Temperature is the key element which regulates VOC production and emissions from vegetation. Synthesis of these compounds is always related to temperature and temperature is the main element regulating enzyme activity of plant cells and the volatility of VOCs. Rising temperature increases internal vaporization pressure of the cells and as a results elements are released from the cells faster. Temperature response of vaporizing compounds is high until we reach temperature which is at the range of 40 to 45 oC degrees. Many VOCs have a special temperature range and annual variation which drives volatility of VOCs.
High temperatures can cause drought and heat stress which increases emissions of VOCs, since VOCs will increase the heat tolerance of the plan cells. On the other hand, exceptionally high temperatures are relatively rare in boreal vegetation zone.
Air humidity follows changes in solar radiation with the certain delay. Sun radiates to the Earth surface but at the same time Earth reflects radiation back to the space. Leaving radiation is more powerful when coverage of cloud surface is higher since clouds reflect solar radiation back to the space. Proportional ratio of leaving radiation increases when relative coverage of the clouds is higher. High variation in radiation proportion causes annual rhythm to the air temperature.
Diurnal variation in solar radiation reflects directly to the air temperature. Solar radiation warms lower part of the atmospheric surface during sunny and cloudy days but air cools down when sun goes down. The heat storage in the soil regulates diurnal variation. Strong peaks in the temperature are typical to the sunny and cloudy days.
Even temperature measurements are relatively easy, it is still partly challenging if temperature accuracy should be under few degrees. Radiation heats the sensor so it should be protect against heat radiation. Temperature also varies spatially and temporary so measurements executed at one point will not present temperature range for the whole forest. Temperature sum is calculated by summing the degrees when daily average temperature is over 5 degrees. Annual temperature sum reached usually over 1300 day degrees (dd) when it is under 600 day degrees in Northern Finland.