def adjust_weather(data, interval, interval_unit):
    """Resample weather data, zero out negative GHI.

    data should be DataFrame from parse_weather
    interval: e.g. 15
    interval_unit: e.g. "Min"
    """
    # Get 15-minute average. Since we want historic data leading up to
    # the time in our interval, use 'left' options
    weather = data.resample('{}{}'.format(interval, interval_unit),
                            closed='right', label='right').mean()

    # Zero-out negative GHI.
    weather['ghi'][weather['ghi'] < 0] = 0

    return weather