As I have stated before, I am love flight and flying, and one of the things I do, every now and then, is to look at planes coming and going on flightradar24.com.
On the previous few days I have seen a number of flights divert or return due to bad weather at the destination: yesterday I saw an Austrian Airlines flight from Vienna to Skopje (Macedonia) return to Vienna, then a Wizz Air flight from Basel to Tuzla (Bosnia & Herzegovina) divert to Timișoara (Romania). The Wizz Air flight took off today around noon and has circled above Tuzla for more than two hours, waiting for the fog to lift so that it could land, and since that did not happen, it diverted yet again, this time to Banja Luka.
All this extra flying means lots and lots of fuel being burned, and in the case of the Wizz Air flight, the aircraft being unable to fly its subsequent legs, all of this amounting to a hefty sum in fuel costs (the A320 burns roughly 2000-2500 kg/h or 2500-3000 l/h while holding) and lost revenue, not to mention passenger food & accomodation.
So nature can and does throw a wrench every now and then in the way of human plans, and when she does, expect delays and costs.