Even though the prominent tree – the same horse chestnut featured in this article – in this image has lost its leaves, and is settling in for the winter, it is still significantly warmer than the sky behind it. This is because of the Greenhouse Effect.
Solar radiation, even on an overcast day, contains mostly visible light and short-wave infrared radiation (the latter between 700 and 1350 nm wavelength). Very little is in long-wave IR, above 8000 nm, to which the FLIR One™ IR camera is most sensitive. Thus the sky, when captured in a long-wave IR image, is usually the coldest area.
That solar radiation warms objects like trees and grass, which then emit long-wave IR which is detected by the camera, hence the trees and grass appear warmer than the sky, and warmer than the inert telegraph pole and smaller post seen to the left of the tree. The halo apparent around the branches of the tree is an artefact resulting from low resolution.
The cattle, here resting on the ground to chew the cud, are significantly warmer than the trees and grass, as they are homeotherms, maintaining a body temperature similar to ours. Even though they have quite a lot of insulating fat, thick hides, and body hair, their surface temperature is the highest in the image.