Thermal wake - AHAC review

=Thermal Wake Studies During the August 21st 2017 Total Solar Eclipse (AHAC 2017)= The total solar eclipse of August 21st 2017 was a great opportunity to study the thermal wake profile neither at daytime nor at nighttime. This event was of great interest because the it provided unique conditions. Two research groups from the University of Minnesota and St. Catherine University flew about 40 temperature sensors (DS18B20) to the near space using weather balloons. The measurements were conducted using a wake boom: a carbon fiber rod with temperature sensors spaced along the 3.5 m boom length giving a maximum end to end sensor spacing of 3.2 m. One factor that made this project challenging was the fact that the ballon was moving through a non-constant temperature environment. After analyzing the data collected during the ascent of the balloon at the eclipse conditions and comparing this set of data to the one that was collected during the ascent at daytime conditions they came into the conclusion that in stratosphere a cooling effect can be observed during the total solar eclipse.

=Applying Newton’s Law of Cooling When The Target Keeps Changing Temperature, Such As In Stratospheric Ballooning Missions (AHAC 2016)= Newton’s Law of Cooling states that the rate of heat loss of a body is directly proportional to the difference in the temperatures between the body and its surroundings provided the temperature difference is small and the nature of radiating surface remains same. A research group from the University of Minnesota and St. Catherine University using a weather ballon managed to explore how the readings from the thermal sensors react under circumstances when the two conditions required for application of Newton’s Law of Cooling are not met using a spreadsheet computer. This because the thermal sensors are in motion during their ascent and descent of atmosphere and the changing environmental conditions influence the thermal decay time “constant” for the thermometer as well. They concluded that the comparison of thermometer decay times at atmospheric pressure and at reduced pressure show that these values are not in fact constant over the wide range of pressures encountered during stratospheric ballooning missions, further complicating analysis of (and correction of) logged temperature data.

=Using Thermocouple, Thermistor, and Digital Sensors to Characterize the Thermal Wake Below Ascending Weather Balloons (AHAC 2015)= A research group from the University of Minnesota and St. Catherine University studied the thermal wake below ascending weather balloons using three different devices: (Neulog)thermocouple, (HOBO)thermistor, and (Arduino-logged DS18B20) digital sensors. This was done by sending a weather balloon in the near space that had attached to it a wake boom. A wake boom is helpful to measure the temperature variations up to 1.5m horizontally. Having the wake boom constructed like an X is very interesting because it gave them the opportunity to compare data gathered from sensors with direct sunlight on them and the data set collected from the other sensors. They concluded that the thermal wake is warmer than the ambient air during daytime ascents, due to solar radiation warming the balloon skin, but colder than ambient air during nighttime ascents, due to adiabatic cooling of the gas inside the balloon.

=Using HOBO data loggers with Air/Water/Soil temperature probes to measure free-air temperature on high-altitude balloon flights (AHAC 2012)= Motivated from the disagreement between results from sensors mounted in different ways and between results from a given sensor between ascent and descent in thermal wake studies, a research group presented some preliminary results from their investigation and suggested methods to collect more meaningful data during the studies of the thermal wakes. The disagreement in the results from different methods comes from (a) whether the sensors are exposed to the sun or shaded in some way, (b) where the sensors are physically located with respect to nearby payload boxes (and the color of those boxes), and (c) whether the sensors are surrounded by a thermal wake trailing below the balloon. Some ways that can make the thermal wake research more conclusive are : better calibration procedures, reconstruction of the boom, more flights, sending the sensors in higher altitudes and to carefully study the geometrical properties of the sensors that are going to be used.