Further reading

GNU Fortran compiler documentation: https://gcc.gnu.org/onlinedocs/gfortran
Wikipedia article on advection: https://en.wikipedia.org/wiki/Advection
Wikipedia article on finite differences: https://en.wikipedia.org/wiki/Finite_ difference

Summary

Building an executable Fortran program consists of the compilation and linking steps.
There are five program units in Fortran: main program, function, subroutine, module, and submodule.
A program begins with a program statement and ends with an end program statement.
In every program, we first declare the data, and executable code comes after.
Use implicit none at the top of your declarative code to enforce explicit declaration of all variables.
There are five built-in data types in Fortran: integer, real, complex, character, and logical.
if blocks are used to test for conditions and take different execution branches depending on their values.
Use the stop statement to abort the program immediately and print a helpful message to the terminal.
do loops are used to iterate over sections of the code a specified number of times.
Start, end, and increment values of a do loop counter can have any integer value.
Fortran’s arithmetic rules are the same as those we learn in school: exponentiation is evaluated first, then multiplication and division are evaluated, and finally addition and subtraction go last; this order can be overruled with parentheses.
print * is an easy way to print the values of any variable or literal constant to the terminal.