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 2025-01-22.

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 8% 5% 5% 4 4 5% 7% 32%	105167
	Night	17% 12% 4 8% 5% 4 4 3 5% 6% 30%	104186
Transparency	Day	29% 46% 8% 11% 6%	108772
	Night	32% 46% 7% 10% 5%	108669
Seeing	Day	19% 13% 27% 23% 13% 4	14696
	Night	21% 12% 28% 26% 12% 1	24245

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% 23% 8% 10% 5% 5% 4 3 4 5% 16%	2597
	11th to 20th	Day	14% 22% 7% 22% 9% 5% 3 2 3 4 9%	2878
	21st to end	Day	23% 19% 8% 12% 7% 5% 4 3 4 4 11%	3133
February	1st to 10th	Day	17% 19% 5% 14% 6% 7% 4 4 4 5% 15%	1710
	11th to 20th	Day	14% 18% 6% 16% 7% 5% 5% 3 4 4 19%	1663
	21st to end	Day	22% 13% 4 13% 8% 5% 5% 2 5% 5% 19%	1486
March	1st to 10th	Day	20% 12% 5% 8% 5% 4 3 3 4 5% 29%	3432
	11th to 20th	Day	11% 20% 5% 7% 5% 4 4 4 5% 6% 29%	3480
	21st to end	Day	18% 12% 4 10% 4 4 4 4 5% 6% 31%	3815
April	1st to 10th	Day	19% 14% 3 7% 5% 4 4 4 4 6% 31%	2251
	11th to 20th	Day	19% 10% 3 6% 4 5% 4 3 5% 5% 36%	2283
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 4 5% 34%	2470
May	1st to 10th	Day	20% 9% 2 7% 5% 4 5% 4 5% 8% 32%	4108
	11th to 20th	Day	14% 10% 3 9% 6% 6% 5% 4 6% 7% 31%	4352
	21st to end	Day	12% 6% 2 5% 4 3 4 4 5% 8% 46%	4837
June	1st to 10th	Day	15% 7% 3 5% 5% 4 4 4 7% 7% 38%	2660
	11th to 20th	Day	12% 5% 2 7% 5% 5% 5% 4 7% 9% 40%	2660
	21st to end	Day	14% 7% 2 5% 4 4 4 4 6% 7% 43%	2660
July	1st to 10th	Day	9% 5% 2 5% 5% 4 5% 4 6% 9% 46%	4440
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 6% 10% 42%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 4 7% 9% 43%	4467
August	1st to 10th	Day	17% 6% 2 6% 4 3 4 4 6% 9% 39%	4104
	11th to 20th	Day	12% 6% 2 7% 5% 5% 5% 4 8% 10% 35%	4044
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	4309
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 4 6% 7% 43%	2280
	11th to 20th	Day	11% 5% 2 5% 4 5% 4 3 6% 7% 47%	2185
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 33%	2039
October	1st to 10th	Day	19% 8% 2 7% 4 4 4 5% 4 7% 35%	1900
	11th to 20th	Day	19% 9% 3 9% 5% 4 5% 4 5% 6% 30%	1900
	21st to end	Day	24% 12% 5% 9% 6% 5% 4 3 5% 5% 23%	2090
November	1st to 10th	Day	15% 10% 3 9% 6% 6% 4 4 5% 6% 33%	1748
	11th to 20th	Day	15% 14% 4 7% 6% 5% 4 3 5% 7% 30%	1600
	21st to end	Day	21% 15% 4 8% 6% 4 4 4 6% 7% 22%	1600
December	1st to 10th	Day	21% 17% 5% 11% 6% 6% 5% 4 6% 6% 14%	2600
	11th to 20th	Day	22% 18% 6% 9% 4 4 4 3 5% 5% 20%	2986
	21st to end	Day	25% 20% 5% 12% 6% 5% 4 3 3 5% 13%	3960
January	1st to 10th	Night	19% 21% 6% 10% 5% 4 4 3 4 5% 19%	4569
	11th to 20th	Night	14% 21% 7% 15% 8% 5% 4 4 6% 6% 11%	4287
	21st to end	Night	26% 20% 6% 12% 6% 4 3 3 2 4 14%	4546
February	1st to 10th	Night	19% 18% 5% 13% 6% 4 4 3 4 4 19%	2470
	11th to 20th	Night	17% 15% 5% 10% 6% 5% 5% 4 6% 5% 22%	2403
	21st to end	Night	19% 15% 4 12% 7% 5% 4 4 5% 5% 19%	1962
March	1st to 10th	Night	21% 12% 3 8% 5% 4 3 3 4 4 33%	3408
	11th to 20th	Night	19% 15% 3 7% 4 4 4 3 5% 6% 30%	3479
	21st to end	Night	17% 8% 2 7% 4 4 3 3 5% 6% 42%	3817
April	1st to 10th	Night	22% 11% 4 7% 5% 4 4 3 4 4 31%	1929
	11th to 20th	Night	19% 8% 3 7% 5% 4 5% 3 5% 6% 35%	1862
	21st to end	Night	17% 9% 3 6% 4 5% 5% 4 6% 5% 35%	1710
May	1st to 10th	Night	22% 10% 3 8% 4 3 4 3 4 6% 32%	2833
	11th to 20th	Night	14% 6% 3 7% 5% 5% 5% 5% 6% 9% 36%	2540
	21st to end	Night	12% 5% 2 6% 4 4 4 4 6% 8% 46%	2738
June	1st to 10th	Night	13% 6% 3 7% 6% 6% 6% 5% 7% 7% 37%	1520
	11th to 20th	Night	11% 5% 2 6% 6% 5% 6% 4 5% 7% 43%	1520
	21st to end	Night	15% 6% 1 5% 4 5% 6% 4 6% 7% 40%	1520
July	1st to 10th	Night	8% 5% 2 7% 5% 5% 4 3 6% 8% 45%	2520
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 46%	2520
	21st to end	Night	9% 4 2 6% 5% 5% 4 5% 6% 7% 47%	3082
August	1st to 10th	Night	10% 7% 2 7% 4 4 5% 4 5% 9% 42%	2825
	11th to 20th	Night	14% 8% 3 6% 4 4 5% 4 6% 8% 39%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 9% 46%	3313
September	1st to 10th	Night	14% 7% 2 5% 4 3 4 3 5% 6% 47%	1900
	11th to 20th	Night	12% 4 2 5% 3 3 3 3 5% 7% 53%	1990
	21st to end	Night	17% 7% 2 7% 4 5% 5% 4 7% 5% 37%	2140
October	1st to 10th	Night	18% 6% 2 7% 5% 4 4 3 5% 7% 39%	2280
	11th to 20th	Night	22% 9% 3 7% 4 5% 5% 3 4 6% 31%	2280
	21st to end	Night	24% 8% 3 8% 4 6% 4 4 6% 6% 28%	2508
November	1st to 10th	Night	17% 10% 3 6% 3 4 4 3 5% 6% 39%	2546
	11th to 20th	Night	15% 13% 3 7% 6% 5% 5% 4 5% 8% 30%	2800
	21st to end	Night	20% 13% 4 9% 6% 4 5% 4 5% 7% 24%	2800
December	1st to 10th	Night	18% 20% 6% 11% 6% 4 5% 4 5% 6% 15%	4580
	11th to 20th	Night	19% 21% 5% 9% 5% 4 4 3 4 6% 20%	5208
	21st to end	Night	26% 17% 5% 9% 5% 5% 4 3 4 5% 15%	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	48% 41% 5% 3 3	2598
	11th to 20th	Day	56% 38% 3 2 1	2878
	21st to end	Day	50% 43% 5% 2	3133
February	1st to 10th	Day	25% 68% 4 3	1710
	11th to 20th	Day	23% 65% 4 6% 1	1672
	21st to end	Day	19% 66% 6% 6% 3	1486
March	1st to 10th	Day	32% 43% 7% 13% 5%	3435
	11th to 20th	Day	31% 44% 13% 9% 3	3480
	21st to end	Day	30% 44% 6% 15% 4 1	3815
April	1st to 10th	Day	22% 66% 3 6% 2	2251
	11th to 20th	Day	18% 66% 4 7% 4 1	2283
	21st to end	Day	20% 66% 4 4 5% 1	2470
May	1st to 10th	Day	32% 42% 8% 11% 5% 1	4120
	11th to 20th	Day	32% 44% 11% 12% 1	4356
	21st to end	Day	22% 43% 8% 16% 10% 3	4837
June	1st to 10th	Day	16% 66% 4 7% 6% 1	2660
	11th to 20th	Day	15% 66% 6% 8% 6%	2660
	21st to end	Day	15% 64% 4 10% 7%	2660
July	1st to 10th	Day	24% 37% 8% 18% 12% 1	4370
	11th to 20th	Day	23% 37% 9% 19% 12%	4365
	21st to end	Day	23% 37% 12% 14% 14% 1	4324
August	1st to 10th	Day	29% 39% 9% 14% 9% 1	4091
	11th to 20th	Day	28% 40% 12% 15% 4	4083
	21st to end	Day	22% 39% 8% 23% 8%	4309
September	1st to 10th	Day	15% 62% 6% 12% 5%	2280
	11th to 20th	Day	12% 61% 5% 12% 9% 1	2185
	21st to end	Day	19% 61% 7% 10% 4	2039
October	1st to 10th	Day	21% 39% 9% 19% 10% 1	3160
	11th to 20th	Day	25% 38% 12% 14% 11%	3140
	21st to end	Day	29% 38% 11% 17% 5%	3428
November	1st to 10th	Day	27% 58% 6% 6% 2 1	1748
	11th to 20th	Day	24% 60% 6% 8% 2	1600
	21st to end	Day	30% 58% 7% 4 1	1600
December	1st to 10th	Day	52% 37% 7% 3	2600
	11th to 20th	Day	49% 35% 8% 6% 2	2986
	21st to end	Day	55% 29% 7% 7% 2	3960
January	1st to 10th	Night	48% 43% 4 5% 1	4569
	11th to 20th	Night	52% 39% 5% 2 1	4287
	21st to end	Night	51% 42% 3 2 1	4546
February	1st to 10th	Night	25% 67% 4 5%	2470
	11th to 20th	Night	23% 65% 4 7% 2	2405
	21st to end	Night	22% 66% 4 5% 2	1962
March	1st to 10th	Night	34% 41% 6% 15% 4	3418
	11th to 20th	Night	33% 45% 10% 8% 4 1	3480
	21st to end	Night	24% 42% 9% 14% 10% 1	3817
April	1st to 10th	Night	24% 65% 3 6% 2	1929
	11th to 20th	Night	18% 66% 4 7% 5% 1	1862
	21st to end	Night	16% 66% 5% 8% 5%	1710
May	1st to 10th	Night	35% 43% 6% 9% 5% 2	2840
	11th to 20th	Night	28% 46% 11% 14% 1	2541
	21st to end	Night	22% 42% 5% 16% 12% 2	2739
June	1st to 10th	Night	16% 67% 3 7% 7% 1	1520
	11th to 20th	Night	15% 66% 4 9% 6% 1	1520
	21st to end	Night	16% 64% 4 8% 7% 1	1520
July	1st to 10th	Night	26% 38% 7% 14% 12% 3	2480
	11th to 20th	Night	25% 37% 8% 20% 9%	2472
	21st to end	Night	24% 38% 11% 14% 11% 1	2984
August	1st to 10th	Night	27% 38% 8% 18% 7% 1	2814
	11th to 20th	Night	33% 40% 10% 14% 3	2812
	21st to end	Night	21% 40% 9% 21% 9%	3313
September	1st to 10th	Night	13% 62% 7% 11% 7% 1	1900
	11th to 20th	Night	12% 61% 6% 11% 9% 1	1990
	21st to end	Night	19% 61% 5% 10% 5%	2140
October	1st to 10th	Night	25% 39% 5% 17% 15%	3800
	11th to 20th	Night	29% 38% 8% 15% 10%	3779
	21st to end	Night	22% 39% 13% 19% 8%	4120
November	1st to 10th	Night	26% 58% 5% 6% 5%	2546
	11th to 20th	Night	25% 59% 5% 9% 3	2800
	21st to end	Night	28% 60% 7% 5% 1	2800
December	1st to 10th	Night	49% 38% 8% 4 1	4580
	11th to 20th	Night	52% 35% 5% 6% 2	5208
	21st to end	Night	54% 30% 7% 8% 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	22% 24% 34% 14% 6% 2	200
	11th to 20th	Day	28% 22% 33% 11% 5% 1	287
	21st to end	Day	25% 21% 29% 19% 6%	300
February	1st to 10th	Day	25% 24% 34% 14% 2	270
	11th to 20th	Day	24% 26% 36% 13% 1	270
	21st to end	Day	29% 22% 34% 14% 1	269
March	1st to 10th	Day	23% 17% 35% 19% 5% 1	460
	11th to 20th	Day	23% 18% 33% 21% 4 1	460
	21st to end	Day	28% 12% 30% 23% 6% 1	506
April	1st to 10th	Day	23% 16% 32% 21% 7% 2	460
	11th to 20th	Day	27% 18% 25% 18% 10% 3	452
	21st to end	Day	23% 14% 30% 22% 8% 2	460
May	1st to 10th	Day	24% 13% 26% 24% 10% 3	463
	11th to 20th	Day	16% 10% 27% 25% 17% 4	487
	21st to end	Day	14% 7% 26% 29% 17% 6%	529
June	1st to 10th	Day	17% 12% 28% 30% 12% 1	490
	11th to 20th	Day	13% 11% 24% 30% 18% 4	490
	21st to end	Day	16% 8% 31% 29% 13% 3	490
July	1st to 10th	Day	9% 6% 21% 32% 25% 6%	485
	11th to 20th	Day	13% 8% 22% 29% 20% 7%	484
	21st to end	Day	10% 5% 28% 32% 19% 6%	532
August	1st to 10th	Day	9% 7% 22% 31% 21% 9%	486
	11th to 20th	Day	13% 5% 22% 28% 22% 10%	479
	21st to end	Day	12% 5% 24% 29% 19% 10%	539
September	1st to 10th	Day	15% 7% 22% 27% 19% 10%	490
	11th to 20th	Day	10% 5% 17% 26% 25% 17%	490
	21st to end	Day	17% 13% 25% 22% 17% 7%	490
October	1st to 10th	Day	18% 13% 25% 19% 19% 6%	490
	11th to 20th	Day	22% 16% 29% 22% 8% 2	490
	21st to end	Day	31% 17% 26% 18% 8%	539
November	1st to 10th	Day	21% 18% 31% 19% 8% 3	339
	11th to 20th	Day	26% 23% 27% 18% 4 3	200
	21st to end	Day	34% 20% 33% 12% 3	200
December	1st to 10th	Day	28% 22% 32% 14% 5% 1	200
	11th to 20th	Day	29% 25% 28% 15% 4	200
	21st to end	Day	29% 25% 30% 12% 3 1	220
January	1st to 10th	Night	27% 21% 31% 15% 6%	800
	11th to 20th	Night	29% 20% 30% 17% 4	800
	21st to end	Night	24% 15% 32% 23% 6%	841
February	1st to 10th	Night	26% 21% 30% 20% 2	760
	11th to 20th	Night	24% 17% 35% 20% 3	743
	21st to end	Night	27% 15% 34% 21% 2	628
March	1st to 10th	Night	22% 13% 35% 25% 5%	760
	11th to 20th	Night	24% 16% 30% 23% 7%	760
	21st to end	Night	22% 10% 32% 25% 10% 1	836
April	1st to 10th	Night	24% 12% 32% 28% 4	760
	11th to 20th	Night	23% 11% 32% 25% 8% 1	738
	21st to end	Night	20% 9% 27% 34% 10% 1	570
May	1st to 10th	Night	21% 6% 24% 32% 14% 2	570
	11th to 20th	Night	13% 6% 28% 40% 13%	406
	21st to end	Night	14% 4 25% 35% 19% 2	411
June	1st to 10th	Night	13% 4 30% 37% 15% 1	380
	11th to 20th	Night	12% 6% 27% 38% 17%	380
	21st to end	Night	14% 6% 25% 40% 14%	380
July	1st to 10th	Night	8% 3 21% 43% 24% 2	370
	11th to 20th	Night	11% 4 22% 40% 21% 2	368
	21st to end	Night	10% 3 22% 39% 24% 2	619
August	1st to 10th	Night	10% 3 16% 41% 27% 3	567
	11th to 20th	Night	9% 5% 17% 33% 32% 3	564
	21st to end	Night	10% 2 21% 37% 26% 4	738
September	1st to 10th	Night	15% 6% 18% 36% 20% 4	760
	11th to 20th	Night	13% 4 18% 28% 30% 6%	760
	21st to end	Night	19% 7% 22% 30% 18% 4	760
October	1st to 10th	Night	19% 7% 26% 27% 19% 3	760
	11th to 20th	Night	25% 14% 25% 24% 9% 2	760
	21st to end	Night	29% 12% 30% 24% 5%	836
November	1st to 10th	Night	22% 12% 29% 26% 10% 1	780
	11th to 20th	Night	22% 17% 34% 18% 8% 2	800
	21st to end	Night	26% 17% 34% 18% 6%	800
December	1st to 10th	Night	31% 20% 28% 16% 5%	800
	11th to 20th	Night	27% 21% 31% 18% 4	800
	21st to end	Night	31% 16% 32% 19% 2	880

Page generated at 2025-01-19 03:06:45. Version ac07

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