There are many considerations: among the prime ones are weather at destination and at the alternate (including forecast changes in the relevant timescale), together with how much excess fuel you have to hold before making a diversion. Other than for US airlines, the decision is normally made by the captain, though sometimes the airline's ops room may give an instruction. In the US, airline despatchers and the airline ops rooms exercise much more detailed control over flights than is the case elsewhere, so they may well call the shots. ATC does not have all the relevant info, such as fuel endurance (though sometimes they will ask), crew duty time considerations, or why particular alternates are nominated. ATC also does not normally declare "airport closed" - usually it's up to the captain to evaluate whether the conditions are within limits for him, his crew, and his aircraft, given the state of the weather, landing aids, runways, and so on.
Last edited by kenparry; 31st May 2024 at 13:36.