Updated: 2026-08-15
For anyone prone to hay fever in London, the key question is always the same: when does the pollen season actually start? The answer, based on the complete daily temperature record for St James's Park kept by the Met Office, is written into the thermal profile of each month. The indicator used here is straightforward and reliable: days where the maximum temperature reaches or exceeds 15°C. That threshold marks the moment when grass and tree pollen become active and airborne concentrations begin to affect allergy sufferers. Using this proxy, the practical onset of the pollen season for allergy sufferers begins in April, when the number of days at or above 15°C starts to become frequent enough to trigger symptoms. In the long run of data from 1912 to 2025, the month that dominates this thermal trigger is not spring at all—it is August, with 3,375 days above the threshold, followed closely by July's 3,350 days.
That single figure is not an outlier; it reflects a seasonal pattern that repeats with remarkable consistency across more than a century of measurements. August, the warmest month on average for the capital, holds the top position for the number of days crossing the 15°C line. But the spread of those days tells the real story of when Londoners should start preparing. The earliest months of the year barely register: January has just 4 days above the threshold, February 53, and even March, often associated with the first tree pollen, still sits at 442. The season truly accelerates in April, when 1,225 days qualify—this is the month when the count becomes meaningful for allergy sufferers, marking the transition from sporadic warm days to a consistent pattern. It then surges in May with 2,542 days. By the time June arrives, the count jumps to 3,152, and the peak of the season—July and August—becomes unmistakable.
Looking at the annual maximum temperatures recorded at St James's Park, the data also shows why recent summers feel so much more intense for allergy sufferers. The year 2022 stands out with an annual maximum of 40.2°C, the highest value in the entire series from 1912 to 2025. That extreme heat does not just make life uncomfortable; it extends the window of days above 15°C well into the autumn, prolonging exposure to pollen. Compare that with 1931, when the annual maximum reached just 25.6°C—a summer that would have produced far fewer qualifying days and a shorter, milder pollen season. The contrast between these years, drawn directly from the Met Office archive, shows how warmer seasons lengthen the allergic period.
The monthly distribution of days above the threshold is best appreciated as a full table, because it shows the slow build, the explosive middle, and the sharp decline. It also reveals when allergy sufferers should start taking precautions:
| Month | Days with max temperature ≥ 15°C | What it means for allergy sufferers |
|---|---|---|
| January | 4 | No practical pollen risk |
| February | 53 | Still dormant for most allergens |
| March | 442 | Occasional warm days, but not yet a consistent trigger |
| April | 1,225 | Season begins here: counts become significant enough to affect sensitive individuals |
| May | 2,542 | Full allergy season underway; regular symptoms expected |
| June | 3,152 | High pollen activity; most sufferers need daily medication |
| July | 3,350 | Peak season—the worst period for hay fever |
| August | 3,375 | Peak season continues; still the highest count on record |
| September | 3,066 | Remains intense; many sufferers still affected |
| October | 1,980 | Gradual decline, but late-season allergens can persist |
| November | 298 | Mostly over for the year |
| December | 31 | Season effectively closed |
The pattern in the table is clear: the pollen season in London does not start with a single day in March or April. It is a gradual ramp that begins in earnest in April, strengthens through May, and reaches its peak in the high summer months of July and August. For allergy sufferers, the practical takeaway is that April is the month to start preparing—this is when the count of days above 15°C becomes high enough to consistently trigger symptoms. The counts for July and August—3,350 and 3,375 respectively—are virtually identical, meaning that across 113 years of records, Londoners have experienced nearly the same number of pollen-friendly days in both months. September still holds a substantial 3,066 days, which explains why hay fever can linger well into the early autumn, especially in warmer years.
What this means for planning is simple. If you are sensitive to pollen and live in or around London, the Met Office data indicates that the risk becomes significant from late April, reaches its maximum in July and August, and does not fully subside until October. The annual maximum temperature series reinforces this: warmer years such as 2022 (40.2°C), 2019 (37.0°C), and 2020 (36.7°C) all fall in the last decade, suggesting that the number of days above 15°C—and therefore the length of the pollen season—has been expanding. Yet the comparison with historical years like 1932 and 1947, both reaching 33.9°C, shows that intense summers are not new; what has changed is their frequency in recent years.
The complete annual series of maximum temperatures at St James's Park, plotted year by year, offers the most direct view of how each summer compares:
The chart shows the complete annual series. The table lists the most notable years; the full series comes from Met Office.
| Year | Annual max temperature (°C) |
|---|---|
| 1990 | 35.3 |
| 2003 | 14.0 |
| 2015 | 34.7 |
| 2018 | 35.0 |
| 2019 | 37.0 |
| 2020 | 36.7 |
| 2022 | 40.2 |
The annual series, while variable, shows a clear warming trend in the most recent decades. Beyond the exceptional 40.2°C of 2022, the years 2019 (37.0°C) and 2020 (36.7°C) sit among the highest values ever recorded at this station, which has been observing continuously since 1912. For pollen sufferers, the implication is direct: more extreme summer heat translates into more days crossing the 15°C threshold, and consequently a longer, more intense pollen season. The Met Office record from St James's Park, covering the period from 1912 to 2025, provides the longest continuous urban temperature series in the United Kingdom, making it the definitive reference for understanding how London's climate—and its pollen calendar—is changing.
Source: Met Office, London St James's Park station, period 1912–2025.
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© 2026 HistoClima · data: Met Office (OGL v3.0) · not affiliated with Met Office · citation permitted with source.