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 2025-01-07.

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% 42%	104895
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	104090
Transparency	Day	38% 7% 12% 23% 16% 3	108500
	Night	39% 6% 11% 23% 17% 3	108625
Seeing	Day	17% 12% 26% 25% 15% 5%	15161
	Night	18% 10% 30% 30% 11% 1	24592

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	15% 6% 2 5% 5% 3 4 3 4 5% 48%	2565
	11th to 20th	Day	15% 10% 2 9% 5% 4 5% 3 4 5% 39%	2683
	21st to end	Day	21% 7% 2 6% 4 4 5% 3 5% 6% 38%	3124
February	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 5% 6% 41%	1710
	11th to 20th	Day	17% 8% 2 5% 4 4 4 4 6% 6% 42%	1786
	21st to end	Day	18% 10% 3 6% 4 3 4 3 5% 5% 39%	1570
March	1st to 10th	Day	25% 5% 2 4 4 3 4 3 4 6% 41%	3197
	11th to 20th	Day	10% 4 2 5% 4 5% 5% 3 5% 8% 50%	3480
	21st to end	Day	21% 6% 2 5% 4 3 4 3 4 5% 44%	3999
April	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 6% 43%	2280
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 45%	2254
	21st to end	Day	17% 5% 2 5% 4 4 5% 4 6% 7% 41%	2280
May	1st to 10th	Day	27% 8% 2 5% 4 4 4 3 5% 7% 30%	3977
	11th to 20th	Day	24% 7% 2 7% 5% 4 5% 3 5% 8% 29%	4398
	21st to end	Day	12% 4 2 6% 3 4 4 4 6% 9% 46%	4823
June	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 10% 40%	2660
	11th to 20th	Day	12% 5% 2 5% 5% 5% 5% 5% 7% 9% 41%	2660
	21st to end	Day	10% 4 1 5% 4 4 5% 5% 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 6% 5% 6% 5% 8% 11% 42%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	4440
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 5% 8% 9% 34%	4625
August	1st to 10th	Day	16% 6% 2 7% 4 4 5% 4 8% 10% 33%	4104
	11th to 20th	Day	12% 4 2 5% 4 5% 5% 4 7% 9% 43%	3760
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2280
	11th to 20th	Day	13% 4 1 5% 4 4 4 3 5% 8% 49%	2271
	21st to end	Day	20% 6% 3 7% 5% 3 4 3 5% 6% 38%	2056
October	1st to 10th	Day	17% 6% 2 5% 3 4 3 3 4 6% 46%	1900
	11th to 20th	Day	15% 5% 2 5% 3 3 4 4 5% 6% 49%	1900
	21st to end	Day	17% 5% 2 5% 4 4 3 3 4 5% 48%	2066
November	1st to 10th	Day	12% 5% 1 6% 3 3 3 3 5% 6% 53%	1761
	11th to 20th	Day	15% 5% 2 4 3 3 4 4 6% 6% 48%	1800
	21st to end	Day	15% 5% 2 4 5% 3 5% 3 5% 5% 48%	1656
December	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 6% 5% 47%	2600
	11th to 20th	Day	21% 8% 2 5% 3 3 3 3 5% 6% 41%	2990
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	3960
January	1st to 10th	Night	18% 6% 2 5% 4 4 3 3 4 6% 45%	4514
	11th to 20th	Night	19% 8% 3 5% 3 3 4 3 4 5% 44%	4276
	21st to end	Night	22% 8% 3 6% 4 3 4 3 4 5% 39%	4532
February	1st to 10th	Night	16% 8% 3 5% 4 4 4 3 6% 6% 41%	2470
	11th to 20th	Night	19% 8% 2 5% 4 3 4 3 4 6% 43%	2279
	21st to end	Night	18% 7% 2 4 4 4 4 4 5% 6% 42%	1884
March	1st to 10th	Night	22% 7% 2 6% 4 3 3 3 4 6% 39%	3652
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 4 5% 5% 51%	3480
	21st to end	Night	17% 8% 3 6% 3 3 3 2 3 4 46%	3631
April	1st to 10th	Night	17% 7% 2 6% 4 3 3 4 5% 6% 42%	1900
	11th to 20th	Night	15% 5% 2 6% 4 4 4 3 5% 7% 44%	1875
	21st to end	Night	20% 5% 2 5% 3 3 4 3 5% 6% 44%	1900
May	1st to 10th	Night	23% 9% 3 8% 4 4 4 4 4 7% 31%	2933
	11th to 20th	Night	25% 8% 4 7% 4 4 3 4 5% 5% 29%	2495
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 10% 44%	2732
June	1st to 10th	Night	14% 6% 2 5% 4 3 5% 4 6% 8% 43%	1520
	11th to 20th	Night	11% 6% 3 6% 6% 5% 5% 4 6% 9% 40%	1520
	21st to end	Night	11% 4 2 5% 4 4 4 5% 8% 10% 42%	1520
July	1st to 10th	Night	7% 6% 3 8% 7% 5% 5% 5% 7% 9% 39%	2520
	11th to 20th	Night	10% 6% 2 6% 5% 5% 5% 5% 8% 11% 36%	2520
	21st to end	Night	13% 6% 3 8% 5% 4 5% 5% 7% 8% 36%	2923
August	1st to 10th	Night	19% 5% 2 6% 5% 4 5% 4 6% 9% 35%	2825
	11th to 20th	Night	13% 6% 2 7% 4 5% 5% 4 5% 7% 41%	3035
	21st to end	Night	11% 5% 2 5% 4 4 4 4 5% 8% 49%	3476
September	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	13% 5% 1 4 3 2 3 3 4 7% 54%	1908
	21st to end	Night	20% 7% 2 7% 4 4 3 4 5% 7% 40%	2120
October	1st to 10th	Night	20% 6% 2 6% 5% 3 3 3 4 5% 43%	2280
	11th to 20th	Night	16% 5% 1 4 3 4 2 2 5% 7% 51%	2280
	21st to end	Night	16% 5% 2 5% 4 3 4 3 4 5% 50%	2532
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2537
	11th to 20th	Night	16% 7% 2 4 3 3 3 3 5% 5% 48%	2600
	21st to end	Night	13% 5% 1 4 5% 4 4 4 6% 6% 47%	2740
December	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 49%	4580
	11th to 20th	Night	23% 5% 2 5% 3 3 3 3 6% 7% 41%	5205
	21st to end	Night	18% 7% 3 5% 4 4 4 4 5% 7% 39%	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	36% 6% 13% 26% 15% 4	2566
	11th to 20th	Day	48% 5% 10% 21% 13% 3	2683
	21st to end	Day	46% 7% 13% 21% 11% 2	3124
February	1st to 10th	Day	42% 7% 13% 24% 12% 2	1710
	11th to 20th	Day	42% 5% 10% 25% 15% 3	1795
	21st to end	Day	46% 5% 10% 21% 14% 4	1570
March	1st to 10th	Day	45% 4 10% 28% 11% 2	3197
	11th to 20th	Day	30% 8% 11% 29% 19% 4	3480
	21st to end	Day	44% 5% 8% 19% 19% 4	3999
April	1st to 10th	Day	40% 7% 12% 20% 16% 5%	2280
	11th to 20th	Day	37% 6% 11% 22% 17% 5%	2254
	21st to end	Day	40% 7% 12% 18% 19% 4	2280
May	1st to 10th	Day	52% 7% 11% 17% 11% 2	4012
	11th to 20th	Day	51% 9% 14% 17% 7% 1	4402
	21st to end	Day	32% 9% 14% 26% 17% 2	4837
June	1st to 10th	Day	36% 10% 13% 24% 15% 2	2660
	11th to 20th	Day	37% 8% 16% 24% 13% 3	2660
	21st to end	Day	32% 9% 16% 23% 18% 1	2660
July	1st to 10th	Day	34% 11% 18% 21% 13% 4	4370
	11th to 20th	Day	35% 8% 18% 26% 11% 2	4365
	21st to end	Day	43% 10% 12% 19% 15% 2	4478
August	1st to 10th	Day	43% 9% 16% 23% 8%	4091
	11th to 20th	Day	35% 9% 13% 29% 14% 1	3797
	21st to end	Day	32% 7% 12% 29% 17% 2	4180
September	1st to 10th	Day	39% 7% 11% 20% 20% 4	2280
	11th to 20th	Day	32% 6% 11% 24% 23% 4	2271
	21st to end	Day	46% 6% 8% 18% 19% 2	2056
October	1st to 10th	Day	35% 5% 9% 18% 28% 5%	3160
	11th to 20th	Day	24% 5% 10% 24% 29% 9%	3140
	21st to end	Day	24% 3 10% 29% 28% 5%	3380
November	1st to 10th	Day	33% 4 13% 26% 20% 4	1761
	11th to 20th	Day	35% 6% 11% 23% 22% 3	1800
	21st to end	Day	38% 5% 11% 31% 13% 3	1656
December	1st to 10th	Day	37% 5% 16% 26% 13% 3	2600
	11th to 20th	Day	44% 4 12% 29% 9% 2	2986
	21st to end	Day	39% 7% 15% 21% 17% 2	3960
January	1st to 10th	Night	40% 5% 11% 25% 15% 4	4514
	11th to 20th	Night	43% 5% 10% 25% 14% 3	4276
	21st to end	Night	47% 6% 8% 26% 11% 1	4532
February	1st to 10th	Night	43% 5% 13% 24% 14% 2	2470
	11th to 20th	Night	44% 4 9% 21% 19% 4	2281
	21st to end	Night	43% 5% 10% 20% 16% 6%	1884
March	1st to 10th	Night	46% 5% 11% 24% 13% 2	3652
	11th to 20th	Night	32% 8% 12% 29% 16% 3	3480
	21st to end	Night	42% 5% 8% 17% 23% 5%	3631
April	1st to 10th	Night	42% 5% 11% 19% 19% 5%	1900
	11th to 20th	Night	39% 7% 12% 20% 18% 5%	1875
	21st to end	Night	39% 7% 10% 20% 18% 6%	1900
May	1st to 10th	Night	51% 8% 10% 18% 10% 2	2963
	11th to 20th	Night	52% 10% 12% 14% 10% 2	2496
	21st to end	Night	35% 10% 15% 21% 15% 4	2739
June	1st to 10th	Night	37% 8% 12% 23% 18% 1	1520
	11th to 20th	Night	38% 9% 13% 24% 14% 2	1520
	21st to end	Night	33% 10% 14% 24% 16% 3	1520
July	1st to 10th	Night	39% 9% 19% 17% 12% 3	2480
	11th to 20th	Night	38% 10% 19% 23% 9% 1	2472
	21st to end	Night	43% 9% 13% 19% 15% 1	2830
August	1st to 10th	Night	46% 10% 17% 19% 8% 1	2814
	11th to 20th	Night	40% 7% 12% 26% 14% 1	3064
	21st to end	Night	33% 7% 11% 28% 20% 2	3476
September	1st to 10th	Night	38% 7% 10% 20% 21% 4	1900
	11th to 20th	Night	31% 6% 9% 25% 25% 4	1908
	21st to end	Night	44% 6% 9% 19% 19% 3	2120
October	1st to 10th	Night	42% 6% 9% 20% 20% 3	3800
	11th to 20th	Night	24% 4 11% 25% 31% 6%	3779
	21st to end	Night	24% 4 11% 31% 24% 6%	4168
November	1st to 10th	Night	32% 5% 11% 25% 20% 6%	2537
	11th to 20th	Night	38% 5% 10% 22% 20% 5%	2600
	21st to end	Night	36% 5% 12% 26% 18% 2	2740
December	1st to 10th	Night	37% 5% 11% 29% 16% 2	4580
	11th to 20th	Night	43% 4 13% 21% 15% 4	5208
	21st to end	Night	42% 8% 11% 20% 18% 1	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	18% 27% 35% 16% 4 1	192
	11th to 20th	Day	20% 36% 26% 18% 1	227
	21st to end	Day	20% 23% 35% 16% 5% 1	297
February	1st to 10th	Day	20% 23% 34% 17% 5%	270
	11th to 20th	Day	24% 23% 37% 14% 1	394
	21st to end	Day	23% 25% 30% 19% 4 1	382
March	1st to 10th	Day	16% 25% 33% 19% 6%	460
	11th to 20th	Day	20% 23% 31% 18% 6% 2	460
	21st to end	Day	22% 17% 36% 19% 6% 1	506
April	1st to 10th	Day	20% 20% 36% 16% 6% 2	460
	11th to 20th	Day	16% 19% 29% 24% 8% 4	452
	21st to end	Day	22% 16% 34% 21% 7% 1	460
May	1st to 10th	Day	27% 14% 28% 21% 9% 2	463
	11th to 20th	Day	19% 10% 29% 28% 12% 2	487
	21st to end	Day	17% 5% 23% 27% 22% 7%	529
June	1st to 10th	Day	12% 7% 28% 28% 18% 7%	490
	11th to 20th	Day	15% 7% 21% 32% 19% 6%	490
	21st to end	Day	10% 6% 24% 35% 18% 7%	490
July	1st to 10th	Day	11% 4 16% 31% 29% 10%	485
	11th to 20th	Day	6% 2 15% 37% 31% 10%	484
	21st to end	Day	11% 3 18% 35% 26% 7%	532
August	1st to 10th	Day	11% 4 16% 33% 26% 9%	486
	11th to 20th	Day	11% 3 14% 32% 29% 10%	479
	21st to end	Day	11% 4 19% 30% 24% 12%	539
September	1st to 10th	Day	16% 7% 18% 23% 24% 11%	490
	11th to 20th	Day	14% 3 20% 29% 22% 12%	490
	21st to end	Day	24% 8% 22% 28% 15% 3	490
October	1st to 10th	Day	21% 5% 25% 26% 14% 8%	490
	11th to 20th	Day	14% 13% 26% 27% 16% 4	490
	21st to end	Day	18% 11% 30% 25% 13% 3	526
November	1st to 10th	Day	14% 10% 34% 25% 14% 3	395
	11th to 20th	Day	14% 18% 36% 23% 10%	400
	21st to end	Day	16% 22% 28% 26% 8%	256
December	1st to 10th	Day	20% 19% 34% 22% 6% 1	200
	11th to 20th	Day	27% 21% 29% 20% 5%	200
	21st to end	Day	22% 19% 27% 23% 8% 1	220
January	1st to 10th	Night	19% 17% 40% 20% 3	785
	11th to 20th	Night	23% 16% 33% 23% 4	760
	21st to end	Night	18% 16% 39% 24% 2	836
February	1st to 10th	Night	20% 14% 39% 25% 2	760
	11th to 20th	Night	23% 21% 38% 17% 1	743
	21st to end	Night	22% 18% 37% 19% 3	628
March	1st to 10th	Night	17% 18% 38% 24% 4	760
	11th to 20th	Night	20% 16% 37% 21% 6% 1	760
	21st to end	Night	21% 10% 41% 23% 5%	836
April	1st to 10th	Night	20% 15% 37% 22% 5%	760
	11th to 20th	Night	15% 13% 33% 31% 7%	751
	21st to end	Night	22% 10% 32% 29% 8%	760
May	1st to 10th	Night	20% 7% 34% 31% 6%	654
	11th to 20th	Night	19% 9% 30% 32% 10%	374
	21st to end	Night	17% 4 20% 38% 19% 2	411
June	1st to 10th	Night	14% 4 24% 39% 18% 1	380
	11th to 20th	Night	12% 2 25% 46% 16%	380
	21st to end	Night	8% 3 24% 47% 18% 1	380
July	1st to 10th	Night	11% 4 17% 44% 22% 2	370
	11th to 20th	Night	7% 1 15% 44% 31% 2	368
	21st to end	Night	9% 2 17% 44% 26% 2	525
August	1st to 10th	Night	10% 2 16% 41% 29% 2	567
	11th to 20th	Night	13% 2 16% 37% 29% 2	712
	21st to end	Night	14% 3 16% 37% 26% 4	836
September	1st to 10th	Night	16% 3 19% 37% 22% 4	760
	11th to 20th	Night	14% 3 14% 37% 27% 4	760
	21st to end	Night	23% 5% 20% 33% 18% 1	760
October	1st to 10th	Night	22% 6% 22% 28% 18% 4	760
	11th to 20th	Night	17% 7% 28% 33% 13% 1	760
	21st to end	Night	18% 9% 32% 29% 10% 1	836
November	1st to 10th	Night	13% 9% 35% 31% 12% 1	780
	11th to 20th	Night	20% 13% 33% 27% 8% 1	800
	21st to end	Night	14% 16% 34% 28% 7% 1	800
December	1st to 10th	Night	21% 15% 34% 22% 7% 1	800
	11th to 20th	Night	19% 14% 38% 24% 5%	800
	21st to end	Night	20% 12% 34% 31% 3	880

Page generated at 2025-01-04 03:23:58. Version ac07

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