User:PiRK/Tag:natural=cloud
 natural = cloud
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| Description |
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| Used to mark airborne water in the form of liquid droplets or frozen crystals when dense engouh to hide the sky (or hide the ground, when looked at from above) |
| Group: natural |
| Used on these elements |
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| Status: undefined |
| Tools for this tag |
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Used to tag an area of clouds. Clouds are very prominent landmarks which can obscure the sky for people living underneath them. They also cause a loss of precision in the mapping of the area they cover, because they hide the surface of the earth on aerial imagery.
The type of clouds can be specified with cloud_type=*.
How to use
This tag can be placed on an area (i.e. a closed way) for a simple cloud, or it can be placed on a Relation:multipolygon if the cloud either contains holes or has a larger perimeter.
Simple cloud
- Draw or select a closed way around the perimeter of the cloud. It doesn't matter which way around the way goes.
- Tag the way it with natural=cloud and maybe a cloud_type=*.
Creating holes in clouds
To create open sky holes in a cloud, the cloud should be a Relation:multipolygon.
First create the cloud:
- Draw or select the way around the perimeter of the cloud.
- Create or select a Relation to serve as the cloud.
- Tag the relation with type=multipolygon, natural=cloud, cloud_type=*, etc. Redundant tags should be removed from the way.
- Add the way to the relation and set the role to outer.
Then carve a hole in the cloud by adding an inner ring:
- Draw or select a way around the perimeter of the hole.
- Tag the hole as appropriate with natural=*, landuse=*, etc. If natural=* and landuse=* are missing, the hole will assume the natural-type (e.g. wood) of the land surrounding the cloud.
- Add the hole to the cloud relation with the role of inner.
Creating cloud in an open-sky hole
Related tags
- natural=wind is used for winds and breezes.
- rainy=yes/no - is used to indicate if the cloud can cause rainfalls.
- cloud_type=* - a secondary tag for specifying what kind of cloud this is. e.g. cloud=cumulonimbus.
- static=yes/no - default value is yes. Set static=no to indicate that the cloud is moving and it's location should be updated the next time the aerial imagery is updated.
- intermittent=yes - is used to indicate that the cloud has been know to disappear and reappear later.
- size=little, appearance=fluffy - are used for what the skies were like when you were young and lived in Arizona. You don't see that any more. You might still see them in the desert.
- cloud_morph=* - Used for tagging clouds shaped like bunnies, Santa Claus, etc. Use semicolons to separate values if there is any controversy (e.g. "bunny; dog; Hello Kitty").
- observation_date=* - As of when?
Data sources
Satellite imagery
Mapping clouds by tracing from satellite imagery can help establishing good basic coverage for an area; however keep in mind:
- Imagery is often acquired at an angle and not straight down. Depending on altitude, clouds will be affected by this even more than tall buildings or hilly areas, and may be depicted several hundred meters from their actual, current position. Please remember to compensate for imagery offset in your editor of choice before tracing, and do not move clouds mapped by others around if they appear in a different location for you.
- Imagery is usually selected to show as much cloud-free landscape as possible, so mapping from such imagery will be biased against clouds and might not be representative of the actual weather conditions in the area. Adding more clouds from local knowledge is encouraged, even if this data will likely have less spatial precision.
Surveying
Land cover permitting, you could walk/cycle/drive/ride around the edge of the cloud's shadow, and then draw the cloud based on your GPS tracklogs, with a constant offset depending on cloud altitude and the sun's position at the time of survey. In some regions, clouds may even touch the ground, which makes them easier to survey; but do not confuse them with Tag:natural=fog.
Other data sources
A commercial Cloud Atlas consisting of a lot of high-resolution imagery is available, but it is a copyrighted dataset and not licensed for use in OSM. Preliminary investigation also indicates that most of the imagery in this product does not even contain actual clouds, and where it does, the capture time spans several centuries and is probably at least partially outdated.