Section 3.6:
Applying Temperature Data
Learning Objective
Summarize several applications of temperature data.
Section Content
There are many useful and practical applications for temperature data. In this section you will learn about important indices that relate to energy use, agriculture, and human comfort. Because the National Weather Service and the U.S. news media still compute and report these temperature variables in Fahrenheit, we will use that scale throughout this discussion.
Degree Days
Three indices have the term degree days as part of their name: heating degree days, cooling degree days, and growing degree days.
Heating Degree Days
A commonly used method for evaluating energy demand is heating degree days. This index starts from the assumption that heating is not required in a building when the daily mean temperature outdoors is 65°F or higher. Simply, each degree of mean temperature below 65°F is counted as 1 heating degree day. Therefore, heating degree days are determined each day by subtracting the daily mean below 65°F from 65°F. Thus, a day with a mean temperature of 50°F has 15 heating degree days, and a day with an average temperature of 65°F or higher has none.
Fuel consumption for a location can be estimated by calculating the total number of heating degrees for an entire year. Figure 3.27 shows the average number of heating degree days for locations throughout the lower 48 states. The amount of fuel required to maintain a certain temperature in a building is proportional to the total heating degree days. This means that doubling the heating degree days usually doubles the fuel consumption; thus, a month with 1000 heating degree days will require twice as much fuel as for a month with 500.
Figure 3.27
Average annual U.S. total heating degree days
When seasonal totals are compared for different places, we can estimate differences in seasonal fuel consumption (Table 3.2). For example, more than five times as much fuel is required to heat a building in Chicago (nearly 6500 total heating degree days) than to heat a similar building in Los Angeles (almost 1300 heating degree days).
Table 3.2
Average Annual Heating and Cooling Degree Days for Selected Cities
Cooling Degree Days
Just as fuel needs for heating can be estimated and compared by using heating degree days, the amount of power required to cool a building can be estimated by using a similar index called cooling degree days. Because the 65°F base temperature is also used in calculating this index, cooling degree days are determined each day by subtracting 65°F from the daily mean. Thus, if the mean temperature for a given day is 80°F, 15 cooling degree days would be accumulated. Mean annual totals of cooling degree days for selected cities are shown in Table 3.2. By comparing the totals for Baltimore and Miami, we can see that the fuel requirements for cooling a building in Miami are almost 2½ times as great as for a similar building in Baltimore.
Growing Degree Days
Another practical application of temperature data is used in agriculture to determine the approximate date when crops will be ready for harvest. This simple index is called growing degree days. The number of growing degree days for a particular crop on any day is the difference between the daily mean temperature and the base temperature of the crop, which is the minimum temperature required for it to grow. For example, the base temperature for growing sweet corn is 50°F, and for peas is 40°F. Thus, on a day when the mean temperature is 75°F, the number of growing degree days for sweet corn is 25, and the number for peas is 35.
Starting with the onset of the growth season, the daily growing degree-day values are added. Thus, if 2000 growing degree days are needed for a crop to mature, it should be ready to harvest when the accumulation reaches 2000. Although many factors important to plant growth are not included in the index, such as moisture conditions and sunlight, this system nevertheless serves as a simple and widely used tool to determine approximate dates of crop maturity.
Indices of Human Discomfort
Summertime weather reports sometimes include the potential harmful effects of high temperatures coupled with high humidity (see Severe & Hazardous Weather Box 3.3). By contrast, in winter when temperatures are low, we are reminded of the effect of strong winds. In the first instance, we are cautioned about heat stress and the possibility of heat stroke, and in the second case we are warned about windchill and the potential dangers of frostbite. These indices are expressions of apparent temperature—the perceived increase or decrease in temperature felt by the human body.
Severe & Hazardous Weather 3.3
Heat Waves
A heat wave is a prolonged period of abnormally hot and usually humid weather that typically lasts from a few days to several weeks. The impact of heat waves on individuals varies greatly, but it can be serious and even deadly. Elderly people are the most vulnerable because heat puts more stress on weak hearts and bodies. People who live in poverty and often cannot afford air conditioning also suffer disproportionately. Studies also show that the temperature at which death rates increase varies from city to city. In Dallas, Texas, a temperature of 103°F is required before the death rate climbs, whereas in San Francisco, the demarcation is 84°F.
Deadly Impacts
Why don’t heat waves elicit the same sense of fear or urgency as tornadoes, hurricanes, and flash floods? One explanation is that oppressive temperatures may occur over many days before a heat wave exacts its toll, rather than causing devastation in just a few minutes or a few hours. Nevertheless, heat waves cause more deaths, on average, than any other weather-related event (Figure 3.D).
Figure 3.D
Average annual U.S. weather-related fatalities for the 10-year period 2006–2015
Heat is the number-one cause of weather-related fatalities.
In the summer of 2003, much of Europe experienced perhaps its worst heat wave in more than a century (Figure 3.E). Based on government records, it was estimated that nearly 35,000 people perished, with France suffering the greatest number of heat-related fatalities—about 14,000.
Figure 3.E
European heat wave
This image is derived from satellite data and shows the difference in daytime land-surface temperatures during the 2003 European heat wave (July 20–August 20) as compared to the four preceding years. The zone of deep red shows where temperatures were 10°C (18°F) hotter than in the other years.
The severity of heat waves is usually greatest in cities because of the urban heat island (see Box 3.2). Large cities do not cool off as much at night during heat waves as rural areas, which can be a critical difference in the amount of heat stress experienced in inner cities. In addition, the stagnant atmospheric conditions usually associated with heat waves trap pollutants in urban areas and add the stresses of severe air pollution to the already dangerous conditions caused by high temperatures.
Heat Waves and Global Warming
Among the possible consequences of global warming is an increase in the frequency and severity of heat waves. A 2013 report by the Intergovernmental Panel on Climate Change states that it is very likely that human activities have contributed to observed changes in temperature extremes since the mid-twentieth century. The report also notes that as we advance through the 21st century, warmer and/or more frequent hot days and nights over most land areas are virtually certain, and it is very likely that heat waves will occur with a higher frequency and have longer durations.
Weather Safety
According to the National Weather Service, heat-related deaths are preventable if you follow a few safety rules:
Limit strenuous outdoor activities when extreme heat is forecast.
Stay hydrated by drinking plenty of fluids.
Check on the elderly and people without air conditioning.
If you must be outside, stay hydrated and take breaks in the shade as often as possible.
Video - Temperatures and Agriculture (Click to watch the video)
The human body is a heat generator that continually releases energy. Anything that influences the rate of heat loss from the body also influences our sensation of temperature, thereby affecting our feeling of comfort. Several factors determine the thermal comfort of the human body, and temperature and humidity are two primary factors.
Heat Stress
Why are hot, muggy days so uncomfortable? Humans, like other mammals, are warm-blooded creatures who maintain a constant body temperature regardless of the temperature of the environment. The main way our bodies prevent overheating is by perspiring. However, this process does little to cool the body unless the perspiration evaporates (the cooling created by the evaporation of perspiration reduces body temperature). Because high humidity impedes evaporation, people are generally more uncomfortable on hot, humid days than on hot, dry days.
One index widely used by the National Weather Service is the heat stress index, or simply the heat index, which combines temperature and humidity to establish the degree of comfort or discomfort.
Examine Figure 3.28, which illustrates that as relative humidity increases, the apparent temperature, and heat stress, increases as well. It is important to note that factors such as the length of exposure to direct sunlight, wind speed, and general health of the individual greatly affect the amount of stress a person will experience. In addition, while a period of hot, humid weather in New Orleans might be reasonably well tolerated by its residents, a similar event in a northern city such as Minneapolis could be dangerous. This is because such weather is more taxing on people who live where these conditions are relatively rare compared to those who are acclimated to living in areas having prolonged periods of heat and high humidity.
Figure 3.28
Heat index expresses apparent temperature
As relative humidity increases, apparent temperature increases as well. For example, if the air temperature is 90°F and the relative humidity is 65 percent, it would “feel like” 103°F.
Windchill
When the wind blows on a cold day, we realize that we would be more comfortable if the wind would stop. A stiff breeze penetrates ordinary clothing and reduces the body’s capacity to retain heat while causing exposed parts of the body to chill rapidly. Not only is cooling by evaporation heightened in this situation, but the wind also acts to carry heat away from the body by constantly replacing warmer air next to the body with colder air. Thus, windchill is the perceived decrease in air temperature felt by the body due to the flow of air.
The U.S. National Weather Service and the Meteorological Services of Canada use the windchill temperature index, which is designed to calculate how the wind and cold feel on human skin (Figure 3.29). The index accounts for wind effects at face level and takes into account body heat-loss estimates. It was tested and refined by exposing human subjects to a chilled wind tunnel. The windchill chart includes a frostbite indicator, which shows where temperature, wind speed, and exposure time produce frostbite (Figure 3.29).
Figure 3.29
Windchill chart
The shaded areas on the chart indicate frostbite danger. Each shaded zone shows how long a person can be exposed before frostbite develops. For example, a temperature of 0°F and a wind speed of 15 miles per hour will produce a windchill temperature of −19°F. Under these conditions, exposed skin can freeze in 30 minutes.
It is worth noting that the windchill temperature is only an estimate of human discomfort. The degree of discomfort felt by different people varies because it is influenced by many factors. Even if clothing is assumed to be the same, individuals vary widely in their responses because of such factors as age, physical condition, state of health, and level of activity. Nevertheless, as a relative measure, the windchill temperature index is useful because it allows people to make more informed judgments regarding the potential harmful effects of wind and cold.
Section Glossary
apparent temperature: The air temperature perceived by a person.
cooling degree day: Each degree of temperature of the daily mean above 65°F. The amount of energy required to maintain a certain temperature in a building is proportional to the cooling degree-days total.
growing degree days: A practical application of temperature data for determining the approximate date when crops will be ready for harvest.
heat stress index (a.k.a., heat index): The apparent temperature on a hot, humid day.
heating degree days: Each degree of temperature of the daily mean below 65°F is counted as one heating degree-day. The amount of heat required to maintain a certain temperature in a building is proportional to the heating degree-days total.
windchill temperature index: A measure of apparent temperature that uses the effects of wind and temperature on the cooling rate of the human body. The windchill index translates the cooling power of the atmosphere with the wind to a temperature under nearly calm conditions.
Section Summary
Heating and cooling degree days are calculations used to evaluate energy demand.
Growing degree days are used to determine the approximate date when crops will be ready to harvest.
Heat stress and windchill indices are uses of temperature data that relate to apparent temperature—the temperature people perceive.
Section Study Questions
Try to answer the following questions on your own, then click the question to see the correct answer.
Distinguish among heating, cooling, and growing degree days.
Heating degree days and cooling degree days are relative measures that allow us to evaluate the weather-produced needs and costs of heating and cooling. Growing degree days is a basic index used by farmers to determine the approximate date when crops will be ready to harvest.
Why does high humidity contribute to summertime discomfort?
High humidity slows down the evaporation of perspiration, thereby slowing down the body’s natural cooling mechanism.
Explain why strong winds make temperatures in winter feel lower than the thermometer reading.
Air in contact with our skin will warm to body temperature. When winter winds are strong, they will move the warmed air away from our skin, replacing it with colder air. These cold, dry winds also increase evaporation, which cools the body as well.