Meteorology Chapter 4

What is the 'wet-bulb depression'?

The temperature difference between the wet bulb and dry bulb (air temperature) is called the wet-bulb depression.

Explain two ways the Relative Humidity can be changed.

1) Changing the air's water vapor content while holding the temperature constant: When water vapor is added to the air, with no change in air temp, the RH increases, and when water vapor is removed from the air, with no change in air temp, the RH decreases. 2) Changing the air temperature with no change in water vapor content: An increase in air temp while holding water vapor contain constant will lower the RH, while a decrease in air temp while holding water vapor contain constant will raise the RH.

In a volume of air, how does the 'actual vapor pressure (E)' differ from the 'saturation vapor pressure (Es)'?

Actual vapor pressure indicates the air's total or current water vapor content, whereas saturation vapor pressure describes how much water vapor is necessary to make the air saturated at any given temperature, or it describes the maximum amount of water vapor the air could hold at a given temperature. The actual vapor pressure and saturation vapor pressure are the same when the air is said to be saturated or having a relative humidity of 100%. When the air is unsaturated, relative humidity values will be less than 100%. In this situation the saturation vapor pressure will always be greater than the actual vapor pressure.

Scientifically, how do the three states of water differ?

As a gas, water vapor molecules move about quite freely, mixing well with neighboring atoms and molecules. As a liquid, the water molecules are closer together (greater density than ice), and so they constantly jostle and bump each other. In the solid state, the molecules arrange themselves into an orderly pattern, with each molecule more or less locked into a rigid position, able to vibrate, but not able to move about freely.

What does the saturation vapor pressure (Es) primarily depend upon?

At higher temperatures, it takes more water vapor to saturate the air. More vapor molecules will exert a greater pressure. Therefore, saturation vapor pressure depends primarily on the air's temperature because warmer air has the potential to hold more water vapor.

Explain why the air on a hot, humid day is less dense than the air on a hot, dry day.

At the same temperature and at the same atmospheric level (pressure), humid air weighs less than dry air. This is due to the fact that water vapor molecules weigh less than the dry air molecules that they are replacing (The molecular weight of water vapor = 18, while the molecular weight of dry air = 29). Therefore if we have two air masses with these weather parameters: 1. Air temp 80F, Dew-Point temp 40F. 2. Air temp 80F, Dew-Point temp 60F. Air mass #2 would be less dense than #1. Why? Since the air temp is the same fore both air masses we must look at water vapor content (dew-point temp). #2 has the highest water vapor content (dew-point temp of 60F) therefore it makes it the least dense (lighter) air mass.

Explain why the condensation process becomes more efficient when the air becomes colder.

Condensation occurs when water vapor molecules condense onto condensation nuclei (aerosols). These nuclei can consist of microscopic bits of dust, smoke, salt, and other solid particles. In warm air, fast moving vapor molecules strike the nuclei with such impact that they simply bounce away. However, if the air is chilled, the molecules move more slowly and are more apt to stick and condense onto the nuclei.

Miami: Air Temp: 85F Dew-Point Temp: 75F Actual Vape Press: 29.6mb Saturate VP: 41.0mb Fargo: Air Temp: 45F Dew-Point Temp: 45F Actual Vape Press: 10.2mb Saturate VP: 10.2mb Chicago: Air Temp: 65F Dew-Point Temp: 50F Actual Vape Press: 12.3mb Saturate VP: 21.0mb Dallas: Air Temp: 95F Dew-Point Temp: 65F Actual Vape Press: 21.0mb Saturate VP: 56.2mb Which city has the capacity to hold the greatest amount of water vapor?

Dallas Has the highest Temp and Saturated Vapor Pressure of all four cities, therefore the highest capacity to hold the most water vapor if all four locations were saturated at their current temperatures

What are the three primary factors that influence the rate of evaporation?

Evaporation occurs when more water molecules are leaving a water surface than are returning. Factors that influence evaporation rates are: 1) Water Temperature - the warmer the water, the greater the rate of evaporation. 2) Wind Speed - Stronger winds will enhance evaporation. 3) Moisture Content of Air Above Water Surface - drier the air, the higher the potential for evaporation.

Miami: Air Temp: 85F Dew-Point Temp: 75F Actual Vape Press: 29.6mb Saturate VP: 41.0mb Fargo: Air Temp: 45F Dew-Point Temp: 45F Actual Vape Press: 10.2mb Saturate VP: 10.2mb Chicago: Air Temp: 65F Dew-Point Temp: 50F Actual Vape Press: 12.3mb Saturate VP: 21.0mb Dallas: Air Temp: 95F Dew-Point Temp: 65F Actual Vape Press: 21.0mb Saturate VP: 56.2mb Which city has the least amount of water vapor content?

Fargo Has the lowest Dew-Point Temp and/or Actual Vapor Pressure of all four cities

Miami: Air Temp: 85F Dew-Point Temp: 75F Actual Vape Press: 29.6mb Saturate VP: 41.0mb Fargo: Air Temp: 45F Dew-Point Temp: 45F Actual Vape Press: 10.2mb Saturate VP: 10.2mb Chicago: Air Temp: 65F Dew-Point Temp: 50F Actual Vape Press: 12.3mb Saturate VP: 21.0mb Dallas: Air Temp: 95F Dew-Point Temp: 65F Actual Vape Press: 21.0mb Saturate VP: 56.2mb Which city has the highest relative humidity?

Fargo Temp and Dew-Point Temp are the same value therefore 100% RH

Why do hot, humid summer days usually feel hotter than hot, dry summer days?

From earlier readings, we learned that the process of evaporation takes energy and therefore is a cooling process. We also learned that the evaporation rate, in part, is dependent upon the moisture content of the air, the drier the air, the higher the evaporation rate. Putting these factors together, we can see that the evaporation rate from out bodies will be slower on hot, humid days versus hot, dry days, therefore we would feel warmer on hot, humid days because the evaporation rate would be slower.

During a summer day, explain why the Relative Humidity values will change significantly while the amount of water vapor remains fairly constant throughout the day.

In many places, the air's total water vapor (moisture) content varies only slightly during an entire day, and so it is the changing air temperature that primarily regulates the daily variation in RH. This would cause the highest RH values to occur during the coldest part of the day, early morning, and the lowest RH values to occur during the warmest part of the day, mid to late afternoon. From this we can see that RH does not tell us how much water vapor is actually in the air, rather, it tells us how close the air is to being saturated at a given temperature.

Calculate the relative humidity (RH) for an air parcel with a dew-point temperature of 45 degrees F and an air temperature of 70 degrees F. You will need to use the appropriate table on 107 (11th Edition) of your lecture book to figure this problem out.

Just knowing the temperatures is not enough information to calculate the exact RH values. But knowing the temperatures, you can go to a table or chart and find the necessary water vapor information to calculate RH. For this example we will use the table on page 107 of the lecture book. First find the saturation vapor pressure for the dew-point temp of 45 degrees F (10.2 mb). Since the dew-point temp is a measure of the actual water vapor content in the air, it will be used for the numerator. The saturation vapor pressure for the air temperature of 70 degrees F (25.0 mb) will be used in the denominator, since it indicates the capacity of water vapor the air could hold at that temp. RH=E/Es x 100% or 10.2 mb/25.0 mb x 100% = 40.8%

Miami: Air Temp: 85F Dew-Point Temp: 75F Actual Vape Press: 29.6mb Saturate VP: 41.0mb Fargo: Air Temp: 45F Dew-Point Temp: 45F Actual Vape Press: 10.2mb Saturate VP: 10.2mb Chicago: Air Temp: 65F Dew-Point Temp: 50F Actual Vape Press: 12.3mb Saturate VP: 21.0mb Dallas: Air Temp: 95F Dew-Point Temp: 65F Actual Vape Press: 21.0mb Saturate VP: 56.2mb Which city has the greatest amount of water vapor content?

Miami Has the highest Dew-Point Temp and/or Actual Vapor Pressure of all four cities

On what instrument for measuring atmospheric moisture can you find a wet and dry bulb?

On an aspirated or sling psychrometer.

An air parcel has a dew-point temp of 30 degrees F and an air temp of 80 degrees F. The actual vapor pressure is 5.6 mb and the saturated vapor pressure is 35.0 mb. What is the Relative Humidity?

RH = actual vapor pressure (E)/saturated vapor pressure (Es) x 100% RH = 5.6 mb/35.0 mb x 100% RH = 16%

So what exactly does the Relative Value tell us?

Relative Humidity only tells us how close an air mass is to saturation (100%). Warm air with a high relative humidity value would contain a lot of moisture but cold air with a high relative humidity would not contain a lot of moisture. Therefore, relative humidity values are not a good indicator of how much moisture is actually in the air.

When the 'Relative Humidity' is given, why is it also important to know the air temperature?

Since relative humidity (RH) values indicate the amount of water vapor in an air mass relative to its temperature, and that warmer air has the capacity to hold more water vapor, a high RH value in a cold air mass could actually contain less actual water vapor than a low RH value in a warm air mass. The following is an example: An air temperature of 10 degrees F with an actual vapor pressure of 1.9 mb and saturated vapor pressure of 2.4 mb would have a RH of 79%. An air temperature of 50 degrees F with an actual vapor pressure of 4.6 mb and saturated vapor pressure of 12.3 mb would have a RH of 37%. The 50 degree F air mass actually contains more moisture than the 10 degree F air mass (4.6 mb vs 1.9 mb of actual vapor pressure) but has a lower RH (37% vs 79%).

How is the difference between dew-point temperature and air temperature (dew-point depression) related to the Relative Humidity?

Since the dew-point temperature is a direct measure of actual water vapor content in the air, the difference between air temperature and dew-point temperature can indicate whether the Relative Humidity value is low or high. When the two are far apart, the Relative Humidity is low, when the two are close together, the Relative Humidity is high.

So what exactly does the Dew-Point Temperature tell us?

The Dew-Point Temperature is the value we want to use to identify how much water vapor is available. When the dew-point temperature is increasing, the amount of water vapor available is increasing.

What does the 'Relative Humidity' represent?

The Relative Humidity (RH) is the ratio of the amount of water vapor actually in the air to the maximum amount of water vapor required for saturation at a particular temperature (and pressure). In other words, it is the ratio of the air's water vapor current content to its potential full capacity.

Explain why it takes longer to cook vegetables in the mountains than at sea level.

The saturation vapor pressure is directly related to the temperature of the water vapor in the bubbles, higher water vapor temps produce higher vapor pressure values. Hence, any change in atmospheric pressure will change the temp at which water boils. An increase in air pressure raises the boiling point, while a decrease in air pressure lowers it. Since pressure decreases with height, the higher you go, the lower the boiling point, and the longer it would take to cook vegetables. Sea-level boiling point 212 degrees F, Denver at 5,000 feet 203 degrees F.

What is the 'wet-bulb temperature'?

This is the lowest temperature that can be reached by evaporating water into the air.

What is the 'dew-point temperature'?

This is the temperature to which air would have to be cooled (with no change in the air pressure or moisture content) for saturation to occur. The dew-point temperature is a good indicator of the air's actual water vapor content. High dew-pint temperatures indicate high water vapor content; low dew-point temperature, low water vapor content. Addition of water vapor into the air increases the dew-point temperature; removing water vapor from the air lowers the dew-point temperature.