You may have often wondered what exactly a weather sounding is. Meteorologists discuss them time and time again, especially during active weather periods. If you have not heard of a sounding or are elementary with understanding them, this blog will convey an in-depth analysis toward understanding how observed weather soundings work.
Above is a sounding analysis that was observed in Tampa Bay, Florida, the morning of August 14th, 2014. First off, you will need to know what an observed sounding is and it is simply put: An observed sounding analysis is a view of the current state of the atmosphere at a given location. Weather balloons that are launched at any particular location, usually Weather Forecast Offices (WFOs) are launched at 12z and 00z every day. On some occasions, additional weather balloons are launched if the threat for hazardous weather is significant enough (i.e. May 20th, 2013, a special weather balloon was launched at 18z / 1pm CDT from the National Weather Service in Norman, Oklahoma, just a few hours before Moore, Oklahoma was impacted by an EF-5 tornado).
The following number labels on the sounding will be discussed below…
1) The first thing that is labeled on the observed sounding is the location code, date and time stamp of which the sounding was observed. TBW is the code for Tampa Bay. 140814 is the date, which translates to 08/14/2014. The next thing listed is the time stamp of the observation. In this case, it was 1200z. The “Z” stands for Zulu Time, or Universal Coordinated Time (UTC); they are the same. For brief reference for Zulu to regular time conversions, here is a table explaining the conversions:
12z = 8am EDT, 7am CDT, 6am MDT, 5am PDT
00z = 8pm EDT, 7pm CDT, 6pm MDT, 5am PDT
And when the transition back to standard time occurs, 12z and 00z relate to:
12z = 7am EST, 6am CST, 5am MST, 4am PST
00z = 7pm EST, 6pm CST, 5pm MST, 4pm PST
2) The second label on the observed sounding is the temperature and moisture profiles. Temperature appears as the red line and dew point appears as the green line. As the balloon ascends into the atmosphere, it records both the temperature and the moisture. You may notice that the temperature on this sounding increases with height (if the temperature is increasing, the red line will move to the right) before decreasing. This is a common occurrence within the boundary layer. The boundary layer for brief reference, is usually within the lowest two kilometers of the atmosphere and is where temperatures are most affected by strong day time heating and night time cooling, as well as winds being affected by friction of the Earth. Not to digress from the thermodynamic profiles on the sounding, but I needed to briefly mention the boundary layer for the next reason. The reason why sometimes the temperature may warm with height is called the Nocturnal Inversion. The Nocturnal Inversion is denoted by an increase in temperature with height and can often form on clear nights with relatively calm winds. When the sun rises, the temperature inversion begins to erode as winds start to pick up again due to an influx in temperature with the sun heating the surface.
3) The lines on the side of the sounding that are in different directions are called wind barbs. As the weather balloon ascends into the atmosphere, it records the wind speed in knots (1kt = 1.15mph). The wind barbs vary if the wind speed is weak, strong or significant. A barb will also face a different direction given the direction of which the wind is blowing. In wind shear environments, low-level winds may be blowing out of the south before turning with height to out of the northwest (sound familiar?). Here is an image that depicts the different wind speeds barbs may present on a sounding:
4) Number four on the sounding is showing the pressure coordinates on the sounding. Each day, at any particular location on Earth, pressures vary. The pressure of 1000 millibars (mb) on the sounding was at a height of only 13 meters, equivalent to about 43 feet. As you ascend into the atmosphere, the pressure begins to lower. Did you ever notice the ear popping feeling when flying or driving high into a mountain? Same thing. The lower the pressure on the surface, the more likely precipitation and[or] storms are occurring within that area of the sounding.
5) On every observed sounding, just to the right of the pressure coordinates will be the heights measured in kilometers. The conversion to meters can be done if necessary. Notice that the 1-kilometer (3,280ft) height is below the 850mb pressure height, which indicates that the 850mb height is roughly just under one mile (5,280ft) above the surface. You can expect the boundary layer height to be roughly at or around the 850mb pressure height, especially at night when the boundary layer height lowers due to radiational cooling on the surface.
6) You’re probably wondering what #6 labels on the sounding, but incidentally there are two things that #6 is showing on the sounding. First, it is showing the LCL and then the LFC. The LCL is called the Lifted Condensation Level, where an air parcel becomes saturated as it rises. When this happens, it is the layer at which clouds form. The LCL on the 12z Tampa Bay sounding was right around 0.7-0.8km, or about a half mile above the ground. Low lifted condensation levels indicate that it was likely precipitating around that time. The LFC is the Level of Free Convection, of which the saturated air parcel becomes warmer than the surrounding air and can therefore rise freely which leads to the development of thunderstorms.
7) The previous information discussed in #6 leads right to #7 and that is the EL. The EL is called the Equilibrium Level and is ALWAYS above the LFC. The EL is the level at which a rising air parcel becomes equivalent to the surrounding temperature of the environment. It is also the height at which thunderstorm updrafts can no longer rise upward, and is usually a good indicator of cloud top heights.
8) Eight labels the Freezing Level of the atmosphere, or the FZL. Simply put, the freezing level is the altitude, or height, at which an air parcel plummets below freezing (0°C / 32°F). The freezing level can be critical during the winter in indicating whether or not precipitation will fall as rain, sleet, snow, or ice pellets.
9) Lastly, #9 labels the 0°C line on the sounding. This type of sounding is called a Skew-T, and the temperature lines are skewed at a 45° angle. It is important to trace where the temperature is at the bottom of the sounding and follow the 45° line to the red or green line to determine the actual dew point or air temperature. I have drawn it out on the following image. To determine the pressure height at which the freezing level is, simply just look over the pressure coordinates on the left side of the sounding and you can get an estimate of the pressure height at which the temperature reaches freezing. The same rule applies on finding any other temperature, including surface temperature. Remember, to determine temperature on a sounding, simply find the region on the red temperature line that you are looking for and trace a 45° line to the bottom of the sounding and you can determine the temperature.
We will have further discussions on other aspects of weather and forecasting in the near future, including how to read and understand hodographs, station plots and much more. Stay tuned!