than 10%.
• At Tullamore, wind speeds exceeded 12 m/s in less than 10% of January days (since P90 is 10 m/s). The probability of exceeding 12 m/s would be lower than 10%.

This comparison underscores the importance of location-specific climatology for realistic planning and risk assessment. The Shear Glass instrument can help visualise the typical wind at various heights for your chosen location.

While the primary monthly climatology chart typically focuses on mean wind speed, it is important to understand the climatology of gusts. Gusts are short-duration peaks in wind speed, often defined as the highest 3-second average within a 10-minute period. They are crucial for many operations as they represent the instantaneous forces that can cause damage or exceed operational limits.

Gusts are inherently more variable than mean wind speeds. The gust factor (ratio of gust speed to mean speed) also exhibits seasonal and diurnal patterns. In Ireland, higher gust factors are commonly observed during convective conditions, such as those associated with unstable air masses in spring and autumn, or during the passage of intense frontal systems in winter.

For example, while a January mean wind speed might be 8 m/s, the associated gust speed could have a P90 of 20 m/s, indicating that 10% of January gusts exceeded this value. In contrast, a July mean wind speed of 4 m/s might have a P90 gust speed of only 10 m/s.

It is critical for users whose operational limits are defined by gust speed to consult gust climatology where available, or to apply appropriate gust factors to mean wind speed climatology, always noting the source and limitations of such factors. The Wind Agent focuses on providing both mean and gust forecasts, and the exceedance fan can be configured for either metric.

The monthly climatology chart is a powerful tool for setting realistic expectations and informing long-term planning. It helps answer questions such as:

• What is the likelihood of encountering strong winds during a particular month? By examining the P90 value, you can estimate the speed that is exceeded only 10% of the time, providing an upper bound for typical conditions.
• When are the calmest periods expected? The months with the lowest median speeds and narrowest P10-P90 ranges indicate periods of generally lighter and more consistent wind.
• How much variability should be anticipated? A wide interquartile range or a large spread between P10 and P90 suggests that wind conditions can fluctuate significantly within that month, requiring more flexible planning.

For example, if planning a marine operation with a strict wind speed limit of 15 m/s, and the climatology shows that for November, the P90 is 18 m/s, this immediately flags November as a month where exceeding the limit is historically common. Conversely, if the P90 for June is 10 m/s, it suggests a much lower historical probability of exceeding the limit. This information can influence project scheduling, resource allocation, and risk assessments, allowing for proactive adjustments rather than reactive responses to adverse conditions.

Climatology is ideally based on long-term records, typically 30 years, as defined by the World Meteorological Organisation (WMO) for standard climate normals (e.g., 1991–2020). Shorter records, while sometimes the only data available, come with significant limitations:

• Representativeness: A short record (e.g., 5–10 years) may not capture the full range of natural variability, including infrequent but impactful events like severe storms or unusually calm periods. It might be biased by a particularly active or quiet meteorological phase.
• Statistical Robustness: Percentiles and medians derived from short records are less statistically robust. Extreme values can disproportionately influence the statistics, leading to an inaccurate representation of the long-term climate.
• Trend Masking: Short records make it difficult to identify long-term climate trends, such as changes in mean wind speed or storm frequency, which might be evident over longer periods.

When reviewing climatological data, always check the period of record. If it is shorter than the WMO standard, interpret the statistics with caution and acknowledge the increased uncertainty. For instance, a 10-year record might show a P90 that is lower than what a 30-year record would reveal if the 10-year period happened to be less blustery than average. The Wind Agent primarily uses ERA5 reanalysis, which provides a consistent, long-term global dataset, mitigating some of these issues for locations without extensive local observations.

The monthly climatology provides the backdrop, but for immediate operational decisions, it must be paired with live, high-resolution forecasts. The climatology tells you what is typical for a given month; the forecast tells you what is expected for the coming hours and days.

How to use them together:

1. Contextualise the forecast: If the forecast for a summer day shows wind speeds at the P90 of the monthly climatology, you know it will be an uncharacteristically blustery day for that time of year. This might prompt extra vigilance or a review of contingency plans.
2. Evaluate forecast anomalies: If a winter forecast shows wind speeds consistently below the P10 of the climatology, it indicates an unusually calm period, which could be beneficial for certain operations but also unusual enough to warrant closer monitoring of model agreement.
3. Inform long-range planning: Use climatology to identify periods that are historically more favourable or challenging for specific operations. Then, as the operational window approaches, switch to detailed hourly forecasts from The Wind Agent, using tools like the ensemble plume and the exceedance fan to assess real-time probabilities against your limits.

The Wind Agent's instrument allows you to view the current forecast alongside historical data, providing a comprehensive picture for decision-making. The Agreement Spine can also show how well recent forecasts have aligned with observations for your location, adding another layer of confidence to your planning.