How I can find annual rainfall? With arrays in C

For a start you will need an array large enough for 53 years, 1970 - 2022, rather than 100. When you get the data use the year after subtracting 1970 as the index to your array, something like:

void SetRainFall(int year, int amount)
{
    year -= 1970;
    annual_rainfalls[year] = amount;
}