import math


def calculate_row_relative_humidity(air_temperature, dew_temperature):
    """
    Computes the relative humidity from air temperature and dew point.
    :param air_temperature: the dry air temperature
    :param dew_temperature: the dew point
    :return: the relative humidity
    """
    positive = {'b': 17.368, 'c': 238.88}
    negative = {'b': 17.966, 'c': 247.15}
    const = positive if air_temperature > 0 else negative
    pa = math.exp(dew_temperature * const['b'] / (const['c'] + dew_temperature))
    rel_humidity = pa * 100. * 1 / math.exp(const['b'] * air_temperature / (const['c'] + air_temperature))
    return rel_humidity