Temperature
Content
Temperature
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.
Temperature
data given as
a monthly
average and
as a daily
average
(Graphs from
Revit)
Dry Bulb Temperature
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
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.”
Degree days are temperature past a threshold, multiplied by
For example…
time
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)
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.
Temperature
data given as
a monthly
average and
as a daily
average
(Graphs from
Revit)
Dry Bulb Temperature
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
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.”
Degree days are temperature past a threshold, multiplied by
For example…
time
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)
