Viewing pollen by satellite

Viewing pollen by satellite

April 6, 2009

The pollen season has started and for hay fever sufferers the greatest challenge comes from birch trees. Remote-sensing can now give better pollen forecasts.

Asthma and allergies, including hay fever, are two of Norway's most common illnesses. One in five Norwegians suffers from these illnesses, and the proportion is increasing.

Birch pollen is the most significant allergy-causing pollen type in Norway. One-fifth of the country is covered by birch forests, while mixed forests containing many birch trees cover another one-fifth. Regrowth of previous grazing land and reduced felling mean the birch is ever advancing.

Pollen forecasts

More accurate pollen forecasting can improve the day-to-day lives of hay fever sufferers. Pollen forecasts contribute to the correct dosing of preventative medicines and assists with the planning of activities.

Today's pollen forecasts are mostly based on measurements from 12 traps distributed throughout Norway. These traps cover large and varied regions. The amount of pollen varies between coastal and inland areas, as well as with altitude and different types of vegetation. This is where satellite-based measurements come in.

It is scarcely possible to see pollen with the naked eye, so is it now possible to see pollen using satellite-based technology? Can this thought generate greater breathing difficulties than the moderate spreading of birch pollen?

Senior Research Scientist Stein Rune Karlsen at Norut Tromsø puts it into context:

"The onset of birch leafing and flowering occurs around the same time. There is an increase in photosynthesis as a result of leafing that can be measured by satellite. Or to put it simply: you can see when the birch forests become green," says Karlsen.

New map

"We interpreted the satellite data and linked this with maps we had previously developed of vegetation types over the entire country. In this way, we have developed a completely new map that shows the start of the birch pollen season in Norway."

Click on the following link to download a poster of the pollen map. Text on map is in Norwegian only (A4format, 6 Mb):

Bjørkepollen plakat

The map of Norway shows the eight-year mean (2000-2007). It shows major differences in the timing of the onset of birch flowering occur from year to year.

The measurements from the traps throughout Norway capture only a small part of the variation in pollen amounts. The satellite data provides a useful correction and a better basis for preparing good and more precise forecasts.

As well as calculating the onset of birch flowering in spring, the map provides a good indication of regional differences throughout the entire pollen season.

What does the map show?

Not surprisingly, the map shows that the onset of birch flowering occurs much earlier in southern parts of Norway than in the north of the country. Similar differences also exist between lowland and mountain areas.

According to the map, in an average year you can expect the onset of the birch flowering season to occur as follows:


  • Before May 1: Lowland areas in Southern Norway
  • May: Lowland areas in Northern Norway and areas of 300-700 m above sea level in Southern Norway
  • June: Lowland areas in the far north as well as areas around and above the forest line in the south


The high mountains in Norway lead to large local differences. In Western Norway, birch and silver birch trees can produce pollen even before April 20, while dwarf birch on mountain plateaus a few hundred metres away can produce pollen two months later.

What the map does not show

But a map cannot tell everything. The map expands on averages. There are major differences in the timing of the onset of birch flowering from year to year, especially along the coast. In 2004 and 2005, there was a relatively late start in most of the country, while in 2002 and 2004 there were major regional differences in Norway. In 2002, the start of the season was early in Northern Norway, while in 2004 it was early in the south of the country.

"There are also large differences in how much pollen each tree produces," says the Tromsø plant ecologist. "For instance, the trap station in Oslo shows on average 10 times more pollen than Tromsø, even though there are more birch trees in Tromsø than Oslo."

"The map doesn't provide details of the amount of pollen, but one can assume that areas with early starts generally get more pollen than areas with late starts."

"In most instances, there was good conformity between the data from the trap stations, observations of birch leafing and the satellite data that was utilised, but this conformity was lacking locally," says Karlsen adding: "We believe the map is about 90 percent correct, but includes local errors spread in between."

Another source of errors is that the map indicates only pollen from local birch and does not provide information about long distance transported pollen. Some years the pollen season can start before the onset of birch flowering in Norway. This occurred particularly in parts of the country in 2005 and 2007. South-easterly winds brought birch pollen, probably from the Baltic States and Poland.

"The next stage of the project is to follow the birch pollen season in real time using satellite data."

Pollen forecast for 2009

The pollen forecasts for 2009 (in Norwegian only) started on February 20 and it is soon the turn of the birch tree. The map will be used as a reference for this year's pollen forecast, but it can also serve as a useful source of reference for hay fever sufferers in general.

The map was developed by Northern Research Institute (Norut) and Hallvard Ramfjord (NTNU/the Norwegian Asthma and Allergy Society NAAF), while the Norwegian Space Centre has made a financial contribution.

The findings from the project "Improved pollen forecasting using MODIS-NDVI satellite data" has recently been published internationally, with the following reference:

Karlsen, S.R., H. Ramfjord, K. A. Høgda, B. Johansen, F. S. Danks & T. E. Brobakk. 2009. A satellite-based map of onset of birch (Betula) flowering in Norway. Aerobiologia. 25:15-25.

Contact person: Senior Research Scientist Stein Rune Karlsen, Norut Tromsø