Computes the variance of corresponding sections over time.

    Returns:
      a list of np arrays.

def _sections_to_variance_sections(self, sections_over_time):
    '''Computes the variance of corresponding sections over time.

    Returns:
      a list of np arrays.
    '''
    variance_sections = []

    for i in range(len(sections_over_time[0])):
      time_sections = [sections[i] for sections in sections_over_time]
      variance = np.var(time_sections, axis=0)
      variance_sections.append(variance)

    return variance_sections