def ia(ta, r, a, b, c):
    """Chicago design storm equation - intensity after peak. Helper for i function.

    Args:
        ta: time after peak in minutes (measured from peak towards end)
        r: time to peak ratio (peak time divided by total duration)
        a: IDF A parameter - can be calculated from getABC
        b: IDF B parameter - can be calculated from getABC
        c: IDF C parameter  - can be calculated from getABC

    Returns:
        Returns intensity in mm/hr.

    """
    return a*((1-c)*ta/(1-r)+b)/((ta/(1-r))+b)**(c+1)