How high do weather balloons fly?
We've taken a bit of "artistic license" in this activity by allowing balloons to climb into the mesosphere 60 km above Earth's surface. Typical weather balloons have an operational ceiling somewhere around 30 km. Special-purpose high-altitude research balloons sometimes reach as high as 35 km or even 45 km. The altitude record for a balloon carrying people is just shy of 35 km. The altitude record for unmanned balloons is 51.8 km.
The minimum altitude for spacecraft is about 100 km; below that level atmospheric drag is strong enough to quickly pluck a satellite from orbit. Regions of the atmosphere between 40 and 100 km are therefore difficult to study; too high for balloons, too low for satellites. Researchers use sub-orbital-sounding rocket flights to probe the mesosphere directly, but such flights last just minutes and thus supply relatively limited data. Because of these difficulties, study of the mesosphere is difficult and less is known about that region than about other layers of the atmosphere. Some scientists jokingly refer to the mesosphere as the "ignorosphere."
Layers of the Atmosphere
Scientists divide the atmosphere up into distinct layers, as follows:
- Troposphere - From ground level up to somewhere between 8 and 16 km (5 and 10 miles, or 26,000 to 53,000 feet), depending on latitude and season. Most of the mass (~80%) of the atmosphere is here and essentially all weather occurs in the troposphere. Temperature decreases with increasing altitude. The tropopause is the name given to the boundary between the top of the troposphere and the bottom of the stratosphere above.
- Stratosphere - Extends from the tropopause to about 50 km (31 km) up. Temperature rises with altitude. Contains the ozone layer, which shields Earth's surface from most solar ultraviolet radiation. Top boundary is called the stratopause.
- Mesosphere - Extends from the stratopause to about 85 km (53 miles). Many meteors burn up here. Temperature decreases with altitude. The coldest temperatures in Earth's atmosphere, about -85° C (-120° F), are found near the top of this layer. The top boundary is called the mesopause. Part of the ionosphere, a series of sub-layers containing higher levels of ionized and thus electrically charged atoms and molecules, is in the mesosphere.
- Thermosphere - From the mesopause to between 500 and 1,000 km (311 to 621 miles) up. Air is very, very thin here. Variations in solar heating due to the Sun's 11-year sunspot cycle and to short-term space weather storms cause the air in this layer to expand and contract; thus the large variation in altitude of the top of this layer (the thermopause). Most of the ionosphere is within the thermosphere. Temperatures increase with altitude, but also vary dramatically over time in response to solar activity. The aurora (Southern and Northern Lights) periodically light up the thermosphere. The top boundary is called the thermopause. Many spacecraft actually orbit within the thermosphere.
- Exosphere - From the thermopause on upward, this layers is not universally recognized as a layer of the atmosphere. The exosphere is essentially the sparse scattering of atmospheric gasses as they gradually thin to the near-vacuum of space.
The greenhouse effect and greenhouse gases
Solar energy of various wavelengths across the electromagnetic spectrum arrives at Earth at the "top" of our planet's atmosphere. Most of that solar energy is in the form of visible light. There is also quite a bit of ultraviolet (UV) and infrared (IR) radiation, and lesser amounts of X-rays and radio waves. Our atmosphere is mostly transparent at visible wavelengths. Although some sunlight is scattered by air molecules or reflected by clouds, most of it passes straight through the atmosphere and impinges upon the land or sea beneath. The atmosphere is not as transparent at ultraviolet (UV) and infrared (IR) wavelengths; most of the UV is absorbed by ozone in the stratosphere, while much of the incoming IR is absorbed by various greenhouse gases (water vapor, carbon dioxide, methane, and others) which are concentrated in the troposphere. Sunlight that strikes the Earth's surface (including oceans) warms the land or water. The warm ground or ocean emits infrared radiation, which carries energy back upward into the atmosphere. However, greenhouse gases quickly absorb much of that outbound IR energy, heating the lower atmosphere. Thus, the troposphere is warmest near ground level where the heating source is nearby, and cooler at higher altitudes as one gets further and further from the warm ground.
The ozone layer, UV radiation, and the stratosphere
Normal oxygen molecules (O2) have two oxygen atoms. Ozone (O3), a special type of oxygen molecule, has three atoms instead of two. UV photons from the Sun hit normal oxygen molecules in the stratosphere. The high-energy photons break the molecular bonds holding the oxygen atoms together, splitting the O2 molecule apart into two separate oxygen atoms (the process is called photodissociation). Some of those individual atoms combine with other oxygen molecules to form ozone (O3) molecules. Over time, ozone accumulates in the stratosphere, forming the ozone layer - a region in the stratosphere with elevated concentrations of ozone.
Ozone is almost opaque at UV wavelengths. The ozone layer absorbs most of the incoming solar UV radiation. Ozone molecules shed the energy they absorbed from UV photons as heat, warming the stratosphere. The intensity of UV radiation is greatest at the top of the stratosphere, where energy from the incoming sunlight hasn't yet been "diluted" by atmospheric absorption. The temperature trend in the stratosphere is, therefore, exactly opposite of that in the troposphere below - the warmest area is at the highest altitudes and temperatures grow cooler as one goes lower and moves away from the main source of heating.
Let's consider an analogy to help us understand the temperature trends in the troposphere and stratosphere. Imagine two giant hot plates as heat sources. One is on the ground, facing upwards. It represents the heating of the atmosphere by IR radiation emitted by the warm ground (which was warmed by incoming sunlight). The second hot plate is at the top of the stratosphere, facing downward. It represents the heat given off by ozone molecules after they have absorbed energy from incoming UV radiation. So where are the warmest and coolest areas in the lower atmosphere? The air nearest the lower "hot plate" (Earth's surface) figures to be warm, with temperature decreasing as one moves upward away from the heat source. Likewise, air near the upper "hot plate" should be warm, with temperature dropping off as one moves downward away from the heat source. It also makes sense that the coolest temperatures in the lower atmosphere should be roughly midway between the two "hot plates". The tropopause, the boundary between the troposphere and the stratosphere, corresponds to this relatively cool spot between the two heating sources. If you look at a graph of temperature versus altitude in the lower atmosphere you can see how this "pair of hot plates" analogy plays out in the real atmosphere.
Why is the mesosphere so cold?
Above the stratopause (the boundary between the top of the stratosphere and the bottom of the mesosphere) temperatures once again decrease with altitude, as was the case in the troposphere. The air is so thin above the stratosphere that relatively few photons of incoming solar radiation (whether visible light, IR, or UV) collide with air molecules. Temperatures in the mesosphere are therefore quite cold, dropping to -85° C (-120° F) near the top of the layer. The "hot plate" (from the previous analogy) near the top of the stratosphere provides some warmth to the mesosphere above, but temperatures quickly cool as one moves higher and away from that heat source as one climbs through the mesosphere.
Air pressure variation with altitude is much simpler than temperature variation. Standard pressure at sea level is defined as 1 atmosphere ( = 1013 millibars = 14.7 lb/in2 = 101.3 kilopascals). Pressure drops steadily with altitude; at roughly 5,500 meters it is down to 1/2 of the sea level value. Rise up another 5,500 meters and the pressure drops by half again, so that pressure at 11 km altitude is roughly a quarter of the sea level value. In fact, the decrease of air pressure with altitude approximately follows an exponential decay curve. Atmospheric scientists use a concept called "scale height" (H) to express the rate of this decay. In Earth's troposphere, the scale height is about 8.4 km. The equation that expresses this trend is:
|P = P0 e – z / H|| |
- P = the air pressure at a given altitude
- P0 = air pressure at sea level
- z = altitude in kilometers
- H = scale height in km
If you have advanced students or like to mix some math into your science lessons, you might want to have your students try to determine the scale height from the data they gather from their balloon flights. This could be a trial-and-error iterative approach, where the students first guess at the scale height, plug it into the equation above, and see how well it matches their data, and then iteratively adjust their hypothesized scale height until it fits their data pretty well. You could also have the students graph their pressure vs. altitude data on semi-log paper; the data should generate a more-or-less straight line, with the slope representing the scale height.
Please note that the equation above is an approximation, though a pretty good one. The actual behavior of the atmosphere is a bit more complex than portrayed by this simple equation. As mentioned above, the scale height in the troposphere is about 8.4 km in the troposphere. If we extend our area of interest higher into the atmosphere, it turns out that the average scale height from sea level to 70 km is about 7.6 km; so scale height does vary with altitude. Any value between 7.5 and 8.5 km that your students determine for scale height would be a pretty good result. Here are some values for pressure at various altitudes, to help you get a feel for this:
|Denver's altitude ("mile high")|
|Half of sea level pressure|
|Top of Mt. Everest|
|10% of sea level pressure|
Data Table for the "Standard Atmosphere"
Scientists sometimes use a concept called the "standard atmosphere" to represent the conditions (such as temperature and pressure) in the "typical" atmosphere. The standard atmosphere removes variations from day to night, across the seasons, at different latitudes, and as weather systems move across regions. The standard atmosphere is more-or-less Earth's average atmosphere, with variations across space and time removed. The table below provides temperature and pressure data versus altitude for the standard atmosphere to a height of 60 kilometers (about 37 miles or 196,850 feet). These are the values used "behind the scenes" in the virtual ballooning software simulation.
|8||-37.0||356.5||35|| -36.7||5.746|| |
|9||-43.5||308||40|| -22.8|| 2.871|
|14||-56.5||141.7|| || || |
This activity was developed by the UCAR Center for Science Education.