helper.rb

def value_for_label object, method
  (object && object.send(method)) || "-"
end

def conditions_map_for conditions
  {
    :dp_c => 'Dew point',
    :rh => 'Relative humidity',
    :feels_like_c => 'feels like',
    :wind_dir_and_speed => 'Wind',
    :rainfall_mm => "Rainfall #{conditions.rainfall_mm_period if conditions}"
  }
end

shorthand.html.haml

  .conditions
    %dl
      - conditions_map_for(@conditions).each do |method,label|
        %dt= label
        %dd= value_for_label @conditions, method