Sunset Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunset Astronomical Society clear sky chart from 2005-11-06 to 2025-04-11.

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% 9% 4 8% 5% 5% 5% 4 6% 7% 34%	112674
	Night	14% 11% 4 9% 5% 4 4 4 5% 7% 34%	115715
Transparency	Day	47% 6% 13% 21% 12% 1	116248
	Night	48% 6% 12% 21% 12% 1	120271
Seeing	Day	16% 10% 29% 25% 14% 6%	16056
	Night	18% 8% 30% 31% 12% 1	26021

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	10% 14% 6% 14% 8% 6% 5% 4 5% 7% 20%	3617
	11th to 20th	Day	11% 15% 7% 18% 12% 9% 6% 4 3 4 10%	3620
	21st to end	Day	16% 17% 7% 15% 5% 4 5% 4 5% 5% 17%	4087
February	1st to 10th	Day	11% 14% 5% 11% 8% 5% 6% 4 6% 7% 23%	1800
	11th to 20th	Day	12% 14% 6% 11% 9% 5% 5% 4 5% 5% 24%	1912
	21st to end	Day	13% 14% 5% 12% 8% 5% 4 4 5% 6% 26%	1650
March	1st to 10th	Day	14% 9% 3 8% 5% 6% 5% 4 5% 6% 36%	4480
	11th to 20th	Day	12% 9% 4 8% 6% 5% 5% 4 5% 8% 33%	4978
	21st to end	Day	18% 11% 4 9% 5% 5% 5% 3 4 5% 29%	5465
April	1st to 10th	Day	17% 12% 4 9% 5% 4 5% 4 5% 6% 29%	2397
	11th to 20th	Day	17% 9% 3 7% 4 3 5% 4 5% 6% 36%	2385
	21st to end	Day	16% 8% 4 7% 5% 6% 5% 3 5% 6% 34%	2470
May	1st to 10th	Day	15% 8% 3 7% 5% 4 5% 4 6% 8% 35%	4082
	11th to 20th	Day	14% 6% 3 6% 5% 4 5% 4 6% 8% 38%	4081
	21st to end	Day	10% 6% 2 6% 4 5% 4 4 6% 8% 46%	4716
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	2731
	11th to 20th	Day	11% 5% 2 5% 5% 5% 5% 4 6% 8% 44%	2850
	21st to end	Day	10% 6% 3 6% 4 6% 6% 5% 7% 9% 40%	2706
July	1st to 10th	Day	10% 6% 2 8% 5% 5% 5% 4 7% 9% 37%	4440
	11th to 20th	Day	6% 4 2 5% 3 5% 5% 4 6% 11% 50%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	4751
August	1st to 10th	Day	10% 5% 2 7% 4 6% 6% 4 7% 8% 41%	4104
	11th to 20th	Day	11% 5% 2 7% 4 4 5% 4 5% 8% 46%	4044
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 7% 9% 42%	4419
September	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 4 6% 7% 44%	2195
	11th to 20th	Day	11% 5% 2 5% 3 4 4 4 6% 9% 47%	2090
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 7% 8% 38%	2090
October	1st to 10th	Day	15% 5% 3 7% 5% 4 4 4 5% 7% 42%	2066
	11th to 20th	Day	19% 9% 3 8% 6% 6% 4 3 5% 7% 31%	1878
	21st to end	Day	20% 10% 3 8% 5% 5% 5% 4 6% 8% 26%	1881
November	1st to 10th	Day	14% 9% 3 7% 5% 5% 4 4 6% 7% 35%	1762
	11th to 20th	Day	14% 13% 4 8% 6% 5% 6% 5% 6% 8% 25%	1800
	21st to end	Day	18% 14% 4 10% 6% 5% 5% 4 5% 6% 23%	1700
December	1st to 10th	Day	13% 16% 5% 11% 7% 6% 6% 4 7% 6% 19%	2472
	11th to 20th	Day	17% 19% 6% 12% 6% 4 4 4 4 5% 20%	2625
	21st to end	Day	27% 20% 6% 11% 5% 4 4 3 4 4 11%	3890
January	1st to 10th	Night	11% 12% 7% 13% 6% 5% 4 4 4 6% 27%	6348
	11th to 20th	Night	12% 14% 7% 18% 9% 8% 6% 4 4 5% 13%	6360
	21st to end	Night	14% 17% 7% 11% 6% 5% 5% 4 6% 7% 19%	6884
February	1st to 10th	Night	13% 14% 5% 11% 7% 5% 5% 4 6% 5% 25%	2600
	11th to 20th	Night	13% 13% 5% 10% 7% 5% 5% 4 5% 6% 26%	2373
	21st to end	Night	15% 14% 5% 11% 5% 4 5% 4 5% 7% 25%	1980
March	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 5% 6% 39%	5373
	11th to 20th	Night	11% 8% 3 7% 4 4 4 4 6% 9% 39%	4981
	21st to end	Night	17% 11% 4 8% 5% 4 4 3 5% 6% 34%	5467
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 3 5% 7% 35%	2020
	11th to 20th	Night	19% 7% 3 6% 4 3 5% 4 4 6% 38%	1750
	21st to end	Night	17% 8% 3 6% 4 4 4 4 5% 6% 38%	1710
May	1st to 10th	Night	12% 8% 3 7% 4 4 5% 4 5% 8% 38%	2813
	11th to 20th	Night	16% 6% 2 7% 5% 4 4 3 6% 7% 41%	2812
	21st to end	Night	12% 6% 2 6% 4 3 4 3 5% 6% 48%	2844
June	1st to 10th	Night	11% 6% 2 6% 4 5% 4 4 5% 7% 45%	1439
	11th to 20th	Night	9% 7% 3 8% 4 4 4 3 5% 10% 45%	1330
	21st to end	Night	8% 5% 1 6% 5% 5% 5% 4 6% 8% 47%	1465
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	2530
	11th to 20th	Night	7% 3 2 5% 4 3 4 3 6% 7% 56%	2530
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 4 6% 8% 48%	2808
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 5% 8% 49%	2825
	11th to 20th	Night	11% 5% 2 4 4 3 4 3 6% 6% 51%	2785
	21st to end	Night	8% 5% 2 6% 3 4 5% 4 6% 8% 48%	3220
September	1st to 10th	Night	9% 5% 1 6% 5% 5% 3 3 6% 6% 50%	1966
	11th to 20th	Night	9% 5% 2 5% 3 4 4 3 5% 8% 54%	2090
	21st to end	Night	13% 7% 3 6% 4 4 5% 4 5% 7% 43%	2090
October	1st to 10th	Night	16% 6% 2 5% 3 4 3 3 6% 7% 46%	2114
	11th to 20th	Night	18% 10% 3 8% 5% 3 4 3 5% 6% 35%	2299
	21st to end	Night	20% 9% 4 8% 5% 4 5% 3 5% 7% 31%	2717
November	1st to 10th	Night	12% 10% 2 6% 5% 4 4 3 5% 7% 42%	2536
	11th to 20th	Night	14% 13% 4 8% 5% 5% 5% 4 7% 7% 29%	2600
	21st to end	Night	18% 14% 4 8% 6% 5% 5% 4 5% 7% 25%	2695
December	1st to 10th	Night	13% 14% 5% 11% 6% 5% 5% 4 6% 8% 23%	4709
	11th to 20th	Night	14% 17% 5% 12% 6% 5% 5% 3 5% 5% 22%	5582
	21st to end	Night	26% 21% 5% 9% 5% 3 3 2 4 4 17%	7070

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	61% 6% 14% 15% 4	3618
	11th to 20th	Day	72% 8% 6% 9% 5%	3620
	21st to end	Day	68% 5% 10% 10% 6% 1	4087
February	1st to 10th	Day	58% 6% 12% 17% 6%	1800
	11th to 20th	Day	60% 6% 11% 15% 8% 1	1922
	21st to end	Day	56% 5% 12% 18% 8% 1	1650
March	1st to 10th	Day	48% 4 14% 20% 10% 4	4480
	11th to 20th	Day	48% 6% 15% 20% 11% 1	4978
	21st to end	Day	56% 4 12% 16% 10% 1	5465
April	1st to 10th	Day	55% 5% 10% 18% 10% 1	2397
	11th to 20th	Day	47% 5% 12% 21% 14% 2	2385
	21st to end	Day	50% 6% 12% 18% 12% 2	2470
May	1st to 10th	Day	45% 6% 12% 20% 15% 2	4120
	11th to 20th	Day	41% 6% 13% 26% 12% 1	4085
	21st to end	Day	36% 5% 12% 22% 22% 3	4729
June	1st to 10th	Day	37% 7% 13% 28% 13% 2	2731
	11th to 20th	Day	37% 6% 12% 24% 19% 2	2850
	21st to end	Day	38% 7% 15% 25% 14% 2	2706
July	1st to 10th	Day	41% 7% 14% 20% 15% 2	4371
	11th to 20th	Day	27% 7% 15% 34% 15% 1	4365
	21st to end	Day	33% 8% 15% 22% 21% 2	4602
August	1st to 10th	Day	38% 8% 12% 27% 15% 1	4091
	11th to 20th	Day	35% 7% 12% 33% 12% 1	4083
	21st to end	Day	36% 8% 15% 27% 13% 1	4418
September	1st to 10th	Day	39% 7% 13% 28% 12% 1	2195
	11th to 20th	Day	33% 5% 15% 30% 15% 1	2090
	21st to end	Day	42% 5% 15% 25% 13%	2090
October	1st to 10th	Day	31% 5% 9% 32% 19% 3	3430
	11th to 20th	Day	39% 5% 13% 30% 13%	3094
	21st to end	Day	50% 6% 13% 22% 7% 1	3081
November	1st to 10th	Day	46% 6% 14% 23% 11%	1762
	11th to 20th	Day	54% 6% 14% 17% 9% 1	1800
	21st to end	Day	60% 5% 15% 14% 5%	1700
December	1st to 10th	Day	61% 9% 17% 11% 2	2472
	11th to 20th	Day	67% 5% 10% 14% 4	2621
	21st to end	Day	76% 5% 7% 9% 4	3890
January	1st to 10th	Night	57% 5% 11% 18% 8% 2	6348
	11th to 20th	Night	69% 8% 9% 9% 5%	6360
	21st to end	Night	61% 6% 12% 13% 7%	6884
February	1st to 10th	Night	58% 6% 10% 18% 8% 1	2600
	11th to 20th	Night	57% 6% 9% 18% 8% 1	2374
	21st to end	Night	58% 4 11% 17% 9%	1980
March	1st to 10th	Night	45% 4 16% 24% 10% 1	5373
	11th to 20th	Night	39% 6% 18% 24% 12% 1	4981
	21st to end	Night	50% 5% 13% 19% 11% 1	5467
April	1st to 10th	Night	49% 4 10% 21% 14% 2	2020
	11th to 20th	Night	47% 5% 8% 20% 17% 3	1750
	21st to end	Night	45% 5% 11% 20% 15% 3	1710
May	1st to 10th	Night	43% 6% 11% 20% 17% 3	2840
	11th to 20th	Night	42% 5% 10% 28% 13% 2	2813
	21st to end	Night	36% 5% 11% 22% 21% 5%	2852
June	1st to 10th	Night	38% 6% 10% 26% 18% 2	1439
	11th to 20th	Night	35% 6% 12% 24% 20% 2	1330
	21st to end	Night	34% 7% 13% 27% 17% 2	1465
July	1st to 10th	Night	36% 6% 12% 26% 13% 7%	2490
	11th to 20th	Night	26% 6% 13% 36% 19% 1	2482
	21st to end	Night	33% 8% 17% 21% 19% 2	2719
August	1st to 10th	Night	32% 6% 15% 31% 15% 1	2814
	11th to 20th	Night	32% 6% 14% 31% 16% 1	2812
	21st to end	Night	32% 7% 13% 31% 16% 1	3220
September	1st to 10th	Night	35% 6% 11% 29% 18% 1	1966
	11th to 20th	Night	30% 5% 12% 35% 18% 1	2090
	21st to end	Night	39% 5% 14% 29% 12% 1	2090
October	1st to 10th	Night	27% 4 11% 34% 23% 1	3530
	11th to 20th	Night	42% 5% 11% 26% 16% 1	3810
	21st to end	Night	45% 5% 11% 29% 11% 1	4467
November	1st to 10th	Night	43% 5% 12% 25% 14% 1	2536
	11th to 20th	Night	52% 5% 13% 19% 9% 1	2600
	21st to end	Night	58% 5% 16% 15% 5% 1	2695
December	1st to 10th	Night	58% 8% 18% 13% 4	4709
	11th to 20th	Night	61% 6% 11% 13% 8% 1	5585
	21st to end	Night	71% 4 9% 8% 8%	7070

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% 14% 33% 24% 10% 2	300
	11th to 20th	Day	19% 13% 39% 22% 5% 1	300
	21st to end	Day	20% 15% 30% 25% 9% 1	330
February	1st to 10th	Day	20% 15% 39% 22% 2 2	300
	11th to 20th	Day	16% 20% 34% 26% 5%	300
	21st to end	Day	26% 17% 37% 17% 2	249
March	1st to 10th	Day	21% 13% 42% 21% 2 1	300
	11th to 20th	Day	17% 13% 33% 25% 10% 1	498
	21st to end	Day	25% 11% 33% 22% 8% 1	550
April	1st to 10th	Day	22% 14% 31% 25% 8% 1	500
	11th to 20th	Day	23% 17% 30% 18% 8% 4	452
	21st to end	Day	16% 16% 30% 23% 12% 2	460
May	1st to 10th	Day	21% 13% 32% 19% 12% 3	463
	11th to 20th	Day	17% 9% 26% 24% 16% 7%	487
	21st to end	Day	14% 7% 26% 27% 19% 8%	665
June	1st to 10th	Day	12% 8% 31% 28% 15% 6%	680
	11th to 20th	Day	13% 8% 30% 28% 15% 6%	680
	21st to end	Day	12% 6% 30% 30% 17% 5%	680
July	1st to 10th	Day	8% 4 25% 28% 22% 13%	671
	11th to 20th	Day	9% 5% 25% 31% 20% 10%	668
	21st to end	Day	9% 3 25% 32% 24% 8%	701
August	1st to 10th	Day	8% 3 22% 24% 26% 16%	486
	11th to 20th	Day	11% 3 18% 26% 29% 13%	479
	21st to end	Day	13% 5% 20% 26% 26% 11%	539
September	1st to 10th	Day	11% 5% 26% 26% 20% 11%	490
	11th to 20th	Day	10% 3 19% 24% 29% 14%	490
	21st to end	Day	16% 11% 19% 27% 18% 9%	490
October	1st to 10th	Day	18% 8% 28% 22% 16% 7%	490
	11th to 20th	Day	21% 15% 28% 26% 9% 1	468
	21st to end	Day	24% 12% 34% 22% 7% 1	330
November	1st to 10th	Day	19% 13% 36% 21% 9% 2	300
	11th to 20th	Day	18% 23% 35% 18% 5% 1	300
	21st to end	Day	26% 11% 39% 18% 6%	250
December	1st to 10th	Day	25% 14% 34% 21% 6% 1	200
	11th to 20th	Day	23% 18% 36% 20% 4 1	200
	21st to end	Day	26% 16% 32% 21% 4 1	310
January	1st to 10th	Night	19% 12% 36% 25% 7% 1	1000
	11th to 20th	Night	22% 11% 36% 25% 6% 1	1000
	21st to end	Night	18% 9% 34% 31% 8%	1053
February	1st to 10th	Night	19% 10% 38% 30% 4	800
	11th to 20th	Night	19% 10% 39% 27% 5%	783
	21st to end	Night	22% 12% 39% 23% 4	660
March	1st to 10th	Night	19% 9% 34% 30% 8% 1	800
	11th to 20th	Night	17% 10% 38% 28% 7% 1	800
	21st to end	Night	22% 10% 31% 31% 5% 1	880
April	1st to 10th	Night	20% 11% 34% 28% 6% 1	620
	11th to 20th	Night	22% 10% 30% 28% 8% 1	565
	21st to end	Night	22% 6% 30% 32% 8% 1	570
May	1st to 10th	Night	15% 7% 32% 36% 9% 1	570
	11th to 20th	Night	20% 4 26% 36% 13% 1	561
	21st to end	Night	14% 2 26% 38% 19% 2	617
June	1st to 10th	Night	12% 5% 27% 40% 15% 1	489
	11th to 20th	Night	11% 7% 23% 43% 16% 1	380
	21st to end	Night	10% 3 23% 44% 19% 1	515
July	1st to 10th	Night	7% 2 19% 42% 27% 3	555
	11th to 20th	Night	8% 2 19% 43% 25% 3	552
	21st to end	Night	9% 2 20% 45% 22% 2	624
August	1st to 10th	Night	8% 1 15% 48% 25% 3	567
	11th to 20th	Night	9% 2 17% 37% 31% 4	564
	21st to end	Night	11% 3 19% 36% 29% 2	627
September	1st to 10th	Night	10% 3 24% 34% 24% 5%	636
	11th to 20th	Night	12% 3 18% 35% 27% 4	760
	21st to end	Night	15% 6% 24% 32% 19% 4	760
October	1st to 10th	Night	19% 6% 25% 31% 16% 2	760
	11th to 20th	Night	22% 11% 27% 30% 9% 1	760
	21st to end	Night	24% 10% 35% 25% 6%	836
November	1st to 10th	Night	18% 11% 32% 28% 11% 1	780
	11th to 20th	Night	20% 13% 36% 23% 8%	800
	21st to end	Night	24% 11% 37% 25% 5%	800
December	1st to 10th	Night	22% 11% 36% 26% 5% 1	877
	11th to 20th	Night	23% 10% 39% 24% 4 1	1000
	21st to end	Night	31% 9% 33% 24% 3	1100

Page generated at 2025-04-08 09:17:23. Version ac07

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