A Fortran developer designs, writes, tests, and debugs programs using the Fortran programming language, a high-level language primarily used for scientific computing and numerical computation. They utilize its array features for data manipulation and its module system to structure large applications. They also improve existing codes for efficiency and speed. Their duties include optimizing algorithms, ensuring accuracy of computations, and maintaining documentation for code modifications. They collaborate with scientists and engineers to translate mathematical models into software, often working on complex problems in fields like physics, engineering, and meteorology.