Dark Horse Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dark Horse Observatory clear sky chart from 2005-11-05 to 2009-12-29.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	15% 5% 5% 5% 5% 5% 7% 37%	16461
	Night	17% 6% 5% 5% 7% 38%	16474
Cloud Cover	Day	15% 6% 5% 5% 5% 7% 38%	16471
	Night	17% 5% 5% 5% 7% 39%	16493
Transparency	Day	7% 15% 25% 17%	13175
	Night	6% 15% 25% 18%	13252
Seeing	Day	9% 27% 26% 16%	1036
	Night	14% 31% 28% 9%	4873

By Month

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	14% 7% 5% 6% 5% 5% 7% 36%	1036
	Night	18% 6% 6% 7% 35%	1663
February	Day	14% 7% 5% 5% 6% 42%	1040
	Night	13% 7% 5% 5% 5% 6% 7% 39%	1367
March	Day	15% 6% 5% 6% 6% 7% 38%	1331
	Night	17% 7% 5% 5% 7% 37%	1347
April	Day	15% 6% 6% 5% 6% 38%	1418
	Night	18% 5% 5% 6% 7% 37%	1186
May	Day	14% 5% 5% 7% 5% 5% 8% 36%	1681
	Night	14% 5% 5% 6% 5% 6% 40%	1033
June	Day	17% 5% 6% 6% 7% 9% 30%	1663
	Night	16% 5% 5% 6% 5% 5% 7% 37%	956
July	Day	9% 5% 5% 8% 6% 7% 9% 35%	1702
	Night	9% 5% 7% 6% 8% 11% 36%	988
August	Day	12% 5% 5% 5% 6% 9% 39%	1525
	Night	15% 5% 5% 5% 6% 42%	1188
September	Day	18% 5% 5% 6% 43%	1375
	Night	15% 6% 5% 5% 48%	1228
October	Day	17% 5% 8% 41%	1213
	Night	18% 6% 7% 43%	1481
November	Day	23% 7% 6% 36%	1260
	Night	23% 6% 6% 6% 32%	1911
December	Day	16% 6% 5% 7% 39%	1217
	Night	18% 5% 5% 8% 37%	2126

Percentage Cloud Cover
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month		Occurrence of each Forecast	Data Points
January	Day	15% 7% 5% 5% 5% 5% 7% 36%	1034
	Night	18% 6% 5% 5% 7% 36%	1664
February	Day	14% 7% 5% 5% 42%	1038
	Night	14% 5% 5% 5% 5% 7% 40%	1358
March	Day	16% 6% 6% 5% 5% 6% 38%	1352
	Night	17% 6% 5% 5% 7% 38%	1371
April	Day	16% 6% 6% 5% 6% 38%	1417
	Night	19% 5% 5% 5% 7% 37%	1186
May	Day	14% 5% 5% 6% 5% 5% 9% 36%	1675
	Night	14% 5% 5% 7% 5% 6% 39%	1027
June	Day	16% 5% 5% 7% 6% 6% 9% 31%	1672
	Night	15% 5% 6% 5% 9% 37%	958
July	Day	9% 5% 6% 8% 5% 7% 9% 35%	1702
	Night	9% 5% 5% 6% 6% 8% 10% 37%	989
August	Day	13% 5% 5% 5% 7% 8% 39%	1519
	Night	15% 5% 5% 6% 42%	1190
September	Day	18% 5% 5% 7% 42%	1374
	Night	15% 5% 5% 6% 48%	1229
October	Day	17% 5% 5% 5% 8% 42%	1213
	Night	19% 5% 7% 44%	1481
November	Day	23% 7% 5% 6% 36%	1258
	Night	23% 7% 5% 6% 32%	1910
December	Day	16% 6% 5% 7% 39%	1217
	Night	18% 5% 5% 5% 5% 8% 37%	2130

Transparency (when cloud cover <= 30%)
Color Meanings: Poor Below Average Average Above average Transparent
Month		Occurrence of each Forecast	Data Points
January	Day	6% 18% 30% 13%	774
	Night	6% 17% 32% 14%	1217
February	Day	5% 18% 33% 12%	849
	Night	5% 17% 30% 16%	1089
March	Day	6% 16% 25% 18%	1036
	Night	7% 16% 24% 17%	1060
April	Day	7% 12% 17% 25% 6%	1109
	Night	6% 13% 19% 23% 6%	927
May	Day	8% 12% 24% 19%	1366
	Night	7% 15% 20% 20% 6%	818
June	Day	11% 20% 28% 8%	1256
	Night	10% 19% 29% 13%	722
July	Day	10% 13% 28% 14%	1108
	Night	11% 18% 22% 17%	659
August	Day	8% 11% 23% 20%	1362
	Night	5% 13% 19% 24%	1052
September	Day	5% 11% 23% 25%	1180
	Night	6% 9% 24% 27%	1080
October	Day	12% 26% 19%	1098
	Night	12% 26% 21%	1335
November	Day	5% 13% 29% 10%	1037
	Night	6% 13% 24% 12%	1515
December	Day	5% 24% 23% 13%	1000
	Night	5% 19% 25% 13%	1778

Seeing (when cloud cover <= 80%)
Color Meanings: Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month		Occurrence of each Forecast	Data Points
January	Night	24% 40% 19%	436
February	Day	28% 41% 18%	53
	Night	27% 38% 17%	397
March	Day	21% 32% 25% 7%	113
	Night	23% 37% 21%	459
April	Day	14% 42% 12% 11%	113
	Night	16% 36% 24% 8%	430
May	Day	9% 29% 22% 18%	117
	Night	6% 32% 38% 8%	281
June	Day	31% 33% 17%	104
	Night	6% 29% 45% 11%	214
July	Day	22% 39% 25%	104
	Night	25% 44% 19%	228
August	Day	15% 30% 18% 16%	116
	Night	17% 39% 21%	421
September	Day	16% 21% 28% 8%	108
	Night	21% 35% 22%	445
October	Day	9% 18% 31% 16%	112
	Night	9% 27% 30% 12%	461
November	Day	9% 31% 23% 9%	96
	Night	15% 28% 25% 7%	528
December	Night	18% 38% 19%	573

Page generated at 2009-12-30 05:20:30. Version ab11