Grass season, hour by hour
Grass pollen is not evenly spread through the day. It arrives in waves, and the waves are predictable enough that shifting a run or a window by three hours can matter more than any daily forecast number.
Where the day's pollen comes from
Grasses release pollen when their flowers open, which for most of the common allergenic species happens in the morning, broadly between seven and ten o'clock, given dry, warming air. Rain or heavy dew delays it; a damp morning that clears by eleven can push the whole release later.
Once the grains are airborne, the atmosphere decides where they go. As the ground warms, rising air lifts pollen up and mixes it through a layer that can be a kilometre or more deep by mid-afternoon. That mixing dilutes concentrations near the ground even though the total amount in the air is still climbing. In the evening, as the surface cools and the mixing layer collapses, what was aloft settles back down into the shallow layer you are breathing.
Which gives the classic two-peak shape:
- A morning peak, roughly late morning, from fresh local release before the air has mixed deeply.
- A dip in the afternoon, when vertical mixing is strongest and the same pollen is spread through a much larger volume.
- An evening peak, often the higher of the two in built-up areas, as pollen that drifted in from surrounding countryside descends.
Town and country run on different clocks
This is the part that surprises people, and it is the basis of the standard European advice on when to ventilate:
| Where you live | Typically worst | Best window to air the flat |
|---|---|---|
| City centre | Evening, roughly 18:00 to midnight | Early morning, roughly 06:00 to 08:00 |
| Countryside | Morning, roughly 06:00 to 12:00 | Evening, after roughly 19:00 |
The logic is straightforward once you see it. In the countryside you are standing next to the source, so you get hit at release time. In a city the nearest large grassland is kilometres away, so the pollen needs the afternoon to travel and the evening collapse to bring it back down to street level. Same pollen, same day, opposite advice.
Treat this as a starting hypothesis, not a law. A park across the road, a river valley funnelling wind, or a mown verge under your window all bend the curve. The point of an hourly forecast is to check the shape where you actually are, rather than assume it.
What weather does to the curve
- Rain scrubs grains out of the air, often within minutes. The relief is real but temporary: a dry, breezy spell afterwards can reload the air fast, and the vegetation has kept flowering the whole time.
- Wind raises concentrations by carrying pollen further and by shaking it loose. Dead calm days concentrate it near the source instead.
- Humidity suppresses release. Grass flowers tend not to open in saturated air, which is why a muggy overcast morning can be quieter than a bright one.
- Thunderstorms are the exception to "rain is good". The gust front ahead of a storm sweeps up pollen and the humidity can rupture grains into fragments small enough to reach the lower airways. If you have asthma as well as hay fever, the half hour before a summer storm breaks is the wrong time to be out.
Using the shape of the day
Once you know your own curve, a few things get easy:
- Move the run, do not cancel it. For most city dwellers, early morning is a far better slot than after work, which is exactly the opposite of most people's habit.
- Ventilate on a timer, not on impulse. Pick the trough and open everything wide for ten minutes rather than leaving a window ajar all evening.
- Take antihistamines ahead of the peak, not after it. Oral antihistamines generally need an hour or so to take effect, so dosing when symptoms are already going is dosing late. Your pharmacist or doctor is the right person to plan the actual regimen with.
- Shower and change before bed on high days. Hair and outdoor clothes carry a surprising load into the bedroom, which then has all night to work on you.
- Dry laundry indoors during peak weeks. A line of washing outside is an efficient pollen collector.
Why hourly beats daily
A daily figure has to summarise all of the above into one word. Two days can both be labelled "moderate" while one had a flat, mild curve all day and the other had a sharp three-hour spike at seven in the evening. If your bad evenings keep landing on days the forecast called unremarkable, the daily number is simply the wrong resolution for the question.
Polly's hourly view shows the curve for your location so you can see the troughs rather than guess at them, and the diary lets you check the pattern against how you actually felt. Related reading: why your allergy app says "low" on a terrible day.
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