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All About Barometers

A barometer is an instrument used to measure atmospheric pressure, or the pressure exerted by the atmosphere on the earth's surface. Everything around us has mass and takes up space, including the air, and these molecules of air are constantly exerting pressure on everything around us. Atmospheric pressure is also one of the basic meteorological elements, and measuring the changes in the pressure can help to predict the weather. There are two commonly used barometers, the mercury barometer and the aneroid barometer.

Mercury Barometer

The Mercury Barometer was invented in 1643 by Evangalista Torricelli, and was formerly called Torricelli's tube. It is comprised of a glass tube about three feet long, closed at one end, filled with mercury, and inverted with the open end in a dish of mercury. The air exerts pressure on the dish of mercury, thus forcing the mercury in the tube to rise and fall as the pressure changes.

Aneroid Barometer

An aneroid barometer is much smaller, constructed of a vacuum box with an internal or external spring, which keeps the box from collapsing. Attached to the spring is a wire, which moves an arrow or pointer on the face of the barometer. As the pressure changes, the spring compresses or loosens, moving the pointer to indicate the pressure reading.

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Wind Chill Chart

The combination of high winds and cold temperatures can make it feel a lot colder than what the thermometer is reading, this is known as the Wind Chill. The wind chill equivalent can be very dangerous if it falls below -25 deg. Fahrenheit. The wind chill index depends on two parameters: Temperature and Wind Speed. To use this chart, locate the temperature along the left-hand column and the wind speed across the top row. The wind chill index is found at the intersection of the two.

TEMPERATURE (°F)
WIND SPEED - MPH
CALM 5 10 15 20 25 30 35
45 43 34 29 26 23 21 20
40 37 28 23 19 16 13 12
35 32 22 16 12 8 6 4
30 27 16 9 4 1 -2 -4
25 22 10 2 -3 -7 -10 -12
20 16 3 -5 -10 -15 -18 -20
15 11 -3 -11 -17 -22 -25 -27
10 6 -9 -18 -24 -29 -33 -35
5 0 -15 -25 -31 -36 -41 -43
0 -5 -22 -31 -39 -44 -49 -52
-5 -10 -27 -38 -46 -51 -56 -58
-10 -15 -34 -45 -51 -59 -64 -67
-15 -21 -40 -51 -60 -66 -71 -74
-20 -26 -46 -58 -67 -74 -79 -82
-25 -31 -52 -65 -74 -81 -86 -89

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Heat Index/Apparent Temperature

The heat index is a measure of the contribution that high humidity makes with abnormally high temperatures in reducing the body's ability to cool itself. For example, the index shows that for an actual air temperature of 100 degrees Fahrenheit and a dewpoint temperature of 77, the effect on the human body would be the same as 120 degrees. Sunstroke and heat exhaustion are likely when the heat index reaches this level. Local offices of NOAA's National Weather Service in most areas of the country will include heat index values in their forecasts and statements when the index is expected to exceed 105 degrees for two days or more. In areas where people are acclimated to high temperatures or high temperatures with high humidity, the heat index typically exceeds this level and offices in these areas will initiate use of the heat index at higher levels.

In the 40-year period from 1936 through 1975, nearly 20,000 people died in prolonged episodes of high heat and humidity in the United States. The three-month, summer heat wave of 1980 claimed at least 1,250 lives when record heat and humidity spread across the southern, central, and eastern portions of the country. But these are direct casualties. No one knows how many other deaths were advanced by heat waves, although the estimates are in the thousands.

The heat index, given in terms of degrees fahrenheit, is sometimes referred to as the "apparent temperature". It describes what hot weather "feels like" to the average person for various combinations of high temperature and humidity. It also includes the dangers posed by heat stress, such as sunstroke, heat exhaustion, heat cramps, and heatstroke.

Normally, the human body cools itself at high temperatures by producing perspiration, which evaporates to carry heat away. High humidity interferes with this process. When the thermal limits of the human body's normal temperature of 98.6 degrees Fahrenheit are exceeded by very much or for very long, the warm-blooded human dies.

There are some steps that can be taken to reduce the danger of experiencing problems associated with a high heat index. Here are a few basic rules:

  1. Do not perform strenuous activities during the hottest part of the day (late morning until late afternoon) - r e l a x !
  2. Wear light colored, light weight clothing. (light colors reflect the sunlight!)
  3. Eat less food (especially less protein).
  4. Drink, drink, drink! - your best bets are water or non-alcoholic beverages.
  5. Find a nice shady place to sit out the heat - stay out of the sun.
  6. Remember that a high heat index also makes life unpleasant for animals, too. If you take your pet for a ride and have to stop somewhere, be sure to leave a window open for ventilation - a closed up vehicle can get very hot very quickly, causing death to locked in pets.

A table is available to help calculate the heat index from the temperature and relative humidity.

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Apparent Temperature/Heat Index Table

Below is the Heat Index (or Apparent Temperature) Table. It measures the effects of humidity and temperature on the human body in a similar fashion to wind chill. Relative humidity is along the top, and the outside temperature is listed along the left hand side. The values in the table are in degrees fahrenheit. A "-1" in the table indicates that the heat index temperature is so great it is off the scale.

TEMPERATURE
RELATIVE HUMIDITY (%)
  0 10 20 30 40 50 60 70 80 90 100
68°F 61 63 63 64 66 66 68 68 70 70 70
70°F 64 64 66 66 68 68 70 70 72 72 73
72°F 66 66 68 68 70 70 72 72 73 73 75
74°F 68 68 70 72 72 73 73 75 75 75 77
76°F 70 72 72 73 73 75 75 77 77 79 79
78°F 75 75 77 77 79 79 81 81 82 84 86
80°F 77 77 79 79 81 81 82 84 86 88 91
82°F 79 79 81 81 82 84 84 88 90 93 97
84°F 79 81 81 82 84 86 88 91 95 99 104
86°F 81 82 82 84 86 88 91 95 99 104 113
88°F 82 84 84 86 88 91 95 99 104 113 127
90°F 84 84 86 88 91 95 99 104 111 124 144
92°F 84 84 86 88 91 97 102 109 120 136 -1
94°F 86 88 90 93 97 100 108 117 131 -1 -1
96°F 90 91 95 99 102 109 120 136 -1 -1 -1
98°F 90 93 97 100 106 115 129 -1 -1 -1 -1
100°F 91 95 99 104 111 120 136 -1 -1 -1 -1
102°F 93 97 100 106 115 129 -1 -1 -1 -1 -1
104°F 95 99 104 109 120 138 -1 -1 -1 -1 -1
106°F 95 100 106 113 126 -1 -1 -1 -1 -1 -1
108°F 97 102 108 117 133 -1 -1 -1 -1 -1 -1
110°F 99 104 111 120 140 -1 -1 -1 -1 -1 -1
112°F 100 103 113 126 147 -1 -1 -1 -1 -1 -1
114°F 102 109 122 138 -1 -1 -1 -1 -1 -1 -1
116°F 104 111 124 145 -1 -1 -1 -1 -1 -1 -1
118°F 106 113 127 153 -1 -1 -1 -1 -1 -1 -1
120°F 108 117 133 -1 -1 -1 -1 -1 -1 -1 -1
122°F 108 118 140 -1 -1 -1 -1 -1 -1 -1 -1

Heat Index 80 - 85: Few people begin to feel uncomfortable.

Heat Index 85 - 90: About half of the people feel uncomfortable.

Heat Index 90 - 104: Sunstroke, heat cramps and heat exhaustion are possible with prolonged exposure and physical activity.

Heat Index 105 - 129: Sunstroke, heat cramps, or heat exhaustion likely. Heatstroke possible with prolonged exposure and physical activity.

Heat Index 130 or Higher: Heatstroke or sunstroke imminent.

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Saturation Vapor Pressure Table

Air Temperature (°C) Saturation Vapor Pressure (E) (mb)
-18 1.5
-15 1.9
-12 2.4
-9 3
-7 3.7
-4 4.6
-1 4.6
2 6.9
4 8.4
7 10.2
10 12.3
13 14.8
16 17.7
18 21
21 25
24 29.6
27 35
29 41
32 48.1
35 56.2
38 65.6
41 76.2
43 87.8
46 101.4
49 116.8
52 134.2

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