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Understanding Soundings

 

Introduction to Glider Flying > Soaring Weather > The Atmosphere > Understanding Soundings

The so-called Skew-T/Log-P (or simply Skew-T for short) is an example of the thermodynamic dia-gram most commonly used in the United States. The Skew-T part of the name comes from the fact that temperature lines on the chart are slanted, while the Log-P is a reminder that pressure does not decrease linearly in the atmosphere. A temperature and dew-point sounding presented on a Skew-T shows a record of the current atmospheric stability, moisture content, and winds versus altitude. Given surface fore-cast temperatures, the potential for thermal soaring, including the likelihood of cumulus and/or over-development can then be forecast. Using Skew-T diagrams to their fullest potential requires practice. [Figure 9-10]

Figure 9-10. Skew-T from an actual sounding.

There are five sets of lines on a standard Skew-T. Other types of thermodynamic charts, for instance the Tephigram often used in Great Britain, have the same lines, but with a somewhat different presentation. The colors and actual number of lines vary, but the main diagram components should always be present. The following discussion refers to the Skew-T in Figure 9-10.

Horizontal blue lines indicate pressure levels and are labeled every 100 millibars (mb) along the left side of the diagram. On this diagram, the approximate height (in feet) of each pressure level in the standard atmosphere is shown on the right. The actual height of each pressure level varies from day to day. Slanted (skewed) blue lines indicate temperature and are labeled every 10°C along the right side of the isotherm. Thin red lines slanted at an angle almost perpendicular to the temperature lines indicate dry adiabats. (An air parcel following a dry adiabat is changing temperature with height at the DALR.) Thin green lines curving in the same direction as, but at a different angle to, the dry adiabats represent saturated adiabats. (An air parcel that is saturated follows a saturated adiabat is changing temperature with height at the SALR.) The thin orange lines slanting in the same direction, but at an angle to the temperature lines rep-resent the ratio of water vapor to dry air, called the mixing ratio. Lines of constant mixing ratio are labeled in grams of water vapor per kilogram of dry air, abbreviated g/kg.

Over the continental United States, several dozen sounding balloons are launched twice daily, at 00 and 12 Universal Coordinated Time (UTC). These sound-ing balloons record temperature, humidity, and winds at several mandatory levels, as well as “significant” levels, where notable changes with height occur. In Figure 9-10, the actual temperature sounding (shown in bold red), the actual dew-point temperature (shown in bold green), and the winds aloft (shown in wind barbs on the right side) for this day are shown.

The basic analysis for forecasting the potential for dry thermals based on a sounding is achieved by answering the question, “At what levels is a parcel of air rising from the surface warmer than the ambient air?” Assume on this day, the surface temperature is forecast to reach 23°C. This point is marked on the Skew-T; the parcel of air at 23°C is warmer than the surrounding air and starts to rise at the DALR. When the parcel has risen along a dry adiabat (parallel to the slanted red line) to 900 mb (3,200 feet), it has cooled to 17.2°C, which is warmer than the surrounding air at 15°C. Continuing upward along the dry adiabat, at about 780 mb (7,100 feet) the air parcel and surrounding air are at the same temperature, and the air no longer rises due to its buoyancy. The Thermal Index (TI) at each level is defined as the temperature of the air parcel having risen at the DALR subtracted from the ambient temperature. Experience has shown that a TI should be -2 for thermals to form and be sufficiently strong for soaring flight. Larger nega-tive numbers favor stronger conditions, while values of 0 to –2 may produce few or no thermals. On this day, with a surface temperature of 23°C, the TI is found to be 15–17.2 or -2.2 at 900 mb, sufficient for at least weak thermals to this level. At 780 mb, the TI is 0, and as mentioned, the expectation is that this would be the approximate top of thermals.

Thermal strength is difficult to quantify based on the TI alone since many factors contribute to thermal strength. For instance, in the above example, the TI at 800 mb (6,400 feet) was –1. The thermal may or may not weaken at this level depending on the thermal size and the amount of vertical wind shear. These factors tend to mix in ambient air and can decrease the thermal strength.

It is important to remember that the TI calculated as above is based on a forecast temperature at the surface. If the forecast temperature is incorrect, the analysis above produces poor results. As a further example, assume that on this day the temperature only reached 20°C. From a point on the surface at 20°C and following a dry adiabat upwards, the TI reaches 0 only 1,000 feet AGL, making the prospects for workable thermals poor. On the other hand, if temperatures reached 25°C on this day, thermals would reach about 730 mb (8,800 feet), be stronger, and have more nega-tive TI values.

The previous analysis of the morning sounding calcu-lated the TI and maximum thermal height based upon a maximum afternoon temperature. In reality, the sounding evolves during the day. It is not untypical for a morning sounding to have an inversion as shown in Figure 9-10. A weaker inversion on another day is shown in Figure 9-11. This sounding was taken at 12 UTC, which is 05 local time (LT) at that location. The surface temperature was 13°C at the time of the sounding. A shallow inversion is seen near the surface with a nearly isothermal (no temperature change) layer above. Two hours after the sounding was taken (07 LT), the surface temperature had risen to only 14°C. By 09 LT, the temperature had risen to 17°C. The line labeled “09” shows how the sounding should look at this time. It was drawn by taking the surface temperature at that time, and following the DALR until TI is 0, which is the same as intercepting the ambient temperature. Lines at other times are drawn in a similar fashion. At 10 LT, the TI becomes 0 at about 2,200 feet AGL, so the first thermals may be starting. By 12 LT, at 25°C, thermals should extend to 4,000 feet AGL. Because of the isothermal layer, ther-mal heights increased steadily until about 16 LT, when temperatures reached 31°C, at which time they reached about 8,000 feet AGL. Understanding this evolution of the convective layer can help predict when thermals will first form, as well as if and when they might reach a height satisfactory for an extended or cross-country flight. [Figure 9-11, on next page]

The analysis presented thus far has neglected the possibility of cumulus clouds, for which the orange slanted mixing ratio lines on the Skew-T need to be considered. The assumption that a rising parcel con-serves its mixing ratio is also needed. For instance, if an air parcel has a mixing ratio of 8 g/kg at the surface, it will maintain that value as it rises in a thermal. Typically, this is true, though factors, such as mixing with much drier air aloft can cause errors.

Refer to the sounding in Figure 9-12. The temperature on this day reached 26°C during the afternoon. In order to determine if cumulus clouds would be present, draw a line from 26°C at the surface parallel to a red dry adiabat as before. Draw a second line from the surface

Figure 9-11. Skew-T from an actual sounding.

dew point temperature parallel to the orange mixing ratio lines. The two lines intersect at a point before the parcel has a zero TI. This is the base of the cumulus, called the convective condensation level (CCL). In this case, cloud base occurs at about 750 mb (8,100 feet). Since the parcel is saturated above this level, it no longer cools at the DALR, but at the SALR. Next, from the CCL, draw a line parallel to a saturated adiabat until it intersects the original sounding temperature curve. This shows the maximum cumulus height, at about 670 mb (11,000 feet). [Figure 9-12]

The above analysis leads to a rule of thumb for esti-mating the CCL. The temperature and dew point converge at about 4.4°F per 1,000 feet of altitude gain. This is the same as saying for every degree of surface temperature and dew point spread in Fahrenheit, multiply by 225 feet to obtain the base of the convective cloud (if any). Since aviation surface reports are reported in degrees Centigrade, convert the data by multiplying every degree of surface temperature and dew point spread in degrees Centigrade by 400 feet. For example, if the reported temperature

Figure 9-12. Skew-T from an actual sounding.

is 28ºC and the reported dew point is 15°C, we would estimate cloud base as (28 – 15) x 400 = 5200 feet AGL.

Notice that in Figure 9-12, the dew point curve shows a rapid decrease with height from the surface value. As thermals form and mixing begins, it is likely that the drier air just above the surface will be mixed in with the moister surface air. A more accurate estimate of the CCL is found by using an average dew point value in the first 50 mb rather than the actual surface value. This refinement can change the analyzed CCL by as much as 1,000 feet.

The second example, Figure 9-11, would only produce dry thermals, even at this day’s maximum temperature of 32°C. Following a line parallel to a mixing ratio line from the surface dew point, the height of any cumulus would be almost 12,000 feet AGL, while at 32°C, thermals should only reach 9,000 feet AGL. The elevated inversion at 9,000 to 10,000 feet AGL effectively caps thermal activity there.

It is also important to recognize the limitations of a sounding analysis. The sounding is a single snapshot of the atmosphere, taken at one time in one location. (This is not absolutely true since the sounding balloon rises at about 1,000 feet per minute (fpm), so it takes about 30 minutes to reach 30,000 feet, during which time it has also drifted with the winds aloft from the launch point). The analysis is limited by how well the sounding is representative of the greater area. This may or may not be a factor depending on the larger-scale weather situation, and in any case, tends to be less valid in regions of mountainous terrain. In addition, the upper air patterns can change during the day due to passing fronts or smaller-scale, upper-air features. For example, local circulation patterns near mountains can alter the air aloft over nearby valleys during the day. A temperature change aloft of only a few degrees also can make a large difference. Despite these limita-tions, the sounding analysis is still an excellent tool for soaring pilots.

In recent years, with the advent of the Internet, sound-ings from numerical weather model forecasts have become available in graphical form, like the Skew-T. Thus, forecast soundings are available for a variety of locations (far more numerous than the observational sounding network) and at many intervals over the fore-cast cycle. The advantage of using model forecast soundings is a dramatic increase in both space and time resolution. For instance, maps of the predicted thermal tops can be made over a large (e.g., multi-state) area from model data spaced every 10 miles or closer. Great detail in the forecast distribution of thermals is avail-able. In addition, model output can be produced far more frequently than every 12 hours. For instance, hourly model soundings can be produced for a location. This is a tremendous potential aide to planning both local and cross-country flights. The disadvantage is that these forecasts are not real data. They are a model forecast of what the real atmosphere should do. Model forecasts of critical items, such as temperatures at the surface and aloft, are often inaccurate. Thus, the model-forecast soundings are only as good as the model forecast. Fortunately, models show continual improvement, so this new tool should become more useful in the future.

Soaring - Atmospheric stability
Soaring - Air masses conducive to thermal soaring
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