Blazny Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blazny 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	13% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	131435
	Night	14% 12% 4 10% 6% 5% 5% 4 5% 6% 29%	131603
Transparency	Day	32% 39% 8% 14% 7% 1	134897
	Night	35% 38% 8% 12% 6% 1	136215
Seeing	Day	20% 17% 33% 20% 8% 2	17531
	Night	20% 13% 31% 26% 9% 1	27118

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	11% 16% 6% 19% 8% 6% 5% 4 5% 6% 13%	4573
	11th to 20th	Day	10% 18% 6% 17% 10% 7% 5% 4 6% 5% 13%	4679
	21st to end	Day	9% 15% 7% 16% 9% 6% 6% 4 4 6% 18%	5156
February	1st to 10th	Day	10% 14% 6% 12% 8% 6% 6% 5% 6% 7% 21%	1817
	11th to 20th	Day	13% 13% 5% 12% 7% 6% 5% 4 5% 6% 24%	1957
	21st to end	Day	10% 14% 6% 14% 7% 5% 5% 3 4 7% 24%	1730
March	1st to 10th	Day	13% 11% 3 9% 5% 5% 4 3 6% 8% 32%	4578
	11th to 20th	Day	13% 13% 4 9% 7% 5% 5% 3 5% 6% 29%	5042
	21st to end	Day	18% 12% 6% 12% 6% 4 5% 3 4 4 25%	5669
April	1st to 10th	Day	17% 12% 4 9% 6% 5% 5% 3 4 6% 29%	2520
	11th to 20th	Day	18% 10% 4 7% 4 4 4 3 4 6% 35%	2670
	21st to end	Day	15% 10% 3 9% 5% 4 4 3 6% 7% 33%	2730
May	1st to 10th	Day	16% 8% 3 6% 4 3 4 3 5% 6% 40%	6018
	11th to 20th	Day	13% 9% 3 7% 4 4 4 3 5% 8% 40%	6122
	21st to end	Day	11% 8% 3 7% 4 4 4 3 5% 8% 43%	7307
June	1st to 10th	Day	13% 6% 2 5% 5% 4 4 4 7% 8% 41%	3000
	11th to 20th	Day	12% 8% 3 7% 5% 4 5% 4 6% 9% 39%	3000
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 8% 40%	3000
July	1st to 10th	Day	14% 7% 2 6% 3 4 5% 4 6% 8% 41%	6731
	11th to 20th	Day	8% 5% 2 6% 4 4 4 4 5% 10% 48%	6360
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 10% 47%	6926
August	1st to 10th	Day	7% 6% 3 7% 5% 5% 5% 5% 7% 9% 42%	4351
	11th to 20th	Day	13% 5% 2 5% 4 5% 4 4 6% 8% 44%	4174
	21st to end	Day	13% 7% 3 6% 3 4 3 3 5% 7% 46%	4642
September	1st to 10th	Day	15% 7% 2 5% 3 4 5% 4 4 7% 43%	2340
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 6% 9% 42%	2200
	21st to end	Day	14% 8% 3 7% 4 5% 4 4 6% 8% 38%	2200
October	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 5% 7% 36%	2156
	11th to 20th	Day	20% 11% 3 10% 7% 5% 4 4 6% 8% 21%	1918
	21st to end	Day	21% 11% 3 9% 7% 6% 6% 4 5% 7% 21%	1980
November	1st to 10th	Day	19% 11% 4 10% 5% 4 5% 3 6% 7% 25%	1852
	11th to 20th	Day	15% 16% 7% 11% 6% 5% 6% 4 5% 5% 19%	1810
	21st to end	Day	20% 20% 5% 11% 7% 6% 5% 3 4 4 14%	1633
December	1st to 10th	Day	13% 19% 5% 12% 7% 6% 6% 5% 6% 5% 15%	2350
	11th to 20th	Day	17% 16% 6% 12% 7% 5% 5% 4 6% 5% 16%	2691
	21st to end	Day	25% 23% 7% 11% 5% 4 3 2 4 4 13%	3553
January	1st to 10th	Night	13% 14% 6% 18% 8% 6% 5% 4 5% 6% 15%	8333
	11th to 20th	Night	10% 18% 8% 16% 9% 7% 6% 5% 5% 5% 12%	8236
	21st to end	Night	9% 13% 6% 15% 8% 7% 7% 5% 6% 7% 17%	9058
February	1st to 10th	Night	9% 16% 6% 12% 8% 6% 5% 5% 5% 5% 23%	2803
	11th to 20th	Night	10% 13% 5% 12% 8% 5% 5% 3 6% 7% 25%	2548
	21st to end	Night	12% 14% 5% 12% 7% 5% 5% 4 5% 5% 25%	2076
March	1st to 10th	Night	16% 10% 3 8% 5% 4 4 3 5% 7% 35%	5482
	11th to 20th	Night	13% 11% 4 10% 6% 5% 4 4 5% 7% 31%	5136
	21st to end	Night	21% 10% 5% 8% 5% 4 4 3 4 5% 30%	5547
April	1st to 10th	Night	18% 11% 4 8% 5% 5% 5% 4 5% 6% 29%	2100
	11th to 20th	Night	20% 9% 4 6% 4 4 4 3 5% 6% 35%	1899
	21st to end	Night	18% 9% 4 7% 6% 4 4 3 5% 6% 35%	1890
May	1st to 10th	Night	16% 7% 2 6% 5% 4 5% 4 5% 6% 39%	4143
	11th to 20th	Night	15% 10% 4 8% 5% 4 4 3 5% 5% 39%	3896
	21st to end	Night	14% 9% 3 8% 4 4 4 3 5% 7% 39%	3588
June	1st to 10th	Night	13% 6% 2 6% 5% 4 4 4 7% 10% 39%	1400
	11th to 20th	Night	12% 7% 2 6% 5% 5% 5% 4 7% 8% 40%	1400
	21st to end	Night	13% 8% 3 6% 4 3 4 3 7% 6% 42%	1400
July	1st to 10th	Night	13% 6% 2 7% 3 4 4 3 6% 8% 43%	3243
	11th to 20th	Night	8% 5% 1 6% 4 3 5% 4 5% 7% 51%	3620
	21st to end	Night	9% 5% 2 4 3 3 4 3 5% 9% 53%	3948
August	1st to 10th	Night	9% 7% 3 6% 5% 5% 5% 4 6% 9% 44%	2788
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 5% 9% 49%	2875
	21st to end	Night	10% 5% 2 6% 4 3 4 2 4 6% 52%	3251
September	1st to 10th	Night	12% 6% 3 6% 4 4 5% 4 5% 7% 45%	2040
	11th to 20th	Night	15% 5% 2 5% 4 3 3 4 5% 8% 46%	2200
	21st to end	Night	17% 7% 3 6% 4 5% 4 4 6% 7% 39%	2200
October	1st to 10th	Night	17% 8% 2 6% 3 4 3 3 5% 7% 41%	2242
	11th to 20th	Night	20% 11% 2 8% 6% 5% 4 4 4 6% 29%	2480
	21st to end	Night	24% 12% 3 9% 6% 4 4 3 4 5% 25%	2860
November	1st to 10th	Night	18% 11% 3 8% 5% 4 4 4 5% 7% 31%	2666
	11th to 20th	Night	17% 15% 5% 10% 6% 5% 5% 4 5% 6% 22%	2809
	21st to end	Night	20% 19% 5% 10% 6% 4 4 3 5% 5% 18%	2983
December	1st to 10th	Night	13% 17% 6% 12% 7% 6% 6% 5% 6% 6% 16%	5060
	11th to 20th	Night	15% 18% 5% 13% 6% 5% 5% 4 5% 6% 18%	5736
	21st to end	Night	24% 23% 8% 12% 4 4 3 2 3 4 12%	7667

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	55% 26% 10% 5% 1 2	4574
	11th to 20th	Day	55% 27% 6% 9% 3	4710
	21st to end	Day	52% 25% 5% 12% 5%	5156
February	1st to 10th	Day	24% 61% 5% 7% 3	1817
	11th to 20th	Day	27% 59% 5% 6% 4	1966
	21st to end	Day	26% 60% 5% 7% 3	1730
March	1st to 10th	Day	36% 32% 12% 10% 9% 2	4580
	11th to 20th	Day	43% 30% 10% 11% 5% 1	5042
	21st to end	Day	48% 30% 6% 9% 6% 1	5669
April	1st to 10th	Day	24% 60% 5% 7% 3 1	2520
	11th to 20th	Day	21% 60% 4 8% 6% 1	2670
	21st to end	Day	19% 60% 5% 9% 6% 1	2730
May	1st to 10th	Day	31% 30% 7% 17% 12% 2	6027
	11th to 20th	Day	29% 31% 8% 18% 14% 1	6126
	21st to end	Day	30% 30% 10% 15% 11% 4	7307
June	1st to 10th	Day	16% 63% 4 10% 6% 2	3000
	11th to 20th	Day	19% 62% 5% 9% 5% 1	3000
	21st to end	Day	17% 61% 6% 10% 6% 1	3000
July	1st to 10th	Day	31% 29% 12% 19% 9%	6659
	11th to 20th	Day	22% 29% 15% 28% 6%	6285
	21st to end	Day	22% 28% 11% 26% 13%	6728
August	1st to 10th	Day	22% 39% 9% 22% 8%	4336
	11th to 20th	Day	26% 38% 9% 21% 6%	4213
	21st to end	Day	26% 38% 12% 19% 5%	4641
September	1st to 10th	Day	19% 59% 8% 10% 4	2340
	11th to 20th	Day	15% 58% 7% 15% 5%	2200
	21st to end	Day	18% 59% 8% 10% 5%	2200
October	1st to 10th	Day	20% 38% 10% 20% 11%	3507
	11th to 20th	Day	29% 38% 9% 22% 2	3095
	21st to end	Day	41% 38% 7% 12% 2	3180
November	1st to 10th	Day	28% 55% 7% 8% 2	1852
	11th to 20th	Day	31% 56% 6% 5% 2	1810
	21st to end	Day	35% 56% 4 4 1	1633
December	1st to 10th	Day	46% 39% 8% 6% 1	2350
	11th to 20th	Day	47% 35% 8% 8% 3	2691
	21st to end	Day	57% 28% 4 6% 5%	3553
January	1st to 10th	Night	54% 25% 9% 7% 4 1	8333
	11th to 20th	Night	57% 26% 7% 5% 4	8280
	21st to end	Night	48% 28% 9% 10% 5%	9058
February	1st to 10th	Night	24% 60% 5% 7% 4	2803
	11th to 20th	Night	23% 59% 5% 9% 2	2550
	21st to end	Night	23% 60% 5% 7% 5%	2076
March	1st to 10th	Night	35% 31% 13% 12% 8% 1	5493
	11th to 20th	Night	38% 30% 13% 12% 5%	5137
	21st to end	Night	42% 30% 7% 13% 8%	5547
April	1st to 10th	Night	24% 60% 4 7% 5%	2100
	11th to 20th	Night	22% 59% 3 9% 5% 1	1899
	21st to end	Night	23% 59% 4 8% 6% 1	1890
May	1st to 10th	Night	31% 31% 7% 16% 12% 3	4150
	11th to 20th	Night	34% 30% 4 20% 11% 1	3897
	21st to end	Night	34% 31% 8% 15% 7% 5%	3589
June	1st to 10th	Night	15% 63% 4 9% 8% 1	1400
	11th to 20th	Night	19% 63% 4 8% 4 2	1400
	21st to end	Night	17% 61% 4 12% 5% 2	1400
July	1st to 10th	Night	31% 28% 10% 18% 11% 1	3207
	11th to 20th	Night	23% 27% 13% 26% 10%	3572
	21st to end	Night	19% 28% 11% 28% 13% 1	3832
August	1st to 10th	Night	25% 38% 7% 20% 10%	2779
	11th to 20th	Night	24% 39% 9% 21% 6% 1	2902
	21st to end	Night	21% 38% 9% 24% 8%	3251
September	1st to 10th	Night	16% 59% 8% 11% 6%	2040
	11th to 20th	Night	15% 59% 7% 13% 6% 1	2200
	21st to end	Night	18% 60% 7% 11% 4	2200
October	1st to 10th	Night	24% 38% 9% 21% 7%	3671
	11th to 20th	Night	24% 38% 13% 20% 6%	4029
	21st to end	Night	34% 39% 12% 14% 2	4610
November	1st to 10th	Night	27% 55% 7% 8% 3	2666
	11th to 20th	Night	27% 57% 8% 6% 2	2809
	21st to end	Night	33% 56% 5% 5% 2	2983
December	1st to 10th	Night	42% 40% 10% 7% 1	5060
	11th to 20th	Night	44% 34% 10% 6% 4	5735
	21st to end	Night	59% 28% 4 7% 3	7667

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% 16% 40% 16% 6%	308
	11th to 20th	Day	22% 18% 42% 12% 5%	330
	21st to end	Day	23% 15% 37% 21% 2 1	363
February	1st to 10th	Day	23% 28% 31% 12% 4 1	330
	11th to 20th	Day	23% 31% 30% 13% 2	330
	21st to end	Day	25% 30% 33% 11% 1	273
March	1st to 10th	Day	23% 26% 34% 14% 2	330
	11th to 20th	Day	25% 20% 35% 15% 4 1	518
	21st to end	Day	28% 22% 26% 17% 6% 1	594
April	1st to 10th	Day	26% 18% 34% 16% 4 1	540
	11th to 20th	Day	29% 22% 25% 14% 8% 2	532
	21st to end	Day	23% 21% 29% 17% 7% 2	540
May	1st to 10th	Day	23% 19% 32% 16% 8% 2	543
	11th to 20th	Day	20% 16% 37% 18% 7% 3	630
	21st to end	Day	15% 16% 35% 23% 8% 2	789
June	1st to 10th	Day	15% 17% 38% 23% 7% 1	730
	11th to 20th	Day	16% 16% 38% 21% 6% 2	730
	21st to end	Day	20% 15% 32% 24% 7% 2	730
July	1st to 10th	Day	10% 11% 39% 27% 9% 3	721
	11th to 20th	Day	13% 12% 36% 26% 10% 4	718
	21st to end	Day	11% 9% 35% 28% 12% 4	794
August	1st to 10th	Day	11% 8% 32% 32% 13% 5%	605
	11th to 20th	Day	14% 8% 24% 29% 17% 9%	519
	21st to end	Day	15% 11% 29% 25% 15% 4	583
September	1st to 10th	Day	16% 15% 26% 24% 15% 5%	530
	11th to 20th	Day	13% 11% 27% 29% 15% 5%	530
	21st to end	Day	18% 14% 24% 30% 12% 2	530
October	1st to 10th	Day	21% 18% 32% 17% 10% 2	530
	11th to 20th	Day	22% 22% 35% 17% 3 1	448
	21st to end	Day	30% 19% 35% 13% 3	363
November	1st to 10th	Day	25% 21% 35% 15% 3 1	330
	11th to 20th	Day	23% 27% 33% 11% 4 1	288
	21st to end	Day	28% 26% 35% 10% 1	220
December	1st to 10th	Day	29% 16% 34% 15% 5% 2	220
	11th to 20th	Day	30% 27% 30% 9% 3	220
	21st to end	Day	32% 23% 30% 10% 4	242
January	1st to 10th	Night	23% 19% 33% 22% 4	1050
	11th to 20th	Night	22% 17% 34% 21% 7%	1050
	21st to end	Night	19% 13% 38% 23% 6%	1155
February	1st to 10th	Night	18% 19% 35% 24% 5%	913
	11th to 20th	Night	18% 20% 33% 23% 6%	823
	21st to end	Night	20% 18% 36% 21% 4	692
March	1st to 10th	Night	18% 14% 41% 20% 6%	840
	11th to 20th	Night	20% 18% 34% 21% 6%	840
	21st to end	Night	20% 14% 34% 26% 6%	908
April	1st to 10th	Night	22% 14% 32% 27% 4 1	630
	11th to 20th	Night	25% 13% 27% 26% 8% 1	623
	21st to end	Night	23% 11% 34% 25% 8%	630
May	1st to 10th	Night	17% 11% 32% 30% 9% 1	630
	11th to 20th	Night	18% 8% 32% 32% 9% 1	609
	21st to end	Night	13% 10% 34% 31% 12%	497
June	1st to 10th	Night	14% 11% 33% 36% 8%	400
	11th to 20th	Night	13% 10% 30% 35% 12% 1	400
	21st to end	Night	17% 9% 29% 34% 11% 1	400
July	1st to 10th	Night	11% 5% 28% 43% 13% 1	420
	11th to 20th	Night	10% 6% 27% 34% 21% 2	582
	21st to end	Night	13% 4 24% 38% 19% 2	657
August	1st to 10th	Night	12% 4 26% 37% 20% 1	597
	11th to 20th	Night	11% 4 22% 34% 27% 3	594
	21st to end	Night	14% 5% 23% 40% 17% 1	660
September	1st to 10th	Night	14% 7% 25% 30% 20% 4	640
	11th to 20th	Night	15% 8% 27% 29% 18% 4	800
	21st to end	Night	20% 8% 25% 29% 15% 3	800
October	1st to 10th	Night	21% 9% 28% 29% 12% 1	800
	11th to 20th	Night	25% 17% 28% 23% 6%	800
	21st to end	Night	31% 12% 29% 23% 5%	880
November	1st to 10th	Night	24% 16% 32% 21% 6% 1	820
	11th to 20th	Night	23% 23% 29% 20% 5%	840
	21st to end	Night	29% 19% 30% 18% 3	883
December	1st to 10th	Night	23% 19% 32% 19% 6% 1	1050
	11th to 20th	Night	27% 18% 34% 15% 5% 1	1050
	21st to end	Night	33% 17% 27% 18% 5%	1155

Page generated at 2026-05-14 01:58:53. Version ac07

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