CSI Astrophysical Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CSI Astrophysical Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	126471
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	122924
Transparency	Day	42% 4 11% 23% 17% 3	129998
	Night	42% 3 10% 23% 18% 3	127484
Seeing	Day	19% 8% 26% 26% 16% 5%	16198
	Night	20% 9% 29% 29% 11% 1	25312

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 6% 2 4 3 3 3 3 6% 5% 51%	3597
	11th to 20th	Day	14% 8% 3 6% 4 4 5% 3 5% 6% 43%	3570
	21st to end	Day	18% 10% 4 7% 4 4 3 3 3 4 40%	4347
February	1st to 10th	Day	16% 8% 2 7% 4 4 4 4 5% 6% 40%	1810
	11th to 20th	Day	15% 8% 2 6% 5% 4 4 2 5% 5% 43%	1948
	21st to end	Day	18% 8% 2 6% 5% 4 4 4 5% 6% 38%	1650
March	1st to 10th	Day	22% 6% 2 4 3 3 3 3 4 6% 44%	4476
	11th to 20th	Day	12% 8% 3 6% 4 4 4 4 6% 7% 42%	4791
	21st to end	Day	17% 6% 2 6% 4 3 4 3 4 6% 43%	5918
April	1st to 10th	Day	18% 9% 2 6% 4 4 4 4 5% 6% 38%	2400
	11th to 20th	Day	17% 6% 2 6% 4 4 4 4 5% 8% 40%	2374
	21st to end	Day	18% 5% 2 6% 4 4 5% 4 6% 7% 40%	2410
May	1st to 10th	Day	27% 8% 2 6% 4 4 4 4 5% 8% 28%	5886
	11th to 20th	Day	24% 9% 3 6% 4 4 3 3 5% 6% 35%	6267
	21st to end	Day	20% 6% 2 6% 4 5% 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	15% 6% 2 5% 5% 4 6% 5% 7% 9% 37%	2800
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 4 6% 8% 38%	2800
	21st to end	Day	10% 6% 2 7% 5% 4 5% 4 7% 10% 40%	2800
July	1st to 10th	Day	9% 6% 3 7% 6% 5% 6% 4 7% 11% 37%	6360
	11th to 20th	Day	10% 7% 2 7% 5% 6% 6% 5% 8% 11% 33%	6360
	21st to end	Day	11% 7% 2 6% 5% 5% 6% 4 6% 9% 38%	6926
August	1st to 10th	Day	18% 6% 3 7% 5% 5% 5% 4 7% 9% 32%	4260
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 6% 8% 43%	3850
	21st to end	Day	9% 5% 2 6% 4 4 4 4 6% 9% 46%	4312
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 4 2 5% 3 3 4 3 5% 8% 51%	2316
	21st to end	Day	18% 7% 2 7% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Day	19% 6% 2 5% 4 3 4 3 5% 6% 43%	2014
	11th to 20th	Day	15% 5% 2 5% 3 3 4 4 5% 6% 49%	1900
	21st to end	Day	19% 5% 2 5% 3 4 4 3 4 6% 45%	1948
November	1st to 10th	Day	15% 5% 2 5% 4 3 2 3 4 5% 53%	1761
	11th to 20th	Day	17% 5% 2 5% 3 3 4 3 5% 5% 49%	1800
	21st to end	Day	16% 7% 2 4 4 3 4 3 6% 7% 45%	1725
December	1st to 10th	Day	15% 7% 2 5% 4 3 3 3 6% 7% 45%	2600
	11th to 20th	Day	23% 7% 2 3 3 2 3 2 4 5% 45%	2986
	21st to end	Day	17% 8% 2 6% 4 4 4 4 5% 7% 39%	3960
January	1st to 10th	Night	13% 6% 2 5% 4 3 3 3 4 6% 50%	6349
	11th to 20th	Night	14% 9% 2 5% 4 3 3 3 4 5% 47%	6315
	21st to end	Night	18% 10% 2 6% 3 3 4 3 5% 6% 40%	6605
February	1st to 10th	Night	17% 9% 3 6% 4 4 4 3 4 6% 41%	2588
	11th to 20th	Night	18% 9% 2 6% 4 4 3 3 5% 5% 42%	2338
	21st to end	Night	19% 8% 2 5% 3 3 3 3 5% 6% 42%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 2 4 5% 43%	5364
	11th to 20th	Night	14% 10% 2 6% 3 3 4 3 4 6% 46%	5165
	21st to end	Night	18% 8% 3 6% 4 4 3 3 5% 5% 40%	5013
April	1st to 10th	Night	19% 7% 3 6% 5% 5% 4 4 5% 6% 38%	2000
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	1975
	21st to end	Night	18% 7% 2 5% 4 3 4 3 4 7% 43%	1988
May	1st to 10th	Night	26% 9% 3 7% 4 4 4 3 4 7% 28%	4054
	11th to 20th	Night	23% 8% 3 6% 4 4 3 3 5% 6% 35%	3668
	21st to end	Night	22% 8% 2 5% 3 4 4 3 5% 8% 34%	3926
June	1st to 10th	Night	16% 7% 2 5% 4 4 4 4 6% 8% 40%	1600
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 7% 9% 39%	1600
	21st to end	Night	10% 5% 2 7% 5% 5% 4 3 7% 11% 42%	1600
July	1st to 10th	Night	9% 6% 3 9% 5% 6% 6% 5% 6% 10% 37%	3600
	11th to 20th	Night	13% 10% 3 7% 5% 5% 5% 4 7% 8% 33%	3600
	21st to end	Night	13% 6% 2 6% 4 4 4 4 6% 8% 42%	3926
August	1st to 10th	Night	22% 6% 2 6% 4 4 5% 4 7% 7% 33%	2852
	11th to 20th	Night	14% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	3197
	21st to end	Night	9% 5% 2 4 3 4 4 4 5% 7% 52%	3586
September	1st to 10th	Night	13% 6% 2 7% 4 4 4 4 5% 5% 44%	2000
	11th to 20th	Night	13% 5% 2 5% 3 3 4 2 6% 8% 51%	2082
	21st to end	Night	18% 7% 3 6% 4 3 3 2 4 7% 43%	2200
October	1st to 10th	Night	18% 6% 2 5% 4 3 4 3 5% 7% 43%	2230
	11th to 20th	Night	14% 4 2 5% 3 3 4 3 4 6% 51%	2280
	21st to end	Night	20% 7% 2 5% 3 2 3 3 4 5% 45%	2649
November	1st to 10th	Night	12% 5% 2 5% 4 4 4 3 4 6% 53%	2537
	11th to 20th	Night	17% 6% 3 5% 4 3 3 2 5% 5% 48%	2600
	21st to end	Night	15% 6% 2 5% 4 4 3 3 5% 7% 46%	2673
December	1st to 10th	Night	15% 8% 2 6% 3 3 3 3 5% 5% 48%	4580
	11th to 20th	Night	25% 7% 2 4 2 3 3 2 4 5% 43%	5208
	21st to end	Night	20% 9% 2 6% 3 4 3 3 4 5% 40%	6996

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	34% 3 7% 32% 22% 3	3598
	11th to 20th	Day	43% 2 8% 23% 17% 6%	3600
	21st to end	Day	50% 2 9% 22% 16% 1	4347
February	1st to 10th	Day	45% 3 13% 24% 14% 2	1810
	11th to 20th	Day	45% 2 8% 26% 17% 3	1957
	21st to end	Day	47% 2 9% 24% 15% 4	1650
March	1st to 10th	Day	42% 2 8% 23% 16% 8%	4479
	11th to 20th	Day	41% 3 10% 27% 15% 4	4791
	21st to end	Day	44% 2 9% 15% 25% 5%	5918
April	1st to 10th	Day	46% 3 9% 21% 17% 5%	2400
	11th to 20th	Day	42% 3 11% 23% 18% 3	2374
	21st to end	Day	42% 3 9% 22% 19% 4	2410
May	1st to 10th	Day	55% 4 14% 16% 10% 1	5897
	11th to 20th	Day	51% 2 9% 23% 14% 1	6271
	21st to end	Day	47% 4 9% 24% 16% 1	6949
June	1st to 10th	Day	41% 4 13% 24% 16% 2	2800
	11th to 20th	Day	43% 4 14% 25% 12% 2	2800
	21st to end	Day	38% 5% 13% 26% 17% 1	2800
July	1st to 10th	Day	41% 8% 13% 22% 16% 1	6290
	11th to 20th	Day	42% 7% 18% 21% 11% 1	6285
	21st to end	Day	40% 6% 10% 21% 22% 1	6728
August	1st to 10th	Day	46% 6% 13% 24% 10% 1	4246
	11th to 20th	Day	38% 4 13% 30% 14% 1	3886
	21st to end	Day	33% 4 11% 28% 22% 2	4312
September	1st to 10th	Day	38% 4 11% 24% 21% 3	2400
	11th to 20th	Day	33% 3 10% 27% 25% 2	2316
	21st to end	Day	46% 3 10% 19% 19% 3	2200
October	1st to 10th	Day	36% 3 11% 22% 23% 6%	3326
	11th to 20th	Day	25% 2 10% 25% 32% 5%	3140
	21st to end	Day	28% 2 13% 26% 27% 4	3186
November	1st to 10th	Day	36% 2 12% 25% 23% 3	1761
	11th to 20th	Day	38% 3 8% 25% 23% 3	1800
	21st to end	Day	39% 2 13% 31% 13% 2	1725
December	1st to 10th	Day	39% 2 15% 28% 15% 2	2600
	11th to 20th	Day	43% 2 12% 26% 15% 3	2986
	21st to end	Day	46% 3 8% 24% 16% 3	3960
January	1st to 10th	Night	36% 2 7% 27% 23% 4	6349
	11th to 20th	Night	40% 2 11% 23% 17% 7%	6360
	21st to end	Night	47% 2 8% 22% 17% 4	6605
February	1st to 10th	Night	45% 3 12% 25% 13% 2	2588
	11th to 20th	Night	45% 3 8% 21% 19% 5%	2340
	21st to end	Night	44% 2 6% 25% 19% 4	1980
March	1st to 10th	Night	44% 2 8% 24% 13% 9%	5374
	11th to 20th	Night	41% 2 13% 25% 15% 4	5166
	21st to end	Night	47% 3 5% 16% 23% 6%	5013
April	1st to 10th	Night	48% 3 8% 19% 17% 5%	2000
	11th to 20th	Night	45% 3 9% 20% 21% 2	1975
	21st to end	Night	42% 3 9% 22% 20% 5%	1988
May	1st to 10th	Night	58% 3 12% 15% 11% 2	4060
	11th to 20th	Night	52% 4 12% 16% 15% 2	3669
	21st to end	Night	48% 3 12% 22% 12% 2	3927
June	1st to 10th	Night	41% 5% 10% 22% 21% 1	1600
	11th to 20th	Night	41% 5% 14% 27% 13% 1	1600
	21st to end	Night	38% 5% 14% 25% 16% 2	1600
July	1st to 10th	Night	42% 7% 14% 21% 14% 2	3560
	11th to 20th	Night	46% 7% 15% 21% 11% 1	3552
	21st to end	Night	38% 5% 13% 23% 19% 1	3809
August	1st to 10th	Night	50% 5% 16% 18% 10% 1	2842
	11th to 20th	Night	42% 5% 10% 30% 11% 1	3227
	21st to end	Night	31% 4 10% 32% 21% 2	3586
September	1st to 10th	Night	40% 3 10% 25% 20% 2	2000
	11th to 20th	Night	33% 3 10% 25% 26% 3	2082
	21st to end	Night	44% 3 7% 23% 21% 2	2200
October	1st to 10th	Night	38% 4 10% 24% 22% 2	3698
	11th to 20th	Night	25% 2 9% 28% 31% 4	3779
	21st to end	Night	30% 3 10% 31% 24% 2	4361
November	1st to 10th	Night	35% 2 10% 25% 23% 4	2537
	11th to 20th	Night	41% 2 7% 25% 20% 5%	2600
	21st to end	Night	39% 2 13% 28% 16% 1	2673
December	1st to 10th	Night	40% 2 12% 31% 13% 2	4580
	11th to 20th	Night	45% 2 10% 24% 16% 4	5208
	21st to end	Night	48% 2 9% 20% 18% 4	6996

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	20% 15% 38% 21% 6% 2	200
	11th to 20th	Day	18% 26% 39% 14% 4	200
	21st to end	Day	23% 12% 37% 19% 8% 1	304
February	1st to 10th	Day	23% 12% 36% 23% 4 1	310
	11th to 20th	Day	25% 17% 36% 21% 2	496
	21st to end	Day	25% 14% 34% 22% 4 1	414
March	1st to 10th	Day	19% 17% 36% 23% 6%	500
	11th to 20th	Day	17% 16% 38% 21% 7% 1	500
	21st to end	Day	24% 11% 34% 22% 7% 1	550
April	1st to 10th	Day	26% 11% 34% 20% 7% 1	500
	11th to 20th	Day	21% 12% 33% 22% 10% 3	492
	21st to end	Day	22% 13% 29% 26% 9% 2	500
May	1st to 10th	Day	28% 8% 29% 22% 9% 4	503
	11th to 20th	Day	23% 5% 29% 27% 14% 3	527
	21st to end	Day	21% 4 18% 26% 24% 7%	573
June	1st to 10th	Day	16% 5% 22% 31% 20% 6%	530
	11th to 20th	Day	16% 4 23% 33% 19% 5%	530
	21st to end	Day	15% 2 19% 35% 24% 6%	530
July	1st to 10th	Day	13% 2 14% 33% 27% 11%	525
	11th to 20th	Day	10% 2 13% 33% 29% 12%	524
	21st to end	Day	14% 2 17% 34% 28% 6%	576
August	1st to 10th	Day	12% 2 15% 33% 28% 10%	526
	11th to 20th	Day	13% 2 13% 33% 30% 9%	519
	21st to end	Day	14% 2 18% 32% 23% 11%	583
September	1st to 10th	Day	16% 3 23% 27% 19% 12%	530
	11th to 20th	Day	16% 2 18% 32% 20% 11%	530
	21st to end	Day	24% 4 22% 28% 17% 5%	530
October	1st to 10th	Day	25% 5% 20% 23% 20% 8%	502
	11th to 20th	Day	17% 7% 29% 24% 20% 3	490
	21st to end	Day	23% 7% 30% 25% 13% 3	464
November	1st to 10th	Day	15% 8% 31% 26% 16% 4	395
	11th to 20th	Day	18% 14% 34% 24% 11%	400
	21st to end	Day	19% 12% 36% 22% 10% 1	325
December	1st to 10th	Day	21% 13% 37% 20% 8% 2	200
	11th to 20th	Day	26% 15% 32% 20% 8%	200
	21st to end	Day	23% 10% 38% 19% 11%	220
January	1st to 10th	Night	18% 18% 39% 21% 3	800
	11th to 20th	Night	24% 15% 34% 23% 4	800
	21st to end	Night	19% 16% 40% 23% 3	880
February	1st to 10th	Night	24% 12% 38% 24% 2	800
	11th to 20th	Night	24% 16% 40% 19% 1	783
	21st to end	Night	23% 16% 33% 25% 3	660
March	1st to 10th	Night	21% 15% 35% 25% 5%	800
	11th to 20th	Night	21% 12% 38% 23% 5% 1	800
	21st to end	Night	21% 11% 38% 24% 6%	880
April	1st to 10th	Night	23% 12% 35% 24% 6%	800
	11th to 20th	Night	22% 7% 35% 29% 6%	791
	21st to end	Night	20% 8% 33% 31% 7% 1	788
May	1st to 10th	Night	26% 9% 27% 29% 9% 1	600
	11th to 20th	Night	22% 7% 31% 29% 10% 1	435
	21st to end	Night	21% 3 16% 38% 18% 3	433
June	1st to 10th	Night	19% 3 23% 37% 17% 1	400
	11th to 20th	Night	16% 3 20% 41% 20% 1	400
	21st to end	Night	11% 2 23% 43% 21% 1	400
July	1st to 10th	Night	14% 3 15% 38% 28% 2	390
	11th to 20th	Night	12% 2 15% 41% 26% 3	388
	21st to end	Night	11% 1 16% 48% 22% 1	438
August	1st to 10th	Night	13% 1 13% 44% 25% 3	547
	11th to 20th	Night	13% 3 13% 36% 31% 4	790
	21st to end	Night	16% 2 13% 38% 27% 4	880
September	1st to 10th	Night	16% 3 21% 36% 20% 5%	800
	11th to 20th	Night	16% 3 16% 37% 23% 5%	800
	21st to end	Night	22% 5% 20% 35% 17% 2	800
October	1st to 10th	Night	22% 5% 25% 26% 20% 3	773
	11th to 20th	Night	17% 8% 27% 35% 12% 1	760
	21st to end	Night	23% 6% 32% 27% 11% 1	836
November	1st to 10th	Night	14% 9% 34% 30% 12% 1	780
	11th to 20th	Night	21% 13% 33% 25% 9%	800
	21st to end	Night	18% 15% 35% 25% 7% 1	800
December	1st to 10th	Night	25% 15% 32% 21% 7% 1	800
	11th to 20th	Night	22% 12% 38% 24% 3	800
	21st to end	Night	24% 11% 36% 26% 3	880

Page generated at 2025-10-01 01:39:01. Version ac07

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