Starlight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	14% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	130819
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 41%	130668
Transparency	Day	42% 6% 11% 21% 15% 4	134424
	Night	40% 6% 12% 22% 16% 4	135133
Seeing	Day	23% 16% 25% 20% 12% 4	16704
	Night	19% 13% 24% 28% 14% 2	26778

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	14% 8% 3 7% 4 4 3 3 5% 6% 43%	5258
	11th to 20th	Day	9% 10% 4 9% 6% 4 4 3 4 4 43%	5280
	21st to end	Day	14% 6% 2 7% 4 4 4 3 5% 5% 45%	5780
February	1st to 10th	Day	12% 8% 3 8% 5% 4 6% 4 4 6% 39%	1890
	11th to 20th	Day	17% 8% 3 7% 4 5% 5% 4 5% 6% 37%	1843
	21st to end	Day	17% 9% 2 6% 4 4 4 4 5% 6% 39%	1814
March	1st to 10th	Day	16% 7% 3 6% 5% 4 4 4 5% 7% 38%	5045
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 5% 7% 48%	5057
	21st to end	Day	13% 8% 3 8% 5% 4 5% 4 5% 5% 39%	5465
April	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 5% 7% 42%	2306
	11th to 20th	Day	14% 7% 2 5% 4 4 4 4 6% 7% 43%	2374
	21st to end	Day	17% 8% 3 6% 5% 4 4 4 5% 6% 38%	2529
May	1st to 10th	Day	14% 7% 3 8% 6% 6% 7% 5% 7% 8% 29%	5862
	11th to 20th	Day	23% 7% 3 7% 5% 5% 5% 4 6% 9% 29%	5861
	21st to end	Day	16% 6% 2 7% 5% 6% 6% 5% 7% 8% 33%	6433
June	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 9% 39%	2775
	11th to 20th	Day	11% 6% 3 7% 5% 5% 6% 5% 7% 10% 35%	2800
	21st to end	Day	8% 5% 3 6% 7% 5% 5% 5% 7% 10% 39%	2693
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 6% 5% 7% 10% 38%	5900
	11th to 20th	Day	8% 5% 2 8% 7% 7% 7% 6% 9% 11% 29%	5900
	21st to end	Day	12% 7% 3 8% 6% 6% 7% 5% 8% 10% 29%	6424
August	1st to 10th	Day	14% 6% 2 8% 5% 5% 6% 5% 8% 10% 30%	4234
	11th to 20th	Day	9% 5% 3 8% 6% 7% 7% 5% 8% 9% 33%	4174
	21st to end	Day	8% 4 2 6% 5% 5% 6% 4 7% 9% 43%	4345
September	1st to 10th	Day	14% 6% 2 6% 4 5% 5% 3 5% 8% 44%	2400
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 3 6% 7% 42%	2218
	21st to end	Day	24% 6% 3 7% 3 5% 3 4 5% 6% 33%	2185
October	1st to 10th	Day	15% 6% 2 6% 4 4 4 2 4 6% 46%	2000
	11th to 20th	Day	16% 5% 2 5% 4 4 3 3 5% 6% 47%	2000
	21st to end	Day	16% 6% 2 5% 4 3 3 3 4 5% 48%	2200
November	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 5% 7% 52%	2057
	11th to 20th	Day	15% 5% 2 5% 4 3 3 4 6% 6% 47%	1893
	21st to end	Day	14% 8% 2 6% 4 4 4 3 5% 6% 45%	1680
December	1st to 10th	Day	12% 9% 2 6% 4 5% 5% 3 4 5% 45%	2680
	11th to 20th	Day	19% 8% 3 6% 3 3 3 3 5% 6% 41%	3070
	21st to end	Day	20% 8% 3 9% 5% 4 4 4 5% 6% 31%	4394
January	1st to 10th	Night	11% 8% 2 7% 5% 5% 5% 4 6% 7% 40%	7635
	11th to 20th	Night	9% 7% 3 8% 5% 4 3 2 4 6% 48%	7680
	21st to end	Night	15% 6% 2 6% 4 4 4 3 4 5% 47%	8401
February	1st to 10th	Night	12% 8% 3 6% 4 5% 4 5% 5% 8% 41%	2730
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 5% 6% 37%	2663
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 38%	1989
March	1st to 10th	Night	18% 9% 3 6% 4 4 4 3 4 5% 41%	5028
	11th to 20th	Night	11% 4 2 5% 3 3 4 4 6% 8% 49%	5058
	21st to end	Night	13% 7% 2 7% 5% 4 4 4 5% 7% 42%	5467
April	1st to 10th	Night	15% 8% 2 6% 5% 5% 4 4 4 6% 40%	2092
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 4 6% 8% 44%	1975
	21st to end	Night	16% 5% 2 5% 4 4 4 3 7% 8% 42%	1888
May	1st to 10th	Night	18% 7% 3 7% 4 4 5% 4 7% 8% 34%	4033
	11th to 20th	Night	23% 9% 3 7% 5% 5% 4 4 6% 7% 29%	4032
	21st to end	Night	16% 6% 3 8% 5% 5% 5% 5% 7% 9% 33%	4421
June	1st to 10th	Night	11% 5% 1 5% 4 4 5% 4 7% 10% 44%	1615
	11th to 20th	Night	11% 5% 2 6% 4 5% 4 4 6% 9% 44%	1600
	21st to end	Night	9% 4 1 4 3 4 5% 6% 8% 10% 44%	1696
July	1st to 10th	Night	8% 6% 2 7% 4 4 5% 4 8% 9% 42%	4058
	11th to 20th	Night	11% 7% 3 9% 6% 6% 5% 5% 7% 9% 31%	4057
	21st to end	Night	11% 9% 3 8% 6% 6% 6% 5% 7% 9% 29%	4427
August	1st to 10th	Night	16% 6% 3 7% 5% 5% 5% 4 6% 8% 35%	2915
	11th to 20th	Night	13% 7% 2 8% 5% 5% 5% 5% 6% 7% 37%	2875
	21st to end	Night	9% 6% 2 6% 4 4 4 4 5% 9% 46%	3520
September	1st to 10th	Night	12% 4 2 6% 4 4 5% 3 5% 7% 49%	2000
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 5% 8% 45%	2178
	21st to end	Night	23% 8% 2 5% 3 4 4 3 5% 5% 37%	2213
October	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Night	14% 6% 2 5% 4 3 4 3 4 8% 47%	2400
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 7% 48%	2640
November	1st to 10th	Night	11% 8% 2 6% 3 3 3 3 4 6% 49%	2461
	11th to 20th	Night	14% 6% 2 5% 3 3 4 4 5% 7% 48%	2719
	21st to end	Night	14% 6% 2 5% 4 4 4 4 5% 6% 45%	2940
December	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 6% 7% 46%	4720
	11th to 20th	Night	18% 9% 3 6% 4 5% 5% 3 4 5% 38%	5345
	21st to end	Night	17% 6% 3 6% 5% 5% 5% 4 6% 7% 35%	6797

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	41% 3 10% 20% 19% 7%	5259
	11th to 20th	Day	43% 3 8% 14% 20% 12%	5280
	21st to end	Day	42% 3 7% 26% 18% 5%	5780
February	1st to 10th	Day	45% 5% 10% 20% 16% 4	1890
	11th to 20th	Day	47% 5% 9% 19% 16% 4	1852
	21st to end	Day	45% 5% 10% 18% 16% 5%	1814
March	1st to 10th	Day	44% 4 9% 19% 15% 8%	5045
	11th to 20th	Day	34% 4 9% 25% 20% 8%	5057
	21st to end	Day	46% 6% 8% 17% 17% 6%	5465
April	1st to 10th	Day	42% 6% 11% 19% 16% 6%	2306
	11th to 20th	Day	39% 6% 11% 22% 18% 4	2374
	21st to end	Day	46% 6% 10% 21% 12% 4	2529
May	1st to 10th	Day	49% 8% 12% 20% 9% 2	5897
	11th to 20th	Day	52% 6% 11% 20% 10% 1	5865
	21st to end	Day	46% 8% 14% 17% 12% 3	6447
June	1st to 10th	Day	40% 8% 14% 23% 14% 1	2775
	11th to 20th	Day	42% 10% 14% 19% 14% 1	2800
	21st to end	Day	38% 11% 14% 23% 12% 2	2693
July	1st to 10th	Day	39% 10% 16% 21% 10% 4	5835
	11th to 20th	Day	42% 13% 18% 20% 6% 1	5831
	21st to end	Day	47% 8% 13% 22% 9%	6240
August	1st to 10th	Day	46% 10% 16% 21% 7%	4221
	11th to 20th	Day	43% 9% 12% 24% 11% 1	4213
	21st to end	Day	36% 9% 16% 26% 12% 1	4345
September	1st to 10th	Day	39% 7% 13% 17% 19% 4	2400
	11th to 20th	Day	40% 6% 9% 23% 19% 3	2218
	21st to end	Day	51% 5% 10% 18% 15% 2	2185
October	1st to 10th	Day	32% 4 9% 20% 27% 8%	3260
	11th to 20th	Day	28% 3 10% 25% 19% 14%	3240
	21st to end	Day	28% 3 9% 29% 19% 13%	3538
November	1st to 10th	Day	33% 4 10% 23% 25% 4	2057
	11th to 20th	Day	37% 5% 9% 20% 23% 7%	1893
	21st to end	Day	39% 5% 9% 23% 20% 3	1680
December	1st to 10th	Day	41% 4 8% 21% 21% 4	2680
	11th to 20th	Day	45% 4 10% 20% 18% 4	3066
	21st to end	Day	51% 5% 12% 17% 14% 1	4394
January	1st to 10th	Night	40% 5% 11% 20% 19% 6%	7636
	11th to 20th	Night	36% 3 9% 21% 22% 9%	7680
	21st to end	Night	39% 3 9% 24% 21% 4	8401
February	1st to 10th	Night	40% 6% 13% 23% 16% 2	2730
	11th to 20th	Night	47% 5% 9% 18% 17% 5%	2665
	21st to end	Night	43% 6% 11% 20% 13% 6%	1989
March	1st to 10th	Night	44% 5% 9% 19% 15% 8%	5028
	11th to 20th	Night	32% 5% 9% 26% 23% 5%	5058
	21st to end	Night	41% 5% 13% 18% 18% 5%	5467
April	1st to 10th	Night	43% 7% 13% 18% 16% 4	2092
	11th to 20th	Night	36% 6% 15% 21% 18% 4	1975
	21st to end	Night	39% 6% 14% 26% 13% 2	1888
May	1st to 10th	Night	46% 10% 14% 19% 9% 2	4060
	11th to 20th	Night	52% 8% 12% 21% 6% 1	4033
	21st to end	Night	44% 9% 18% 21% 5% 3	4429
June	1st to 10th	Night	33% 11% 14% 26% 15%	1615
	11th to 20th	Night	37% 9% 17% 23% 12% 2	1600
	21st to end	Night	30% 11% 19% 23% 14% 1	1696
July	1st to 10th	Night	37% 10% 18% 23% 10% 2	4015
	11th to 20th	Night	47% 11% 18% 18% 6%	4006
	21st to end	Night	47% 10% 16% 18% 8%	4297
August	1st to 10th	Night	45% 9% 18% 23% 5%	2904
	11th to 20th	Night	42% 8% 16% 22% 12% 1	2902
	21st to end	Night	34% 7% 17% 30% 10% 1	3520
September	1st to 10th	Night	35% 8% 14% 20% 19% 4	2000
	11th to 20th	Night	38% 6% 10% 27% 17% 2	2178
	21st to end	Night	47% 6% 11% 19% 15% 2	2213
October	1st to 10th	Night	38% 4 9% 20% 23% 5%	3920
	11th to 20th	Night	30% 4 10% 19% 25% 12%	3899
	21st to end	Night	24% 3 12% 27% 25% 9%	4252
November	1st to 10th	Night	36% 4 9% 22% 23% 6%	2461
	11th to 20th	Night	35% 5% 9% 23% 23% 5%	2719
	21st to end	Night	38% 6% 10% 22% 19% 4	2940
December	1st to 10th	Night	37% 4 10% 26% 20% 2	4720
	11th to 20th	Night	47% 4 12% 17% 16% 3	5348
	21st to end	Night	44% 7% 14% 20% 15% 1	6797

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	35% 28% 23% 10% 3 1	330
	11th to 20th	Day	38% 28% 24% 8% 1	330
	21st to end	Day	39% 28% 25% 7% 2	363
February	1st to 10th	Day	30% 33% 26% 8% 2	330
	11th to 20th	Day	50% 23% 19% 7% 1	330
	21st to end	Day	38% 23% 26% 12% 1	367
March	1st to 10th	Day	30% 24% 25% 18% 4	540
	11th to 20th	Day	31% 25% 24% 14% 4 1	528
	21st to end	Day	28% 24% 27% 15% 5% 1	550
April	1st to 10th	Day	30% 26% 21% 16% 6%	500
	11th to 20th	Day	25% 20% 27% 18% 9% 2	492
	21st to end	Day	27% 21% 24% 17% 9% 2	500
May	1st to 10th	Day	26% 20% 28% 14% 9% 3	503
	11th to 20th	Day	27% 17% 25% 18% 9% 4	527
	21st to end	Day	20% 13% 23% 24% 13% 6%	573
June	1st to 10th	Day	15% 13% 27% 25% 14% 6%	530
	11th to 20th	Day	14% 11% 28% 22% 17% 7%	530
	21st to end	Day	9% 10% 30% 30% 16% 5%	530
July	1st to 10th	Day	13% 5% 22% 28% 21% 11%	525
	11th to 20th	Day	6% 4 26% 32% 23% 9%	524
	21st to end	Day	14% 6% 24% 26% 20% 10%	576
August	1st to 10th	Day	18% 7% 21% 23% 20% 11%	526
	11th to 20th	Day	14% 5% 22% 27% 22% 10%	519
	21st to end	Day	12% 6% 18% 32% 20% 13%	583
September	1st to 10th	Day	16% 8% 21% 23% 22% 10%	530
	11th to 20th	Day	14% 5% 20% 30% 22% 9%	530
	21st to end	Day	31% 11% 20% 22% 13% 3	530
October	1st to 10th	Day	20% 8% 23% 27% 18% 4	530
	11th to 20th	Day	21% 15% 25% 22% 14% 2	530
	21st to end	Day	25% 16% 29% 20% 9% 2	583
November	1st to 10th	Day	20% 16% 32% 21% 9% 2	535
	11th to 20th	Day	27% 20% 28% 19% 5% 1	348
	21st to end	Day	33% 18% 29% 15% 4 1	220
December	1st to 10th	Day	27% 30% 26% 11% 4 1	220
	11th to 20th	Day	32% 21% 25% 15% 5% 1	220
	21st to end	Day	34% 22% 28% 12% 4	322
January	1st to 10th	Night	24% 25% 30% 18% 3	840
	11th to 20th	Night	25% 20% 29% 24% 3	840
	21st to end	Night	21% 22% 37% 18% 2	924
February	1st to 10th	Night	20% 22% 32% 23% 2	840
	11th to 20th	Night	26% 25% 32% 16% 1	823
	21st to end	Night	27% 20% 35% 17% 2	692
March	1st to 10th	Night	23% 19% 32% 23% 3	840
	11th to 20th	Night	27% 19% 28% 21% 5% 1	829
	21st to end	Night	21% 19% 33% 22% 6%	880
April	1st to 10th	Night	21% 22% 27% 25% 5%	800
	11th to 20th	Night	18% 16% 30% 26% 10%	791
	21st to end	Night	20% 14% 27% 31% 8%	688
May	1st to 10th	Night	18% 10% 27% 30% 14% 1	600
	11th to 20th	Night	21% 10% 21% 31% 15% 2	591
	21st to end	Night	18% 4 18% 35% 23% 3	650
June	1st to 10th	Night	11% 3 18% 40% 25% 2	415
	11th to 20th	Night	14% 4 16% 41% 26% 2	400
	21st to end	Night	10% 3 18% 43% 22% 3	496
July	1st to 10th	Night	12% 1 14% 38% 29% 5%	585
	11th to 20th	Night	8% 2 11% 34% 39% 5%	582
	21st to end	Night	9% 3 14% 37% 34% 4	657
August	1st to 10th	Night	16% 3 10% 36% 33% 3	597
	11th to 20th	Night	13% 3 10% 37% 33% 4	594
	21st to end	Night	11% 4 13% 36% 32% 4	840
September	1st to 10th	Night	14% 4 12% 37% 27% 7%	800
	11th to 20th	Night	18% 4 14% 31% 29% 4	800
	21st to end	Night	26% 7% 18% 29% 20% 1	800
October	1st to 10th	Night	18% 9% 16% 31% 23% 4	800
	11th to 20th	Night	19% 14% 23% 29% 15% 1	800
	21st to end	Night	19% 14% 24% 31% 11% 1	880
November	1st to 10th	Night	17% 11% 27% 33% 10% 2	820
	11th to 20th	Night	22% 15% 27% 27% 8% 1	840
	21st to end	Night	20% 18% 26% 28% 7% 1	840
December	1st to 10th	Night	24% 19% 28% 24% 4	840
	11th to 20th	Night	21% 18% 28% 28% 4	840
	21st to end	Night	22% 17% 27% 30% 3	924

Page generated at 2026-03-15 02:23:55. Version ac07

Copyright © 2026 Attilla Danko.