Blazer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Blazer Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	127573
	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	124398
Transparency	Day	38% 7% 11% 21% 18% 5%	131327
	Night	37% 6% 10% 21% 20% 7%	128718
Seeing	Day	16% 10% 23% 24% 18% 8%	16561
	Night	16% 8% 24% 29% 20% 4	26197

By Month

Percentage Cloud Cover SE (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% 10% 4 6% 2 3 3 2 3 5% 44%	3717
	11th to 20th	Day	16% 6% 3 7% 3 2 2 2 3 6% 52%	4372
	21st to end	Day	21% 10% 3 5% 3 3 3 2 4 5% 41%	5082
February	1st to 10th	Day	19% 11% 3 6% 4 3 3 3 4 6% 40%	2000
	11th to 20th	Day	22% 8% 2 6% 4 4 3 3 4 5% 40%	1948
	21st to end	Day	21% 8% 3 6% 4 4 3 2 5% 4 40%	1650
March	1st to 10th	Day	22% 8% 3 7% 5% 4 5% 3 4 4 36%	4480
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 4 6% 6% 44%	4657
	21st to end	Day	14% 7% 3 7% 4 4 4 4 5% 7% 40%	5967
April	1st to 10th	Day	13% 6% 2 5% 5% 4 3 4 5% 7% 47%	2400
	11th to 20th	Day	14% 6% 2 5% 4 4 3 2 5% 6% 48%	2374
	21st to end	Day	10% 5% 2 6% 4 5% 4 3 6% 10% 46%	2400
May	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 7% 8% 34%	5585
	11th to 20th	Day	15% 7% 3 8% 5% 5% 6% 5% 7% 8% 30%	5861
	21st to end	Day	13% 7% 2 7% 5% 5% 5% 4 6% 8% 38%	6433
June	1st to 10th	Day	9% 6% 2 7% 5% 6% 6% 4 8% 10% 38%	2600
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 4 8% 11% 42%	2738
	21st to end	Day	7% 5% 2 5% 5% 4 6% 5% 7% 9% 45%	2800
July	1st to 10th	Day	5% 5% 2 8% 6% 7% 7% 6% 8% 11% 36%	6360
	11th to 20th	Day	7% 4 1 5% 5% 6% 7% 7% 10% 14% 34%	6135
	21st to end	Day	9% 7% 3 8% 6% 7% 7% 6% 9% 12% 28%	6023
August	1st to 10th	Day	3 3 1 5% 4 4 5% 5% 6% 9% 54%	3905
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 10% 46%	3849
	21st to end	Day	7% 4 2 5% 4 5% 5% 3 7% 8% 50%	4312
September	1st to 10th	Day	9% 5% 3 7% 5% 5% 4 4 7% 8% 44%	2400
	11th to 20th	Day	12% 5% 2 7% 5% 5% 4 3 5% 7% 45%	2219
	21st to end	Day	10% 6% 2 5% 4 3 5% 4 4 6% 51%	2200
October	1st to 10th	Day	9% 5% 2 5% 3 4 3 3 4 6% 55%	2200
	11th to 20th	Day	9% 4 1 4 4 3 3 3 4 6% 58%	2102
	21st to end	Day	15% 5% 1 5% 3 3 4 3 4 5% 53%	2124
November	1st to 10th	Day	11% 5% 2 4 4 4 3 3 4 6% 53%	1852
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 6% 54%	1890
	21st to end	Day	16% 7% 2 5% 3 3 4 4 5% 6% 46%	1890
December	1st to 10th	Day	18% 9% 3 5% 3 4 4 3 5% 5% 42%	3020
	11th to 20th	Day	20% 9% 3 6% 4 3 3 2 4 4 41%	3463
	21st to end	Day	19% 8% 3 5% 3 3 4 3 5% 5% 41%	4565
January	1st to 10th	Night	18% 10% 3 7% 3 3 3 2 4 5% 42%	6423
	11th to 20th	Night	17% 9% 4 7% 3 2 2 2 3 4 48%	5756
	21st to end	Night	21% 10% 2 6% 3 3 3 2 3 4 44%	6116
February	1st to 10th	Night	19% 8% 3 6% 3 3 3 3 4 5% 42%	2401
	11th to 20th	Night	22% 8% 2 5% 5% 3 3 3 3 4 41%	2338
	21st to end	Night	20% 7% 3 6% 5% 3 4 3 5% 6% 39%	1980
March	1st to 10th	Night	21% 10% 3 7% 4 4 4 3 5% 5% 35%	5373
	11th to 20th	Night	12% 7% 2 4 3 3 3 3 5% 7% 50%	5317
	21st to end	Night	19% 7% 3 7% 5% 4 3 2 3 4 43%	4962
April	1st to 10th	Night	13% 6% 2 5% 4 4 5% 4 5% 7% 46%	2000
	11th to 20th	Night	13% 4 2 5% 3 3 4 3 6% 7% 49%	1975
	21st to end	Night	11% 5% 2 6% 4 4 4 4 4 8% 49%	2000
May	1st to 10th	Night	14% 7% 3 7% 4 4 4 4 5% 8% 40%	4306
	11th to 20th	Night	18% 9% 3 8% 5% 4 5% 3 6% 6% 34%	4032
	21st to end	Night	11% 6% 2 6% 4 4 5% 4 7% 10% 39%	4421
June	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 43%	1800
	11th to 20th	Night	5% 4 2 4 4 4 5% 4 6% 11% 52%	1654
	21st to end	Night	5% 4 2 5% 4 5% 5% 4 7% 10% 49%	1600
July	1st to 10th	Night	5% 5% 2 7% 5% 6% 6% 5% 8% 12% 39%	3618
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 5% 8% 13% 40%	3827
	21st to end	Night	7% 7% 3 8% 6% 5% 7% 6% 10% 12% 28%	4810
August	1st to 10th	Night	2 2 1 4 3 4 4 4 7% 10% 60%	3244
	11th to 20th	Night	4 3 1 5% 4 4 5% 4 7% 11% 53%	3200
	21st to end	Night	6% 3 2 5% 3 4 4 4 7% 9% 54%	3586
September	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 8% 48%	2000
	11th to 20th	Night	11% 6% 2 5% 4 4 3 3 7% 8% 46%	2160
	21st to end	Night	8% 4 2 5% 3 4 4 4 7% 7% 53%	2200
October	1st to 10th	Night	8% 4 2 4 2 3 3 2 4 6% 61%	2200
	11th to 20th	Night	11% 5% 2 5% 3 2 4 2 4 6% 56%	2293
	21st to end	Night	16% 6% 2 4 2 3 2 2 4 5% 55%	2715
November	1st to 10th	Night	11% 5% 2 4 5% 3 3 3 4 5% 55%	2666
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 4 7% 54%	2730
	21st to end	Night	16% 6% 2 4 4 3 3 3 4 5% 48%	2730
December	1st to 10th	Night	21% 6% 2 4 3 2 3 3 4 6% 46%	4380
	11th to 20th	Night	19% 9% 2 6% 3 4 3 2 3 4 44%	4952
	21st to end	Night	21% 8% 3 5% 3 4 3 3 5% 6% 38%	6633

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	44% 3 4 27% 19% 3	3718
	11th to 20th	Day	36% 3 5% 25% 27% 4	4372
	21st to end	Day	47% 3 6% 18% 20% 6%	5082
February	1st to 10th	Day	47% 4 7% 19% 20% 4	2000
	11th to 20th	Day	48% 3 5% 18% 19% 6%	1958
	21st to end	Day	47% 4 6% 13% 23% 7%	1650
March	1st to 10th	Day	50% 5% 7% 12% 18% 8%	4480
	11th to 20th	Day	39% 5% 7% 17% 19% 12%	4657
	21st to end	Day	42% 5% 9% 22% 18% 4	5969
April	1st to 10th	Day	36% 6% 10% 22% 19% 7%	2400
	11th to 20th	Day	36% 5% 7% 23% 21% 7%	2374
	21st to end	Day	34% 6% 10% 25% 22% 3	2400
May	1st to 10th	Day	44% 9% 12% 24% 10% 2	5620
	11th to 20th	Day	47% 9% 15% 20% 8% 1	5865
	21st to end	Day	41% 9% 11% 22% 15% 2	6447
June	1st to 10th	Day	38% 10% 13% 20% 16% 2	2600
	11th to 20th	Day	32% 9% 15% 24% 18% 2	2738
	21st to end	Day	33% 10% 17% 21% 16% 4	2800
July	1st to 10th	Day	36% 12% 17% 23% 10% 1	6291
	11th to 20th	Day	33% 14% 26% 22% 4 1	6060
	21st to end	Day	43% 13% 22% 18% 4	5850
August	1st to 10th	Day	23% 9% 13% 34% 21% 1	3893
	11th to 20th	Day	31% 9% 16% 27% 15% 3	3885
	21st to end	Day	29% 9% 13% 22% 23% 4	4311
September	1st to 10th	Day	36% 8% 13% 19% 19% 5%	2400
	11th to 20th	Day	38% 7% 11% 17% 21% 5%	2219
	21st to end	Day	33% 6% 10% 22% 21% 8%	2200
October	1st to 10th	Day	20% 4 5% 15% 28% 27%	3590
	11th to 20th	Day	18% 3 5% 18% 32% 25%	3407
	21st to end	Day	26% 4 4 21% 34% 11%	3414
November	1st to 10th	Day	33% 3 8% 19% 28% 9%	1852
	11th to 20th	Day	33% 4 7% 20% 26% 10%	1890
	21st to end	Day	39% 5% 6% 19% 24% 7%	1890
December	1st to 10th	Day	45% 4 4 19% 24% 3	3020
	11th to 20th	Day	48% 3 7% 19% 18% 6%	3460
	21st to end	Day	44% 4 8% 23% 19% 1	4565
January	1st to 10th	Night	46% 3 5% 21% 23% 2	6423
	11th to 20th	Night	42% 3 4 19% 25% 7%	5756
	21st to end	Night	46% 3 4 18% 23% 7%	6116
February	1st to 10th	Night	46% 3 6% 19% 20% 6%	2401
	11th to 20th	Night	48% 4 5% 17% 19% 7%	2339
	21st to end	Night	46% 6% 5% 16% 19% 9%	1980
March	1st to 10th	Night	51% 4 7% 13% 17% 8%	5373
	11th to 20th	Night	33% 5% 7% 17% 21% 16%	5317
	21st to end	Night	46% 4 5% 21% 18% 6%	4962
April	1st to 10th	Night	36% 6% 11% 22% 19% 7%	2000
	11th to 20th	Night	34% 4 8% 21% 22% 10%	1975
	21st to end	Night	33% 6% 11% 24% 22% 5%	2000
May	1st to 10th	Night	41% 9% 13% 21% 15% 2	4336
	11th to 20th	Night	49% 8% 13% 20% 9% 2	4033
	21st to end	Night	36% 10% 13% 22% 16% 2	4429
June	1st to 10th	Night	33% 10% 13% 23% 19% 2	1800
	11th to 20th	Night	26% 9% 15% 27% 20% 4	1654
	21st to end	Night	28% 11% 16% 22% 19% 4	1600
July	1st to 10th	Night	33% 12% 22% 22% 9% 1	3580
	11th to 20th	Night	31% 12% 28% 23% 5% 1	3782
	21st to end	Night	42% 14% 23% 17% 4 1	4669
August	1st to 10th	Night	16% 8% 14% 37% 23% 2	3232
	11th to 20th	Night	24% 9% 20% 29% 16% 2	3230
	21st to end	Night	25% 8% 16% 21% 23% 6%	3586
September	1st to 10th	Night	34% 7% 16% 21% 17% 6%	2000
	11th to 20th	Night	34% 7% 13% 20% 21% 6%	2160
	21st to end	Night	28% 6% 10% 24% 24% 8%	2200
October	1st to 10th	Night	18% 3 6% 17% 31% 25%	3590
	11th to 20th	Night	19% 3 5% 15% 32% 27%	3727
	21st to end	Night	23% 3 4 24% 30% 17%	4375
November	1st to 10th	Night	31% 4 8% 18% 26% 13%	2666
	11th to 20th	Night	31% 4 7% 19% 29% 11%	2730
	21st to end	Night	39% 4 5% 21% 23% 9%	2730
December	1st to 10th	Night	41% 4 5% 22% 25% 3	4380
	11th to 20th	Night	45% 4 6% 18% 23% 4	4954
	21st to end	Night	44% 5% 8% 23% 17% 2	6633

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	32% 22% 29% 15% 2	330
	11th to 20th	Day	30% 23% 29% 14% 3 1	330
	21st to end	Day	27% 23% 37% 9% 2 1	363
February	1st to 10th	Day	26% 23% 33% 14% 3	300
	11th to 20th	Day	36% 21% 32% 9% 1 1	300
	21st to end	Day	31% 23% 35% 9% 1	249
March	1st to 10th	Day	28% 20% 34% 13% 4 1	300
	11th to 20th	Day	24% 21% 37% 14% 3 1	340
	21st to end	Day	24% 16% 35% 18% 6%	550
April	1st to 10th	Day	21% 18% 33% 19% 9% 1	500
	11th to 20th	Day	21% 16% 30% 20% 11% 2	492
	21st to end	Day	17% 12% 30% 27% 13% 1	500
May	1st to 10th	Day	18% 11% 26% 29% 12% 4	503
	11th to 20th	Day	13% 7% 31% 26% 14% 8%	527
	21st to end	Day	11% 6% 25% 29% 18% 11%	573
June	1st to 10th	Day	12% 5% 19% 29% 23% 12%	530
	11th to 20th	Day	7% 3 15% 34% 26% 14%	668
	21st to end	Day	9% 3 14% 29% 30% 14%	730
July	1st to 10th	Day	8% 2 10% 32% 33% 15%	721
	11th to 20th	Day	9% 3 10% 22% 32% 25%	623
	21st to end	Day	10% 2 9% 24% 33% 23%	576
August	1st to 10th	Day	9% 2 10% 27% 33% 20%	526
	11th to 20th	Day	8% 2 11% 27% 32% 19%	519
	21st to end	Day	13% 2 10% 27% 29% 19%	583
September	1st to 10th	Day	10% 3 11% 37% 30% 10%	530
	11th to 20th	Day	11% 3 13% 26% 32% 15%	530
	21st to end	Day	14% 4 16% 30% 28% 8%	530
October	1st to 10th	Day	12% 5% 23% 32% 22% 7%	530
	11th to 20th	Day	10% 10% 29% 37% 12% 1	432
	21st to end	Day	21% 13% 26% 28% 10% 1	363
November	1st to 10th	Day	21% 10% 28% 28% 11% 2	330
	11th to 20th	Day	15% 17% 38% 23% 6% 1	330
	21st to end	Day	19% 19% 36% 17% 8% 1	330
December	1st to 10th	Day	30% 19% 32% 15% 3 1	330
	11th to 20th	Day	25% 20% 32% 15% 8%	330
	21st to end	Day	27% 18% 33% 17% 4	363
January	1st to 10th	Night	23% 17% 33% 22% 4	1024
	11th to 20th	Night	24% 16% 31% 23% 6%	895
	21st to end	Night	22% 15% 38% 23% 2	924
February	1st to 10th	Night	25% 12% 32% 25% 5%	801
	11th to 20th	Night	27% 17% 31% 23% 2	783
	21st to end	Night	22% 19% 33% 23% 1	660
March	1st to 10th	Night	22% 16% 30% 27% 5%	800
	11th to 20th	Night	19% 12% 35% 26% 8% 1	797
	21st to end	Night	20% 11% 34% 25% 10%	660
April	1st to 10th	Night	16% 15% 31% 26% 11% 1	600
	11th to 20th	Night	14% 9% 35% 28% 11% 2	593
	21st to end	Night	15% 9% 31% 29% 15% 1	600
May	1st to 10th	Night	12% 4 30% 32% 21% 2	600
	11th to 20th	Night	14% 3 24% 32% 21% 5%	591
	21st to end	Night	10% 2 17% 34% 31% 6%	650
June	1st to 10th	Night	8% 3 14% 35% 33% 8%	600
	11th to 20th	Night	5% 2 13% 30% 42% 9%	600
	21st to end	Night	6% 3 9% 31% 40% 12%	600
July	1st to 10th	Night	5% 1 7% 29% 45% 13%	585
	11th to 20th	Night	7% 2 8% 25% 40% 19%	582
	21st to end	Night	7% 1 8% 25% 44% 15%	657
August	1st to 10th	Night	5% 5% 29% 45% 15%	597
	11th to 20th	Night	7% 1 9% 30% 41% 12%	594
	21st to end	Night	10% 1 11% 29% 39% 10%	660
September	1st to 10th	Night	10% 2 12% 33% 36% 6%	600
	11th to 20th	Night	12% 4 12% 29% 38% 5%	760
	21st to end	Night	9% 3 16% 33% 36% 4	800
October	1st to 10th	Night	10% 5% 20% 33% 28% 5%	800
	11th to 20th	Night	14% 7% 22% 37% 19% 2	800
	21st to end	Night	19% 7% 20% 34% 18% 2	880
November	1st to 10th	Night	13% 9% 21% 35% 19% 2	820
	11th to 20th	Night	13% 10% 32% 32% 12% 1	840
	21st to end	Night	20% 9% 30% 29% 11% 1	840
December	1st to 10th	Night	25% 10% 28% 29% 8%	840
	11th to 20th	Night	23% 12% 29% 29% 7%	840
	21st to end	Night	24% 14% 30% 25% 6%	924

Page generated at 2026-01-29 00:21:45. Version ac07

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