Yellow rain is caused by pine tree pollen according to investigation of many years in Changsha.

根据对长沙黄雨的多年观测，确定黄雨是松树花粉所致。

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Significantly, this line marks the southern limit of the birch tree, a plant whose pollen is one of the causes of hay fever in northern Europe.

值得注意的是，这条线标志着桦树的南部界限，而桦树的花粉是引发北欧花粉热的一个原因。

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Its ability to distinguish more than 170 odours in the wild is vital for smelling nectar, pollen, water, tree resin and pheromones.

它能在野外辨别170多种气味。这种能力对闻花蜜、花粉、水、树脂和信息素的至关重要。

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The spring allergy capitals are based on tree pollen counts, while the fall cities are ranked on ragweed pollen counts from September through November of the previous years.

CNN: Worst U.S. cities for allergy sufferers

While people are tempted to say they have spring tree pollen allergies, it is helpful to know the exact types of trees you are sensitive to -- that way you can watch the pollen counts more efficiently.

CNN: Ah-choo! 4 questions for the allergist

"If you have a choice to plan an outdoor event versus an indoor event, people with spring allergies, who have tree pollen allergies, would be better off choosing indoor activities during the peak pollen season, " he said.

CNN: Predicting allergy season eludes experts