A strong wind can greatly increase heat loss at low temperatures. The perceived temperature tries to give a numerical coefficient to this effect, based on the impact of the wind on the exposed skin. What you need to calculate the perceived temperature is a measurement of the temperature and wind speed. Both are available in the weather forecast, and you can measure the wind speed at home with nothing more complicated than small plastic cups and straws.
Steps
Method 1 of 3: Calculate the Perceived Temperature on Your Own
Step 1. Measure the temperature, T
Use a thermometer or look at the outside temperature on a weather forecast website. You can measure it in Fahrenheit or Celsius, but read the next step carefully to know which one to use for the perceived temperature.
The perceived temperature is not defined for temperatures below 10 ° C (50 ºF). If the temperature is higher, the wind does not have much effect on the perceived temperature
Step 2. Find or measure wind speed, V
You can find the estimated wind speed for your area on almost any weather forecast website, or by searching for "wind speed + (your city name)". If you own an anemometer or make one by following the instructions below, you can measure the wind speed yourself. If the temperature measurement is in Fahrenheit, use the wind speed measurement in miles per hour (mph). If you use degrees Celsius, use kilometers per hour (km / h). If necessary, search online for a website to convert miles per hour to km / h.
- If you are using an official wind speed measurement taken at a height of 10 m (33 ft), multiply it by 0.75 to get a rough estimate of the wind speed at 1.5 m (5 ft), the height average of the human face.
- Wind below 5 km / h (about 3 mph) does not have a significant effect on the perceived temperature.
Step 3. Enter these values into the formula
Different formulas have been proposed for calculating the perceived temperature over the years and in different areas, but here we will use the one in force in Great Britain, the United States and Canada, created by an international team of researchers. Insert the values into the formula below, replacing T with the temperature and V with the wind speed:
- If you use ºF and mph: perceived temperature = 35.74 + 0.6215 T. - 35, 75 V.0, 16 + 0, 4275 TV0, 16
- If you use ºC and km / h: perceived temperature = 13, 12 + 0, 6215 T. - 11, 37 V.0, 16 + 0, 3965 TV0, 16
Step 4. Suitable for sunlight
The bright sun can raise the perceived temperature by +5.6 to +10 ºC (+10 to +18 ºF). There is no official formula for calculating this effect, but keep in mind that sunlight will make the air seem warmer than the perceived temperature formula would suggest.
Step 5. Understand the perceived temperature
Perceived temperature is a concept invented to describe how wind increases heat loss on exposed skin. In extreme conditions this can become a determining factor in how quickly freezing takes place: at a perceived temperature below -28 ºC (-19 ºF), freezing takes place on exposed skin within 15 minutes or less. Below -50 ºC (-58 ºF), exposed skin can freeze within 30 seconds.
Method 2 of 3: Use a Perceived Temperature Calculator
Step 1. Find a perceived temperature calculator online, for example these (in English):
the US National Weather Service, freemathhelp.com, or onlineconversion.com.
These calculators use the new formula for perceived temperature adopted by the United States and other countries in 2001. If you want to use others, make sure they are based on this formula, as the old one can give misleading results
Step 2. Look for the temperature and air velocity
Both of this information is typically available in weather forecasts on websites, TV and radio channels, and in newspapers.
Step 3. Multiply the wind speed by 0.75
Unless the forecast specifies the wind speed at ground level, multiply the speed by 0.75 to get a more accurate estimate of the wind speed at face height.
This estimate is based on the standard measure of wind speed at a height of 10 m (33 ft), under average atmospheric conditions. Using wind speed measured at a height of 1.5m (5ft) is more accurate, but not easily found without your own anemometer
Step 4. Enter the value into the calculator
Make sure you choose the units (such as mph or ºC) in which the measurements are written. Click on "OK" or a similar button, and you should get the perceived temperature.
Method 3 of 3: Measure the Wind Speed
Step 1. Consider whether or not to purchase an anemometer
An anemometer is a tool for measuring wind speed: you can buy it online, or you can make a simple one yourself in about half an hour by following the steps below. If you buy one, skip to the step where you count the rotations - or go directly to read the wind speed, if the instrument has a digital display.
Step 2. Make holes in small plastic cups
Take four small plastic cups and make a hole in each one, about 1.5 cm below the rim. Take a fifth glass, and drill four equally spaced holes in it, about 6mm below the rim, then drill a fifth hole in the center of the bottom.
You can use the tip of a pencil to punch the holes if you don't have anything sharper
Step 3. Build half of the basic shape
Insert a plastic straw into one of the single-hole glasses, about 2.5cm. Insert the other side of the straw into the two holes of the five-hole glass. Insert the free part of the straw into another of the single-hole glasses. Turn the two single-hole glasses so they point in opposite directions along the same plane as the straw. Secure the straw to the glass with a stapler.
Step 4. Complete the basic shape
Repeat with another straw, inserting it into the two remaining holes of the central five-hole glass. Rotate the last two glasses until the opening of each is close to the base of the other. In other words, the glass on the top points to the right, the one on the right points down, the one on the bottom points to the left, and the one on the left points up. Secure the straws to the glasses with a stapler.
Step 5. Make a base for the anemometer
Slide the two straws in until all four glasses are the same distance from the center. Insert a small pin at the intersection of the two straws, and then insert the garment with the eraser of a pencil through the hole in the base of the central cup, and push it gently into the pin. Now you can hold the anemometer by the tip of the pencil, and use it to measure the wind speed.
Step 6. Count the number of rotations the anemometer performs
Keep the anemometer upright in a windy area. Look at one of the glasses (mark it with a marker if that makes it easier for you to follow) and count the number of times it rotates. Use a timer, or ask a friend to check a clock to calculate 15 seconds, and stop when the time runs out. Multiply the count by four to get the number of revolutions per minute (rpm).
For greater accuracy, count the number of rotations in 60 seconds (so without multiplying)
Step 7. Calculate the circumference
Measure the distance from one edge of the anemometer to the other to find the diameter of the rotation, d. The circumference of the circle is equal to π d. This is the distance traveled with one revolution.
If you don't have a calculator, you can use 3, 14 as an estimate of π, or even just 3 for a rough estimate
Step 8. Calculate the wind speed
Convert the calculated circumference to a unit most useful for measuring wind speed (kilometers or miles). Multiply the result by the calculated rpm to get the distance traveled in one minute. Multiply the result by 60 to get the distance traveled in one hour (km / h or mph). Here are the formulas in Anglo-Saxon and metric units:
- Anglo-Saxons: (_ circumference _ inches / revolutions) * (1/12 ft / inches) * (1/5280 miles / ft) * (_ rpm _ revolutions / minute) * (60 minutes / hour) = _ wind speed _ in miles per hour.
- Metrics: (_ circumference _ centimeters / revolutions) * (1 / 100,000 kilometers / centimeters) * (_ rpm _ revolutions / minute) * (60 minutes / hour) = _ wind speed _ in kilometers per hour.
Advice
- Wind cools people and things much faster than still air, but it does not cause the indoor temperature to drop below the outdoor temperature. Simply put, perceived temperature is useful when talking about people or animals, but not about inanimate objects that don't produce their own heat.
- Apparent temperature (the degree of heat loss) is also affected by humidity, air pressure, physical exertion, and the natural difference between individuals. There is no commonly used formula that also considers these factors.
