Originally Posted by Deadshade
Reply With QuoteStep 6, precipitations:
General explanations:
What cause rain? : When the air gets colder. Colder air masses have a smaller moisture retention capacity than the hotter ones, and it fall to the ground.
· Rising air in low pressure areas: the Intertropical convergence zone (ITCZ) and the polar front.
· Polar front, when hot air masses encounter cold air masses. This is called a cold front.
· Orographic lift: Air rising above the mountains. Side facing the dominant winds is rainy.
General tips
· Precipitation tends to decrease when moving away from the source of water.
· In summer, some islands and peninsulas are drier than inland locations but wetter in winter. Ex: Nova Scotia.
· If the winds are blowing directly from sea to land, there are more precipitations.
· Inland precipitation follows the directions of the winds. See the Asian monsoon in Mongolia.
· Precipitation categories spread a lot over flatlands.
· Size and shape of wet systems are influenced by dry systems. It bends and is push it away. Ex: Africa ITCZ.
· Higher latitudes tend to be drier than those close to the equator.
Transition map, it's not mandatory but it's meant to help figuring out where it rains.
Temporary green/wet and white/dry map
I recommend making maps like the ones below. For the sake of simplification, we can classify the different areas on the planet, according to whether it’s a dry air mass or a wet one mostly by using just the air pressure and wind directions. Here are the different zones. Green is for rainy areas, white is dry.
January and July
explanation pressure winds and rain Jan.png explanation pressure winds and rain July.png
1,2,3,4 are roughly the same thing. They are dry.
5 and 6 are pretty similar but have a different cause. They are wet.
7 It’s just to represent the fact that the cold polar air is drier but not completely dry either.
1· High pressure systems (H, for short) are dry and the areas receiving dry air are dry as well.
2· The equatorward east side of the oversea H is usually really dry.
3· Equatorward west of H is dry
4· Equatorward west side tend to be dry in winter. Dry but can have some rain at right angle from the sea. It never rain with cold currents. Orographic rain possible. In summer, they make the weather drier as in Brazil.
5· The west and poleward sides of H are wet. This is where the cold air from the poles and the hot air from the tropics collide. Strom formations are more common. They don’t travel as far west in winter as in summer because the continents have a higher air pressure.
6· Low pressure systems (L) are wet if there is water available and if the winds are not blocked by mountains. Inland winds are dry even if they are located in a low pressure area as it’s the case in North Africa in January.
7· In summer especially, the areas receiving the cold air from the pole are drier.
Mapping the precipitation using the 6 categories
Now, time to map the precipitation. I would recommend using the color scheme provided to paint it if you intend to use the script later.
There are 6 categories for precipitations, from 0 to 5:
Category 0: The first one is pink/magenta and is actually a penalty I added later to get a more precise results. It can have a huge impact on colder areas.
Category 1: The second category is transparent.
The other colors are as indicated in the document, 6 (whitish blue) being the wettest.
color rain.png
''Painting''
1· ITCZ (number 6 on the white/green map). It can cover rather large area. Cover the whole area until stopped by mountains.
It has the highest precipitations (5), the size depend on the winds and the moisture they carry. More moisture=larger wet area.
For example, Africa ITCZ is smaller than in South America presumably, because it receives half of its winds from the dry Sahara.
Ideally the ITCZ is centered on the center of the low pressure, following the direction of the winds
Information: the Amazonian forest generates 50-80% of its own rain with its own transpiration. Less trees means less precipitation.
January and July
jan itcz.png jul itcz.png
2· High latitudes dominant Westerlies: located mostly on west coasts. Number 6
Winter: Poleward of 30
Category 4 is on coasts at right angle mostly, on peninsulas and small islands.
Category 3 covers a distance of 10 to 15 degrees inland from the source of water. (1 degree is about 111,11km at the equator)
Category 2 is covering a large area 2000 or 3000 km from the sea.Summer: Poleward of 40-45
Category 4 is at right angle with the sea.
Category 3 is almost the default at mid latitudes as far as 2000-2500 km inland.
Category 2 is near the poles instead of 3.
Outside the poles, Category 2 is only a transition to the dry areas.
January and July
jan westerlies.png jul westerlies.png
3· Extratropical storm path: Located in mid to high latitudes, west of oceanic high pressure centers. Number 5
Explanation: The High over the water pushes hot air toward the land and toward the pole, clashing with the colder air coming from the pole. The clash is a cold front, a rapid cooling of the air, generating the precipitations. Area is smaller in winter due to the H overland.
When the Westerlies become dominant, this influence gets weaker.Winter: poleward of 20-25
Category 4 is on coasts if direct winds, can go 10 degrees inland near tropics, otherwise it’s marginal.
Peninsulas and direct onshore winds are wetter are also category 4.
Category 3 covers a distance of 10 to 15 degrees inland from the source of water.
Category 2 is a much shorter transition: 5 degrees or less.Summer: poleward of 25 more or less
Category 4 can go10-15 degrees inland but the distance gets considerable shorter by getting close to the polar circle (60-66).
Past 40-45 of latitude: Category 4 becomes less common on coast but still possible inland
Category 3 is the default category at mid latitudes.
Category 2 is near the poles instead of 3.
Outside the poles, Category 2 is only a transition to the dry areas.
January and July
jan east coast storm.png jul east coast storm.png
4· Winter Monsoon: near the sea in winter, on the east coasts. Represented by 1,3,4
Need winds from ocean, strength of precipitation depend on the angle between the sea and the land . High at right angle but low or null if parallel (see graphic).
Precipitation decrease quickly when moving away from the coast.
Edit About China: there is a semi permanent low pressure system over the southern part of the country in winter possibly similar to what happens over North America = instability. It would be more appropriate in the category above.
January and july
jan winter monsoon.png jul winter monsoon.png
Poles summer: hotter water means more precipitations, with the eastern side usually drier than the west because of the dominant wind direction and warmer waters.
Orographic lift effect
Precipitations fall because the cooling of the air saturates it with water. The water only falls if the air is saturated.
Notes
· Precipitations are centred on the flanks not on the ridges. (Actually, in Iraq, it's centered on the plains)
· Large areas have a larger effect and a rapid increase in elevation has also a bigger effect.
· High pressure zones with cold currents do not generate an Orographic lift: example Atacama Desert.
-Polar winds do not have orographic lift either.
Tip: when painting the precipitation related to the orographic lift, I've found it was easy to do it as follow. Select one level of precipitation and look at the elevation graphic to see at which altitude the precipitations should increase and use the paint tool on that color. This way, you will cover all the map in 1 click. Do this for the different altitude level if necessary.
Color altitudes for reference (see graphic)
Green: 0-1 km (the different colors don't have an impact below 1000, they could but it's just too complicated)
color altitude.png
Explanation: Take each category at a time and increasing the precipitation at these places only if the altitude is high enough.
When the default precipitation category at sea level is x (0-1 km), increase the precipitation.
Every ridge provoking an Orographic rain effect lowers the % moisture and makes the remaining air drier. It also means that in order to re-saturate the air of water, colder temperatures are needed.
I'm not sure what the mountain on the second line is for. Ignore it. I will try to find what it means.
altitude rain.png
Magenta, category 0
· The last part is to add the magenta penalty for the really dry places. This category is useful for moderate/cold areas that have low temperature but also low precipitations.
Without this some steppes appeared as lush forests.
· Should be put only where no other colors are present.
· Less likely at higher altitudes.
· Spread out on plains but close with rugged terrain.
Final results:
January
jan precipitation final.png
July
jul precipitation final.png
