Half Moon Bay pollen forecast — James Johnston House neighborhood
Pollen Today
Today (20 September) pollen in James Johnston House, Half Moon Bay is high; ragweed is high; grass and mugwort are low.
Pollen Tomorrow
Tomorrow (21 September 2026) levels are similar to today.
7-Day Trend
Over the next week grass and ragweed will decrease; mugwort will remain stable.
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Pollen Risk
Today - 20 September
| Pollen species | Level | Value |
|---|---|---|
| Alder (Alnus) | None | 0/100 |
| Birch (Betula) | None | 0/100 |
| Hazel (Corylus) | None | 0/100 |
| Cypress (Cupressaceae) | None | 0/100 |
| Grass (Poaceae) | Low | 17/100 |
| Willow (Salicaceae) | None | 0/100 |
| Mugwort (Artemisia) | Low | 2/100 |
| Ragweed (Ambrosia) | High | 57/100 |
Tomorrow - 21 September
| Pollen species | Level | Value | 7-Day Trend |
|---|---|---|---|
| Alder (Alnus) | None | 0/100 | — |
| Birch (Betula) | None | 0/100 | — |
| Hazel (Corylus) | None | 0/100 | — |
| Cypress (Cupressaceae) | None | 0/100 | — |
| Grass (Poaceae) | Low | 25/100 | |
| Willow (Salicaceae) | None | 0/100 | — |
| Mugwort (Artemisia) | Low | 10/100 | |
| Ragweed (Ambrosia) | High | 65/100 |
Local Environmental Conditions
Here, just around the area, sycamore and coast live oak thrive along the damp bank, both common streamside trees here. Located within the local area, this area has this park's mix of turf and landscaped trees contributes a modest grass and ragweed pollen load. Comparable land cover in surrounding areas tends to produce a similar pollen mix overall.
Average temperatures range from about 11°C in March to 17°C in September, and most of the 551 mm annual rainfall falls in the cooler months around March. Pollen activity typically picks up as temperatures climb out of the March low, from around April onward.
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Frequently Asked Questions About Pollen in California
Is pollen high in Half Moon Bay right now?
On 20 September 2026, overall pollen levels in Half Moon Bay are high. Ambrosia is the main contributor to airborne pollen, while artemisia and grass remain at lower levels
Will pollen levels change in Half Moon Bay tomorrow?
On 21 September 2026, pollen conditions in Half Moon Bay will be similar to today, with ambrosia at high levels, artemisia at low levels, grass at low levels
Is pollen expected to decrease in Half Moon Bay this week?
According to the five-day outlook, ambrosia and grass pollen levels are expected to gradually decrease, suggesting improving conditions later in the week.
Which pollen types are currently active in Half Moon Bay?
On 20 September 2026, Ambrosia is the dominant pollen source in Half Moon Bay. artemisia and grass are also present but at lower levels.
When is pollen season in Half Moon Bay?
How often is the Half Moon Bay pollen forecast updated?
The Half Moon Bay pollen forecast is updated four times daily. See the timestamp below for the most recent update.
Does pollen in Half Moon Bay come from nearby or far away?
Studies show that most pollen exposure (often 70–90%) comes from nearby sources, such as local trees and vegetation. A good pollen forecast therefore depends on accurately representing local vegetation. Pollen can travel long distances, but this usually matters only early in the season or during specific wind events.
Are pollen levels in Half Moon Bay based on measurements or forecasts?
Pollen data is often misunderstood. Most "pollen levels" are not real-time measurements, but forecasts.
Direct pollen measurements are typically delayed by 1-2 days because they require manual analysis. To show current conditions in Half Moon Bay, modern forecasts combine recent measurements, weather data, and information about local vegetation.
This allows us to estimate pollen levels for today and the coming days, not just report the past.
Airmine’s atmospheric monitoring platform was developed with support from the European Space Agency (ESA)’s Business Applications and Space Solutions (BASS) team, using satellite and Earth observation technologies to improve emissions monitoring.
Understanding the Scale
The 0–100 scores represent risk levels, not direct measurements of pollen or air pollutant concentrations. Higher scores indicate higher risk. Pollen and air quality use different thresholds.
Pollen
| None | 0 | No or negligible risk of exposure. Pollen/mold exposure is expected to be minimal. |
| Low | 0-25 | Low risk of exposure. Small amounts of pollen or mold spores may be present. |
| Moderate | 25-50 | Moderate risk of exposure. Pollen/mold exposure is expected to be noticeable. |
| High | 50-75 | High risk of exposure. Elevated pollen/mold exposure is expected. |
| Very High | 75-100 | Very high risk of exposure. Pollen/mold exposure is expected to be at or near its highest levels. |
Get More in the Airmine Pollen app
Get 15-day pollen forecasts with morning, afternoon and evening detail, plus 5-day air quality forecasts. Explore interactive pollen risk maps, estimated pollen season start, route profiles and a symptoms diary.
Features and forecast availability vary by region. Currently covering Europe, the contiguous US, southern Canada, South Africa, Australia, Indonesia (Java & Bali), Brazil, Paraguay, Uruguay and the Buenos Aires area.
Developed with support from the European Space Agency (ESA) Business Applications and Space Solutions (BASS).
