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Dew Point, Humidity and Barometric Pressure

Objective: Students will be able to measure dew point and relative humidity using appropriate tools and methodology.

Water vapor is water that exists in the atmosphere in the gaseous state. The amount of water vapor in the atmosphere can vary greatly from day to day and across geographic areas. Typically, water volume in the air can vary up to about 4% by volume and it primarily comes from evaporated seawater. Evaporation is a key step in the water cycle, where water in the liquid state is converted to the gaseous state. As there is an increase in temperature, the rate of evaporation increases.

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Additionally, having a greater surface area of water in an area and higher wind velocity nearby water surfaces will result in more evaporation.

What is Humidity?

Humidity refers to the amount of water vapor in the atmosphere. At any given temperature, the atmosphere can only absorb a limited amount of water before it is fully saturated. This is the saturation point. At the specified temperature within a region, when saturation is reached, additional water vapor will condense into visible moisture. Refer to the table below to observe the maximum water vapor content in the atmosphere that can exist by temperature.

Source: Engineering Toolbox


If the air reaches its saturation point, it is said to be saturated. Relative humidity measures the proportion of water relative to the maximum amount of water that is possible at the saturation point temperature. Therefore, if the air contains only one quarter of the water needed to be fully saturated, its relative humidity, expressed as a percent, is 25%.

Temperature change plays a role in measuring this percent. If the temperature in a region increases and the amount of moisture remains the same, the relative humidity would lower. Conversely, if the temperature in a region decreases but the moisture content remains the same, the relative humidity percentage would be higher.

What is Dew Point?

If you watch the weather report on a local news network, you will probably hear the term, “dew point,” when referring to atmospheric moisture. The dew point is the temperature at which the moisture in the air becomes totally saturated. When this temperature is reached, any additional water vapor above the maximum moisture content will condense into a liquid. When the temperature reaches the dew point temperature, fog near the ground surface or clouds in the atmosphere will form.

The term dew point refers to a measurement of the absolute humidity of an area. While relative humidity was a proportion, absolute humidity refers to 100% water vapor in the atmosphere at that specific temperature. If the dew point in a region is above 65˚F, then it will feel sticky and uncomfortable outside. The higher the dew point, the more moisture is in the air in that region!

How do we Measure Humidity and Dew Point?

To measure relative humidity, a hygrometer is used. There are multiple types of hygrometers, but we will focus on one particular type, referred to as a psychrometer (sling psychrometer) which can be used to measure relative humidity and dew point. This device consists of two thermometers, attached to a device that can be swung in a circular motion. One of the thermometers is encased with a wick (fabric material) that is dipped in water before use. The purpose of the bare (dry-bulb) thermometer is to measure the current local air temperature. The wick-covered one (wet-bulb) measures a temperature that is lower, due to evaporation of the water from the bulb when swinging it in a circular motion. The wet-bulb temperature will only equal the dry-bulb temperature if the air has reached its saturation point (dew point).

The sling psychrometer is used by swinging the device in a circular motion for a period of time. Then, the two temperatures are recorded and data tables are used to measure the relative humidity and dew point. If the air contains more moisture, the wet-bulb value will be higher. Conversely, the air will be drier (less moist) when the wet-bulb temperature is lower.

Lab Part 1: Measuring Dew Point and Relative Humidity

Materials: Sling Psychrometer, Distilled Water, Timer, Data Tables (A, B)


  1. Wet the wick on the wet-bulb thermometer. We will be doing one set of data collection tests inside an air-conditioned room and one outside. You will only wet the wick one time per location (indoors and outdoors).
  2. We will take an average of four recordings – you will swing the psychrometer in 30 second intervals and record the temperatures of each thermometer after each interval. Please do not re-wet the wick in between each interval. Fill in the data table below for each recording.
  3. Find the average dry-bulb and wet-bulb temperatures after all data is collected. Subtract the wet-bulb from the dry-bulb, which will give you the “depression.”
  4. Refer to your charts – Table A and Table B are used to calculate relative humidity and dew point. The only values you will use for these tables are the dry-bulb temperature and the depression that you calculated. The left column will be the dry-bulb temperature and the top will be the depression. Find the point where the two values meet on the table to determine your results. Note: If you are confused at this step, carefully review the pre-lab video under the content tab, lab 9.
  5. Repeat these steps twice – once for indoors measurements and once for outdoors measurements.

Data Collection Tables:

INDOORS MEASUREMENTS Dry-Bulb Temperature (˚C) Wet-Bulb Temperature (˚C)
First 30 Seconds
Second 30 Seconds
Third 30 Seconds
Fourth 30 Seconds
Average Values

Calculated Depression: ________________

Relative Humidity: _________________

Dew Point: __________________

OUTDOORS MEASUREMENTS Dry-Bulb Temperature (˚C) Wet-Bulb Temperature (˚C)
First 30 Seconds
Second 30 Seconds
Third 30 Seconds
Fourth 30 Seconds
Average Values

Calculated Depression: ________________

Relative Humidity: _________________

Dew Point: __________________



Lab Part 2: Barometric Pressure

Objective: Students will be able to read a barometer, understand units of barometric pressure and isobars, and use weather maps to develop a barograph.

The barometer is an instrument used to measure the weight of air that presses down on us from above. This value is referred to as barometric pressure. The original barometer was produced in the 1600s by Evangelista Torricelli. A mercury barometer has a vertical glass tube, closed on one end, sitting in an open mercury-filled basin. The mercury within the tube adjusts until the weight of the column is equal to the atmospheric force exerted on the reservoir. Barometers may also be water-based or consist of vacuum pump oil as well.

If your region is experiencing a period of cold, dense and heavy air, then the atmosphere will press on the mercury in the column more, and the barometer measurement is referred to as “rising.” Conversely, if your region experiences warmer, lighter air, then the barometer will be described as “falling.” At sea level, the common atmospheric pressure is 29.92 inches of mercury (expressed as Hg). 29.92 inch of Hg (or 760 mm Hg) is “one atmosphere” of pressure. In aviation and television weather reports, pressure is given in inches of mercury (“Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps.

In the lab, we will simply practice reading a barometer and record it on the diagram to the right. Draw a mark to indicate the current level. Then, write the measurement in inches of Hg, number of atmospheres, and millibars below.

________________   __________________   __________________

What are Isobars and Pressure Gradients?

Air pressure varies from region to region and temporally (over time). To interpret weather conditions, meteorologists must consider air pressure in their measurements and predictions. On a weather map, pressure is displayed using isobars. Isobars are lines on a weather map that connect points of equal pressure.

The spaces in between isobars indicate the amount of change over a distance. If isobars are close together, this indicates that the pressure changes rapidly over distance. Conversely, if the isobars are spread apart, the change is much more gradual. The amount of change of pressure over a distance is referred to as a pressure gradient.

Today, you will be drawing isobars using the following charts.

Rules for Drawing Isobars


1. Draw your isobars in a neat and smooth line instead of a jagged line.

2. An isobar should begin and end at an edge of the map, or, alternatively, loop around and close on itself.

3. An isobar should never branch or fork.

4. An isobar cannot touch or cross other lines.

5. Isobars should be drawn at equal intervals. (1000, 996, 992)

6. Each Isobar line should be labeled.


Chart 1: Complete the 1008 mb isobar by continuing the line. Hint: The 1008 mb isobar must not cross through areas of lower pressure. For example, a 1008 mb isobar would NOT go between the 1007 and 1006 points.



Chart 2: Draw the 988 mb isobar. Cross through the 988 mb points and continue the line where appropriate.

Chart 3: Follow the rules for drawing isobars to complete the chart below. Start with 1024 mb and draw isobars every 4 mb.

For the final part of this lab activity, you will construct a barogram using isobars on actual weather maps. Use the set of weather maps from the Miami Herald for the Month of March 1993. This has been provided by your instructor in the content tab (lab 9). Using the weather maps provided, analyze the barometric pressures for Miami and New York City for the month of March, 1993.


Use a red pencil to represent Miami and a blue pencil to represent New York city. Plot the pressure for each day in March on the blank barogram. Complete the graph by connecting dots across each dataset.

Once you are done, think critically about the changes that occur throughout the graph that you produced. Write a paragraph explaining what weather changes might have occurred throughout the month of March in both regions.

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