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 2024-10-24.

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% 3 8% 5% 5% 5% 4 6% 7% 35%	100871
	Night	14% 10% 4 8% 5% 4 4 3 5% 7% 35%	97901
Transparency	Day	47% 6% 13% 21% 12% 1	100665
	Night	49% 6% 12% 21% 11% 1	97780
Seeing	Day	16% 10% 29% 25% 15% 6%	15545
	Night	18% 8% 30% 31% 12% 1	25300

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% 17% 7% 11% 7% 5% 5% 4 5% 6% 19%	2527
	11th to 20th	Day	14% 17% 8% 18% 13% 8% 4 3 3 3 8%	2530
	21st to end	Day	21% 20% 6% 13% 6% 5% 4 3 4 4 12%	2849
February	1st to 10th	Day	10% 14% 5% 11% 8% 5% 6% 4 6% 7% 23%	1710
	11th to 20th	Day	11% 14% 5% 11% 9% 5% 5% 4 5% 5% 25%	1813
	21st to end	Day	13% 14% 5% 11% 8% 5% 4 4 5% 6% 26%	1570
March	1st to 10th	Day	16% 11% 4 8% 5% 5% 5% 3 4 6% 32%	3120
	11th to 20th	Day	12% 10% 5% 9% 7% 6% 5% 4 5% 6% 32%	3478
	21st to end	Day	18% 12% 4 9% 5% 5% 5% 4 4 5% 30%	3815
April	1st to 10th	Day	17% 12% 4 9% 5% 4 5% 3 5% 6% 29%	2277
	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	21% 10% 3 8% 5% 5% 5% 4 6% 8% 26%	1809
November	1st to 10th	Day	13% 8% 3 7% 6% 5% 4 4 6% 7% 36%	1672
	11th to 20th	Day	13% 13% 5% 9% 6% 5% 6% 5% 6% 8% 26%	1710
	21st to end	Day	18% 14% 4 9% 6% 5% 4 4 5% 6% 24%	1615
December	1st to 10th	Day	13% 16% 6% 12% 7% 5% 6% 4 5% 5% 22%	1444
	11th to 20th	Day	14% 16% 5% 11% 6% 4 4 4 5% 6% 24%	1675
	21st to end	Day	28% 18% 5% 11% 6% 5% 5% 3 5% 5% 10%	2719
January	1st to 10th	Night	15% 16% 7% 11% 6% 5% 4 3 4 5% 23%	4428
	11th to 20th	Night	13% 16% 8% 17% 10% 8% 5% 3 4 5% 13%	4440
	21st to end	Night	19% 19% 7% 12% 6% 5% 4 3 5% 5% 15%	4814
February	1st to 10th	Night	13% 14% 5% 10% 7% 6% 4 4 6% 5% 25%	2470
	11th to 20th	Night	13% 13% 5% 10% 7% 5% 5% 4 5% 7% 26%	2252
	21st to end	Night	16% 14% 5% 11% 5% 4 5% 4 4 7% 26%	1884
March	1st to 10th	Night	14% 9% 3 7% 5% 4 4 3 4 6% 40%	3733
	11th to 20th	Night	10% 7% 3 7% 5% 5% 4 4 6% 8% 41%	3481
	21st to end	Night	15% 11% 4 8% 6% 4 4 3 5% 6% 33%	3817
April	1st to 10th	Night	16% 11% 3 6% 5% 4 4 3 5% 7% 35%	1919
	11th to 20th	Night	19% 7% 3 6% 4 3 5% 4 4 6% 38%	1744
	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	21% 10% 4 8% 5% 4 4 3 4 7% 30%	2614
November	1st to 10th	Night	12% 10% 2 6% 5% 3 4 4 5% 7% 42%	2406
	11th to 20th	Night	14% 13% 4 8% 4 5% 5% 4 7% 7% 29%	2470
	21st to end	Night	18% 14% 4 8% 6% 5% 5% 4 5% 7% 26%	2560
December	1st to 10th	Night	16% 15% 5% 11% 6% 5% 5% 4 5% 6% 21%	2733
	11th to 20th	Night	14% 14% 5% 12% 6% 5% 5% 4 6% 6% 23%	3522
	21st to end	Night	28% 20% 5% 9% 5% 4 3 2 4 4 15%	4944

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	65% 5% 14% 12% 4	2528
	11th to 20th	Day	78% 7% 6% 7% 2	2530
	21st to end	Day	75% 4 9% 8% 4	2849
February	1st to 10th	Day	59% 6% 12% 17% 6%	1710
	11th to 20th	Day	59% 5% 11% 15% 8% 1	1823
	21st to end	Day	56% 5% 12% 18% 7% 1	1570
March	1st to 10th	Day	53% 4 12% 23% 7% 1	3120
	11th to 20th	Day	51% 6% 12% 19% 10% 2	3478
	21st to end	Day	57% 4 12% 16% 10% 2	3815
April	1st to 10th	Day	55% 5% 10% 18% 11%	2277
	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	42% 6% 10% 26% 14% 2	2066
	11th to 20th	Day	52% 6% 14% 21% 8%	1878
	21st to end	Day	55% 6% 15% 16% 7% 1	1809
November	1st to 10th	Day	46% 6% 14% 23% 11%	1672
	11th to 20th	Day	54% 5% 15% 17% 8% 1	1710
	21st to end	Day	60% 5% 15% 14% 5%	1615
December	1st to 10th	Day	64% 7% 12% 15% 2	1444
	11th to 20th	Day	60% 5% 15% 16% 4	1671
	21st to end	Day	77% 5% 8% 7% 3	2719
January	1st to 10th	Night	62% 5% 12% 13% 6% 1	4428
	11th to 20th	Night	72% 7% 9% 9% 4	4440
	21st to end	Night	69% 6% 11% 10% 3	4814
February	1st to 10th	Night	58% 6% 10% 19% 7% 1	2470
	11th to 20th	Night	57% 6% 9% 18% 8% 1	2253
	21st to end	Night	58% 4 10% 17% 10%	1884
March	1st to 10th	Night	46% 3 16% 27% 8% 1	3733
	11th to 20th	Night	38% 6% 15% 23% 16% 2	3481
	21st to end	Night	51% 5% 14% 20% 9% 1	3817
April	1st to 10th	Night	49% 4 10% 21% 14% 2	1919
	11th to 20th	Night	46% 5% 8% 20% 17% 3	1744
	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	38% 4 12% 28% 16% 1	2114
	11th to 20th	Night	51% 5% 11% 22% 10% 1	2299
	21st to end	Night	55% 5% 12% 19% 9% 1	2614
November	1st to 10th	Night	42% 5% 12% 26% 14% 1	2406
	11th to 20th	Night	52% 5% 14% 20% 8% 1	2470
	21st to end	Night	57% 5% 16% 15% 5%	2560
December	1st to 10th	Night	62% 6% 12% 15% 5%	2733
	11th to 20th	Night	59% 7% 14% 12% 7% 1	3525
	21st to end	Night	73% 5% 10% 8% 4	4944

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	19% 14% 30% 25% 10% 2	270
	11th to 20th	Day	20% 13% 37% 24% 6% 1	270
	21st to end	Day	21% 13% 29% 26% 10%	297
February	1st to 10th	Day	19% 15% 40% 23% 2 2	270
	11th to 20th	Day	16% 19% 34% 26% 5%	270
	21st to end	Day	26% 16% 39% 17% 2	225
March	1st to 10th	Day	21% 11% 43% 21% 2 1	270
	11th to 20th	Day	17% 12% 33% 26% 11% 1	458
	21st to end	Day	25% 10% 34% 23% 8% 1	506
April	1st to 10th	Day	20% 14% 31% 25% 9% 1	460
	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% 13% 33% 22% 7% 1	306
November	1st to 10th	Day	18% 13% 36% 22% 9% 2	270
	11th to 20th	Day	16% 24% 36% 18% 6% 1	270
	21st to end	Day	26% 11% 40% 18% 6%	225
December	1st to 10th	Day	24% 14% 35% 20% 6% 1	180
	11th to 20th	Day	22% 17% 37% 19% 4 1	180
	21st to end	Day	25% 16% 35% 21% 4	279
January	1st to 10th	Night	20% 12% 36% 24% 7% 1	950
	11th to 20th	Night	23% 11% 35% 25% 6% 1	950
	21st to end	Night	18% 8% 34% 32% 8%	1001
February	1st to 10th	Night	18% 10% 37% 30% 4	760
	11th to 20th	Night	19% 10% 38% 28% 5%	743
	21st to end	Night	23% 12% 38% 23% 4	628
March	1st to 10th	Night	19% 9% 33% 31% 8% 1	760
	11th to 20th	Night	17% 9% 38% 29% 7% 1	760
	21st to end	Night	22% 10% 31% 31% 6% 1	836
April	1st to 10th	Night	21% 10% 34% 28% 6% 1	589
	11th to 20th	Night	22% 10% 30% 27% 9% 1	563
	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	25% 10% 35% 24% 6%	805
November	1st to 10th	Night	17% 11% 31% 28% 11% 1	740
	11th to 20th	Night	21% 13% 36% 23% 8%	760
	21st to end	Night	24% 10% 36% 25% 5%	760
December	1st to 10th	Night	23% 10% 36% 26% 5% 1	833
	11th to 20th	Night	22% 10% 39% 24% 4 1	950
	21st to end	Night	31% 9% 34% 24% 3	1045

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