Sunday, December 11, 2016

Ten Data from a Near Space Mission

Nope, there's no error in the title; data is plural for datum. Here are ten examples of data collected during a typical near space mission. Each is easily captured using a programmable flight computer.

Temperature is something everyone understands. Best of all, it's easily measured with a LM335 temperature sensor connected to a microcontroller with analog-to-digital capability. The LM335 is a temperature-controlled Zener diode that produces a voltage proportional to its temperature. The voltage increases by 100 mV for every 100 kelvin increase in temperature. Theoretically, it produces zero volts at absolute zero and 5.00 volts at 500 kelvins (441 degrees F). Most people are aware that air temperature decreases with increasing altitude (in fact, at a rate of 1 degree F for every 300 foot increase in altitude in the Troposphere). What many people are not aware of is that the air temperature begins increasing again with altitude once one enters the Stratosphere. In fact, the switch in the lapse rate is what delineates the Stratosphere for the Troposphere. The air temperature of the Troposphere decreases with increasing altitude because the ground warms the lowest layer of the atmosphere. As you move away from this source of heat, the air temperature decreases. The air temperature in the Stratosphere increases with increasing altitude because solar ultraviolet and ozone warms the upper layers of the atmosphere. As you approach this source of heat, the air temperature increases. 

Everyone is aware of the fact that air pressure decreases with increasing altitude. And anyone who has brought a sealed bag of potato chips to a mountain excursion has seen the sealed bag expand and becomes stiffer as he or she drives higher. Earth's gravity pulling down on the gas molecules making up the atmosphere creates air pressure. Gravity makes the air pile up on the ground creating more air pressure at the surface and less the higher one goes. As a balloon climbs higher, it experiences a decrease in air pressure the higher it climbs. The rate at which the air pressure decreases with altitude is a function of three factors, the average mass of the molecules making up the atmosphere, the air temperature, and the acceleration due to gravity. A convenient description for the rate of an atmosphere's decrease in air pressure is called the atmosphere's Scale Height (H). By definition, Scale Height is the change in altitude needed to decrease the air pressure by a factor e, or 2.718... In the case of Earth, the Scale Height is 8.5 km, 5.2 miles, or 27,375 feet. Another way to look at the lapse rate or Earth's atmospheric pressure is that it decreases by half for every 18,000 foot change in altitude.                  

Relative Humidity is a comparison between the amount of water vapor dissolved in the atmosphere (absolute humidity in units of grams water per kilogram of atmosphere) and the amount of water vapor the atmosphere can hold in solution at its current temperature and pressure. In other words, relative humidity is a measure of the atmosphere's level of saturation. The atmosphere can hold less and less water vapor at higher and higher altitudes. This is because the atmosphere's density decreases with increasing altitude. The relative humidity however tends to remain constant with altitude except when near sources of water. The Earth's surface is one large source of water in the form of lakes, streams, and moist ground. At altitude, the relative humidity spikes near clouds. Above the clouds however, there are no large reservoirs of water for the atmosphere to draw upon and therefore the relative humidity tends to be low. Water vapor absorbs infrared light and this is one reason infrared observatories are built on tall mountain tops. 

Cosmic Rays are not actually rays. Rays are electromagnetic radiation and they consist of photons, or particles of light. Photons have no mass; they travel at the speed of light and are the subatomic particle responsible for carrying the electromagnetic force between charged objects. Eighty-five percent of Cosmic Rays consist of protons and 12% of alpha particles, or helium nuclei. A small percentage of detected cosmic rays are mesons, but those are the product of the collisions between cosmic rays and molecules of gas in the atmosphere. Very rarely, gamma rays (a form of electromagnetic radiation) are found in Cosmic Rays. The source of Cosmic Rays appears to be supernovae explosions. The strong magnetic fields associated with the explosion of massive stars are capable to accelerating some of the supernova atoms to high energies. The Cosmic Rays then travel within the galaxy until they strike Earth's atmosphere. Physicists refer to them as primary Cosmic Rays as they enter the atmosphere. A collision between a primary Cosmic Ray and a molecule of nitrogen or oxygen in the atmosphere creates a shower of subatomic particles called secondary Cosmic Rays. The secondary Cosmic Ray can create addition showers of subatomic particles that are lower in energy than the original particle. Eventually, most Cosmic Rays are absorbed by the atmosphere as they gain electrons and turn into ordinary molecules.             

Physicists divide Ultraviolet radiation into three broad ranges, UV-A (315 nm - 400 nm), UV-B (280 nm - 315 nm), and UV-C (100 nm - 280 nm). UV-C is used as a germicidal, in other words it destroys cells and is therefore very dangerous to humans. Fortunately, it's entirely blocked high in the atmosphere by the ozone layer inside the stratosphere. The majority of UV-B is blocked in the ozone layer, but some does reach the surface and gives tans and sunburns. UV-A is the band of ultraviolet most likely to reach the surface and it too gives people sun tans and sun burn. UV-B is more dangerous than UV-A because of the large amount of energy packed inside of its photons. However, even UV-A is dangerous in excess amounts. The UV-B Flux chart from data recorded on this near space mission shows an increase in the flux once the balloon has climbed above 30,000 feet. At this point, the balloon has entered the ozone layer and so there's less ozone to block UV-B from the sun. But this isn't the only reason it's increasing. The sun's increasing elevation above the horizon is a second factor for creating an increase in UV-B flux. As the sun climbs higher, its light travels through less air and therefore less ozone. Finally, the jaggedness of the data is due to spinning and swinging of the BalloonSat carrying this sensor. Sometimes the data was collected when the sensor faces more directly towards the sun and at other times when the sensor was pointed more away from the sun. 

Viewing the ground below with a thermal imager shows surface temperature variations across a large swath of land. A person could collect the same data by carrying a thermometer across 100 square miles of land. In this image, yellow and white represent the warmest locations and blue and black the coldest. The Owyhee Mountains are a desert mountain range. It's dry grasses warm quickly at sunrise and therefore appear yellow in this image. The neighboring farmlands are well-irrigated, leafy green, and therefore cooler. The red streak visible in the mountains is a valley receiving some sheltering from the rising sun. A balloon took this image at an altitude of 94,400 feet.

Even better is to record images of the ground in visible light and thermal infrared at the same time. This image taken at 13,000 feet shows the Snake River and two of its many islands. The river's water is warmer than the surface temperature of the islands and their foliage. The neighboring farmland is cooler than the open desert next to it and a lot cooler than the Snake River. There's a structure, perhaps a barn on the edge of the farm that shows up as the white spot in the visible image. It's apparently much cooler this morning because it appears as a small blue spot in the thermal image.

A thermal imager shows just how cold clouds can be. This image taken at 42,000 feet includes clouds that appear black to the thermal imager. Clouds are water vapor condensed into water droplets. They occur where the temperature of the atmosphere is at the dew point. Unless there's fog on the ground, the dew point is colder than the surface air temperature. Since the air temperature decreases with increasing altitude, we can expect clouds to be colder than the ground.    

Near infrared is the portion of the electromagnetic spectrum bordering between the infrared and visible red. The atmosphere is very good at scattering blue light from the sun through a process called Rayleigh Scattering. It occurs because molecules of oxygen and nitrogen are roughly the size of a blue photon's wavelength. In the case of red or even infrared light, the wavelength is too long for the smaller molecules comprising the atmosphere to affect them very strongly. Unless there is a lot of atmosphere, as there is when looking at the horizon, very little red or infrared light is scattered by Rayleigh Scattering. This picture of the horizon was taken at an altitude of 38,000 feet. The distance to the horizon is 250 miles, yet this image shows mountains nearly all the way to the horizon in crisp detail. The sky is black because no infrared has been scattered out of the sunlight, except very close to the horizon. 

A picture taken in infrared from an altitude of 72,000 feet shows details of the ground otherwise invisible in visible light. If this was a regular color picture, the ground would have a bluish cast to it and that blue haze would blur out some of the details. Since it is infrared, we see sharp relief in the Owyhee Mountains (left side), some of the erosion of the mountains towards the Snake River Valley, the Snake River and several of its many islands, and Lake Lowell. Farmland still with crops appears white. That's because chlorophyll is reflective to red and infrared light. Near infrared images is a good way to estimate the health of plants since healthy plants are rich in chlorophyll.                

No comments:

Post a Comment