A Brief Preview of KWP
Exploring the weather and climate of Kerbin through interactive gameplay.
Riding the Mid-latitude Jet Stream
Jet Stream Flight
The upper air chart below highlights the approximate location of the flights depicted in the images below. A fixed-wing aircraft, with a rear-mounted turbojet, was flown from the KSC toward the northeast until the jet-stream was encountered around 20-30°N and 55-65°W.
When flying into the wind the ground speed was much slower than the true airspeed. When flying parallel to the surface and directly into the wind the difference in ground speed and airspeed is the wind speed. In this example, the highest speed attained flying upwind was just below Mach 1.While increased airflow over the wings resulted in more lift, it also produced more drag preventing the thrust of the engine from overcoming the sound barrier.
Flying upwind (into the wind)
Turning the plane around 180 degrees reduced drag and true airspeed, as now the aircraft flew with the wind. This reduction in drag allowed the engine to push through the sound barrier and reach a speed of Mach 1.08 (note the aerodynamic effects visible in the image above). In contrast to the upwind flight, the ground speed is now much greater than the true airspeed. This is because the aircraft is flying in an atmosphere that is itself moving with respect to the ground. In this example, the aircraft is flying parallel to the surface with a tailwind and no crosswind. This simplifies the calculation of the ground speed which, in this case, is reduced to the sum of the true airspeed and the wind speed.
Flying downwind (with the wind)
No Weather (KWP Toggled Off)
In the stock atmosphere, this aircraft is able to pass through the transonic region and reach speeds well over Mach 1. This difference in performance is due to the difference in pressure profiles between the stock atmosphere and KWP. In the stock atmosphere, the scale height is around ~5.6 km. In KWP, the scale height is similar to Earth's ~8.5 km. Consequently, pressure decreases with height more slowly with height in KWP. In both the upwind and downwind examples, the pressure at 10-km is around 260 hPa and the air density is ~0.4 kg/m. In the stock atmosphere, the pressure at 10-km is only 180 hPa and the air density is merely 0.28 kg/m. This difference in atmosphere structure makes it more difficult to break the sound barrier and produce single stage to orbit (SSTO) space planes when using KWP. If this challenge is overly burdensome KWP can be turned off by clicking the Toggle KWP button in the GUI.
Stock Atmosphere (KWP Off)
Launch and Re-entry in the Tropics
A simple three-stage orbital rocket, carrying Jebediah Kerman, launches on a warm humid day from the KSC. For this launch atmospheric data were retrieved from MPAS point weather data, as opposed to MPAS climatology data. As the rocket initiates a gravity turn to the east it encounters westerly winds of ~10.5 m/s. Since the rocket is traveling vertically as well as horizontally the crosswind component is significant. In this case, the crosswind represents the component of the wind perpendicular to the rocket to the vessel's flight path. Since the wind is mostly horizontal (vertical wind is in mm/s) and the rocket's flight path is mostly vertical the crosswind component is large. Nevertheless, the rocket also experiences a tailwind since a component of its velocity is eastward which is downwind.
SRB recovery using the FMRS mod. With KWP enabled the falling SRB drifts with the wind toward the south-southeast. The effect of the wind on the flight path of the SRB is visible in the navball, which shows the retrograde marker pointing in the direction the wind is coming from (NNW). When the KWP is toggled off and the game defaults to the stock atmosphere the parachute straightens out and the retrograde marker returns to the vertical. With no wind, the SRB falls straight down.
Stock Atmosphere (KWP Off)
KWP Enabled (Weather)
Atmospheric Interface (Re-entry)
At 65-km atmospheric pressure is around 0.01 hPa (in stock) and 0.15 hPa (in KWP). This difference in atmospheric pressure in the mesosphere results in higher dynamic pressure and an earlier onset of visual (thermodynamic) effects. While this effect appears dramatic the difference in external (shock) heating is not substantial. In KWP the ambient temperature at 65-km is around 22 K warmer than in stock. This difference in ambient temperature accounts for the difference in external (shock) temperature.
KWP Enabled (Weather)
Stratospheric Easterlies (Re-entry)
Depending on the phase of the quasi-biennial oscillation winds in the tropical stratosphere may be easterly or westerly. In this case, stratospheric winds are easterly (out of the east). As the capsule pierces through the lower stratosphere it travels eastward into the wind. With KWP enabled these headwinds to contribute to an increase in atmospheric drag and true airspeed. Around 34-km ASL, the external (shock) temperature and ambient temperature is colder in KWP than in stock. At this altitude, the atmospheric pressure in KWP (~6.17 hPa) is triple that of the stock atmosphere (~2.04 hPa). This difference in pressure and atmospheric density results in increased dynamic pressure and drag.
KWP Enabled (Weather)
The snapshots above capture Bill Kerman descending in a command pod through 1-km ASL. With KWP enabled the parachute bends with the wind experiencing lift and causing the capsule to drift toward the west-northwest (WNW). The drifting of the falling capsule is captured by the navball which shows the retrograde marker off-vertical and aligned with the direction the wind is coming from (ESE). In the stock atmosphere, the command pod falls straight down (retrograde marker remains vertical). In the absence of wind the command pod and parachute experience no lift.
KWP Enabled (Weather)
Polar Orbiter: Climate and Weather Monitoring
A three-stage orbital rocket prepared to launch from the KSC. For this launch atmospheric data were retrieved from the MPAS climatology. The uncrewed rocket carried a satellite to be deployed in a polar orbit for weather and climate monitoring. On the day of launch, onshore winds were relatively light (< 5 m/s) and out of the northeast with temperatures at a balmy 27°C (81°F).
At ignition, four solid rocket boosters roared to life, and the craft lifted off the pad. In less than a minute, the rocket reached 5600 m ASL (~500 hPa). As sea level pressure is around 1000 hPa the 500 hPa level effectively divides the atmosphere in half, with half the mass of the atmosphere lying above and below this level. Thus, by the time the rocket reached 500 hPa it had already passed through half of the atmosphere (by mass).
Just before reaching 500 hPa, the fuel supply of the solid rocket boosters was exhausted. The boosters decoupled and fell back to Kerbin, littering the area around the KSC with rocket debris.
As the second stage engine ignited the rocket initiated a gravity turn to the north-northwest. The westward component of this trajectory accounted for the rotation of the planet below, reducing the fuel necessary to inject into polar orbit. Around 20-km ASL the rocket pierced through the tropical tropopause where temperatures reached a minimum of ~200 K (-73°C or -100°F).
In space (>70 km ASL), the second stage was decoupled and the rocket's fairing was deployed. The third and final stage engine ignited sending the payload satellite into a polar orbit with an apoapsis and periapsis of around 250-km ASL.
Once in a stable orbit sensor packages were deployed. Data from KWP allowed climate monitoring of Kerbin. Below, the satellite passes over the Kerbal Space Center (KSC), which is located on a square patch of light green land along the southeastern coast of the "Africa" like continent below. At the KSC the climatological conditions, at the time of day of the satellite's passing, were light winds. Low values of OLR (outgoing longwave radiation), at tropical latitudes, suggest clouds above the KSC are typically high level (e.g. cirrus). With an average precipitation of 1.1 mm/hr, precipitation at the KSC, when it occurs, is typically light to moderate in intensity. Nevertheless, the average precipitable water is around 19.5 mm, suggesting the lower atmosphere is ripe with moisture.
Passing over the desert biome, the satellite captures climatological conditions near the DLS. At the Desert Launch Site (DLS) precipitation is rare as the average precipitation rate is less than 0.1 mm/hr. While air temperatures are cooler at the DLS than at the KSC, the DLS is located at a higher elevation. Winds are stronger and the air is drier at the DLS, relative to the KSC. Low humidity is not confined to the surface layer. The amount of precipitable water in the atmospheric column above the DLS is less than half that observed at the KSC. Unsurprisingly, cloud cover at the DLS is frequently sparse.
Here the polar orbiter passes over the grasslands home to the Woomerang Launch Site (WLS). In this region, the climate is cold, windy, and wet. Average surface temperatures hover around 249 K (-24°C or -11°F). Winds near the WLS are typically quite strong, averaging 7.4 m/s (26 km/h, 17 mph). With an average liquid precipitation rate of 0.8 mm/hr and temperatures well below freezing, light to moderate snowfall is not uncommon at the WLS. In comparison to the DLS and KSC, mean sea level pressure is notably lower. Since the WLS is located in the mid-latitude storm tracks low-pressure systems frequently pass through the region, dropping the average sea level pressure.
Below, the satellite approaches the north pole, passing above the northern ice shelf. This polar desert is characterized by extreme cold and little to no precipitation, with an average surface temperature of 205 K (-68°C or -91°F) and an average precipitation rate of less than 0.1 mm/hr. Frequent cloud cover coupled with windy surface conditions makes this a challenging environment for any kerbal, especially pilots.
East of the KSC continent, the satellite orbits the open ocean and adjacent coastline. This region is notable due to the significant temperature contrast between the mountainous terrain along the coast and the warm ocean waters off the coast. Over the ocean, KWP reports sea surface temperature instead of surface air temperature. At this latitude, sea surface temperatures are relatively warm ~285 K (12°C or 53°F), compared to land surface temperatures which, along the mountains coast, dip as low as 246 K (-27°C or -17°F). The large gap in temperature between the land and ocean is paralleled by a large gap in moisture content. Over the ocean, the precipitable water is five times that over land, highlighting how the moisture capacity of the air increases nonlinearly with increasing temperature. Together, the difference in temperature and moisture content accounts for the difference in precipitation rate over land and the ocean. With respect to wind, average surface wind speeds are higher over the ocean than over land, where friction with the land surface slows the wind.