Sky Bright Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sky Bright Observatory clear sky chart from 2005-11-06 to 2026-07-01.

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% 6% 2 6% 4 4 4 4 6% 7% 41%	137880
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	134465
Transparency	Day	41% 5% 11% 23% 17% 3	141409
	Night	41% 5% 10% 23% 18% 4	139023
Seeing	Day	21% 15% 29% 22% 10% 3	17124
	Night	19% 13% 31% 27% 9% 1	26387

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 6% 3 4 4 4 5% 6% 47%	4661
	11th to 20th	Day	16% 7% 2 7% 3 4 3 3 4 5% 46%	4747
	21st to end	Day	16% 6% 2 5% 4 4 5% 3 5% 7% 43%	5780
February	1st to 10th	Day	15% 8% 3 5% 6% 4 4 4 5% 7% 41%	1890
	11th to 20th	Day	16% 8% 3 5% 5% 3 4 3 5% 6% 41%	2022
	21st to end	Day	17% 9% 3 7% 4 4 3 3 5% 6% 40%	1730
March	1st to 10th	Day	21% 6% 2 4 3 4 3 2 4 5% 45%	4576
	11th to 20th	Day	11% 7% 2 6% 4 5% 5% 4 5% 6% 46%	5042
	21st to end	Day	18% 5% 2 5% 4 4 4 3 5% 6% 43%	5955
April	1st to 10th	Day	17% 7% 2 5% 4 4 4 3 5% 7% 40%	2520
	11th to 20th	Day	15% 6% 2 5% 4 5% 4 4 6% 7% 43%	2494
	21st to end	Day	17% 5% 3 6% 4 4 4 4 6% 7% 41%	2520
May	1st to 10th	Day	23% 7% 2 6% 4 4 5% 3 5% 7% 34%	7497
	11th to 20th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 39%	8221
	21st to end	Day	25% 6% 2 6% 3 3 4 3 5% 7% 35%	9060
June	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 9% 40%	2940
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 7% 8% 40%	2940
	21st to end	Day	11% 5% 2 6% 4 4 6% 4 6% 9% 41%	2940
July	1st to 10th	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 10% 37%	6360
	11th to 20th	Day	12% 6% 3 8% 6% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 39%	6849
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	4205
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	3850
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 46%	4312
September	1st to 10th	Day	13% 5% 2 6% 5% 5% 4 3 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 1 4 4 3 3 3 5% 7% 51%	2354
	21st to end	Day	18% 7% 3 7% 4 4 4 3 5% 6% 38%	2200
October	1st to 10th	Day	17% 6% 2 5% 3 3 4 4 5% 5% 46%	2026
	11th to 20th	Day	16% 5% 1 4 3 3 4 4 5% 6% 49%	2000
	21st to end	Day	17% 6% 2 5% 3 3 3 3 5% 5% 47%	2114
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 2 4 7% 53%	1851
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 7% 47%	1890
	21st to end	Day	16% 5% 2 6% 3 4 3 4 5% 5% 47%	1780
December	1st to 10th	Day	13% 7% 2 5% 4 3 4 3 7% 7% 44%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 3 2 4 5% 45%	3066
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 7% 40%	4048
January	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 5% 6% 47%	8234
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 48%	8136
	21st to end	Night	16% 8% 2 5% 4 3 4 3 5% 5% 45%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 5% 7% 41%	2730
	11th to 20th	Night	18% 7% 2 6% 4 4 3 4 5% 6% 41%	2482
	21st to end	Night	17% 8% 2 6% 4 2 4 3 5% 6% 42%	2076
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 3 4 5% 45%	5484
	11th to 20th	Night	11% 9% 2 5% 3 3 4 3 4 5% 50%	5122
	21st to end	Night	16% 7% 3 6% 4 4 4 4 4 5% 42%	5218
April	1st to 10th	Night	18% 7% 2 7% 4 4 5% 4 5% 5% 41%	2100
	11th to 20th	Night	16% 6% 2 7% 4 4 4 3 5% 6% 42%	2075
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 7% 44%	2100
May	1st to 10th	Night	20% 7% 3 7% 4 4 5% 4 6% 8% 32%	5439
	11th to 20th	Night	20% 8% 3 7% 4 3 4 3 5% 7% 37%	4645
	21st to end	Night	26% 7% 3 7% 4 4 3 3 5% 7% 33%	5114
June	1st to 10th	Night	14% 5% 2 5% 5% 5% 3 3 6% 9% 44%	1680
	11th to 20th	Night	12% 7% 2 6% 6% 4 5% 4 5% 7% 42%	1680
	21st to end	Night	9% 5% 2 5% 4 5% 6% 4 8% 10% 41%	1680
July	1st to 10th	Night	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 37%	3600
	11th to 20th	Night	13% 8% 3 8% 5% 4 5% 5% 7% 10% 32%	3600
	21st to end	Night	12% 6% 2 6% 5% 4 5% 4 6% 8% 43%	3986
August	1st to 10th	Night	21% 6% 2 6% 4 4 5% 4 6% 9% 33%	2915
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 5% 7% 42%	3192
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 8% 50%	3586
September	1st to 10th	Night	14% 5% 3 7% 5% 3 4 3 5% 6% 46%	2000
	11th to 20th	Night	12% 5% 1 5% 2 3 4 4 5% 6% 53%	2045
	21st to end	Night	20% 6% 2 6% 3 3 3 4 5% 7% 40%	2200
October	1st to 10th	Night	19% 5% 2 5% 3 3 4 2 5% 5% 46%	2370
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	18% 6% 2 5% 3 3 3 3 4 6% 47%	2725
November	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 4 7% 53%	2667
	11th to 20th	Night	17% 5% 2 5% 4 3 4 3 4 5% 48%	2730
	21st to end	Night	14% 5% 2 5% 4 4 5% 4 6% 6% 45%	2835
December	1st to 10th	Night	14% 7% 2 5% 4 3 4 3 4 7% 48%	4720
	11th to 20th	Night	22% 7% 2 5% 3 3 3 3 4 7% 41%	5348
	21st to end	Night	19% 7% 3 6% 3 4 4 3 5% 6% 41%	7150

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	38% 3 9% 28% 20% 2	4662
	11th to 20th	Day	41% 4 6% 23% 21% 6%	4777
	21st to end	Day	41% 4 12% 28% 14% 1	5780
February	1st to 10th	Day	43% 5% 12% 25% 12% 2	1890
	11th to 20th	Day	45% 5% 9% 23% 15% 3	2031
	21st to end	Day	46% 3 9% 23% 15% 4	1730
March	1st to 10th	Day	42% 3 8% 23% 16% 7%	4579
	11th to 20th	Day	37% 4 9% 29% 16% 5%	5042
	21st to end	Day	42% 4 9% 17% 23% 6%	5955
April	1st to 10th	Day	43% 5% 9% 22% 16% 5%	2520
	11th to 20th	Day	39% 6% 12% 23% 17% 4	2494
	21st to end	Day	42% 6% 9% 21% 19% 4	2520
May	1st to 10th	Day	49% 5% 12% 18% 15% 1	7507
	11th to 20th	Day	45% 5% 9% 24% 15% 1	8225
	21st to end	Day	48% 6% 8% 22% 14% 2	9061
June	1st to 10th	Day	38% 8% 11% 24% 17% 2	2940
	11th to 20th	Day	40% 8% 12% 24% 13% 3	2940
	21st to end	Day	39% 7% 12% 23% 17% 2	2940
July	1st to 10th	Day	41% 9% 14% 18% 16% 2	6290
	11th to 20th	Day	46% 8% 18% 18% 10% 1	6285
	21st to end	Day	41% 7% 9% 20% 22% 1	6652
August	1st to 10th	Day	49% 7% 12% 22% 10% 1	4192
	11th to 20th	Day	39% 7% 12% 26% 14% 1	3886
	21st to end	Day	34% 6% 11% 27% 20% 2	4312
September	1st to 10th	Day	39% 6% 10% 20% 21% 3	2400
	11th to 20th	Day	33% 6% 8% 25% 25% 4	2354
	21st to end	Day	48% 5% 8% 17% 20% 2	2200
October	1st to 10th	Day	36% 4 9% 18% 29% 5%	3299
	11th to 20th	Day	26% 3 10% 24% 30% 8%	3240
	21st to end	Day	25% 3 11% 27% 28% 5%	3391
November	1st to 10th	Day	33% 4 12% 26% 22% 3	1851
	11th to 20th	Day	38% 5% 8% 24% 21% 3	1890
	21st to end	Day	39% 4 11% 31% 13% 2	1780
December	1st to 10th	Day	38% 4 15% 27% 14% 2	2680
	11th to 20th	Day	43% 4 10% 30% 11% 2	3066
	21st to end	Day	41% 6% 11% 23% 18% 1	4048
January	1st to 10th	Night	38% 4 11% 25% 17% 5%	8234
	11th to 20th	Night	40% 3 8% 23% 19% 7%	8181
	21st to end	Night	41% 3 7% 26% 21% 3	8401
February	1st to 10th	Night	44% 5% 12% 24% 13% 3	2730
	11th to 20th	Night	44% 5% 9% 20% 18% 5%	2484
	21st to end	Night	43% 5% 8% 23% 16% 5%	2076
March	1st to 10th	Night	43% 4 8% 23% 13% 9%	5494
	11th to 20th	Night	36% 6% 12% 26% 15% 5%	5123
	21st to end	Night	43% 5% 7% 16% 24% 5%	5218
April	1st to 10th	Night	44% 5% 9% 19% 18% 5%	2100
	11th to 20th	Night	42% 5% 10% 20% 19% 4	2075
	21st to end	Night	40% 6% 9% 21% 20% 5%	2100
May	1st to 10th	Night	49% 6% 11% 20% 13% 2	5445
	11th to 20th	Night	47% 6% 11% 23% 12% 1	4646
	21st to end	Night	52% 6% 9% 20% 13% 1	5115
June	1st to 10th	Night	37% 7% 10% 23% 20% 2	1680
	11th to 20th	Night	41% 8% 11% 25% 13% 2	1680
	21st to end	Night	36% 8% 15% 24% 15% 2	1680
July	1st to 10th	Night	43% 8% 15% 20% 12% 1	3560
	11th to 20th	Night	45% 7% 19% 19% 8% 1	3552
	21st to end	Night	39% 6% 12% 22% 19% 1	3868
August	1st to 10th	Night	48% 7% 17% 18% 9% 1	2904
	11th to 20th	Night	41% 5% 12% 28% 13% 1	3222
	21st to end	Night	33% 5% 10% 28% 22% 2	3586
September	1st to 10th	Night	39% 6% 11% 20% 21% 3	2000
	11th to 20th	Night	31% 5% 11% 24% 25% 4	2045
	21st to end	Night	44% 5% 9% 20% 19% 2	2200
October	1st to 10th	Night	38% 4 9% 23% 22% 3	3877
	11th to 20th	Night	25% 3 9% 27% 29% 6%	3899
	21st to end	Night	27% 3 10% 31% 25% 4	4398
November	1st to 10th	Night	32% 5% 11% 24% 22% 5%	2667
	11th to 20th	Night	39% 5% 8% 22% 20% 6%	2730
	21st to end	Night	38% 5% 11% 26% 18% 2	2835
December	1st to 10th	Night	37% 4 12% 31% 14% 3	4720
	11th to 20th	Night	45% 3 10% 24% 15% 4	5348
	21st to end	Night	45% 4 10% 20% 18% 3	7150

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% 27% 34% 11% 3 1	220
	11th to 20th	Day	33% 26% 30% 10% 1	237
	21st to end	Day	26% 29% 29% 12% 4	363
February	1st to 10th	Day	26% 25% 35% 11% 4	330
	11th to 20th	Day	33% 21% 34% 11% 2	510
	21st to end	Day	27% 21% 35% 15% 2	446
March	1st to 10th	Day	28% 19% 30% 18% 4 1	540
	11th to 20th	Day	24% 23% 33% 15% 3 1	540
	21st to end	Day	28% 22% 31% 15% 4 1	594
April	1st to 10th	Day	29% 20% 32% 12% 6% 1	540
	11th to 20th	Day	21% 21% 33% 18% 6% 1	532
	21st to end	Day	26% 16% 31% 20% 6% 1	540
May	1st to 10th	Day	29% 19% 29% 14% 7% 1	543
	11th to 20th	Day	23% 14% 31% 23% 8% 2	567
	21st to end	Day	23% 11% 24% 25% 14% 3	617
June	1st to 10th	Day	15% 14% 27% 28% 12% 3	570
	11th to 20th	Day	17% 12% 31% 26% 11% 3	570
	21st to end	Day	14% 9% 30% 29% 14% 4	570
July	1st to 10th	Day	11% 4 24% 34% 21% 6%	525
	11th to 20th	Day	9% 3 23% 36% 24% 5%	524
	21st to end	Day	12% 5% 26% 36% 16% 5%	576
August	1st to 10th	Day	13% 5% 27% 32% 18% 6%	526
	11th to 20th	Day	14% 5% 20% 34% 20% 7%	519
	21st to end	Day	13% 7% 29% 27% 15% 8%	583
September	1st to 10th	Day	16% 9% 27% 25% 17% 6%	530
	11th to 20th	Day	16% 7% 23% 31% 16% 8%	530
	21st to end	Day	26% 12% 21% 26% 10% 5%	530
October	1st to 10th	Day	23% 9% 26% 21% 14% 7%	530
	11th to 20th	Day	20% 16% 28% 22% 11% 3	530
	21st to end	Day	20% 16% 33% 18% 12% 2	535
November	1st to 10th	Day	20% 14% 31% 22% 12% 2	425
	11th to 20th	Day	22% 21% 30% 21% 5%	430
	21st to end	Day	22% 19% 34% 18% 6% 1	320
December	1st to 10th	Day	25% 25% 36% 7% 5%	220
	11th to 20th	Day	34% 19% 32% 12% 3	220
	21st to end	Day	29% 21% 29% 13% 7%	242
January	1st to 10th	Night	21% 20% 37% 19% 4	840
	11th to 20th	Night	25% 19% 30% 23% 3	840
	21st to end	Night	18% 19% 39% 22% 2	924
February	1st to 10th	Night	21% 17% 38% 23% 2	840
	11th to 20th	Night	25% 22% 35% 17% 1	823
	21st to end	Night	23% 19% 37% 19% 2	692
March	1st to 10th	Night	22% 19% 33% 22% 4	840
	11th to 20th	Night	21% 20% 35% 18% 5%	840
	21st to end	Night	22% 16% 40% 17% 5%	924
April	1st to 10th	Night	22% 19% 37% 17% 5%	840
	11th to 20th	Night	20% 15% 36% 25% 4	831
	21st to end	Night	21% 13% 34% 28% 5%	840
May	1st to 10th	Night	21% 13% 36% 24% 6%	693
	11th to 20th	Night	19% 12% 35% 29% 5%	420
	21st to end	Night	20% 6% 26% 35% 11% 2	455
June	1st to 10th	Night	16% 6% 30% 40% 7% 1	420
	11th to 20th	Night	14% 7% 30% 40% 9% 1	420
	21st to end	Night	11% 4 29% 45% 10%	420
July	1st to 10th	Night	9% 4 28% 41% 16% 2	390
	11th to 20th	Night	7% 4 25% 42% 20% 2	388
	21st to end	Night	10% 3 29% 39% 18% 1	461
August	1st to 10th	Night	13% 3 18% 43% 21% 2	597
	11th to 20th	Night	14% 4 19% 39% 21% 2	785
	21st to end	Night	14% 4 21% 37% 22% 2	880
September	1st to 10th	Night	16% 4 27% 35% 16% 2	800
	11th to 20th	Night	15% 4 21% 35% 21% 4	800
	21st to end	Night	24% 5% 22% 32% 17% 1	800
October	1st to 10th	Night	21% 8% 23% 27% 17% 3	800
	11th to 20th	Night	18% 11% 27% 31% 12% 1	800
	21st to end	Night	21% 12% 33% 25% 9% 1	880
November	1st to 10th	Night	14% 12% 34% 29% 10% 1	820
	11th to 20th	Night	20% 17% 30% 25% 8%	840
	21st to end	Night	18% 17% 33% 25% 7% 1	840
December	1st to 10th	Night	24% 15% 34% 20% 6% 1	840
	11th to 20th	Night	21% 16% 34% 24% 5%	840
	21st to end	Night	22% 16% 33% 26% 2	924

Page generated at 2026-06-28 02:10:45. Version ac07

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