When Is Allergy Season? How Meteorologists Do the Math

Allergy season might seem far off but now is actually the time to start thinking about the next year, especially if you’re a provider of services to allergy sufferers. But how do we know when allergy season is likely to hit? Let’s explore some of the ways meteorologists work this out.

1) Understanding Longer Growing Seasons & Changing Plant Zones

Predicting allergy season requires understanding trends around plant growing seasons and pollen production cycles, which are closely connected:

Rising Temperatures Impact Growing Seasons

As summers become longer in our warming climate, springtime shifts to earlier, fall season starts later, and the winter season also gets shorter. As growing seasons get longer, so do our allergy seasons.

Pollen-Producing Plants Moving into New Areas

Due to shifting plant zones, we also now see new plants invading new regions. As a result, people can experience allergies in different ways and discover sensitivities to new types of pollen they might not have been exposed to a lot before.

2) Keeping Track of What’s Happening on the Ground

Citizen science programs help scientists monitor plant reactions to changing environmental conditions via active reporting on seasonal changes such as flowering and leaf-shedding.

For example, in the US, the National Phenology Network deploys the Nature’s Notebook program, enabling individuals from across the country to observe and report changes in plants and animals via an app.

Nature's Notebook
(Image from Nature’s Notebook page on the app store)

In addition to citizen collaborations, governments across the world also utilize pollen monitoring stations. These stations trap pollen using adhesive strips and slides, which observers then examine under a microscope to count and categorize pollen according to plant groups.

In the US, Phenology Network scientists compare station findings with reports on flowering from the app at varying region sizes around monitoring stations, thus establishing a framework of when a season reaches its ebb and flow.

3) Building Seasonal Predictive Models

By processing massive amounts of data from a wide range of sources, and comparing and contrasting different forecast methodologies, meteorologists have created models to predict when allergy season might hit in certain locations. To understand how this forecasting works, it helps to understand the difference between ‘weather’ and ‘climate’ forecasting:

  • Weather forecasting, also called deterministic forecasting, involves predicting specific weather events on certain days, based on what’s happening in the atmosphere when the model starts running. Simply put, weather conditions today help predict the conditions tomorrow.
  • Climate forecasting also called probabilistic forecasting because of the uncertainty associated with long-range weather forecasting, involves predicting changes in climate weeks and even months into the future based on ‘boundary conditions’ – in other words, variables like sea surface temperatures, soil moisture levels, and snow cover.

Some of the factors involved in the latter long-range climate forecasting which would be needed to predict allergy season ahead of time might include:

A –  The Impact of La Niña & El Niño Events

A lot of seasonal forecast predictability comes from sea surface temperatures rising or dropping below normal average. El Niño and La Niña events refer to periods of unusually warm or unusually cold sea surface temperatures in parts of the Pacific Ocean.

During a La Niña event, for example, the tropical Pacific Ocean cools down more than normal, resulting in less precipitation and less upward moving air. This makes winter dryer and warmer over the southern US and cooler over northern regions. A shorter winter makes the growing season, and with it, allergy season, start earlier.

Sea Surface Temperatures During a 2021 La Niña Event

Sea Surface Temperature Visual

(As featured by Dr. Jan Dutton of World Climate Service in Climate-allergy link webinar)

 B – Leveraging Past Data For Future Forecasts

Meteorologists would also look at average temperatures that normally occur during La Niña and El Niño event months from past years to predict likely changes in the climate for the near future according to these historical patterns.

Analog Forecast

(As featured by Dr. Jan Dutton of World Climate Service in Climate-allergy link webinar)

C – Local Growing Season Data

Meteorologists can leverage information from county-level weather databases to track local growing seasons as they extend or shorten: For example, in the very southern US, the growing season might start in January or early February, while in the Pacific Northwest it could arrive as late as June.

D – Higher CO2 Pollution Equals More Pollen

Higher CO2 emissions translate to more pollen production. In fact, according to Climate Central, pollen production doubled over the last century due to rising CO2 levels, and failure to curb these emissions could see concentrations double again by 2060.

Putting Method into Practice: What’s Predicted for 2022?

As the growing season lengthens we witness higher pollen counts in general. But climate events such as La Niña, which we’re now experiencing, increase the likelihood of warmer than average temperatures across the southeastern United States.

For 2022, this means a higher likelihood of an earlier start to the growing season across the region, and we can also assume spring will likely see an early start to pollen season across these regions as well.

Explore this Topic Further

The content in this explainer is based on our dedicated December 2021 webinar session – moderated by Nancy Colleton and featuring leading meteorologists Bernadette Woods Placky, Dr. Theresa Crimmins, and Dr. Jan Dutton which explored this topic in much more detail.

Share This Post
Avatar photo
Luthien Melchior

Senior Content Manager @ BreezoMeter