The Role of Aviation Meteorology in Achieving HAPS Flight



1. Introduction to Aviation Meteorology

Conventional aircrafts fly within the troposphere, also known as the convection zone, which extends from the Earth's surface to an altitude of 10 to 16 kilometers. The convection zone, as its name suggests, is a layer of the atmosphere where convection occurs. The Earth's surface is heated by solar radiation, but the amount of heating varies depending on the latitude due to differences in the angle of solar incidence. To equalize this temperature distribution, convection takes place, resulting in the formation of winds, clouds, and jet streams, which are the characteristics of the convection zone.

Since aircrafts generate lift by catching wind on their wings, they are greatly affected by convection currents. However, convection can be disrupted by various conditions such as terrain, and this disruption manifests itself as aircraft turbulence. In addition, strong winds can cause the aircraft to be swept away, requiring extra thrust to compensate.

Various other weather phenomena such as cumulonimbus clouds, typhoons, and thunderstorms also occur due to convective activity, all of which pose a significant threat to aircraft. Aviation weather is thus important to the safety and efficiency of aviation.

2. Features of HAPS Aircraft

When people think of an “aircraft”, they often imagine a large plane carrying many passengers and cargo, flying at high speeds. In reality, large passenger jets can weigh around 300 tons and fly at speeds of approximately 900 km/h with powerful jet engines.

How about HAPS aircraft? These aircraft have a completely different design and performance to optimize their gliding characteristics. They are lightweight, with long wingspan similar to gliders, and have speed comparable to automobiles. The motors are powered by solar energy, which is a significant departure from conventional aircraft. The most significant difference is that HAPS aircraft fly in the stratosphere, an environment with pressure approximately one twentieth of the surface level, using only solar power for up to several months. These aircrafts are essentially self-sustained and highly environmentally friendly.

In the next chapter, let's explore in more detail the specific environments in which these aircraft operate, with their unique designs and performance tailored to those conditions.

3. Unique Weather Conditions

The temperature on the Earth's surface where everyone lives varies greatly depending on location and altitude, but according to the international standard of atmosphere, the atmospheric temperature is defined as 15°C (59°F). Generally, as altitude increases, the temperature decreases, and it can drop to nearly -100°C (-148°F). It then begins to rise again near the boundary with the stratosphere, known as the tropopause.

HAPS aircraft, which travel between the surface and the stratosphere, need to have a structure capable of withstanding temperature differences ranging from 15°C to -100°C. Additionally, there are regions where the surface temperature exceeds 30°C, which further expands the range of temperature variations that the aircraft must endure.

The troposphere also contains jet streams. These winds can vary significantly depending on the region, but they can exceed speeds of 200 km/h (124 mph). HAPS aircraft operate at speeds comparable to automobiles, so they can quickly be swept away by jet streams. Especially the atmosphere near jet streams can be turbulent, causing fluctuations. This poses a significant threat to HAPS aircraft, which are slow-moving and have specialized structures.

Additionally, it is essential for HAPS aircraft to always fly in a manner that efficiently collects solar energy. Therefore, the aircraft's attitude and flight path are significantly affected by wind speed. HAPS aircraft must fly in harmony with the harsh atmospheric environment.

4. Is the Stratosphere Truly “Stratified”?

The stratosphere is a layer of the atmosphere that ranges from about 10 to 16 km, reaching up to approximately 50 km in altitude. HAPS aircraft are designed to operate around an altitude of 20 km. As the name "stratosphere" suggests, it is a layer where the atmosphere at different altitudes does not mix, meaning there is no convection and the atmosphere is said to be very stable. Unlike the troposphere, there is no formation of clouds in the stratosphere.

As previously mentioned, HAPS aircraft have specialized performance characteristics and structures. Therefore, flying in the cloud-free and stable stratosphere would be considered ideal. However, is the stratosphere truly stable?

In reality, there can be a certain degree of turbulence and strong winds, and the presence of thunder and lightning phenomena is also becoming understood. HAPS aircraft are required to maintain a stable position to ensure a communication area on the ground. If the aircraft sways significantly, the ground communication area may change, and there is also concern about potential damage to the onboard communication equipment. Therefore, it's important to understand the weather conditions in the stratosphere. However, there are significantly fewer aircraft flying in the stratosphere, and specialized weather observations are not being conducted, so the detailed stratospheric conditions are still not well understood.

20 km from on the ground may seem to be a short distance, the stratosphere at an altitude of 20 km remains a largely unexplored space for humanity.

5. Various Threats

In flying HAPS aircraft, we have discussed factors such as wind, temperature, and thunder and lightning phenomena. However, there are other phenomena that require attention, including volcanic ash, ultraviolet rays, and ozone.

During major volcanic eruptions, ash and aerosols released from the volcano can remain in the stratosphere for several months to years, potentially affecting the long-term flight of HAPS. Additionally, due to the high altitudes, measures need to be taken to counteract the effects of ultraviolet rays and ozone on the aircraft's materials.

6. The Key to HAPS Success: Meteorology

HAPS aircraft face various challenges as they ascend from the hot surface to the extreme cold temperatures, encountering jet streams and turbulence along the way. They must navigate through these conditions and reach the stratosphere, where they are exposed to ultraviolet radiation and operate at a pressure of 1/20th of the surface level. Despite their speeds comparable to automobiles, HAPS aircraft must withstand the wind and provide stable services in a designated area for an extended period.

Indeed, it is not an overstatement to say that the key to the success of HAPS lies in meteorology. At Softbank, we are dedicated to researching meteorological phenomena as we strive for the commercialization and stable operation of HAPS for various applications.

By understanding and addressing the unique meteorological challenges associated with HAPS operations, we aim to establish a reliable and sustainable commercial service that can benefit various industries and society as a whole.

Research Areas