Exercise Set 2 – A little bit of atmospheric physics [Total 40 marks]
Complete the table below by filling in the temperature in °C, °F, or K, as needed.
Indoor air temperature Coldest wind chill (N.S.) Boiling point of nitrogen Surface of the Sun
°C 20°C 6500°C
K 77.5 K
2. Imagine a puddle of water, 1 cm deep. Direct sun rays are used to evaporate the water at 0°C. How long will it take to evaporate the water completely? [The latent heat of vaporisation for water is 540 cal. g-1 at 100°C and 600 cal. g-1 at 0°C. The solar constant is 1.97 cal.cm-2 .min-1 , or 1372 W .m-2. Remember that one cubic centimetre (1 cm3) of water = 1 g, so you can consider the puddle to be made up of lots of cubes of water, each 1 cm deep, and 1 cm2 on their top surfaces.]
How long will it take at 33°C? (You will need to estimate the latent heat of vapourisation at 33°C.)
Comment on these results. Why do you think the calculations show it should take only slightly longer on a cold day than on a hot day, when you know well enough that a shallow puddle can lie around for hours in the winter, but will be gone in minutes in the summer? What other factors might come into play? Why does the River Nile not evaporate to dryness when it flows from Khartoum to Cairo through the Sahara Desert?
3. Construct a graph of January, July and annual temperatures against latitude for a north-south transect through North America from Corpus Christi, Texas, to Churchill, Manitoba. As best as you can fit a straight line through each set of data.
Corpus Christi to Churchill climatic data transect – Graph
Comment on any significant trends – why do you think the winter data display a steeper slope than the summer data (refer to Figure 2.24 in the text).
4. Recall that relative humidity is a function of the actual vapour pressure and the saturation vapour pressure (text, p. 106):
where VP = vapour pressure, and W = mixing ratio. The subscript s refers to the saturation vapour pressure or mixing ratio. Compare the actual water content of the air over the sunny Sahara with that of a blizzard in Baffin Bay. In the Sahara, use a temperature of 40°C and a RH of 10%. Use mixing ratios (W/Ws) for your calculations, so that you will end up with water content in g/kg, which is more meaningful than mb here. Calculate the mixing ratio by reversing the above equation and solving for W. Determine the saturation mixing ratio by referring to Table 4-1 in the text. Then do the same for the Baffin Bay blizzard, where the air temperature is -10°C and the RH = 100%.
5. In the tropical rain forest latitudes, intense solar heating causes air to rise along a zone sometimes referred to as the “thermal equator” or the “inter-tropical convergence zone”. You know that the active tropospheric part of the atmosphere is a maximum of 18 km at the equator. Suppose the air at sea level has a temperature of 30°C. What would be its temperature at 18 km assuming it rises by the dry adiabatic lapse rate?
Obviously (I hope) the air never gets that cold! Why not?
6. On the map below are shown the pressure values for a set of weather stations. On a weather chart, pressure is usually shown as a three-digit number, which at first bears no obvious relation to the millibar or kilopascal or inches of mercury units at all! For this exercise we will work in millibars. The numbers reported represent the last two digits and first decimal place in millibars – the leading 9 or 10 is simply dropped off. In other words, the pressure in Toronto shows 101 – insert either a 9 or 10 in front to get the pressure (910.1 or 1010.1 mb). Meteorologists are well aware that a pressure of 910 or so mb would only be found in the eye of an extreme hurricane (See Figure 6-3, p.163), while 1010 mb is just slightly below average sea level pressure. No hurricane was obvious in Toronto that day so we can conclude the pressure was 1010.1 mb. As a general rule, atmospheric pressure doesn’t exceed the range of 960 to 1040 mb, so “high numbers” (above 500) on the map below need a ‘9’ in front of them, while “low numbers” (below 500) need a ’10’.
Click here for a page-sized version of this map to print.
Draw contours on the map at 4 mb intervals. Begin by identifying the area of lowest pressure (remember numbers will be in the 800’s and 900’s), and encircle all the numbers lower than 988 (i.e., 880) with the 988 isobar; then do all the numbers lower than 992 mb (i.e., 920’s on the map) with the 992 mb isobar, and so on until you complete the map. Label each isobar with the millibar value as you go. Notice that pressure refers to the centre of the symbol, not where the pressure values are actually written. If you are unfamiliar to drawing contours, you might find the flash animation at http://ees.acadiau.ca/~raeside/2753/contours.swf useful.
b. Identify the areas of high and low pressure with a large H and a large L.
c. Solid symbols indicate the sky was overcast (possibly raining too) at the time of reading; unfilled symbols indicate at least half the sky was clear. What general relationship do you see between atmospheric pressure and the nature of sky cover?
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