Temperature varies throughout the day and throughout the year and is the most obvious metric to consider for passive heating and cooling design.
Two basic aspects of temperature are dry bulb temperature and wet bulb temperature. From these metrics you can learn about both the air temperature and the humidity.
Dry Bulb Temperature
Temperature data given as a monthly average and as a daily average
(Graphs from Revit)
Dry bulb temperature is simply the temperature of the air. It does not consider moisture. It is measured in degrees Celsius, degrees Fahrenheit, or Kelvin and can be measured with a thermometer exposed to the air. It is commonly referred to as the air temperature and is reported in basic weather reports.
Wet Bulb Temperature
Wet bulb temperature is the air temperature that takes into account the cooling potential of evaporation. It is measured by exposing a moistened thermometer bulb to air flow (wrapping a thermometer bulb in wet cloth and swinging it in the air). The evaporation of the moisture depends on the humidity of the air (think about how slowly it takes wet hair to dry on a humid day). Similar to the dry bulb temperature, wet bulb temperature can be measured in degrees Celsius, degrees Fahrenheit, or Kelvin.
Together, dry bulb and wet bulb temperatures can describe humidity.
Degree days are temperature past a threshold, multiplied by time
To get a sense for the heating and cooling requirements for your building site, a comfortable temperature range needs to be set. This range, often referred to as the “comfort zone,” can then be compared to the building site’s actual temperatures over time. When the site’s temperature is outside of the comfort zone, it is measured in heating or cooling “degree days.”
- If the weather is an average of one degree warmer than comfortable for one day, we say the building needs one “degree day” worth of cooling to stay comfortable.
- If the weather is an average of ten degrees warmer for one day, or is one degree warmer for ten days, then the building needs ten cooling degree days.
- If the weather is ten degrees below the comfortable minimum for a day, then the building needs ten heating degree days.
Degree days are not just useful to estimate heating and cooling needs; they also help make comparisons between buildings more fair. A building in a mild climate like San Francisco will need less heating and cooling energy than a building in a cold climate like Moscow, even if the Moscow building is much better built. Comparing the energy intensities of different buildings with the heating and cooling degree days at each site helps make these comparisons more accurate representations of how efficiently the buildings are designed.
This graph from Montreal shows that in January there are almost 800 heating degree days to stay comfortable
(Graph from Ecotect)
Reading Temperature Charts
Monthly Design Temperatures
Of course, temperatures are not always consistent at the same time of day or year. Designs that always achieve occupant comfort must consider extreme circumstances as well as average conditions.
Monthly temperature design data graph
The graph above shows not only monthly temperatures averaged from historical climate data, but also two levels of uncommon extremes.
The green boxes show the historical averages of monthly highest and lowest dry bulb temperatures; your site is virtually guaranteed to experience these temperatures in these months, so they must be designed for.
The extensions of the boxes show extreme temperatures only recorded 1% of the time in historical data. Your site is not likely to experience these often, but for a robust design, you should consider them.
You can see that this site has significantly less variation than the previous graph. In April, a peak temperature of 64° F has occurred, but generally the temperature range will be between 48° and 57° F for the month.
Diurnal Weather Averages
Diurnal temperature data shows daily cycles of temperature and radiation on the site. The data typically includes dry bulb temperature, wet bulb temperature, direct solar radiation, and diffuse solar radiation as a daily average for each month. From this data you can study the difference between dry bulb and wet bulb temperatures (relative humidity indicator), the difference between nighttime temperatures and daytime temperatures (known as the diurnal swing), and solar radiation patterns in addition to much more.
This diurnal weather chart from Revit is from Nashville, TN.
Annual Temperature Bins
For some sites, heating will dominate your design requirements most of the year; for other sites cooling will dominate. You can quickly see what temperatures are experienced at your site most often by looking at a histogram of temperature throughout the year. This is referred to as temperature bins.
Annual temperature bins for a colder site, mostly requiring heating for comfort. The graph also displays the frequency of wet bulb and dry bulb temperatures, suggesting the climate does not have humid summers.
(Graph from Autodesk Revit)