Half Moon Bay pollen forecast - today, tomorrow and the week ahead
Pollen Today
Today (20 September) pollen in Half Moon Bay is high; ragweed is high; grass and mugwort are low.
Pollen Tomorrow
Tomorrow (21 September 2026) mugwort remains low, ragweed remains high, grass increasing.
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 | 23/100 |
| Willow (Salicaceae) | None | 0/100 |
| Mugwort (Artemisia) | Low | 2/100 |
| Ragweed (Ambrosia) | High | 63/100 |
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) | Moderate | 30/100 | |
| Willow (Salicaceae) | None | 0/100 | — |
| Mugwort (Artemisia) | Low | 10/100 | |
| Ragweed (Ambrosia) | High | 70/100 |
Local Environmental Conditions
Half Moon Bay's setting on a foggy coastal terrace south of San Francisco supports coastal scrub, Monterey cypress, and artichoke and pumpkin farmland as the leading local vegetation, all factoring into the pollen count through the year. Wind patterns across the area help determine how far pollen travels from these source plants. As with most of the region, counts tend to build gradually rather than spike overnight.
The pollen calendar in Half Moon Bay opens with olive and acacia starting January then eucalyptus follows, running through February. Grass pollen builds through March and April, and sagebrush and mugwort extend exposure into early autumn. The Southern California Mediterranean climate advances the season, and onshore Pacific breezes can periodically disperse pollen, a defining feature of the local season.
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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 different from today, with artemisia at low levels, ambrosia at high levels and grass increasing
Is pollen expected to decrease in Half Moon Bay this week?
According to the five-day outlook, grass and ambrosia 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).
