The elongated shape of the city and its isolation by high mountains, have a great influence on climate issues, because the movement of the air masses is from north to south through the transverse cracks Băniţa-Merisor and Surduc-Lainici. The mountains stop the movement of the air masses, so that they stop the air refresh in the depression. Because of that and of the industrialization, the streets, the trees and the grass in the city took a specific aspect of large coalmining regions. City landscape can be compared to that in the Ruhr, where stagnant fog covers the city and stays over because of the surrounding peaks.
There is also in Petrosani another interesting thermal phenomenon, the temperature inversion. This process can be described as stagnations and cooling of the air coming down from the heights of the mountains to the city. Under the influence of those stagnations and cooling, the temperatures exceeds minus 30 ° C (-31.4 ° C in January 14th 1893), or lower, while in the Parang station situated 900-1000 m above, the lowest temperatures were maximum – 24 ° C. Thus, the frosts in Petrosani are stronger than in the surrounding heights, but not longer than there. Although there is a slowdown of the frosts in Parang, that does not mean that their duration is reduced because the days considered cold, with temperatures below -10 ° C are more numerous on the mountain than in Petrosani, where there is sharp cooling because of the radiation effect. Also due to the cold air accumulation, we can have hoar and spring frosts in May.
By its geographical position and altitude, the climate in Petrosani should be sub- mountainous, but judging by the temperature of the hottest month (July average temperature being 16.7 ° C) and average annual temperature (6.8º), it is more a mountain climate, of law altitude. Average temperatures for July are 3-4 ° C and the annual, 2-3 ° C higher than sub-mountainous climate. Warm days with average temperatures above 10 ° C are lower in Petrosani than in other regions considered colder and the number of summer days with temperature above 25° C is 50 days. The average annual precipitation amounts are 700-800 mm / year.
Days with high cloudiness reach over 200 per year. Because industrial activity is intense and the atmosphere is most polluted, the rains are very frequently. Dust and smoke particles function as condensation nuclei, so mists and drizzles are particularly common in the autumn.
Petrosani has a central-European flora type with arctic-alpine elements in the high parts of the mountains and Mediterranean infiltrations in lower areas and special ecological conditions on chalks especially.
The vegetation is diverse: from alder groves and coppices along watercourses, often bordering ruffled buckthorn and raspberry bushes, to beech forests on hillsides and coniferous forests above them. The largest share in Petrosani vegetation is occupied by deciduous (oak, beech and oak) and meadows. Beech forests mixed with resinous (fir tree, spruce) are met on the slopes of the mountains; here we also find alpine meadows.
Fauna consists of forest animals such as stag, bear, deer and also, badger, fox, wolf, lynx, grouse, capercaillie and others.
Alpine field is populated with typical elements such as chamois, griffon vulture, eagle, etc. Mountain waters were colonized by indigenous trout that lives together withother species such as grayling and broad snout.