3.4: Applications

Applications of Temperature

As previously mentioned, air temperature is often considered to be the most important weather variable to the general public. When you open a weather app, chances are that the first thing you notice is the air temperature. The key reason for this is that air temperature is essential to human comfort. Our bodies, which are constantly metabolizing, strive to maintain an average body temperature of 98.6°F. If it’s too cold outside, our bodies lose heat faster than they can generate it, and begin to cool below 98.6°F. This can cause hypothermia. On the other hand, if it’s too hot outside, our bodies lose heat too slowly, causing our temperature to rise. This causes heat exhaustion, and in extreme cases, heat stroke.

Because temperature is so essential to our comfort, we often resort to using heating or cooling devices when temperatures fall outside our comfort zone. Utility companies (such as Pacific Gas and Electric in Northern California) are aware of this and closely monitor weather conditions to ensure that they are generating enough energy to meet weather-related demand. There are numerous tools they use to do this, but in this investigation, we will focus on Heating and Cooling degree days. We will also investigate other important temperature metrics, e.g., growing degree days, wind chill, and the heat index temperature.

Heating and Cooling Degree Days

To forecast the amount of energy typically required for heating or cooling, energy companies use heating and cooling degree days as a key metric. Here’s how they work:
Average Temperature: The average between the day’s daytime high temperature and the nighttime low temperature.

• For example, if the high temperature is 85°F and the low temperature is 65°F. The average would be 75°F. You can calculate the average by adding the high and low temperatures and then dividing the sum by 2.
• When the calculated average temperature is not a whole number, we round up to the next whole number. For example, if the high temperature is 76°F and the low temperature is 67°F, the average temperature would be 71.5°F, which would round up to 72°F.

Heating Degree Days: One heating degree day (HDD) is accumulated for every degree that the day’s average temperature is below 65°F. For example, if the high temperature is 70°F, and the low temperature is 50°F, that would yield an average temperature of 60°F. If we subtract 65°F from 60°F, we get a temperature of -5°F. This would equate to 5 Heating Degree Days for that day. 

18. On May 17, 2019, the weather station at San Jose airport recorded a high temperature of 62°F and a low temperature of 48°F. The average temperature on that day would be:
    1. 55°F
    2. 54°F
    3. 56°F
    4. 53°F

19. Therefore, on May 17, 2019, San Jose airport accumulated _____ HDDs. 
    1. 11
    2. 5
    3. 10
    4. 9

Now, let’s compare two cities, Chicago, in northeastern Illinois, and the coastal town of Eureka, in northwestern California. Figure \(\PageIndex{1}\) graphs the monthly average temperatures for both Chicago and Eureka, with a line representing a constant 65°F monthly temperature for the entire year. 

Figure \(\PageIndex{1}\): Average monthly temperatures for Chicago and Eureka with a constant 65°F temperature line. Table \(\PageIndex{1}\) presents the data in a tabular form. (CC BY 4.0; Alicia Mullens)

Table \(\PageIndex{1}\): Average monthly temperatures for Chicago, IL, and Eureka, CA in °F.

Month	Chicago	Eureka
Jan	27	50
Feb	32	50
Mar	42	51
Apr	53	53
May	64	55
Jun	72	57
Jul	75	58
Aug	74	58
Sep	66	57
Oct	54	55
Nov	42	52
Dec	30	50

20. Based on Figure \(\PageIndex{1}\) or Table \(\PageIndex{1}\), _____ has more months per year when it accumulates HDDs. Remember, we don't need to calculate the daily HDDs, just the number of months when HDDs will be accumulated based on the data.
    1. Eureka
    2. Chicago

21. Eureka likely has substantially lower variation in average temperature because, compared to Chicago, Eureka is:
    1. Much closer to the ocean
    2. At a much lower altitude
    3. Both of these

Cooling Degree Days are accumulated whenever the daily average temperature at a location rises above 65°F. On these days, energy is used to cool a household rather than to warm it. For example, if the high temperature is 83°F, and the low is 68°F, the average temperature for this day would be 75.5°F, which would be rounded up to 76°F. 76°F – 65°F = 11°F above 65°F, which would equal 11 CDDs.

22. Based on \(\PageIndex{1}\) or Table \(\PageIndex{1}\), Chicago has _____ months when it accumulates CDDs.
    1. 0
    2. 2
    3. 4
    4. 6

23. On the other hand, Eureka has _____ months when it accumulates CDDs:
    1. 0
    2. 2
    3. 4
    4. 6 

This suggests that inland locations, such as Chicago, IL, or St. Louis, MO, have temperatures that exhibit a strong seasonal pattern, characterized by cold winters and warm summers. Meanwhile, coastal locations like Eureka or San Francisco, CA, have temperatures that remain relatively stable year-round, fluctuating only slightly. Similarly, due to warmer average temperatures, locations in the southern half of the United States will accrue more CDDs and fewer HDDs. Meanwhile, due to cooler average temperatures, locations in the northern half of the United States will accrue more HDDs and fewer CDDs.

Wind Chill

While temperature is essential to human comfort, other factors, such as wind, can also greatly impact how the temperature feels to the human body. Wind does not directly affect air temperature, but it does affect how the air temperature feels to our bodies. Our bodies warm up the air immediately above our skin, creating a thin “blanket” layer of warmth that keeps us warm. On windy days, this layer of warmth is blown away, forcing our bodies to radiate more heat, thus making it feel cooler. This may be a pleasant surprise on hot days, but it could create potentially dangerous conditions on cold days. To quantify this, the National Weather Service has developed the Wind Chill Index, which provides Wind Chill Equivalent Temperatures (or WETs) for given temperatures and wind speeds. Figure \(\PageIndex{2}\) is a WET conversion chart commonly used by the National Weather Service to warn residents about wind chill. To use the chart, first start with air temperature on the “x” axis, and then travel down that column until you intersect the row for your given wind speed. 

Figure \(\PageIndex{2}\): A Wind Chill chart shows how the wind makes you feel colder. Table \(\PageIndex{2}\) presents the WET conversion in a tabular form. (Public Domain; Understanding Wind Chill via NWS)

Table \(\PageIndex{2}\): Wind Chill Equivalent Temperatures (°F) by NWS

Wind speed (mph)↓	Temperature (°F)
Calm	40	35	30	25	20	15	10	5	0	-5	-10	-15	-20	-25	-30	-35	-40	-45
5	36	31	25	19	13	7	1	-5	-11	-16	-22	-28	-34	-40	-46	-52	-57	-63
10	34	27	21	15	9	3	-4	-10	-15	-22	-28	-35	-41	-47	-53	-59	-66	-72
15	32	26	19	13	6	0	-7	-13	-19	-26	-32	-39	-45	-51	-58	-64	-71	-77
20	30	24	17	11	4	-2	-9	-15	-22	-29	-35	-42	-48	-55	-61	-68	-74	-81
25	29	23	16	9	3	-4	-11	-17	-24	-31	-37	-44	-51	-58	-64	-71	-78	-84
30	28	22	15	8	1	-5	-12	-19	-26	-33	-39	-46	-53	-60	-67	-73	-80	-87
35	28	21	14	7	0	-7	-14	-21	-27	-34	-41	-48	-55	-62	-69	-76	-82	-89
40	27	20	13	6	-1	-8	-15	-22	-28	-36	-43	-50	-57	-64	-71	-78	-84	-91
45	26	19	12	5	-2	-9	-16	-23	-29	-37	-44	-51	-58	-65	-72	-79	-86	-93
50	26	19	12	4	-3	-10	-17	-24	-30	-38	-45	-52	-60	-67	-74	-81	-88	-95
55	25	18	11	4	-3	-11	-18	-25	-31	-39	-46	-54	-61	-68	-75	-82	-89	-97
60	25	17	10	3	-4	-11	-19	-26	-32	-40	-48	-55	-62	-69	-76	-84	-91	-98

For example, at 25°F (first row) and 30 mph (left column), the WET would be 8°F. Note that WETs are only provided for cold air temperatures that could cause frostbite (freezing, discoloration, and potential loss of external limbs) or hypothermia. Frostbite on exposed skin can occur within a few minutes, based on wind chill values:

• 30 minutes or less: Wind chill below -20°F
• 10 minutes or less: Wind chill below -35°F
• 5 minutes or less: Wind chill below -45°F

24. On March 11, 2019, the weather station at De Anza College experienced a low temperature of 40°F and a peak wind of 15 mph. Assuming that the peak wind happened at the same time as the low temperature, the wind chill at De Anza College would be:
    1. 40°F
    2. 32°F
    3. Both of these

Heat Index Temperature

The final human comfort metric we’ll investigate is the Heat Index Temperature. When our body temperature increases, we sweat to cool ourselves. The cooling happens because as sweat evaporates from our bodies, it cools us. However, when relative humidity is high (we’ll cover relative humidity more in depth in Investigation 4), it is more difficult for water to evaporate, resulting in less cooling. As a result, air temperatures feel hotter than they are. This can be summed up in the common phrase: “It’s not the heat, it’s the humidity.” However, just as wind chill does not affect air temperature, humidity does not either. Rather, it affects how the temperature feels by decreasing the amount of evaporation that can happen. Figure \(\PageIndex{3}\) is a Heat Index chart, which works similarly to the Wind Chill chart in Figure \(\PageIndex{2}\), but we replace wind speed with relative humidity instead. 

Figure \(\PageIndex{3}\): A Heat Index chart shows how higher humidity makes you feel warmer. Table \(\PageIndex{3}\) provides the data in a tabular form. (Public Domain; What is the Heat Index via NWS)

Table \(\PageIndex{3}\): Heat Index Temperatures (°F) by NWS

Relative Humidity (%) ↓ Temp (°F) →	80	82	84	86	88	90	92	94	96	98	100	102	104	106	108	110
40	80	81	83	85	88	91	94	97	101	105	109	114	119	124	130	136
45	80	82	84	87	89	93	96	100	104	109	114	119	124	130	137
50	81	83	85	88	91	95	99	103	108	113	118	124	131	137
55	81	84	86	89	93	97	101	106	112	117	124	130	137
60	82	84	88	91	95	100	105	110	116	123	129	137
65	82	85	89	93	98	103	108	114	121	128	136
70	83	86	90	95	100	105	112	119	126	134
75	84	88	92	97	103	109	116	124	132
80	84	89	94	100	106	113	121	129
85	85	90	96	102	110	117	126	135
90	86	91	98	105	113	122	131
95	86	93	100	108	117	127
100	87	95	103	112	121	132

25. A high Relative Humidity can make relatively mild temperatures unbearable. For example, a temperature of 84°F and a relative humidity of 95% would yield a heat index of:
    1. 90°F
    2. 100°F
    3. 98°F
    4. 102°F

Desert Cities, like Phoenix, Arizona, are typically reprieved from the worst of summer conditions by their low relative humidity values. For example, at 4 pm on June 26, Phoenix had a temperature of 100°F, and a Relative Humidity of 13%. While this calculation cannot be done using the condensed version of the Heat Index in Figure \(\PageIndex{3}\) or Table \(\PageIndex{3}\), you can use a heat index calculator (on the NOAA Heat Index webpage) to determine that Phoenix has a heat index of 95°F. In this case, the heat index of Phoenix is lower than the actual air temperature! At the same time, New Orleans, Louisiana, had an air temperature of 92°F and a relative humidity of 45%. This yields a heat index value of 96°F, which is warmer than Phoenix, even if the air temperature in New Orleans was comparatively lower.