Lab 6 Weather Instruments(3)
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University of Arkansas *
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Feb 20, 2024
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LAB 10: WEATHER INSTRUMENTS 100 points
Due exactly one week after your scheduled lab start time.
LEARNING OUTCOMES
At the end of this lab, you will be able to
1.
Describe atmospheric properties and their influence on weather.
2.
Measure atmospheric processes and calculate weather indexes.
3.
Identify relationships between atmospheric properties.
4.
Discover and download atmospheric.
INTRODUCTION
This lab will introduce you to ways of measuring atmospheric properties. Radiation
Energy from the Sun, in the form of shortwave radiation, is the ultimate driving force of weather. Shortwave radiation comes from the sun, enters our atmosphere, and several things can happen to it. This shortwave radiation can be absorbed by clouds or gasses in our atmosphere, or it can reach earth’s surface. While some surfaces at Earth’s surface will absorb this energy, much of it gets reflected back into the sky as longwave radiation. The proportion of radiation that gets reflected off a surface is known as albedo
. To measure radiation, scientists often use a radiometer, which when pointed at a light source, gives you the amount of light energy it’s receiving. Temperature Temperature is the average kinetic energy of molecules in a substance such as the atmosphere. There are three temperature scales used worldwide: Fahrenheit (°F), Celsius (°C), and Kelvin (K).
Scientists work with Celsius and Kelvin and measure these values with thermometers. Mathematical equations are used to convert between the scales:
°F = (°C * 1.8) + 32
°C = (°F – 32) / 1.8
K = °C + 273.15 °C = K – 273.15
Atmospheric
Moisture
Water vapor in the atmosphere is a function of temperature. As the atmosphere gets cooler, the amount of water vapor it can hold decreases, and in turn, as the atmosphere gets warmer, 1
it can hold more water vapor/moisture. Atmospheric moisture is commonly measured in two ways: relative humidity and dew point. Relative humidity
is a ratio (expressed in %) of the current amount of moisture in the air to the maximum amount of moisture the air could hold. It
describes how close the atmosphere is to being completely saturated, and since clouds are fully
saturated, their RH is at 100%. The higher the value, the closer the air is to total saturation, meaning when you drive to school and its foggy, the RH is at or near 100% as well. The dew point of the air is the temperature it must be cooled to become saturated (or attain 100% relative humidity). Therefore, the closer dew point and air temperature are to one another, the higher the relative humidity. Combining relative humidity and air temperature yields a “feels like” temperature known as the heat index. There are many ways to measure atmospheric moisture, but you will use a sling-psychrometer to calculate relative humidity and dew point. The sling-psychrometer contains two thermometers; One of the thermometers contains a wetted sock or sleeve on its bulb. Hence, this thermometer is known as the wet-bulb. The other thermometer is your normal, unmodified thermometer, and it is known as the dry-bulb. The wet-bulb aids in the measurement of the air’s moisture content by measuring the cooling effect of water evaporating from the sleeve. The more moisture there is in the atmosphere, the less water evaporates from the sleeve, resulting in a relatively higher the wet-bulb temperature than if the
humidity were lower.
Barometric Pressure and Wind
Air has mass and weight. The weight of air leads to air pressure and is directly related to temperature. Because the Sun does not heat the surface of the Earth evenly, we see differences
in air pressure. These differences in air pressure cause movement of the air (wind), and the larger the pressure differences, the faster the wind blows. Combining air temperature and wind
speed yields a “feels like” temperature known as the wind chill index. Air moves from areas of higher pressure to areas of lower pressure. The common term for describing the difference in pressures is the pressure gradient, which takes the change in pressure divided by the distance between two points. Pressure gradients can describe pressure conditions in three dimensions and is not only measured horizontally. Low pressure air is relatively light (less dense and often warmer) and ascends in the atmosphere taking any water vapor with it. If this water vapor cools to the dew point, then clouds and precipitation will develop. Air at high pressure is relatively heavy (denser = more air molecules in a given volume)
and descends in the atmosphere. This warming dries the air, and normally allows for clear conditions. Therefore, low pressure systems tend to bring cloudy/wet weather, while high-
pressure systems tend to bring clear/dry conditions.
Air pressure is measured with a barometer. Wind speed and/or direction are measured with several devices. The first of which is called an anemometer, it only measures the wind speed. The second device is termed a wind vane, and it only measures wind direction. The third device can measure both speed and direction and is termed the aerovane. The aerovane is widely used
today in most weather stations.
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LAB 10: WEATHER INSTRUMENTS Name: _____________________________
Section: _____________________________
Take measurements, perform the following calculations, and answer the questions [100 pts].
Part I – Weather Measurements
In this part of the lab exercise, you will go outside and take measurements with a psychrometer
to measure air moisture. Dip the sock/sleeve located on one of the thermometers into a cup of water. Travel outside, and one person per group gets the privilege of slinging/twirling the psychrometer around with the other keeping time. Sling the psychrometer for one minute and take preliminary temperature readings on the thermometers (then answer #1 and #2). Sling the
psychrometer for an additional 30 seconds and again take the wet-bulb reading (answer #3). If there is no change, you may stop and record your final values for both dry- and wet-bulb readings. If the reading is still changing, then continue slinging the psychrometer for 30 seconds
until it is steady and record wet and dry readings when the temperatures have stabilized. Remember to use correct units (Don’t put the temperature in Celsius if it asks for Fahrenheit)!
[2 pts] Record your preliminary dry-bulb temperature below: 1.
Dry-bulb Temperature (°F): __________________ [6 pts] Record your first wet-bulb temperature (time = 60 seconds) and additional wet-bulb temperatures (time = 30 seconds each). Record additional measurements only if the temperature changes drastically between your first and second readings
:
2.
First Wet-bulb Temperature (°F):
_________________
3.
Second Wet-bulb Temperature (°F):
_________________
Additional Wet-bulb T (
if needed
, °F): ____________
Additional Wet-bulb T (
if needed
, °F): ____________
[2 pts] Use the radiometer to make measurements about the amount of radiation (UV level) coming towards the Earth’s surface from the Sun:
4.
Incoming Energy (direct sun): ___________________
5.
Incoming Energy (shade/covered area): ___________________
[2 pts] Recalling from our discussion in class would dark pavement or grass be hotter to the touch on a sunny day?
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[2 pts] Recalling from our discussion in class would dark pavement or grass have a higher albedo (the proportion of radiation that gets reflected off a surface)?
[4 pts] Use the anemometer to measure the wind speed and direction: 6.
Wind speed (mph): _________________
7.
Wind Direction: __________________ [2 pts] Use the barometer to measure air pressure: (you’ll have to convert units)
8.
Air pressure (in of mercury): _________________________
Part II – Weather Calculations
Calculate the final wet-bulb depression, where the wet-bulb depression is found by subtracting the wet-bulb temperature from the dry-bulb temperature (i.e., wet-bulb depression = T
dry
- T
wet
). Then, using your weather measurements, determine the relative humidity with Table 1 and the dew point with Table 2.
9.
[2 pts] Wet-Bulb Depression (°F): __________________
10. [2 pts] Relative Humidity (%) ______________________ (use Table 1)
11. [2 pts] Dew Point (°F): ____________________________ (use Table 2)
4
Table 1: Use this table to obtain a Relative Humidity
in percent. Use your measurement of air temperature (i.e., the dry-bulb temperature) and the calculated wet-bulb depression to find the
approximate relative humidity.
5
Table 2: Use this table to obtain a dew point temperature
in degrees Fahrenheit. Use your measurement of air temperature (i.e., the dry-bulb temperature) and the calculated relative humidity to find the approximate dew point temperature.
With values you have measured and calculated, determine the heat index using Table 3, and answer the following questions about heat index. If a heat index does not exist for your data, explain why instead of providing a heat index value.
12. [4 pts] Heat Index (°F): _______________
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13. [2 pts] If the relative humidity is 30% and the air temperature is 100°F, what is the heat index in °F?
14. [2 pts] If the relative humidity is 60% and the air temperature is 91°F, what is the heat index in °F?
Table 3: Use this table to find the heat index
. First, determine whether the temperature is below 80°F. A heat index is not calculated for temperatures below 80°F. If the temperature is at
or above 80°F, then use the air temperature (i.e., the dry-bulb temperature) and the relative humidity to identify the approximate heat index value in degrees Fahrenheit. With measurements of the temperature and wind speed, we could calculate the wind chill index using Table 4. Determine the wind chill index, if it exists for your measurements, and then
give the wind chill index. If a wind chill does not exist for your data, explain why instead of providing a value. Please use a local weather source to approximate the current wind speeds (e.g., weather app on your phone).
15. [2 pts] Current approximate wind speed (mph): __________________
[1 pt] Source: 16. [4 pts] Wind Chill Index (°F): __________________
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17. [2 pts] If the wind speed is 24 mph and the air temperature is 20°F, what is the wind chill
index in °F?
18. [2 pts] If the wind speed is 33 mph and the air temperature is 15°F, what is the wind chill
index in °F?
Table 4: Use this table to calculate the wind chill index
. Wind chill indices are not calculated for temperatures greater than 40°F. If the temperature is at or below 40°F, then use the air temperature (i.e., the dry-bulb temperature) and the wind speed to determine the approximate
wind chill index value in degrees Fahrenheit.
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Part 3 – Measurement interpretation
Research the current weather data for Fayetteville, AR using the Drake Field weather station:
National Weather Service : Observed Weather for past 3 Days : Fayetteville, Drake Field
(This is weather data for Drake Field in Fayetteville)
19. [20 pts] Compare the current weather data obtained on the Internet for Drake Field to your observations from the UA campus. Explain differences in temperature, dew point, relative humidity, air pressure, wind speed and direction. Why might there be differences? Include these measurements in your answer.
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[32 pts] Use what you have learned about temperature and moisture to answer the following questions. 20. [5 pts] What time of day should usually have the highest relative humidity?
21. [5 pts] What time of day is usually the hottest?
22. [5 pts] Describe types of weather that often occur when relative humidity is 100%.
23. [5 pts] Convert 212°F to °C. Show your work.
24. [5 pts] Convert 25°C to °F. Show your work.
25. [5 pts] Convert 300K to °C. Show your work.
26. [5 pts] Which of these temperatures is hotter: 100°F, 100°C, or 100K? Why? (Use a common scale to support your answer)
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