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 2026-05-02.

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% 8% 42%	131795
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	130704
Transparency	Day	38% 7% 12% 23% 16% 3	135355
	Night	39% 6% 11% 23% 17% 4	135215
Seeing	Day	17% 13% 27% 25% 14% 5%	16876
	Night	18% 10% 30% 30% 11% 1	26336

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	13% 5% 2 6% 5% 4 4 3 5% 5% 48%	4661
	11th to 20th	Day	15% 8% 2 7% 4 3 3 3 3 4 48%	4997
	21st to end	Day	15% 5% 2 5% 4 5% 6% 4 6% 7% 43%	5780
February	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 6% 42%	1890
	11th to 20th	Day	17% 8% 2 5% 4 4 4 4 6% 6% 40%	1980
	21st to end	Day	17% 10% 3 6% 4 3 4 3 5% 6% 40%	1730
March	1st to 10th	Day	20% 4 2 4 3 3 4 3 4 6% 47%	4687
	11th to 20th	Day	12% 7% 2 5% 4 4 4 3 4 6% 48%	5090
	21st to end	Day	18% 5% 2 6% 4 4 4 3 5% 6% 43%	5860
April	1st to 10th	Day	16% 8% 2 5% 4 4 4 4 5% 6% 43%	2520
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 44%	2494
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 7% 41%	2520
May	1st to 10th	Day	25% 7% 2 5% 4 4 4 4 6% 9% 32%	5727
	11th to 20th	Day	25% 6% 2 6% 4 4 4 3 5% 8% 32%	6318
	21st to end	Day	18% 5% 2 6% 4 4 4 4 6% 9% 38%	6934
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2800
	11th to 20th	Day	13% 5% 2 5% 5% 5% 5% 5% 6% 9% 40%	2800
	21st to end	Day	10% 4 2 5% 4 4 5% 5% 7% 10% 44%	2800
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 5% 8% 10% 42%	6360
	11th to 20th	Day	11% 6% 2 7% 5% 5% 5% 5% 8% 11% 33%	6360
	21st to end	Day	9% 5% 2 7% 5% 5% 5% 4 7% 10% 41%	6653
August	1st to 10th	Day	16% 6% 2 7% 4 4 5% 4 8% 10% 34%	4234
	11th to 20th	Day	12% 5% 2 5% 4 5% 5% 4 7% 9% 42%	3881
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 49%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 4 1 4 4 4 4 3 5% 8% 49%	2390
	21st to end	Day	19% 7% 3 7% 5% 3 4 3 5% 6% 38%	2164
October	1st to 10th	Day	16% 6% 2 5% 3 4 3 3 4 6% 47%	2000
	11th to 20th	Day	16% 5% 2 5% 3 3 4 3 5% 6% 48%	2000
	21st to end	Day	16% 5% 2 5% 4 4 3 3 4 5% 48%	2175
November	1st to 10th	Day	12% 5% 1 6% 3 3 3 3 5% 6% 54%	1851
	11th to 20th	Day	14% 5% 2 4 3 3 4 4 6% 6% 48%	1890
	21st to end	Day	15% 5% 2 4 5% 3 5% 3 5% 5% 47%	1739
December	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 6% 5% 47%	2680
	11th to 20th	Day	20% 8% 2 5% 3 3 4 3 5% 6% 41%	3070
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	4048
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 45%	8233
	11th to 20th	Night	15% 7% 3 5% 3 3 3 3 4 5% 50%	7948
	21st to end	Night	15% 7% 2 5% 3 3 4 3 5% 5% 47%	8401
February	1st to 10th	Night	16% 8% 3 6% 4 4 4 3 6% 6% 41%	2730
	11th to 20th	Night	20% 8% 2 5% 4 3 4 3 4 6% 43%	2525
	21st to end	Night	17% 8% 2 5% 4 4 4 4 5% 6% 42%	2076
March	1st to 10th	Night	18% 6% 2 5% 4 3 3 2 4 6% 47%	5382
	11th to 20th	Night	11% 7% 2 5% 4 3 3 3 4 5% 51%	5090
	21st to end	Night	15% 7% 3 6% 4 4 4 3 4 5% 44%	5312
April	1st to 10th	Night	17% 7% 3 6% 4 4 3 4 5% 5% 42%	2100
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 8% 44%	2075
	21st to end	Night	19% 5% 2 5% 3 3 4 3 5% 6% 44%	2100
May	1st to 10th	Night	22% 9% 3 7% 5% 4 5% 4 5% 7% 29%	4220
	11th to 20th	Night	24% 8% 3 6% 4 4 3 4 6% 7% 31%	3575
	21st to end	Night	21% 7% 2 6% 3 4 4 3 5% 8% 38%	3920
June	1st to 10th	Night	14% 6% 2 6% 4 3 5% 4 6% 8% 43%	1600
	11th to 20th	Night	12% 6% 3 6% 6% 5% 5% 4 6% 9% 40%	1600
	21st to end	Night	11% 5% 2 5% 4 4 4 5% 8% 10% 42%	1600
July	1st to 10th	Night	8% 7% 3 7% 6% 5% 5% 5% 7% 9% 38%	3598
	11th to 20th	Night	12% 7% 3 8% 6% 5% 5% 5% 8% 11% 30%	3597
	21st to end	Night	11% 5% 3 7% 4 4 4 5% 7% 8% 43%	4193
August	1st to 10th	Night	18% 5% 2 6% 5% 4 5% 4 6% 9% 35%	2915
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 4 5% 7% 41%	3133
	21st to end	Night	11% 5% 2 5% 3 4 4 4 5% 8% 50%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 4 3 3 5% 6% 47%	2000
	11th to 20th	Night	13% 5% 2 5% 3 2 4 3 4 7% 53%	2009
	21st to end	Night	20% 7% 2 7% 4 4 3 4 5% 7% 38%	2232
October	1st to 10th	Night	19% 6% 2 6% 5% 3 3 3 4 5% 44%	2400
	11th to 20th	Night	17% 4 1 5% 3 4 2 2 4 6% 51%	2400
	21st to end	Night	16% 5% 2 5% 3 3 4 3 4 5% 50%	2665
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2667
	11th to 20th	Night	16% 7% 2 5% 3 3 3 2 5% 5% 48%	2730
	21st to end	Night	14% 6% 1 4 5% 4 4 4 6% 6% 47%	2877
December	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 4 7% 48%	4720
	11th to 20th	Night	22% 6% 2 5% 3 3 3 3 6% 7% 41%	5345
	21st to end	Night	18% 7% 3 5% 4 4 4 4 5% 7% 39%	7150

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	37% 4 10% 25% 20% 3	4662
	11th to 20th	Day	41% 4 6% 23% 19% 7%	4997
	21st to end	Day	39% 5% 12% 29% 13% 2	5780
February	1st to 10th	Day	41% 7% 13% 24% 13% 2	1890
	11th to 20th	Day	44% 5% 10% 24% 15% 3	1989
	21st to end	Day	45% 5% 10% 23% 14% 4	1730
March	1st to 10th	Day	38% 4 9% 25% 17% 6%	4687
	11th to 20th	Day	34% 6% 10% 27% 17% 6%	5090
	21st to end	Day	41% 5% 9% 18% 21% 6%	5860
April	1st to 10th	Day	40% 7% 12% 19% 16% 6%	2520
	11th to 20th	Day	38% 6% 12% 22% 17% 5%	2494
	21st to end	Day	40% 6% 12% 20% 18% 4	2520
May	1st to 10th	Day	48% 7% 15% 18% 11% 2	5759
	11th to 20th	Day	49% 8% 11% 21% 10% 1	6322
	21st to end	Day	40% 8% 12% 23% 15% 1	6949
June	1st to 10th	Day	36% 10% 13% 24% 16% 2	2800
	11th to 20th	Day	38% 8% 15% 23% 13% 3	2800
	21st to end	Day	32% 9% 16% 23% 18% 1	2800
July	1st to 10th	Day	35% 10% 17% 20% 15% 3	6290
	11th to 20th	Day	41% 10% 19% 21% 8% 1	6285
	21st to end	Day	37% 9% 10% 22% 21% 1	6463
August	1st to 10th	Day	43% 9% 16% 23% 9%	4221
	11th to 20th	Day	35% 9% 13% 28% 13% 1	3918
	21st to end	Day	32% 7% 12% 29% 18% 2	4312
September	1st to 10th	Day	38% 7% 11% 21% 20% 4	2400
	11th to 20th	Day	33% 5% 11% 25% 22% 4	2390
	21st to end	Day	47% 6% 9% 18% 18% 2	2164
October	1st to 10th	Day	35% 5% 9% 18% 28% 5%	3260
	11th to 20th	Day	25% 5% 10% 24% 28% 9%	3240
	21st to end	Day	24% 3 10% 29% 28% 5%	3489
November	1st to 10th	Day	32% 4 13% 26% 21% 4	1851
	11th to 20th	Day	35% 6% 11% 23% 22% 3	1890
	21st to end	Day	38% 5% 10% 31% 13% 3	1739
December	1st to 10th	Day	37% 5% 15% 26% 13% 3	2680
	11th to 20th	Day	43% 4 12% 29% 9% 2	3066
	21st to end	Day	39% 7% 15% 21% 17% 2	4048
January	1st to 10th	Night	39% 4 11% 24% 17% 4	8234
	11th to 20th	Night	38% 4 8% 23% 19% 8%	7948
	21st to end	Night	39% 4 8% 28% 19% 3	8401
February	1st to 10th	Night	43% 5% 12% 23% 14% 2	2730
	11th to 20th	Night	44% 4 9% 21% 18% 4	2527
	21st to end	Night	43% 5% 10% 20% 16% 6%	2076
March	1st to 10th	Night	40% 4 9% 25% 12% 10%	5382
	11th to 20th	Night	33% 7% 13% 26% 15% 6%	5090
	21st to end	Night	42% 5% 8% 17% 24% 4	5312
April	1st to 10th	Night	42% 5% 11% 19% 19% 4	2100
	11th to 20th	Night	39% 6% 12% 20% 19% 5%	2075
	21st to end	Night	39% 7% 10% 20% 18% 5%	2100
May	1st to 10th	Night	52% 8% 11% 16% 10% 3	4250
	11th to 20th	Night	49% 9% 11% 19% 11% 1	3576
	21st to end	Night	43% 8% 12% 21% 13% 2	3927
June	1st to 10th	Night	38% 8% 11% 23% 19% 1	1600
	11th to 20th	Night	39% 9% 14% 23% 14% 1	1600
	21st to end	Night	34% 10% 14% 24% 16% 3	1600
July	1st to 10th	Night	41% 9% 16% 18% 14% 2	3560
	11th to 20th	Night	46% 10% 19% 18% 7% 1	3552
	21st to end	Night	36% 8% 13% 23% 20% 1	4070
August	1st to 10th	Night	46% 9% 17% 19% 9% 1	2904
	11th to 20th	Night	40% 7% 12% 26% 14% 1	3162
	21st to end	Night	32% 7% 11% 27% 21% 2	3586
September	1st to 10th	Night	38% 7% 10% 21% 21% 4	2000
	11th to 20th	Night	32% 6% 9% 25% 24% 4	2009
	21st to end	Night	45% 6% 10% 19% 18% 3	2232
October	1st to 10th	Night	42% 6% 8% 20% 20% 3	3920
	11th to 20th	Night	25% 4 11% 25% 30% 6%	3899
	21st to end	Night	24% 4 11% 31% 24% 6%	4301
November	1st to 10th	Night	32% 5% 11% 25% 21% 6%	2667
	11th to 20th	Night	38% 5% 9% 22% 20% 6%	2730
	21st to end	Night	36% 5% 12% 26% 18% 2	2877
December	1st to 10th	Night	37% 5% 12% 29% 16% 2	4720
	11th to 20th	Night	43% 4 12% 21% 15% 4	5348
	21st to end	Night	42% 8% 11% 20% 17% 1	7150

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	17% 26% 37% 16% 4	220
	11th to 20th	Day	20% 35% 28% 15% 1	278
	21st to end	Day	18% 23% 38% 14% 5% 1	363
February	1st to 10th	Day	19% 24% 35% 18% 4	330
	11th to 20th	Day	25% 25% 35% 14% 1	468
	21st to end	Day	22% 23% 32% 19% 4	446
March	1st to 10th	Day	18% 25% 32% 19% 6%	540
	11th to 20th	Day	20% 22% 31% 18% 6% 1	540
	21st to end	Day	20% 19% 36% 18% 5% 1	594
April	1st to 10th	Day	22% 20% 35% 16% 6% 2	540
	11th to 20th	Day	16% 20% 30% 24% 8% 3	532
	21st to end	Day	21% 16% 32% 21% 9% 1	540
May	1st to 10th	Day	26% 14% 29% 21% 9% 2	507
	11th to 20th	Day	20% 10% 30% 26% 12% 2	527
	21st to end	Day	18% 6% 24% 26% 20% 6%	573
June	1st to 10th	Day	12% 6% 29% 28% 18% 7%	530
	11th to 20th	Day	16% 7% 22% 32% 18% 5%	530
	21st to end	Day	10% 6% 23% 34% 19% 8%	530
July	1st to 10th	Day	11% 3 16% 31% 28% 10%	525
	11th to 20th	Day	6% 2 15% 37% 30% 11%	524
	21st to end	Day	11% 3 17% 35% 27% 7%	576
August	1st to 10th	Day	12% 4 16% 33% 25% 10%	526
	11th to 20th	Day	13% 3 13% 32% 29% 10%	519
	21st to end	Day	10% 4 19% 31% 24% 12%	583
September	1st to 10th	Day	16% 7% 18% 25% 24% 11%	530
	11th to 20th	Day	15% 3 19% 29% 23% 12%	530
	21st to end	Day	23% 8% 21% 28% 16% 4	530
October	1st to 10th	Day	20% 5% 24% 26% 15% 10%	530
	11th to 20th	Day	15% 15% 25% 26% 16% 4	530
	21st to end	Day	18% 12% 31% 24% 12% 3	569
November	1st to 10th	Day	14% 11% 34% 25% 13% 2	425
	11th to 20th	Day	14% 19% 36% 22% 9%	430
	21st to end	Day	17% 22% 29% 25% 8%	279
December	1st to 10th	Day	20% 19% 34% 21% 5%	220
	11th to 20th	Day	25% 20% 31% 19% 5%	220
	21st to end	Day	22% 20% 28% 21% 7% 1	242
January	1st to 10th	Night	19% 18% 40% 20% 3	840
	11th to 20th	Night	23% 17% 34% 23% 4	840
	21st to end	Night	18% 16% 41% 24% 2	924
February	1st to 10th	Night	19% 14% 39% 25% 2	840
	11th to 20th	Night	24% 20% 37% 18% 1	823
	21st to end	Night	21% 18% 38% 20% 3	692
March	1st to 10th	Night	18% 17% 38% 23% 4	840
	11th to 20th	Night	19% 16% 37% 22% 6%	840
	21st to end	Night	20% 11% 42% 22% 5%	924
April	1st to 10th	Night	21% 16% 36% 22% 5%	840
	11th to 20th	Night	16% 13% 34% 31% 6%	831
	21st to end	Night	21% 10% 32% 30% 8%	840
May	1st to 10th	Night	20% 7% 35% 32% 6%	693
	11th to 20th	Night	19% 9% 30% 33% 9%	394
	21st to end	Night	18% 4 21% 37% 18% 2	433
June	1st to 10th	Night	14% 4 24% 39% 18% 1	400
	11th to 20th	Night	13% 2 24% 45% 16%	400
	21st to end	Night	8% 3 23% 47% 19% 1	400
July	1st to 10th	Night	12% 4 16% 44% 23% 2	390
	11th to 20th	Night	7% 1 14% 43% 32% 2	388
	21st to end	Night	9% 2 16% 43% 27% 2	553
August	1st to 10th	Night	10% 3 16% 41% 29% 2	597
	11th to 20th	Night	13% 3 16% 37% 29% 2	750
	21st to end	Night	14% 3 17% 37% 25% 4	880
September	1st to 10th	Night	16% 3 19% 37% 22% 4	800
	11th to 20th	Night	14% 3 14% 37% 27% 4	800
	21st to end	Night	23% 5% 20% 34% 18% 1	800
October	1st to 10th	Night	21% 6% 22% 28% 19% 5%	800
	11th to 20th	Night	18% 8% 28% 33% 12% 1	800
	21st to end	Night	18% 9% 33% 28% 10% 1	880
November	1st to 10th	Night	13% 10% 34% 31% 11% 1	820
	11th to 20th	Night	20% 13% 33% 26% 7% 1	840
	21st to end	Night	15% 15% 34% 28% 7% 1	840
December	1st to 10th	Night	21% 15% 34% 22% 7% 1	840
	11th to 20th	Night	19% 14% 38% 24% 5%	840
	21st to end	Night	21% 12% 34% 30% 3	924

Page generated at 2026-04-29 02:21:41. Version ac07

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