Chapter 4 questions Flashcards
Basically, 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 are the primary factors that influence evaporation?
Evaporation occurs when more water molecules are leaving a water surface than returning. Factors that influence evaporation are: (1) water temperature, the warmer the water, the greater the rate of evaporation; (2) wind speed, winds will enhance evaporation; (3) moisture content of air above water surface, drier the air, the higher the potential for evaporation.
Explain why condensation occurs primarily when the air is cooled.
Condensation occurs when water vapor molecules condense onto condensation nuclei. 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 to the nuclei.
How does condensation differ from precipitation?
Condensation will produce small cloud droplets that are visible, but too small to fall out of the sky. Precipitation occurs when cloud particles, liquid or solid, grow to a size large enough that enables them to fall to the earth’s surface.
In a volume of air, how does the ‘actual vapor pressure’ differ from the ‘saturation vapor pressure’?
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 can 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 have a relative humidity of 100%.
When the air is unsaturated , relative humidity values will be less than 100% and the saturation vapor pressure will be greater than the actual vapor pressure.
What does saturation vapor pressure primarily depend upon?
At higher air 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 temperature.
What does the ‘relative humidity’ represent?
The relative humidity (RH) is the ratio of the amount of water vapor actual in the air to the maximum amount of water vapor required for saturation at that particular temperature (and pressure). Or in other words, it is the ratio of the air’s water vapor current content to its possible full capacity.
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 that pertains to the pervious question.
An air temperature of 10˚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˚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˚F air mass actual contains more moisture than the 10˚F air mass (4.6 mb vs 1.9 mb) but has a lower RH (37% vs 79%)
Explain two ways the relative humidity may be changed.
By changing the air’s water vapor content, as water vapor is added to the air, with no change in air temp, the RH increases, and, as water vapor is removed from the air, the RH decreases
By changing the air temperature, with no change in water vapor content, an increase in air temp lowers the RH, while a decrease in air temp raises the RH
Explain why, during a summer day, the relative humidity will change.
In many places, the air’s total 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.
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 our 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 less.
Explain why the air on a hot, humid day is less dense than 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)
What is the ‘dew-point temperature’?
This is the temperature to which air would have to be cooled (with no change in air pressure or moisture content) for saturation to occur. The dew-point is a good indicator of the air’s actual water vapor content. High dew-points indicate high water vapor content; low dew-points, low water vapor content. Addition of water vapor to the air increases the dew-point temperature; removing water vapor lowers the dew-point temperature.
How is the difference between dew point and air temperature (dew point depression) related to the relative humidity?
Since the dew point is a direct measure of actual water vapor in the air, the difference between air temperature and dew point can indicate whether the relative humidity 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.
