1. Why the Temperature Stops Dropping at Airplane Height
When you look out of an airplane window, the sky is a deep blue, and it is a very cold world outside. Around the height where these airplanes fly, the nature of the air changes. According to the Japan Meteorological Agency, the layer from the ground up to about 10–16 km (6–10 miles) is called the troposphere, where the temperature drops as you go higher. Above that, the temperature drop becomes smaller, or it may even start to rise. This boundary is called the tropopause [1].
The center of the jet stream, where strong winds blow, is usually at about 10 km (6 miles) and is strongest near the tropopause [1]. NASA explains that the stratosphere is the layer where jet airliners fly [3]. However, airliners fly near the lower edge of the stratosphere, and depending on the altitude, latitude, and season, they may fly in the troposphere instead.
2. How Ozone Heats the Stratosphere by Absorbing Ultraviolet Light
The main reason the temperature rises in the stratosphere is that ozone absorbs ultraviolet light and heats the atmosphere. The Japan Meteorological Agency explains that high ozone concentration, where ozone absorbs ultraviolet light and heats the atmosphere, is the main cause of the stratosphere [1].
How high does the temperature go in the stratosphere? According to NOAA data, the average temperature at the lower boundary is about -51°C (-60°F), and at the top of the stratosphere, it is at most about -15°C (5°F) [2]. Even though it gets warmer as you go up, it does not get hot like it is on the ground. It simply means it is "warmer than the layer below."
In short, the warmth of the stratosphere is a change in the direction of temperature. It happens when energy from ultraviolet light from above is added to the flow of air that is cooling as it goes up from the ground.
3. How the Height of the Troposphere Differs by Latitude
The entrance to the stratosphere is not at the same height everywhere on Earth. According to NOAA data, the height of the troposphere is about 18–20 km (11–12 miles) at the equator, about 9 km (5.6 miles) at 50 degrees north or south latitude, and just under 6 km (3.7 miles) at the poles [2].
If you divide the equator's height of about 18 km by the pole's height of about 6 km, the difference is about 3 times. This shows that the height of the entrance differs greatly between areas near the equator and the poles.
The Japan Meteorological Agency writes "10–16 km" and NOAA writes the lower edge of the stratosphere is "6–20 km" because these heights change depending on location and season [1][2]. In this article, we use "about 10 km (6 miles)" as a rough guide for areas near Japan.
4. How Rubber Balloons Carry Instruments About 30 km Up Twice a Day
The Japan Meteorological Agency launches balloons with instruments called radiosondes twice a day (at 9:00 and 21:00 in Japan). They measure temperature, humidity, wind direction, and wind speed along with pressure (altitude) up to a height of about 30 km (18.6 miles) [4]. This means observations reach the stratosphere every day.
According to the Fukuoka Regional Meteorological Observatory, the rubber balloon has a diameter of about 1.6 m (5.2 ft) on the ground and rises at a speed of 300–400 m (980–1,300 ft) per minute. At a height of about 30 km, it expands to a diameter of about 8 m (26 ft) and bursts. The sonde then descends slowly with a parachute [5].
If the diameter goes from 1.6 m to 8 m, it is 5 times larger. High in the sky, the air is thin, and the force pushing from outside is small, so the balloon expands greatly.
5. How an Ultra-Thin 2.8 μm Balloon Reached 53.7 km
JAXA's large balloon BS13-08 was launched on September 20, 2013, and recorded an altitude of 53.7 km (33.4 miles). It was an ultra-thin balloon with a volume of 80,000 m³ and a skin thickness of 2.8 μm, surpassing the record from May 2002 (altitude 53.0 km, skin thickness 3.4 μm) [6]. Since 1 μm is 1/1,000 of a millimeter, 2.8 μm is 0.0028 mm.
The 2013 balloon reached its highest point in 2 hours and 42 minutes [6].
The 2002 balloon flew from the Sanriku Balloon Observatory on May 23, 2002, and exceeded the previous world record of 51.8 km by 1.2 km. This update was announced as the first in about 30 years [7].
Between the two records, the skin thickness changed from 3.4 μm to 2.8 μm, and the altitude changed from 53.0 km to 53.7 km.
53.7 km is near the upper edge of the stratosphere, and just above it is the entrance to another layer. This height is about 1.8 times higher than the point reached by the Japan Meteorological Agency's balloons (about 30 km).
6. Try It Yourself: Watch a Rubber Balloon Expand as You Inflate It
Take one rubber balloon and inflate it slowly. As it inflates, the skin stretches and the color becomes lighter. This helps you imagine how the bag of an observation rubber balloon stretches and becomes thinner. The ultra-thin balloons that set records are different from observation balloons.
Balloons can burst suddenly. Keep them away from your face and do this with an adult.
You can read about how skin thickness and altitude records changed in the news from JAXA (ISAS) [6][7]. For the mechanism of the stratosphere, the Japan Meteorological Agency's materials are a good starting point [1], and the Japan Meteorological Agency's page on upper-air observations explains the observation system [4].