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 2026-08-15.

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%	142867
	Night	13% 10% 4 9% 5% 5% 4 4 5% 7% 34%	139204
Transparency	Day	45% 6% 12% 22% 13% 1	146394
	Night	47% 6% 12% 21% 13% 1	143741
Seeing	Day	16% 10% 29% 25% 14% 6%	17983
	Night	18% 8% 30% 31% 12% 1	27718

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	12% 14% 7% 14% 8% 6% 5% 4 5% 7% 18%	4691
	11th to 20th	Day	10% 15% 8% 19% 12% 9% 6% 3 3 4 12%	4710
	21st to end	Day	13% 15% 6% 13% 7% 6% 5% 4 5% 5% 20%	5300
February	1st to 10th	Day	11% 14% 5% 11% 8% 5% 6% 4 6% 7% 24%	1890
	11th to 20th	Day	12% 14% 6% 11% 9% 6% 5% 4 5% 6% 24%	2010
	21st to end	Day	12% 14% 5% 12% 7% 5% 4 4 5% 6% 25%	1730
March	1st to 10th	Day	14% 9% 3 8% 5% 6% 5% 4 5% 6% 36%	4580
	11th to 20th	Day	12% 10% 4 8% 6% 5% 5% 4 5% 8% 33%	5088
	21st to end	Day	18% 11% 4 9% 5% 5% 5% 3 4 5% 29%	5586
April	1st to 10th	Day	17% 12% 4 9% 5% 5% 5% 3 5% 6% 29%	2517
	11th to 20th	Day	16% 9% 3 8% 4 3 5% 4 5% 6% 36%	2639
	21st to end	Day	15% 9% 3 7% 5% 6% 5% 3 5% 7% 34%	2730
May	1st to 10th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	7616
	11th to 20th	Day	13% 7% 3 8% 5% 4 5% 4 6% 8% 37%	7626
	21st to end	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	8827
June	1st to 10th	Day	11% 6% 2 5% 4 5% 5% 5% 6% 9% 42%	3019
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	3150
	21st to end	Day	9% 6% 2 6% 4 6% 6% 5% 7% 9% 40%	2990
July	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 4 6% 10% 41%	8280
	11th to 20th	Day	5% 4 2 5% 4 4 4 3 6% 10% 52%	8280
	21st to end	Day	6% 5% 2 6% 4 4 4 4 6% 10% 48%	8872
August	1st to 10th	Day	10% 5% 2 7% 4 5% 6% 4 7% 8% 41%	4364
	11th to 20th	Day	11% 5% 2 7% 4 4 5% 4 5% 8% 45%	4226
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	4559
September	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 5% 7% 44%	2310
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 6% 9% 47%	2200
	21st to end	Day	14% 7% 2 6% 5% 5% 5% 4 6% 8% 38%	2200
October	1st to 10th	Day	15% 5% 3 6% 5% 4 4 4 5% 7% 43%	2174
	11th to 20th	Day	19% 9% 3 7% 5% 6% 4 3 5% 7% 32%	1977
	21st to end	Day	19% 9% 3 8% 5% 5% 6% 4 6% 8% 27%	1980
November	1st to 10th	Day	14% 9% 3 7% 5% 5% 4 4 6% 7% 35%	1852
	11th to 20th	Day	14% 13% 4 9% 6% 5% 6% 4 6% 8% 25%	1890
	21st to end	Day	18% 14% 4 10% 6% 6% 5% 4 5% 6% 23%	1785
December	1st to 10th	Day	13% 16% 5% 11% 7% 6% 6% 4 7% 6% 19%	2548
	11th to 20th	Day	17% 19% 6% 12% 6% 4 4 4 4 5% 20%	2695
	21st to end	Day	27% 20% 6% 11% 5% 4 4 3 4 4 11%	3976
January	1st to 10th	Night	13% 13% 7% 13% 6% 5% 4 3 4 6% 25%	8232
	11th to 20th	Night	10% 16% 8% 18% 9% 8% 6% 4 4 5% 12%	8280
	21st to end	Night	11% 15% 6% 12% 7% 5% 5% 4 6% 7% 21%	8904
February	1st to 10th	Night	13% 13% 5% 11% 7% 5% 4 4 6% 5% 26%	2730
	11th to 20th	Night	13% 13% 5% 10% 7% 5% 5% 4 5% 6% 27%	2495
	21st to end	Night	15% 14% 5% 11% 5% 4 5% 4 5% 7% 26%	2076
March	1st to 10th	Night	14% 9% 3 8% 5% 4 4 3 5% 6% 39%	5493
	11th to 20th	Night	12% 8% 3 7% 4 4 4 4 6% 9% 38%	5091
	21st to end	Night	16% 11% 4 8% 5% 4 4 3 5% 6% 34%	5588
April	1st to 10th	Night	16% 11% 3 6% 4 4 4 3 5% 7% 35%	2121
	11th to 20th	Night	19% 7% 3 6% 4 4 4 4 5% 6% 38%	1930
	21st to end	Night	16% 8% 3 6% 4 4 4 4 5% 7% 38%	1890
May	1st to 10th	Night	12% 8% 3 8% 5% 4 5% 4 5% 7% 39%	5237
	11th to 20th	Night	14% 7% 3 7% 5% 4 4 3 6% 7% 41%	5234
	21st to end	Night	15% 6% 2 5% 3 3 4 4 5% 6% 47%	5331
June	1st to 10th	Night	11% 6% 2 6% 4 5% 3 4 5% 7% 46%	1591
	11th to 20th	Night	9% 7% 3 8% 4 4 4 3 6% 10% 43%	1470
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	1619
July	1st to 10th	Night	9% 7% 2 6% 5% 4 5% 4 6% 8% 44%	4708
	11th to 20th	Night	5% 3 2 5% 4 3 4 3 6% 9% 56%	4707
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 5% 8% 48%	5227
August	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 5% 8% 49%	3005
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 6% 6% 51%	2911
	21st to end	Night	8% 5% 2 6% 3 4 5% 4 6% 8% 49%	3322
September	1st to 10th	Night	9% 5% 1 6% 5% 4 3 4 6% 6% 51%	2070
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 5% 8% 54%	2200
	21st to end	Night	13% 7% 3 6% 4 4 5% 4 5% 7% 43%	2200
October	1st to 10th	Night	16% 5% 2 5% 3 4 3 3 5% 7% 47%	2225
	11th to 20th	Night	17% 10% 3 8% 5% 3 4 3 4 6% 36%	2420
	21st to end	Night	19% 9% 4 8% 5% 4 5% 3 5% 7% 32%	2860
November	1st to 10th	Night	12% 10% 2 7% 5% 4 4 3 5% 7% 41%	2666
	11th to 20th	Night	14% 13% 4 8% 5% 5% 5% 4 7% 7% 29%	2730
	21st to end	Night	17% 15% 4 8% 6% 5% 5% 4 5% 7% 25%	2830
December	1st to 10th	Night	13% 14% 5% 11% 6% 6% 5% 4 6% 8% 23%	4853
	11th to 20th	Night	14% 17% 5% 12% 6% 6% 5% 3 5% 5% 22%	5732
	21st to end	Night	26% 20% 5% 9% 5% 3 3 2 4 4 17%	7226

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	63% 6% 13% 14% 3	4692
	11th to 20th	Day	72% 7% 5% 9% 6%	4710
	21st to end	Day	63% 5% 11% 13% 8% 1	5300
February	1st to 10th	Day	58% 6% 12% 18% 6%	1890
	11th to 20th	Day	59% 6% 11% 15% 8% 1	2020
	21st to end	Day	57% 5% 12% 18% 8% 1	1730
March	1st to 10th	Day	48% 4 14% 20% 10% 4	4580
	11th to 20th	Day	48% 6% 15% 19% 11% 1	5088
	21st to end	Day	57% 4 12% 16% 10% 1	5586
April	1st to 10th	Day	55% 5% 10% 18% 10% 1	2517
	11th to 20th	Day	48% 5% 12% 21% 14% 2	2639
	21st to end	Day	49% 6% 13% 19% 11% 2	2730
May	1st to 10th	Day	46% 6% 12% 21% 14% 1	7651
	11th to 20th	Day	44% 6% 11% 23% 15% 1	7630
	21st to end	Day	38% 5% 9% 23% 22% 3	8841
June	1st to 10th	Day	36% 7% 12% 29% 14% 2	3019
	11th to 20th	Day	38% 6% 13% 24% 18% 1	3150
	21st to end	Day	37% 7% 15% 25% 14% 2	2990
July	1st to 10th	Day	37% 7% 15% 24% 14% 2	8211
	11th to 20th	Day	27% 6% 12% 36% 19% 1	8205
	21st to end	Day	30% 7% 14% 28% 21% 1	8678
August	1st to 10th	Day	37% 8% 12% 27% 15% 1	4351
	11th to 20th	Day	35% 7% 13% 33% 12% 1	4265
	21st to end	Day	36% 8% 15% 28% 13% 1	4558
September	1st to 10th	Day	38% 7% 13% 29% 12% 1	2310
	11th to 20th	Day	33% 5% 15% 31% 15% 1	2200
	21st to end	Day	42% 5% 15% 25% 13%	2200
October	1st to 10th	Day	31% 5% 9% 33% 19% 3	3538
	11th to 20th	Day	39% 5% 13% 30% 13%	3193
	21st to end	Day	49% 7% 13% 22% 8% 1	3180
November	1st to 10th	Day	46% 6% 14% 23% 11%	1852
	11th to 20th	Day	54% 5% 15% 17% 8% 1	1890
	21st to end	Day	60% 5% 14% 15% 5%	1785
December	1st to 10th	Day	61% 9% 17% 11% 2	2548
	11th to 20th	Day	67% 5% 10% 14% 4	2691
	21st to end	Day	76% 5% 7% 8% 4	3976
January	1st to 10th	Night	58% 6% 11% 16% 8% 2	8233
	11th to 20th	Night	70% 7% 9% 9% 5%	8280
	21st to end	Night	59% 6% 11% 15% 8%	8904
February	1st to 10th	Night	58% 6% 10% 19% 7% 1	2730
	11th to 20th	Night	56% 6% 9% 19% 8% 1	2496
	21st to end	Night	58% 4 11% 18% 9%	2076
March	1st to 10th	Night	45% 4 16% 24% 10% 1	5493
	11th to 20th	Night	40% 5% 18% 24% 12% 1	5091
	21st to end	Night	51% 5% 13% 19% 11% 1	5588
April	1st to 10th	Night	49% 4 10% 21% 14% 2	2121
	11th to 20th	Night	46% 5% 8% 21% 17% 3	1930
	21st to end	Night	44% 6% 11% 21% 15% 3	1890
May	1st to 10th	Night	43% 6% 11% 19% 19% 2	5264
	11th to 20th	Night	42% 5% 11% 27% 13% 2	5235
	21st to end	Night	37% 5% 7% 25% 22% 5%	5341
June	1st to 10th	Night	37% 5% 9% 26% 20% 2	1591
	11th to 20th	Night	37% 7% 12% 24% 19% 2	1470
	21st to end	Night	34% 7% 13% 27% 17% 2	1619
July	1st to 10th	Night	35% 7% 13% 24% 15% 5%	4670
	11th to 20th	Night	23% 6% 12% 36% 21% 1	4662
	21st to end	Night	33% 7% 13% 26% 19% 1	5111
August	1st to 10th	Night	32% 6% 14% 31% 16% 1	2994
	11th to 20th	Night	32% 6% 14% 31% 15% 1	2938
	21st to end	Night	32% 6% 13% 32% 16% 1	3322
September	1st to 10th	Night	34% 6% 11% 30% 18% 1	2070
	11th to 20th	Night	29% 5% 12% 35% 18% 1	2200
	21st to end	Night	40% 5% 14% 28% 12% 1	2200
October	1st to 10th	Night	27% 4 11% 34% 23% 1	3641
	11th to 20th	Night	42% 5% 10% 26% 17% 1	3931
	21st to end	Night	44% 5% 11% 29% 11% 1	4610
November	1st to 10th	Night	43% 5% 12% 25% 14% 1	2666
	11th to 20th	Night	51% 6% 14% 20% 9% 1	2730
	21st to end	Night	58% 5% 16% 15% 5% 1	2830
December	1st to 10th	Night	58% 7% 17% 14% 4	4853
	11th to 20th	Night	61% 6% 11% 13% 9% 1	5735
	21st to end	Night	71% 4 9% 8% 8%	7226

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% 13% 34% 24% 9% 2	330
	11th to 20th	Day	18% 16% 40% 21% 5% 1	330
	21st to end	Day	19% 16% 30% 26% 8% 1	363
February	1st to 10th	Day	20% 15% 37% 23% 3 2	330
	11th to 20th	Day	16% 20% 34% 25% 5%	330
	21st to end	Day	24% 18% 39% 17% 2	273
March	1st to 10th	Day	22% 13% 40% 22% 2 1	330
	11th to 20th	Day	19% 13% 33% 24% 10% 1	538
	21st to end	Day	25% 11% 34% 22% 7% 1	594
April	1st to 10th	Day	22% 15% 31% 24% 8% 1	540
	11th to 20th	Day	22% 17% 30% 19% 8% 4	532
	21st to end	Day	16% 16% 31% 23% 12% 2	540
May	1st to 10th	Day	19% 15% 33% 19% 12% 3	543
	11th to 20th	Day	16% 10% 28% 24% 15% 7%	567
	21st to end	Day	15% 8% 29% 25% 17% 7%	767
June	1st to 10th	Day	11% 8% 31% 29% 15% 5%	780
	11th to 20th	Day	13% 9% 32% 27% 14% 5%	780
	21st to end	Day	11% 6% 30% 32% 16% 5%	780
July	1st to 10th	Day	8% 4 25% 30% 21% 12%	771
	11th to 20th	Day	8% 5% 25% 32% 21% 9%	768
	21st to end	Day	8% 3 26% 31% 24% 7%	807
August	1st to 10th	Day	8% 3 21% 27% 25% 16%	566
	11th to 20th	Day	11% 4 17% 27% 29% 13%	535
	21st to end	Day	13% 5% 20% 26% 25% 11%	583
September	1st to 10th	Day	11% 5% 25% 26% 20% 12%	530
	11th to 20th	Day	10% 3 18% 24% 29% 16%	530
	21st to end	Day	16% 10% 18% 27% 19% 9%	530
October	1st to 10th	Day	17% 8% 28% 22% 17% 8%	530
	11th to 20th	Day	21% 14% 28% 26% 10% 1	507
	21st to end	Day	22% 12% 35% 22% 8% 1	363
November	1st to 10th	Day	19% 13% 36% 22% 8% 2	330
	11th to 20th	Day	18% 24% 35% 17% 5% 1	330
	21st to end	Day	26% 11% 39% 17% 6%	275
December	1st to 10th	Day	25% 13% 34% 20% 7% 1	220
	11th to 20th	Day	22% 20% 35% 19% 4 1	220
	21st to end	Day	26% 16% 34% 20% 4 1	341
January	1st to 10th	Night	19% 12% 36% 25% 6% 1	1050
	11th to 20th	Night	22% 11% 36% 25% 6%	1050
	21st to end	Night	17% 9% 35% 31% 8%	1105
February	1st to 10th	Night	18% 10% 37% 30% 4	840
	11th to 20th	Night	19% 10% 39% 27% 5%	823
	21st to end	Night	23% 12% 38% 23% 4	692
March	1st to 10th	Night	19% 10% 34% 30% 7% 1	840
	11th to 20th	Night	18% 10% 37% 28% 6% 1	840
	21st to end	Night	21% 10% 32% 30% 5% 1	924
April	1st to 10th	Night	21% 12% 34% 27% 6% 1	651
	11th to 20th	Night	23% 11% 29% 27% 9% 1	623
	21st to end	Night	22% 7% 30% 32% 8% 1	630
May	1st to 10th	Night	14% 7% 34% 36% 8% 1	630
	11th to 20th	Night	20% 5% 28% 34% 13% 1	621
	21st to end	Night	15% 2 27% 37% 18% 1	683
June	1st to 10th	Night	11% 5% 28% 41% 14% 1	541
	11th to 20th	Night	11% 7% 25% 42% 15%	420
	21st to end	Night	10% 3 23% 44% 19% 1	569
July	1st to 10th	Night	8% 2 19% 42% 26% 3	615
	11th to 20th	Night	7% 2 18% 44% 25% 3	612
	21st to end	Night	8% 2 21% 45% 21% 2	690
August	1st to 10th	Night	8% 1 15% 48% 25% 3	627
	11th to 20th	Night	9% 2 17% 38% 30% 4	606
	21st to end	Night	11% 3 19% 37% 29% 2	660
September	1st to 10th	Night	10% 3 24% 33% 25% 5%	670
	11th to 20th	Night	12% 3 17% 35% 28% 5%	800
	21st to end	Night	15% 6% 23% 33% 19% 4	800
October	1st to 10th	Night	19% 6% 25% 32% 18% 2	800
	11th to 20th	Night	22% 11% 27% 30% 9% 1	800
	21st to end	Night	23% 10% 35% 26% 6%	880
November	1st to 10th	Night	18% 12% 31% 27% 11% 1	820
	11th to 20th	Night	19% 13% 36% 24% 8%	840
	21st to end	Night	24% 11% 37% 24% 4	840
December	1st to 10th	Night	21% 11% 36% 26% 5% 1	921
	11th to 20th	Night	22% 11% 39% 24% 4 1	1050
	21st to end	Night	30% 10% 33% 24% 3	1155

Page generated at 2026-08-12 03:37:15. Version ac07

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