Blue Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Sky Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	18% 7% 2 6% 4 4 4 4 5% 7% 39%	126636
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 39%	123741
Transparency	Day	29% 39% 7% 14% 10% 2	130156
	Night	29% 38% 7% 14% 11% 2	128295
Seeing	Day	23% 15% 32% 20% 7% 2	16324
	Night	22% 12% 30% 26% 9% 1	25152

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	14% 6% 2 7% 5% 5% 5% 4 7% 7% 38%	3597
	11th to 20th	Day	15% 7% 3 6% 4 3 4 3 4 6% 47%	3570
	21st to end	Day	16% 7% 2 5% 4 3 4 3 4 4 47%	3960
February	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 6% 38%	1820
	11th to 20th	Day	14% 7% 2 6% 5% 4 3 5% 6% 7% 41%	1948
	21st to end	Day	15% 11% 3 6% 6% 3 4 3 5% 6% 38%	1650
March	1st to 10th	Day	23% 10% 2 5% 4 5% 5% 3 5% 5% 33%	4476
	11th to 20th	Day	18% 10% 3 5% 4 4 4 3 5% 6% 39%	4813
	21st to end	Day	28% 8% 2 5% 3 3 4 3 5% 6% 32%	5712
April	1st to 10th	Day	20% 9% 2 7% 5% 4 4 3 4 6% 37%	2400
	11th to 20th	Day	18% 6% 3 6% 5% 4 5% 4 6% 7% 37%	2423
	21st to end	Day	20% 7% 3 7% 5% 4 4 3 4 6% 37%	2600
May	1st to 10th	Day	31% 9% 3 7% 4 4 3 3 5% 6% 24%	5886
	11th to 20th	Day	21% 8% 3 8% 5% 4 4 3 5% 7% 32%	5986
	21st to end	Day	20% 6% 2 7% 4 5% 5% 4 6% 7% 35%	6949
June	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 6% 8% 33%	2800
	11th to 20th	Day	16% 6% 2 5% 4 4 4 4 7% 8% 40%	2800
	21st to end	Day	18% 7% 2 6% 5% 5% 5% 5% 5% 8% 34%	2800
July	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 9% 33%	6360
	11th to 20th	Day	9% 7% 2 6% 4 4 5% 4 7% 10% 40%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 8% 44%	6926
August	1st to 10th	Day	14% 6% 3 6% 5% 5% 6% 5% 6% 8% 37%	4280
	11th to 20th	Day	16% 8% 3 6% 5% 5% 4 4 6% 7% 37%	4175
	21st to end	Day	8% 4 2 4 4 4 5% 4 7% 10% 47%	4349
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 7% 7% 48%	2380
	11th to 20th	Day	13% 4 2 5% 4 3 3 2 4 5% 54%	2200
	21st to end	Day	16% 6% 2 6% 4 3 4 3 5% 7% 46%	2200
October	1st to 10th	Day	20% 6% 3 5% 3 3 4 3 5% 5% 43%	2200
	11th to 20th	Day	16% 5% 2 7% 3 4 3 2 5% 7% 45%	1938
	21st to end	Day	22% 5% 2 5% 4 4 3 4 5% 6% 41%	1980
November	1st to 10th	Day	16% 5% 2 6% 3 3 3 3 4 6% 49%	1851
	11th to 20th	Day	18% 6% 2 4 3 3 3 3 4 6% 48%	1863
	21st to end	Day	21% 7% 2 5% 3 3 3 3 4 6% 43%	1800
December	1st to 10th	Day	15% 9% 2 6% 3 3 5% 3 4 7% 45%	2638
	11th to 20th	Day	19% 6% 2 5% 3 3 3 3 5% 6% 47%	2986
	21st to end	Day	22% 9% 2 5% 4 3 3 3 5% 5% 37%	3960
January	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 7% 44%	6349
	11th to 20th	Night	13% 9% 3 9% 5% 4 4 4 5% 6% 38%	6315
	21st to end	Night	15% 9% 3 6% 5% 4 4 4 5% 5% 40%	6996
February	1st to 10th	Night	15% 10% 3 8% 4 4 4 4 5% 5% 38%	2577
	11th to 20th	Night	14% 9% 2 7% 5% 4 4 4 5% 6% 40%	2338
	21st to end	Night	16% 10% 2 5% 5% 4 5% 3 5% 6% 41%	1980
March	1st to 10th	Night	25% 9% 2 5% 3 4 4 3 4 5% 35%	5364
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 3 5% 6% 37%	5145
	21st to end	Night	26% 8% 3 5% 4 3 3 4 6% 6% 32%	5203
April	1st to 10th	Night	20% 8% 2 6% 4 4 5% 3 5% 6% 36%	2000
	11th to 20th	Night	20% 8% 2 6% 3 4 3 4 4 6% 39%	1926
	21st to end	Night	21% 7% 3 6% 4 3 4 3 5% 6% 38%	1800
May	1st to 10th	Night	33% 10% 3 6% 4 4 4 3 4 6% 23%	4054
	11th to 20th	Night	16% 9% 3 8% 5% 6% 5% 4 6% 8% 28%	3950
	21st to end	Night	20% 8% 3 8% 5% 4 4 3 5% 7% 32%	3926
June	1st to 10th	Night	20% 8% 3 6% 5% 5% 4 3 5% 6% 35%	1600
	11th to 20th	Night	19% 6% 2 5% 4 4 5% 4 7% 9% 36%	1600
	21st to end	Night	19% 8% 2 7% 4 4 5% 4 5% 8% 36%	1600
July	1st to 10th	Night	13% 7% 3 6% 5% 5% 5% 4 7% 9% 37%	3600
	11th to 20th	Night	12% 8% 2 6% 5% 5% 5% 4 7% 9% 38%	3600
	21st to end	Night	12% 4 2 6% 4 4 4 4 5% 6% 49%	3926
August	1st to 10th	Night	21% 6% 2 7% 4 5% 5% 3 5% 6% 34%	2835
	11th to 20th	Night	14% 8% 3 8% 4 5% 5% 4 7% 8% 34%	2872
	21st to end	Night	11% 3 1 5% 4 3 4 4 6% 7% 51%	3546
September	1st to 10th	Night	12% 6% 2 5% 4 3 4 3 5% 7% 49%	2019
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 5% 6% 52%	2200
	21st to end	Night	16% 5% 2 5% 4 3 3 3 5% 7% 46%	2200
October	1st to 10th	Night	20% 6% 3 6% 4 4 3 4 4 6% 42%	2200
	11th to 20th	Night	16% 5% 2 6% 3 4 5% 3 5% 7% 44%	2456
	21st to end	Night	22% 6% 2 6% 4 3 3 4 5% 5% 39%	2860
November	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 4 6% 47%	2667
	11th to 20th	Night	17% 7% 2 5% 4 3 4 3 4 6% 46%	2692
	21st to end	Night	17% 7% 3 5% 4 4 3 3 4 7% 44%	2600
December	1st to 10th	Night	16% 10% 2 6% 4 3 4 3 5% 6% 41%	4541
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	19% 10% 3 6% 4 4 4 3 4 5% 39%	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	32% 28% 6% 20% 13% 1	3598
	11th to 20th	Day	28% 28% 7% 20% 14% 3	3600
	21st to end	Day	31% 29% 4 14% 17% 4	3960
February	1st to 10th	Day	18% 62% 5% 8% 6% 2	1820
	11th to 20th	Day	18% 61% 3 7% 9% 2	1957
	21st to end	Day	16% 61% 4 8% 8% 2	1650
March	1st to 10th	Day	41% 29% 5% 8% 9% 8%	4479
	11th to 20th	Day	34% 30% 10% 16% 7% 3	4813
	21st to end	Day	42% 29% 3 10% 11% 5%	5712
April	1st to 10th	Day	21% 61% 2 7% 6% 2	2400
	11th to 20th	Day	17% 63% 4 8% 7% 2	2423
	21st to end	Day	20% 62% 3 7% 8% 1	2600
May	1st to 10th	Day	47% 32% 8% 9% 4	5897
	11th to 20th	Day	40% 31% 11% 13% 5% 1	5990
	21st to end	Day	36% 31% 10% 14% 9%	6949
June	1st to 10th	Day	17% 62% 3 11% 6%	2800
	11th to 20th	Day	15% 62% 5% 9% 8% 1	2800
	21st to end	Day	18% 61% 4 10% 7% 1	2800
July	1st to 10th	Day	37% 28% 9% 15% 10%	6290
	11th to 20th	Day	28% 30% 11% 18% 12% 1	6285
	21st to end	Day	29% 27% 9% 23% 12% 1	6728
August	1st to 10th	Day	30% 38% 9% 16% 6%	4266
	11th to 20th	Day	31% 38% 10% 16% 5%	4214
	21st to end	Day	18% 39% 10% 20% 13% 1	4349
September	1st to 10th	Day	13% 58% 6% 13% 9% 1	2380
	11th to 20th	Day	17% 57% 3 12% 11% 1	2200
	21st to end	Day	17% 57% 3 14% 9% 1	2200
October	1st to 10th	Day	28% 37% 5% 16% 13% 2	3590
	11th to 20th	Day	18% 36% 5% 21% 18% 2	3128
	21st to end	Day	20% 36% 4 23% 15% 2	3180
November	1st to 10th	Day	19% 53% 6% 12% 8% 1	1851
	11th to 20th	Day	18% 55% 3 8% 14% 2	1863
	21st to end	Day	18% 57% 6% 10% 9% 1	1800
December	1st to 10th	Day	25% 36% 8% 24% 7%	2638
	11th to 20th	Day	27% 32% 9% 12% 14% 6%	2986
	21st to end	Day	37% 27% 5% 15% 12% 5%	3960
January	1st to 10th	Night	28% 27% 8% 26% 10% 2	6349
	11th to 20th	Night	34% 28% 9% 17% 10% 3	6360
	21st to end	Night	35% 29% 5% 13% 13% 5%	6996
February	1st to 10th	Night	19% 62% 5% 7% 6% 2	2577
	11th to 20th	Night	17% 60% 3 8% 9% 2	2340
	21st to end	Night	18% 61% 3 9% 6% 3	1980
March	1st to 10th	Night	40% 29% 6% 8% 10% 7%	5374
	11th to 20th	Night	35% 30% 11% 12% 10% 2	5146
	21st to end	Night	40% 29% 3 8% 16% 5%	5203
April	1st to 10th	Night	21% 62% 3 6% 8% 2	2000
	11th to 20th	Night	18% 62% 3 8% 7% 1	1926
	21st to end	Night	18% 61% 4 7% 9% 1	1800
May	1st to 10th	Night	48% 31% 8% 7% 5%	4060
	11th to 20th	Night	39% 32% 12% 12% 6%	3951
	21st to end	Night	41% 31% 9% 13% 6% 1	3927
June	1st to 10th	Night	19% 62% 3 10% 6%	1600
	11th to 20th	Night	15% 64% 5% 10% 6% 1	1600
	21st to end	Night	17% 60% 4 12% 7% 1	1600
July	1st to 10th	Night	31% 28% 10% 19% 11%	3560
	11th to 20th	Night	32% 29% 11% 14% 13%	3552
	21st to end	Night	26% 27% 10% 25% 12%	3809
August	1st to 10th	Night	36% 37% 7% 12% 7% 1	2825
	11th to 20th	Night	31% 39% 10% 13% 6%	2899
	21st to end	Night	17% 38% 10% 19% 16%	3546
September	1st to 10th	Night	14% 57% 6% 13% 9% 1	2019
	11th to 20th	Night	17% 56% 3 12% 10% 1	2200
	21st to end	Night	18% 56% 4 14% 8% 1	2200
October	1st to 10th	Night	27% 37% 9% 16% 10% 1	3590
	11th to 20th	Night	14% 37% 6% 19% 22% 3	3992
	21st to end	Night	21% 36% 6% 23% 13% 2	4610
November	1st to 10th	Night	19% 53% 4 14% 8% 2	2667
	11th to 20th	Night	19% 54% 3 10% 12% 2	2692
	21st to end	Night	17% 56% 7% 12% 8% 1	2600
December	1st to 10th	Night	29% 36% 5% 22% 8%	4541
	11th to 20th	Night	33% 32% 7% 10% 17% 2	5208
	21st to end	Night	37% 27% 5% 16% 13% 2	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	25% 26% 31% 16% 3	200
	11th to 20th	Day	28% 23% 35% 12% 3 1	200
	21st to end	Day	20% 25% 36% 14% 4	220
February	1st to 10th	Day	29% 22% 34% 13% 2	382
	11th to 20th	Day	30% 24% 31% 13% 3	496
	21st to end	Day	26% 23% 32% 16% 3	414
March	1st to 10th	Day	30% 21% 32% 14% 2	500
	11th to 20th	Day	26% 25% 32% 12% 4	500
	21st to end	Day	30% 21% 32% 11% 5% 2	550
April	1st to 10th	Day	31% 23% 28% 11% 5% 1	500
	11th to 20th	Day	31% 22% 26% 15% 5% 1	492
	21st to end	Day	28% 16% 35% 14% 6% 2	500
May	1st to 10th	Day	32% 18% 31% 14% 4 1	503
	11th to 20th	Day	21% 17% 38% 17% 6% 1	527
	21st to end	Day	21% 13% 35% 23% 6% 1	573
June	1st to 10th	Day	23% 12% 38% 19% 8% 1	530
	11th to 20th	Day	17% 17% 36% 22% 7% 1	530
	21st to end	Day	20% 12% 35% 24% 6% 2	530
July	1st to 10th	Day	18% 6% 36% 27% 10% 3	525
	11th to 20th	Day	16% 6% 32% 33% 11% 2	524
	21st to end	Day	13% 7% 36% 30% 12% 2	576
August	1st to 10th	Day	14% 7% 30% 30% 14% 4	526
	11th to 20th	Day	17% 6% 32% 27% 11% 6%	519
	21st to end	Day	13% 8% 35% 26% 14% 4	583
September	1st to 10th	Day	18% 8% 30% 28% 11% 7%	530
	11th to 20th	Day	19% 7% 31% 24% 12% 8%	530
	21st to end	Day	19% 10% 33% 25% 7% 6%	530
October	1st to 10th	Day	24% 11% 26% 24% 10% 4	530
	11th to 20th	Day	22% 14% 28% 23% 10% 3	451
	21st to end	Day	26% 15% 30% 23% 6%	462
November	1st to 10th	Day	21% 18% 30% 21% 9% 2	425
	11th to 20th	Day	28% 21% 29% 18% 5%	421
	21st to end	Day	30% 19% 27% 19% 5% 2	400
December	1st to 10th	Day	20% 23% 34% 17% 5% 1	225
	11th to 20th	Day	31% 21% 33% 13% 3	200
	21st to end	Day	28% 20% 29% 18% 5% 1	220
January	1st to 10th	Night	21% 20% 35% 21% 3	800
	11th to 20th	Night	25% 17% 35% 20% 3	800
	21st to end	Night	20% 20% 38% 19% 4	880
February	1st to 10th	Night	23% 17% 36% 22% 2	800
	11th to 20th	Night	23% 20% 38% 16% 3	783
	21st to end	Night	22% 18% 33% 23% 3	660
March	1st to 10th	Night	28% 17% 34% 19% 2	800
	11th to 20th	Night	23% 19% 33% 20% 6%	800
	21st to end	Night	26% 15% 32% 21% 5%	880
April	1st to 10th	Night	26% 17% 32% 21% 4	800
	11th to 20th	Night	24% 14% 32% 22% 7%	742
	21st to end	Night	25% 13% 33% 25% 5%	600
May	1st to 10th	Night	27% 9% 30% 25% 9% 1	600
	11th to 20th	Night	22% 9% 32% 29% 7% 1	561
	21st to end	Night	23% 7% 27% 33% 10% 1	433
June	1st to 10th	Night	24% 5% 26% 32% 12% 1	400
	11th to 20th	Night	22% 5% 26% 35% 13% 1	400
	21st to end	Night	21% 3 26% 37% 12%	400
July	1st to 10th	Night	17% 3 20% 44% 15% 1	390
	11th to 20th	Night	14% 4 18% 43% 21% 1	388
	21st to end	Night	14% 3 19% 46% 16% 1	438
August	1st to 10th	Night	15% 3 20% 41% 20% 1	543
	11th to 20th	Night	15% 5% 22% 38% 19% 2	592
	21st to end	Night	16% 3 23% 35% 21% 2	853
September	1st to 10th	Night	15% 4 26% 35% 20% 1	800
	11th to 20th	Night	16% 6% 25% 29% 21% 3	800
	21st to end	Night	19% 4 27% 30% 18% 2	800
October	1st to 10th	Night	23% 9% 24% 27% 16% 2	800
	11th to 20th	Night	19% 10% 29% 28% 13% 1	800
	21st to end	Night	26% 12% 30% 24% 8%	880
November	1st to 10th	Night	20% 12% 32% 28% 7% 1	820
	11th to 20th	Night	22% 16% 32% 22% 6% 1	829
	21st to end	Night	22% 19% 30% 26% 4	800
December	1st to 10th	Night	25% 16% 35% 20% 5%	800
	11th to 20th	Night	24% 19% 30% 23% 3 1	800
	21st to end	Night	24% 16% 33% 23% 3	880

Page generated at 2025-11-15 00:56:01. Version ac07

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