helper.rb

def v1(value)
  value || "-"
end

def v2(object, method)
  object && value = object.send(method) ? value : "-"
end

longhand.html.haml

  .conditions
    - if @conditions
      %dl
        %dt Dew point  
        %dd= v1 @conditions.dp_c 
        %dt Relative humidity 
        %dd= v1 @conditions.rh
        %dt feels like
        %dd= v1 @conditions.feels_like_c
        %dt Wind 
        %dd= v1 @conditions.wind_dir_and_speed
        %dt="Rainfall #{@conditions.rainfall_mm_period}"
        %dd= v1 @conditions.rainfall_mm
    - else
      %dl
        %dt Dew point  
        %dd= v1 nil
        %dt Relative humidity 
        %dd= v1 nil
        %dt feels like
        %dd= v1 nil
        %dt Wind 
        %dd= v1 nil
        %dt="Rainfall #{@conditions.rainfall_mm_period}"
        %dd= v1 nil

shorthand.html.haml

  .conditions
    %dl
      %dt Dew point  
      %dd= v2 @conditions, :dp_c 
      %dt Relative humidity 
      %dd= v2 @conditions, :rh
      %dt feels like
      %dd= v2 @conditions, :feels_like_c
      %dt Wind 
      %dd= v2 @conditions, :wind_dir_and_speed
      %dt="Rainfall #{@conditions.rainfall_mm_period if @conditions}"
      %dd= v2 @conditions, :rainfall_mm