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 2025-02-25.

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	15% 6% 2 6% 4 4 4 4 6% 7% 41%	108598
	Night	16% 7% 2 6% 4 3 4 3 5% 6% 43%	110067
Transparency	Day	41% 6% 11% 23% 17% 3	112174
	Night	40% 5% 10% 23% 18% 3	114654
Seeing	Day	21% 14% 29% 22% 11% 3	15322
	Night	19% 13% 31% 27% 9% 1	24715

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 5% 3 3 3 3 4 6% 51%	3597
	11th to 20th	Day	14% 8% 3 8% 4 4 3 4 4 6% 43%	3639
	21st to end	Day	19% 8% 3 6% 4 3 4 3 4 5% 43%	4466
February	1st to 10th	Day	15% 8% 3 5% 5% 4 4 4 5% 7% 40%	1800
	11th to 20th	Day	16% 8% 2 5% 5% 3 4 3 5% 6% 41%	1923
	21st to end	Day	17% 8% 3 7% 4 4 3 3 4 7% 39%	1610
March	1st to 10th	Day	26% 7% 2 5% 3 4 3 2 4 5% 38%	3116
	11th to 20th	Day	10% 5% 2 5% 4 5% 4 4 6% 7% 48%	3447
	21st to end	Day	22% 6% 2 5% 3 3 4 2 4 5% 44%	4064
April	1st to 10th	Day	16% 7% 3 5% 4 4 4 3 5% 7% 40%	2280
	11th to 20th	Day	15% 7% 2 4 4 5% 4 4 6% 7% 43%	2254
	21st to end	Day	17% 5% 3 6% 4 4 5% 4 6% 7% 40%	2280
May	1st to 10th	Day	27% 9% 3 6% 4 3 5% 3 5% 7% 28%	4033
	11th to 20th	Day	24% 8% 3 7% 5% 4 4 4 5% 6% 29%	4398
	21st to end	Day	12% 5% 2 5% 4 3 5% 4 6% 10% 43%	4837
June	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 8% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 9% 39%	2660
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 4 7% 9% 41%	2660
July	1st to 10th	Day	7% 5% 2 8% 5% 7% 7% 5% 8% 11% 35%	4440
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	12% 7% 3 8% 6% 6% 6% 5% 7% 8% 32%	4765
August	1st to 10th	Day	17% 7% 3 8% 5% 5% 5% 4 7% 9% 32%	4076
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 7% 9% 41%	3730
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 9% 46%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 5% 4 3 6% 7% 44%	2280
	11th to 20th	Day	13% 5% 1 4 4 3 3 3 5% 7% 51%	2237
	21st to end	Day	18% 7% 3 7% 4 4 4 3 5% 6% 38%	2090
October	1st to 10th	Day	18% 6% 2 5% 3 3 4 4 4 6% 45%	1925
	11th to 20th	Day	15% 5% 1 4 3 3 4 4 5% 6% 50%	1900
	21st to end	Day	17% 6% 2 5% 3 3 3 3 5% 5% 47%	2009
November	1st to 10th	Day	13% 6% 2 5% 3 3 3 2 4 6% 53%	1761
	11th to 20th	Day	15% 6% 2 5% 4 3 3 3 5% 7% 48%	1800
	21st to end	Day	16% 5% 2 5% 3 4 3 3 5% 5% 48%	1695
December	1st to 10th	Day	13% 7% 2 5% 4 3 4 3 7% 7% 45%	2600
	11th to 20th	Day	20% 9% 2 4 3 3 3 2 4 5% 45%	2986
	21st to end	Day	16% 6% 2 5% 4 4 5% 4 6% 7% 41%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 5% 5% 51%	6349
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 4 5% 49%	6244
	21st to end	Night	18% 8% 3 5% 4 3 4 3 5% 6% 42%	6490
February	1st to 10th	Night	17% 8% 2 6% 4 3 4 3 5% 7% 41%	2600
	11th to 20th	Night	18% 7% 2 6% 4 4 3 4 5% 6% 41%	2361
	21st to end	Night	17% 8% 2 6% 4 2 4 3 5% 6% 43%	1939
March	1st to 10th	Night	23% 10% 3 5% 3 3 3 3 4 5% 37%	3724
	11th to 20th	Night	11% 6% 2 6% 3 3 4 3 5% 6% 50%	3497
	21st to end	Night	18% 8% 3 6% 4 3 3 2 4 4 44%	3567
April	1st to 10th	Night	17% 6% 2 7% 4 4 5% 4 5% 6% 41%	1900
	11th to 20th	Night	15% 7% 2 7% 4 4 4 3 5% 6% 43%	1875
	21st to end	Night	18% 6% 2 5% 3 3 4 3 5% 7% 43%	1900
May	1st to 10th	Night	23% 8% 3 7% 4 3 4 3 6% 7% 30%	2930
	11th to 20th	Night	23% 10% 3 7% 4 3 4 3 4 6% 32%	2501
	21st to end	Night	14% 6% 2 6% 4 4 4 4 6% 10% 42%	2738
June	1st to 10th	Night	15% 6% 2 5% 5% 5% 3 3 6% 8% 43%	1520
	11th to 20th	Night	11% 7% 2 6% 6% 4 5% 4 5% 6% 43%	1520
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 8% 10% 42%	1520
July	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 8% 9% 38%	2520
	11th to 20th	Night	11% 5% 3 6% 5% 4 5% 5% 8% 11% 38%	2520
	21st to end	Night	15% 7% 2 7% 5% 4 5% 5% 6% 8% 35%	2785
August	1st to 10th	Night	21% 6% 2 6% 4 4 5% 4 6% 9% 33%	2825
	11th to 20th	Night	13% 7% 3 6% 4 4 5% 4 5% 7% 42%	3092
	21st to end	Night	10% 5% 3 5% 3 4 4 4 5% 8% 49%	3476
September	1st to 10th	Night	14% 5% 2 7% 5% 3 4 3 5% 6% 46%	1900
	11th to 20th	Night	13% 5% 1 5% 2 3 4 4 5% 6% 53%	1943
	21st to end	Night	19% 6% 2 6% 3 3 3 3 5% 7% 41%	2090
October	1st to 10th	Night	20% 5% 2 5% 3 3 4 3 5% 5% 44%	2251
	11th to 20th	Night	15% 4 2 4 3 3 3 3 5% 6% 52%	2280
	21st to end	Night	18% 6% 2 5% 3 3 3 3 4 6% 47%	2589
November	1st to 10th	Night	11% 5% 1 5% 4 3 3 3 4 6% 54%	2537
	11th to 20th	Night	17% 5% 2 5% 4 3 4 3 4 5% 48%	2600
	21st to end	Night	14% 5% 2 5% 4 4 5% 4 6% 7% 45%	2700
December	1st to 10th	Night	14% 7% 2 5% 4 3 4 3 4 7% 48%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 7% 41%	5208
	21st to end	Night	19% 7% 3 6% 3 4 4 3 5% 6% 41%	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	35% 3 8% 26% 24% 3	3598
	11th to 20th	Day	43% 5% 7% 22% 17% 5%	3669
	21st to end	Day	45% 5% 11% 25% 14% 2	4466
February	1st to 10th	Day	44% 5% 12% 25% 12% 2	1800
	11th to 20th	Day	44% 5% 9% 24% 16% 3	1932
	21st to end	Day	47% 3 10% 21% 14% 5%	1610
March	1st to 10th	Day	49% 4 9% 27% 9% 2	3119
	11th to 20th	Day	33% 5% 10% 32% 18% 3	3447
	21st to end	Day	44% 4 7% 19% 21% 4	4064
April	1st to 10th	Day	43% 6% 9% 22% 16% 5%	2280
	11th to 20th	Day	39% 6% 12% 23% 17% 4	2254
	21st to end	Day	42% 6% 9% 20% 19% 4	2280
May	1st to 10th	Day	55% 6% 10% 17% 10% 2	4046
	11th to 20th	Day	53% 8% 11% 18% 8% 1	4402
	21st to end	Day	34% 9% 11% 26% 18% 2	4837
June	1st to 10th	Day	39% 8% 11% 25% 15% 2	2660
	11th to 20th	Day	40% 8% 12% 25% 12% 3	2660
	21st to end	Day	38% 8% 12% 23% 17% 1	2660
July	1st to 10th	Day	42% 10% 15% 18% 14% 2	4370
	11th to 20th	Day	40% 8% 16% 21% 14% 1	4365
	21st to end	Day	47% 8% 10% 18% 15% 1	4613
August	1st to 10th	Day	49% 7% 12% 22% 9% 1	4063
	11th to 20th	Day	39% 7% 12% 26% 15% 1	3766
	21st to end	Day	34% 6% 11% 27% 20% 2	4180
September	1st to 10th	Day	39% 6% 10% 19% 22% 3	2280
	11th to 20th	Day	33% 6% 8% 24% 25% 4	2237
	21st to end	Day	47% 5% 8% 17% 21% 2	2090
October	1st to 10th	Day	37% 4 9% 17% 29% 5%	3198
	11th to 20th	Day	25% 3 10% 24% 30% 8%	3140
	21st to end	Day	25% 3 12% 27% 28% 5%	3286
November	1st to 10th	Day	34% 4 12% 26% 22% 3	1761
	11th to 20th	Day	38% 4 8% 25% 22% 3	1800
	21st to end	Day	38% 4 12% 31% 13% 2	1695
December	1st to 10th	Day	37% 4 15% 27% 15% 2	2600
	11th to 20th	Day	43% 4 10% 30% 11% 2	2986
	21st to end	Day	41% 6% 11% 22% 18% 1	3960
January	1st to 10th	Night	36% 4 7% 25% 22% 6%	6349
	11th to 20th	Night	39% 3 9% 24% 18% 7%	6289
	21st to end	Night	43% 4 8% 24% 19% 2	6490
February	1st to 10th	Night	44% 5% 13% 24% 13% 2	2600
	11th to 20th	Night	44% 5% 8% 20% 18% 5%	2363
	21st to end	Night	43% 5% 8% 22% 17% 5%	1939
March	1st to 10th	Night	50% 4 8% 23% 13% 2	3734
	11th to 20th	Night	34% 7% 11% 27% 17% 3	3498
	21st to end	Night	44% 5% 6% 17% 23% 5%	3567
April	1st to 10th	Night	43% 5% 9% 20% 18% 5%	1900
	11th to 20th	Night	42% 5% 10% 21% 19% 4	1875
	21st to end	Night	41% 6% 8% 20% 20% 5%	1900
May	1st to 10th	Night	52% 7% 9% 19% 11% 1	2936
	11th to 20th	Night	54% 7% 13% 15% 10% 2	2502
	21st to end	Night	37% 8% 14% 22% 16% 3	2739
June	1st to 10th	Night	38% 8% 10% 22% 19% 2	1520
	11th to 20th	Night	39% 9% 11% 26% 13% 1	1520
	21st to end	Night	35% 8% 15% 25% 15% 2	1520
July	1st to 10th	Night	41% 10% 19% 17% 11% 2	2480
	11th to 20th	Night	38% 7% 18% 24% 11% 1	2472
	21st to end	Night	46% 7% 13% 19% 14% 1	2696
August	1st to 10th	Night	48% 7% 18% 17% 9% 1	2814
	11th to 20th	Night	41% 5% 12% 28% 14% 1	3122
	21st to end	Night	33% 5% 10% 28% 22% 2	3476
September	1st to 10th	Night	39% 7% 11% 19% 22% 3	1900
	11th to 20th	Night	31% 5% 11% 24% 26% 4	1943
	21st to end	Night	43% 6% 8% 21% 20% 2	2090
October	1st to 10th	Night	39% 4 10% 23% 22% 2	3758
	11th to 20th	Night	24% 3 9% 28% 30% 6%	3779
	21st to end	Night	27% 3 10% 31% 24% 4	4262
November	1st to 10th	Night	33% 5% 11% 24% 22% 6%	2537
	11th to 20th	Night	40% 5% 8% 22% 20% 6%	2600
	21st to end	Night	38% 5% 12% 26% 17% 2	2700
December	1st to 10th	Night	37% 4 12% 31% 14% 3	4580
	11th to 20th	Night	45% 3 10% 23% 14% 4	5208
	21st to end	Night	45% 4 10% 20% 18% 3	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	26% 27% 32% 11% 4 1	200
	11th to 20th	Day	33% 26% 29% 10% 1	215
	21st to end	Day	27% 25% 30% 13% 5%	330
February	1st to 10th	Day	27% 26% 35% 10% 3	300
	11th to 20th	Day	34% 22% 32% 11% 2	471
	21st to end	Day	27% 21% 35% 14% 3	398
March	1st to 10th	Day	25% 21% 30% 20% 3 1	460
	11th to 20th	Day	22% 25% 33% 15% 3 2	460
	21st to end	Day	28% 19% 30% 17% 4 1	506
April	1st to 10th	Day	26% 22% 32% 13% 6% 1	460
	11th to 20th	Day	20% 21% 34% 18% 6% 2	452
	21st to end	Day	27% 15% 31% 20% 5% 2	460
May	1st to 10th	Day	29% 18% 28% 16% 8% 2	463
	11th to 20th	Day	23% 14% 29% 25% 8% 2	487
	21st to end	Day	17% 10% 24% 28% 16% 4	529
June	1st to 10th	Day	15% 14% 27% 28% 12% 4	490
	11th to 20th	Day	14% 11% 31% 28% 13% 3	490
	21st to end	Day	14% 10% 30% 29% 13% 4	490
July	1st to 10th	Day	11% 4 24% 34% 21% 6%	485
	11th to 20th	Day	8% 3 24% 38% 23% 5%	484
	21st to end	Day	12% 6% 28% 34% 15% 5%	532
August	1st to 10th	Day	13% 5% 26% 32% 18% 6%	486
	11th to 20th	Day	13% 6% 20% 34% 20% 6%	479
	21st to end	Day	14% 7% 28% 27% 15% 9%	539
September	1st to 10th	Day	16% 10% 26% 26% 17% 6%	490
	11th to 20th	Day	15% 7% 23% 29% 17% 8%	490
	21st to end	Day	27% 13% 21% 25% 9% 5%	490
October	1st to 10th	Day	24% 9% 26% 21% 13% 6%	490
	11th to 20th	Day	17% 16% 29% 23% 11% 3	490
	21st to end	Day	19% 15% 33% 18% 12% 2	496
November	1st to 10th	Day	19% 12% 32% 23% 12% 2	395
	11th to 20th	Day	21% 21% 32% 21% 5%	400
	21st to end	Day	21% 20% 34% 18% 7% 1	295
December	1st to 10th	Day	25% 26% 37% 7% 6%	200
	11th to 20th	Day	35% 19% 31% 13% 4	200
	21st to end	Day	28% 20% 30% 14% 7%	220
January	1st to 10th	Night	21% 21% 36% 19% 4	800
	11th to 20th	Night	24% 19% 30% 24% 3	800
	21st to end	Night	18% 19% 40% 22% 2	880
February	1st to 10th	Night	22% 17% 38% 22% 2	800
	11th to 20th	Night	26% 23% 35% 16% 1	783
	21st to end	Night	23% 20% 36% 19% 2	647
March	1st to 10th	Night	21% 19% 34% 21% 4	760
	11th to 20th	Night	21% 21% 34% 18% 5%	760
	21st to end	Night	22% 16% 39% 18% 5%	836
April	1st to 10th	Night	21% 19% 37% 18% 5%	760
	11th to 20th	Night	20% 14% 36% 25% 5%	751
	21st to end	Night	20% 13% 35% 27% 5%	760
May	1st to 10th	Night	21% 13% 35% 24% 6%	627
	11th to 20th	Night	19% 12% 34% 29% 5% 1	380
	21st to end	Night	17% 5% 26% 37% 12% 2	411
June	1st to 10th	Night	17% 6% 28% 40% 7% 1	380
	11th to 20th	Night	12% 6% 31% 40% 10% 1	380
	21st to end	Night	10% 3 30% 46% 11%	380
July	1st to 10th	Night	9% 5% 29% 41% 15% 2	370
	11th to 20th	Night	7% 4 26% 42% 20% 2	368
	21st to end	Night	10% 3 29% 39% 18% 1	438
August	1st to 10th	Night	13% 3 19% 43% 21% 2	567
	11th to 20th	Night	14% 4 20% 39% 21% 3	745
	21st to end	Night	14% 4 20% 37% 22% 2	836
September	1st to 10th	Night	16% 4 27% 35% 16% 2	760
	11th to 20th	Night	15% 4 21% 35% 21% 4	760
	21st to end	Night	24% 5% 23% 30% 17% 1	760
October	1st to 10th	Night	22% 8% 23% 27% 16% 3	760
	11th to 20th	Night	17% 11% 28% 32% 12% 1	760
	21st to end	Night	21% 11% 33% 25% 9% 1	836
November	1st to 10th	Night	14% 11% 34% 29% 11% 1	780
	11th to 20th	Night	20% 16% 31% 25% 8% 1	800
	21st to end	Night	18% 17% 34% 24% 7% 1	800
December	1st to 10th	Night	25% 15% 33% 20% 6% 1	800
	11th to 20th	Night	20% 16% 35% 25% 5%	800
	21st to end	Night	22% 16% 33% 27% 2	880

Page generated at 2025-02-22 06:51:37. Version ac07

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