IR diagram of jet - just for fun

It is a general wisdom that IRST is of limited usefulness due to its sensitivity to adverse weather conditions. However, most modern stealth fighters (excepting the F-35 and J-31 tactical bombers) are intended to operate at high altitudes – above 50.000 ft – where ambient temperatures range from -30 to -60 degrees Celsius, which helps provide excellent contrast. Air at this altitude is also very dry, with 99,8% of the atmospheric water being below 45.000 ft. Combined with low air density and low aerosoil content, this means that there is very little atmospheric absorption of IR radiation. This applies especially to the longwave band, but detection capability is significantly improved in most bands as can be seen from the image below.