Geography 103: Weather
In this set of exercises you will learn how to read, plot and interpret sounding data. These data contain information on how temperature, pressure and humidity vary with height as you go up through the atmosphere. One of the most common diagrams used to show these data is a Stuve diagram. This is what you will be plotting.
We will use the sounding data from the Miramar Naval Station shown on the explanation page. You can access and download the Excel file, by selecting this link (KNKX_072104_sounding_data.xls). Since you will be using this same data set for all these exercises, it might be useful to download it and save it on your computer, or print it out.
You will also need a blank Stuve diagram on which to plot the data. This can be downloaded or printed from this link (Blank Stuve Diagram)
Exercise 1: Plot the temperature sounding.
Using the blank Stuve diagram plot the temperature vs pressure. These are in columns D and B of the Excel file. Put a dot or X at each data point and then connect these with straight lines. Use the vertical gray lines for temperature and the horizontal gray lines for pressure. (Try to ignore all the other lines on the plot for now!)
Exercise 2: Water vapor and mixing ratios.
Put a circle on the plot marking the spot where temperature is 24°C and pressure is 950 mb. Can you see that this lies on the sloping (blue) line marked 20? This means that for a temperature of 24°C and a pressure of 950 mb, the saturation mixing ratio is 20 g H2O/kg dry air.
Using the data from the Excel file put a cross on the plot marking the dewpoint temperature at a pressure of 950 mb. This is about 9°C. Can you see that this point lies between the sloping (blue) lines marked 7 and 10? This means that for a dewpoint temperature of 9°C and a pressure of 950 mb, the actual mixing ratio is about 8 g H2O/kg dry air.
Is the air saturated or unsaturated at 950 mb?
Exercise 3: Rising dry air.
Let’s start an unsaturated (“dry”) parcel of air rising from 500m or so above the ground (950 mb). Look at your sounding plot (or the Excel file). What is its temperature? What rate does dry air cool off at as it rises?
On your Stuve diagram use a colored pen to mark the starting point for the air parcel at a temperature of 22 °C and a pressure of 950 mb. What is the altitude? Now mark a point 1000 m higher up with a temperature of 10°C less. (Remember it cools off at about 10°C for every 1000 m that it rises.) Now draw a line connecting your two points. (It should be parallel to the sloping gray lines.) Extend your line upwards until it reaches the point where it approaches the (blue) mixing ratio line that has a value of 7, and stop just before you reach it (so that the mixing ratio is actually 8 rather than 7). What altitude is the air parcel at? What is the air pressure at this point?
This altitude is the lifting condensation level. It corresponds to the point where the moisture contained in the rising air parcel has reached saturation. From this point on the air parcel will rise at the saturated or moist adiabatic lapse rate.
Exercise 4: Rising saturated air.
Now the air is saturated. Continue your colored line upwards following (or parallel to) the closest dashed red line (saturated adiabat).
Does it look like the yellow line in the Understanding Stuve Diagram explanation?
Written by Helen Cox, 2004.