Blue Braille Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blue Braille Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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% 11% 4 9% 5% 5% 4 4 5% 7% 31%	132171
	Night	16% 12% 4 9% 5% 5% 4 3 5% 6% 30%	130615
Transparency	Day	35% 38% 8% 12% 7% 1	135765
	Night	38% 38% 7% 11% 6% 1	135107
Seeing	Day	20% 13% 27% 23% 12% 4	16400
	Night	21% 12% 28% 27% 11% 1	25935

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	17% 19% 7% 13% 5% 5% 4 3 4 5% 18%	4661
	11th to 20th	Day	12% 23% 8% 22% 9% 5% 4 3 3 3 8%	5167
	21st to end	Day	15% 16% 7% 15% 10% 7% 5% 3 4 4 12%	5780
February	1st to 10th	Day	17% 18% 5% 14% 7% 7% 5% 4 4 5% 15%	1890
	11th to 20th	Day	15% 18% 7% 15% 7% 5% 4 3 4 4 18%	1843
	21st to end	Day	22% 13% 4 13% 7% 5% 5% 3 5% 5% 19%	1639
March	1st to 10th	Day	18% 12% 6% 11% 5% 4 3 3 4 5% 29%	5042
	11th to 20th	Day	11% 17% 5% 8% 5% 5% 5% 4 5% 6% 31%	5090
	21st to end	Day	20% 13% 4 9% 4 3 4 3 4 6% 29%	5586
April	1st to 10th	Day	19% 14% 3 7% 5% 4 4 4 4 6% 30%	2488
	11th to 20th	Day	19% 11% 3 6% 4 5% 4 3 5% 5% 35%	2527
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 4 5% 34%	2730
May	1st to 10th	Day	22% 9% 3 7% 5% 5% 4 4 5% 7% 30%	6018
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 4 5% 7% 31%	6326
	21st to end	Day	20% 8% 2 6% 4 4 4 3 5% 7% 37%	6949
June	1st to 10th	Day	15% 7% 3 6% 5% 4 4 4 7% 7% 39%	2800
	11th to 20th	Day	12% 5% 2 7% 5% 5% 5% 4 7% 9% 39%	2800
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 8% 44%	2800
July	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 4 6% 9% 42%	6360
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 47%	6425
August	1st to 10th	Day	17% 6% 2 6% 4 3 4 4 6% 9% 40%	4234
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 8% 10% 35%	4174
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	4445
September	1st to 10th	Day	14% 6% 2 5% 4 5% 4 4 6% 7% 43%	2400
	11th to 20th	Day	11% 5% 2 5% 4 5% 4 3 6% 7% 48%	2300
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	2146
October	1st to 10th	Day	19% 8% 2 7% 4 4 4 5% 4 7% 37%	2000
	11th to 20th	Day	19% 9% 3 9% 5% 5% 5% 4 5% 6% 30%	2000
	21st to end	Day	24% 12% 5% 8% 6% 5% 5% 3 5% 5% 23%	2200
November	1st to 10th	Day	15% 10% 3 8% 6% 6% 4 4 5% 7% 33%	1837
	11th to 20th	Day	15% 14% 4 7% 6% 5% 4 3 6% 7% 30%	1680
	21st to end	Day	21% 15% 4 8% 6% 4 4 4 6% 7% 21%	1680
December	1st to 10th	Day	20% 18% 5% 11% 6% 6% 4 4 6% 6% 14%	2680
	11th to 20th	Day	21% 18% 6% 9% 4 4 4 3 5% 5% 20%	3066
	21st to end	Day	25% 20% 5% 12% 6% 4 4 3 3 4 13%	4048
January	1st to 10th	Night	17% 18% 6% 12% 5% 4 4 3 4 5% 23%	8234
	11th to 20th	Night	13% 20% 7% 17% 7% 5% 4 3 5% 5% 13%	7703
	21st to end	Night	17% 16% 6% 15% 8% 6% 4 3 4 6% 15%	8401
February	1st to 10th	Night	18% 17% 5% 12% 7% 4 5% 3 4 4 19%	2730
	11th to 20th	Night	17% 15% 5% 11% 6% 5% 5% 3 6% 5% 22%	2663
	21st to end	Night	18% 16% 4 12% 7% 5% 4 4 5% 5% 18%	2161
March	1st to 10th	Night	18% 11% 5% 8% 6% 5% 4 3 4 5% 31%	5018
	11th to 20th	Night	15% 12% 3 6% 4 4 4 3 5% 7% 35%	5089
	21st to end	Night	19% 11% 3 7% 4 4 4 3 5% 5% 36%	5588
April	1st to 10th	Night	21% 11% 4 8% 6% 4 4 3 4 5% 30%	2131
	11th to 20th	Night	19% 9% 3 7% 5% 4 5% 3 5% 6% 34%	2060
	21st to end	Night	17% 9% 3 6% 4 5% 6% 4 6% 5% 36%	1890
May	1st to 10th	Night	24% 11% 3 9% 4 4 4 3 4 5% 30%	4143
	11th to 20th	Night	13% 6% 3 7% 5% 5% 5% 5% 6% 8% 37%	3684
	21st to end	Night	17% 6% 3 7% 4 4 4 4 5% 7% 38%	3926
June	1st to 10th	Night	13% 6% 2 7% 6% 6% 5% 4 7% 7% 38%	1600
	11th to 20th	Night	11% 5% 3 6% 6% 5% 5% 4 6% 7% 42%	1600
	21st to end	Night	14% 6% 2 5% 4 5% 6% 4 6% 7% 41%	1600
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 4 4 6% 9% 45%	3600
	11th to 20th	Night	10% 5% 2 7% 4 5% 4 4 6% 8% 46%	3600
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	4423
August	1st to 10th	Night	10% 7% 2 7% 4 4 5% 4 6% 9% 42%	2915
	11th to 20th	Night	13% 8% 3 6% 4 4 5% 4 6% 8% 39%	2875
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	3418
September	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 5% 6% 47%	2000
	11th to 20th	Night	12% 3 2 5% 3 3 3 3 5% 7% 54%	2095
	21st to end	Night	17% 7% 2 7% 4 5% 5% 4 7% 5% 37%	2253
October	1st to 10th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 41%	2400
	11th to 20th	Night	22% 9% 3 7% 4 5% 5% 3 4 6% 32%	2400
	21st to end	Night	23% 8% 3 8% 4 5% 4 4 5% 6% 29%	2640
November	1st to 10th	Night	17% 10% 3 6% 4 4 4 3 5% 6% 39%	2677
	11th to 20th	Night	15% 13% 3 7% 6% 5% 5% 4 5% 7% 29%	2940
	21st to end	Night	20% 13% 4 9% 5% 4 5% 4 5% 7% 23%	2940
December	1st to 10th	Night	18% 20% 6% 11% 6% 4 5% 3 5% 6% 15%	4720
	11th to 20th	Night	19% 21% 5% 9% 5% 4 4 3 4 6% 21%	5348
	21st to end	Night	26% 17% 5% 10% 5% 5% 4 3 4 5% 15%	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	58% 25% 6% 5% 5%	4662
	11th to 20th	Day	72% 22% 3 2 1	5201
	21st to end	Day	60% 25% 7% 6% 2	5780
February	1st to 10th	Day	29% 62% 4 4 1	1890
	11th to 20th	Day	28% 59% 5% 7% 2	1852
	21st to end	Day	24% 61% 6% 6% 3	1639
March	1st to 10th	Day	41% 31% 10% 12% 7%	5045
	11th to 20th	Day	37% 31% 14% 13% 4 1	5090
	21st to end	Day	40% 31% 5% 17% 7%	5586
April	1st to 10th	Day	27% 60% 4 7% 3	2488
	11th to 20th	Day	22% 61% 4 9% 4 1	2527
	21st to end	Day	22% 60% 5% 7% 5% 1	2730
May	1st to 10th	Day	42% 31% 8% 11% 6% 4	6027
	11th to 20th	Day	38% 32% 9% 15% 6%	6330
	21st to end	Day	38% 31% 7% 15% 8% 2	6949
June	1st to 10th	Day	18% 63% 4 7% 7% 1	2800
	11th to 20th	Day	17% 63% 6% 8% 5%	2800
	21st to end	Day	15% 61% 5% 11% 8%	2800
July	1st to 10th	Day	28% 27% 9% 20% 15% 1	6290
	11th to 20th	Day	29% 27% 10% 19% 14% 1	6285
	21st to end	Day	25% 27% 11% 20% 16% 1	6240
August	1st to 10th	Day	28% 38% 10% 14% 9% 1	4221
	11th to 20th	Day	29% 39% 12% 16% 4	4213
	21st to end	Day	22% 38% 8% 23% 8%	4445
September	1st to 10th	Day	15% 59% 6% 14% 6%	2400
	11th to 20th	Day	12% 58% 5% 14% 9% 1	2300
	21st to end	Day	20% 58% 7% 10% 4	2146
October	1st to 10th	Day	21% 38% 9% 20% 11% 1	3260
	11th to 20th	Day	25% 37% 12% 14% 11% 1	3240
	21st to end	Day	30% 37% 10% 17% 6%	3538
November	1st to 10th	Day	29% 56% 6% 7% 2	1837
	11th to 20th	Day	26% 57% 6% 9% 3	1680
	21st to end	Day	32% 56% 7% 4 1	1680
December	1st to 10th	Day	53% 36% 7% 4 1	2680
	11th to 20th	Day	50% 34% 8% 6% 2	3066
	21st to end	Day	56% 28% 7% 7% 2	4048
January	1st to 10th	Night	58% 26% 4 9% 3	8234
	11th to 20th	Night	67% 24% 4 3 2	7744
	21st to end	Night	63% 25% 4 5% 3	8401
February	1st to 10th	Night	28% 61% 4 6% 1	2730
	11th to 20th	Night	27% 59% 5% 7% 2	2665
	21st to end	Night	28% 61% 4 5% 3	2161
March	1st to 10th	Night	42% 30% 7% 12% 7% 1	5028
	11th to 20th	Night	34% 31% 13% 14% 7% 1	5090
	21st to end	Night	37% 30% 8% 13% 11% 1	5588
April	1st to 10th	Night	28% 59% 3 6% 3	2131
	11th to 20th	Night	22% 60% 5% 8% 5% 1	2060
	21st to end	Night	19% 60% 4 11% 6%	1890
May	1st to 10th	Night	46% 30% 5% 8% 7% 4	4150
	11th to 20th	Night	34% 34% 10% 16% 6%	3685
	21st to end	Night	35% 31% 6% 17% 9% 1	3927
June	1st to 10th	Night	17% 63% 3 8% 7% 1	1600
	11th to 20th	Night	17% 62% 5% 9% 6% 1	1600
	21st to end	Night	17% 61% 5% 8% 8% 1	1600
July	1st to 10th	Night	30% 28% 9% 18% 14% 2	3560
	11th to 20th	Night	30% 28% 8% 22% 12% 1	3552
	21st to end	Night	27% 27% 10% 17% 18% 1	4293
August	1st to 10th	Night	27% 37% 9% 17% 8% 2	2904
	11th to 20th	Night	33% 39% 10% 15% 4	2902
	21st to end	Night	21% 39% 10% 21% 8%	3418
September	1st to 10th	Night	14% 59% 7% 12% 7% 1	2000
	11th to 20th	Night	12% 58% 6% 14% 9% 1	2095
	21st to end	Night	20% 58% 5% 11% 6% 1	2253
October	1st to 10th	Night	24% 38% 5% 17% 16%	3920
	11th to 20th	Night	29% 37% 8% 15% 11%	3899
	21st to end	Night	23% 38% 13% 19% 8%	4252
November	1st to 10th	Night	28% 55% 5% 7% 5%	2677
	11th to 20th	Night	26% 56% 5% 9% 3	2940
	21st to end	Night	30% 57% 7% 5% 1	2940
December	1st to 10th	Night	49% 37% 7% 5% 1	4720
	11th to 20th	Night	53% 34% 5% 6% 2	5348
	21st to end	Night	54% 29% 7% 8% 2	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	22% 25% 34% 13% 5% 1	220
	11th to 20th	Day	29% 23% 32% 10% 4 1	316
	21st to end	Day	26% 22% 30% 17% 5%	363
February	1st to 10th	Day	25% 24% 33% 16% 2	330
	11th to 20th	Day	26% 24% 35% 14% 1	330
	21st to end	Day	30% 22% 33% 14% 1	322
March	1st to 10th	Day	24% 18% 34% 18% 5% 1	540
	11th to 20th	Day	25% 19% 31% 19% 5% 1	540
	21st to end	Day	29% 14% 30% 22% 5% 1	594
April	1st to 10th	Day	25% 16% 32% 19% 6% 2	540
	11th to 20th	Day	28% 17% 25% 17% 9% 3	532
	21st to end	Day	24% 14% 29% 23% 9% 2	540
May	1st to 10th	Day	24% 13% 29% 22% 9% 3	543
	11th to 20th	Day	17% 11% 28% 25% 15% 4	551
	21st to end	Day	16% 8% 26% 28% 16% 6%	573
June	1st to 10th	Day	17% 12% 29% 29% 12% 1	530
	11th to 20th	Day	12% 12% 25% 30% 18% 4	530
	21st to end	Day	15% 7% 32% 29% 13% 3	530
July	1st to 10th	Day	9% 6% 22% 32% 25% 6%	525
	11th to 20th	Day	13% 8% 22% 29% 21% 6%	524
	21st to end	Day	10% 5% 28% 32% 19% 6%	576
August	1st to 10th	Day	8% 7% 21% 31% 23% 10%	526
	11th to 20th	Day	13% 5% 21% 28% 22% 10%	519
	21st to end	Day	12% 5% 25% 29% 19% 10%	583
September	1st to 10th	Day	15% 7% 21% 26% 20% 11%	530
	11th to 20th	Day	9% 5% 16% 25% 26% 18%	530
	21st to end	Day	16% 12% 24% 23% 18% 7%	530
October	1st to 10th	Day	17% 12% 24% 20% 19% 7%	530
	11th to 20th	Day	22% 16% 30% 23% 8% 2	530
	21st to end	Day	30% 17% 27% 18% 8%	583
November	1st to 10th	Day	21% 18% 31% 19% 7% 2	368
	11th to 20th	Day	26% 24% 25% 17% 5% 3	220
	21st to end	Day	34% 20% 32% 11% 3	220
December	1st to 10th	Day	28% 22% 30% 15% 5%	220
	11th to 20th	Day	28% 26% 28% 15% 4	220
	21st to end	Day	29% 25% 29% 12% 3 1	242
January	1st to 10th	Night	27% 22% 31% 15% 5%	840
	11th to 20th	Night	30% 21% 30% 16% 3	840
	21st to end	Night	24% 16% 34% 21% 5%	924
February	1st to 10th	Night	26% 20% 30% 21% 3	840
	11th to 20th	Night	24% 18% 34% 20% 3	823
	21st to end	Night	28% 15% 35% 20% 2	692
March	1st to 10th	Night	21% 15% 34% 25% 5%	840
	11th to 20th	Night	24% 16% 31% 23% 6%	840
	21st to end	Night	23% 11% 32% 24% 9% 1	924
April	1st to 10th	Night	24% 13% 33% 26% 4	840
	11th to 20th	Night	24% 11% 31% 25% 8% 1	816
	21st to end	Night	20% 9% 27% 35% 10%	630
May	1st to 10th	Night	21% 6% 24% 33% 13% 2	630
	11th to 20th	Night	13% 6% 28% 39% 13%	441
	21st to end	Night	15% 4 26% 34% 19% 2	433
June	1st to 10th	Night	13% 4 29% 39% 15% 1	400
	11th to 20th	Night	13% 6% 26% 39% 17%	400
	21st to end	Night	14% 5% 25% 41% 15%	400
July	1st to 10th	Night	7% 3 21% 43% 24% 2	390
	11th to 20th	Night	11% 4 21% 41% 22% 2	388
	21st to end	Night	10% 3 21% 40% 24% 2	652
August	1st to 10th	Night	9% 3 16% 41% 28% 3	597
	11th to 20th	Night	9% 5% 16% 34% 33% 3	594
	21st to end	Night	10% 3 21% 38% 25% 3	777
September	1st to 10th	Night	15% 6% 18% 36% 21% 4	800
	11th to 20th	Night	13% 4 17% 28% 32% 6%	800
	21st to end	Night	19% 7% 21% 31% 19% 4	800
October	1st to 10th	Night	19% 7% 25% 27% 19% 3	800
	11th to 20th	Night	25% 14% 26% 25% 8% 2	800
	21st to end	Night	28% 12% 30% 24% 5%	880
November	1st to 10th	Night	23% 12% 29% 26% 9% 1	820
	11th to 20th	Night	22% 17% 33% 18% 8% 1	840
	21st to end	Night	26% 17% 33% 18% 6%	840
December	1st to 10th	Night	31% 20% 27% 17% 5%	840
	11th to 20th	Night	26% 21% 31% 17% 3	840
	21st to end	Night	31% 16% 32% 18% 2	924

Page generated at 2026-05-14 01:58:54. Version ac07

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