SWAOG Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SWAOG Dark Site clear sky chart from 2005-11-06 to 2025-10-19.

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% 4 5% 4 6% 7% 37%	126679
	Night	14% 9% 3 8% 5% 4 4 3 5% 6% 39%	123292
Transparency	Day	44% 6% 12% 23% 13% 2	130187
	Night	44% 6% 11% 22% 15% 2	127845
Seeing	Day	16% 11% 28% 26% 15% 5%	17138
	Night	17% 9% 30% 31% 12% 1	26920

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% 11% 6% 13% 7% 6% 5% 4 5% 6% 25%	3617
	11th to 20th	Day	10% 11% 5% 13% 5% 5% 5% 5% 7% 4 29%	3620
	21st to end	Day	15% 15% 7% 10% 5% 4 3 3 4 6% 28%	4185
February	1st to 10th	Day	16% 13% 5% 9% 7% 6% 5% 4 5% 5% 26%	1820
	11th to 20th	Day	14% 11% 4 9% 6% 6% 3 3 4 6% 35%	1948
	21st to end	Day	15% 13% 5% 10% 5% 4 3 3 6% 6% 31%	1650
March	1st to 10th	Day	17% 12% 3 6% 4 3 3 3 3 5% 41%	4480
	11th to 20th	Day	9% 7% 3 6% 5% 4 5% 4 6% 8% 43%	4709
	21st to end	Day	19% 13% 6% 12% 6% 5% 5% 4 5% 6% 21%	5967
April	1st to 10th	Day	17% 14% 3 9% 5% 4 4 4 5% 6% 30%	2400
	11th to 20th	Day	16% 10% 3 7% 4 5% 4 4 5% 6% 36%	2374
	21st to end	Day	20% 8% 2 8% 5% 5% 4 3 5% 6% 34%	2477
May	1st to 10th	Day	16% 8% 3 6% 5% 4 6% 4 6% 8% 34%	5862
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 6% 7% 41%	6224
	21st to end	Day	13% 7% 2 6% 4 5% 4 4 7% 9% 37%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 6% 8% 46%	2800
	11th to 20th	Day	8% 6% 2 5% 5% 5% 5% 4 7% 8% 45%	2800
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 7% 40%	2800
July	1st to 10th	Day	9% 6% 2 7% 6% 5% 6% 5% 8% 10% 36%	6360
	11th to 20th	Day	8% 5% 3 6% 5% 4 5% 4 7% 11% 42%	6360
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 5% 7% 9% 37%	6924
August	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 7% 9% 45%	4446
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 8% 10% 42%	3849
	21st to end	Day	7% 5% 2 6% 4 4 4 3 7% 10% 48%	4312
September	1st to 10th	Day	10% 6% 2 5% 4 5% 4 3 5% 8% 48%	2400
	11th to 20th	Day	10% 5% 1 4 4 3 4 4 5% 7% 54%	2290
	21st to end	Day	9% 6% 3 6% 4 4 4 3 6% 8% 46%	2200
October	1st to 10th	Day	12% 7% 3 6% 4 3 3 4 5% 7% 47%	2118
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 37%	1980
	21st to end	Day	18% 9% 3 8% 4 3 4 4 4 6% 37%	1895
November	1st to 10th	Day	12% 8% 2 7% 4 3 4 4 4 7% 44%	1762
	11th to 20th	Day	15% 12% 4 8% 5% 4 5% 4 4 6% 32%	1800
	21st to end	Day	18% 11% 4 10% 4 5% 3 3 4 5% 34%	1715
December	1st to 10th	Day	9% 10% 3 8% 5% 4 3 3 5% 6% 44%	2610
	11th to 20th	Day	12% 14% 5% 11% 5% 6% 6% 5% 5% 6% 24%	3008
	21st to end	Day	25% 22% 7% 12% 5% 4 4 3 3 4 11%	3982
January	1st to 10th	Night	12% 13% 6% 12% 5% 5% 4 3 4 5% 31%	6348
	11th to 20th	Night	12% 15% 6% 12% 7% 6% 5% 4 6% 4 23%	6360
	21st to end	Night	16% 14% 6% 9% 5% 4 3 3 4 4 32%	6789
February	1st to 10th	Night	15% 15% 4 8% 6% 4 3 3 4 6% 32%	2578
	11th to 20th	Night	13% 11% 4 8% 5% 4 4 4 3 6% 37%	2338
	21st to end	Night	17% 10% 4 9% 4 3 3 3 4 5% 37%	1980
March	1st to 10th	Night	17% 11% 4 6% 3 4 4 2 4 5% 41%	5373
	11th to 20th	Night	11% 7% 3 6% 4 4 4 3 5% 6% 47%	5294
	21st to end	Night	18% 11% 5% 11% 5% 4 4 3 5% 6% 27%	4962
April	1st to 10th	Night	16% 10% 3 6% 5% 4 4 3 5% 6% 39%	2000
	11th to 20th	Night	19% 10% 2 5% 5% 4 3 3 4 5% 40%	1975
	21st to end	Night	18% 7% 3 8% 5% 3 4 4 6% 7% 35%	1921
May	1st to 10th	Night	17% 8% 3 7% 5% 4 4 4 5% 7% 36%	4033
	11th to 20th	Night	12% 6% 2 6% 4 4 4 5% 6% 9% 44%	3684
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 8% 39%	3942
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 8% 50%	1600
	11th to 20th	Night	8% 5% 2 5% 5% 5% 4 4 7% 10% 46%	1600
	21st to end	Night	8% 7% 2 7% 6% 4 5% 4 6% 7% 44%	1600
July	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	3618
	11th to 20th	Night	7% 6% 2 5% 4 4 4 4 6% 9% 49%	3617
	21st to end	Night	8% 5% 2 7% 5% 5% 5% 4 7% 8% 43%	3947
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 5% 7% 9% 49%	2706
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 6% 8% 50%	3200
	21st to end	Night	8% 4 2 5% 3 3 3 3 6% 9% 53%	3586
September	1st to 10th	Night	9% 5% 2 5% 3 3 2 3 4 7% 56%	2000
	11th to 20th	Night	8% 4 2 4 4 3 3 4 5% 7% 56%	2088
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 6% 7% 48%	2200
October	1st to 10th	Night	9% 5% 2 6% 4 3 3 4 5% 6% 53%	2279
	11th to 20th	Night	16% 7% 2 5% 4 3 4 3 4 6% 46%	2377
	21st to end	Night	20% 9% 3 6% 3 4 3 3 4 7% 38%	2703
November	1st to 10th	Night	13% 7% 3 6% 3 3 3 3 5% 6% 49%	2536
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 3 5% 6% 37%	2600
	21st to end	Night	17% 10% 3 8% 5% 4 4 3 5% 5% 37%	2683
December	1st to 10th	Night	12% 8% 3 7% 4 4 4 3 4 6% 43%	4580
	11th to 20th	Night	17% 14% 5% 9% 5% 4 3 3 4 6% 32%	5199
	21st to end	Night	24% 18% 6% 10% 5% 5% 4 3 4 5% 16%	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	59% 5% 12% 17% 7%	3618
	11th to 20th	Day	54% 5% 10% 11% 16% 5%	3620
	21st to end	Day	57% 4 12% 18% 8%	4185
February	1st to 10th	Day	58% 6% 10% 15% 8% 2	1820
	11th to 20th	Day	51% 5% 10% 23% 9% 2	1958
	21st to end	Day	53% 6% 11% 20% 8% 2	1650
March	1st to 10th	Day	47% 5% 9% 24% 13% 3	4480
	11th to 20th	Day	37% 5% 12% 33% 11% 1	4709
	21st to end	Day	63% 7% 11% 12% 6% 1	5969
April	1st to 10th	Day	52% 6% 10% 21% 10%	2400
	11th to 20th	Day	46% 7% 11% 21% 11% 3	2374
	21st to end	Day	50% 6% 12% 18% 12% 2	2477
May	1st to 10th	Day	45% 9% 11% 22% 13% 1	5897
	11th to 20th	Day	41% 7% 10% 26% 15% 1	6228
	21st to end	Day	39% 8% 12% 24% 13% 4	6949
June	1st to 10th	Day	35% 6% 12% 27% 18% 2	2800
	11th to 20th	Day	35% 7% 11% 25% 19% 3	2800
	21st to end	Day	39% 9% 13% 22% 15% 2	2800
July	1st to 10th	Day	39% 8% 14% 25% 13% 1	6291
	11th to 20th	Day	34% 7% 15% 28% 14% 1	6285
	21st to end	Day	39% 8% 17% 20% 14% 2	6728
August	1st to 10th	Day	33% 8% 17% 29% 12% 1	4431
	11th to 20th	Day	33% 8% 12% 31% 15%	3885
	21st to end	Day	31% 7% 12% 31% 17% 2	4311
September	1st to 10th	Day	33% 6% 13% 25% 18% 5%	2400
	11th to 20th	Day	29% 6% 11% 29% 22% 3	2290
	21st to end	Day	36% 6% 9% 26% 21% 2	2200
October	1st to 10th	Day	25% 4 7% 29% 33% 2	3443
	11th to 20th	Day	31% 4 12% 25% 26% 1	3220
	21st to end	Day	41% 4 10% 27% 15% 3	3095
November	1st to 10th	Day	39% 6% 13% 24% 15% 2	1762
	11th to 20th	Day	52% 4 11% 21% 10% 1	1800
	21st to end	Day	54% 5% 11% 22% 8%	1715
December	1st to 10th	Day	41% 4 16% 30% 9%	2610
	11th to 20th	Day	58% 6% 12% 16% 6% 1	3005
	21st to end	Day	78% 4 7% 7% 4	3982
January	1st to 10th	Night	56% 5% 9% 19% 10%	6348
	11th to 20th	Night	62% 5% 10% 9% 13% 1	6360
	21st to end	Night	56% 4 11% 17% 11% 1	6789
February	1st to 10th	Night	55% 4 11% 18% 11% 2	2578
	11th to 20th	Night	49% 5% 9% 23% 13% 1	2339
	21st to end	Night	49% 4 12% 19% 12% 3	1980
March	1st to 10th	Night	46% 5% 10% 23% 11% 5%	5373
	11th to 20th	Night	37% 5% 11% 30% 14% 3	5294
	21st to end	Night	56% 6% 9% 19% 8% 1	4962
April	1st to 10th	Night	45% 5% 11% 24% 13% 2	2000
	11th to 20th	Night	46% 5% 8% 22% 15% 4	1975
	21st to end	Night	46% 7% 10% 23% 11% 3	1921
May	1st to 10th	Night	47% 7% 12% 17% 14% 3	4060
	11th to 20th	Night	34% 10% 13% 26% 13% 4	3685
	21st to end	Night	41% 8% 11% 22% 15% 3	3950
June	1st to 10th	Night	31% 7% 13% 26% 19% 4	1600
	11th to 20th	Night	32% 7% 12% 25% 21% 3	1600
	21st to end	Night	37% 9% 14% 21% 17% 3	1600
July	1st to 10th	Night	39% 7% 14% 22% 15% 3	3580
	11th to 20th	Night	30% 7% 13% 29% 20% 1	3572
	21st to end	Night	35% 9% 15% 22% 17% 2	3832
August	1st to 10th	Night	28% 10% 17% 26% 18% 1	2697
	11th to 20th	Night	29% 8% 14% 30% 18% 1	3230
	21st to end	Night	27% 6% 12% 33% 18% 3	3586
September	1st to 10th	Night	28% 6% 12% 28% 23% 3	2000
	11th to 20th	Night	27% 5% 8% 32% 25% 2	2088
	21st to end	Night	34% 8% 11% 26% 19% 3	2200
October	1st to 10th	Night	22% 4 7% 36% 29% 2	3721
	11th to 20th	Night	33% 5% 8% 23% 29% 2	3876
	21st to end	Night	40% 4 9% 25% 19% 3	4453
November	1st to 10th	Night	37% 5% 9% 27% 18% 4	2536
	11th to 20th	Night	47% 5% 12% 21% 13% 2	2600
	21st to end	Night	49% 4 11% 23% 12%	2683
December	1st to 10th	Night	42% 5% 14% 28% 10% 1	4580
	11th to 20th	Night	55% 5% 13% 17% 10% 1	5201
	21st to end	Night	70% 6% 8% 11% 5%	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	20% 12% 34% 26% 7%	300
	11th to 20th	Day	19% 16% 40% 21% 4 1	300
	21st to end	Day	22% 18% 30% 23% 6% 1	330
February	1st to 10th	Day	22% 18% 37% 19% 4	300
	11th to 20th	Day	23% 17% 37% 19% 5%	300
	21st to end	Day	24% 15% 41% 17% 3	249
March	1st to 10th	Day	25% 18% 37% 18% 2	300
	11th to 20th	Day	21% 21% 35% 17% 6% 1	305
	21st to end	Day	25% 17% 32% 19% 6% 2	550
April	1st to 10th	Day	25% 13% 32% 23% 6%	500
	11th to 20th	Day	20% 21% 27% 20% 10% 2	492
	21st to end	Day	23% 13% 31% 23% 8% 2	500
May	1st to 10th	Day	21% 12% 29% 26% 10% 2	503
	11th to 20th	Day	20% 10% 28% 25% 13% 4	682
	21st to end	Day	15% 8% 31% 22% 17% 7%	789
June	1st to 10th	Day	10% 9% 28% 29% 18% 5%	730
	11th to 20th	Day	11% 7% 30% 29% 19% 5%	730
	21st to end	Day	12% 8% 30% 30% 14% 6%	730
July	1st to 10th	Day	10% 3 18% 30% 23% 16%	721
	11th to 20th	Day	10% 6% 22% 32% 20% 10%	718
	21st to end	Day	8% 3 24% 34% 23% 7%	794
August	1st to 10th	Day	7% 3 17% 37% 25% 12%	691
	11th to 20th	Day	11% 5% 18% 27% 27% 13%	519
	21st to end	Day	6% 3 19% 31% 29% 13%	583
September	1st to 10th	Day	11% 7% 21% 28% 26% 8%	530
	11th to 20th	Day	10% 2 16% 28% 32% 12%	530
	21st to end	Day	12% 10% 24% 26% 21% 7%	530
October	1st to 10th	Day	13% 8% 26% 29% 20% 5%	448
	11th to 20th	Day	16% 21% 31% 22% 9% 1	324
	21st to end	Day	28% 14% 33% 17% 6% 2	330
November	1st to 10th	Day	16% 16% 35% 21% 8% 4	300
	11th to 20th	Day	23% 19% 33% 18% 6% 2	300
	21st to end	Day	25% 12% 34% 23% 7%	300
December	1st to 10th	Day	16% 18% 39% 19% 7% 1	300
	11th to 20th	Day	18% 16% 39% 21% 6%	300
	21st to end	Day	29% 19% 32% 16% 3	330
January	1st to 10th	Night	20% 13% 38% 23% 5%	1000
	11th to 20th	Night	22% 13% 38% 21% 5% 1	1000
	21st to end	Night	21% 12% 35% 29% 3	1100
February	1st to 10th	Night	24% 11% 37% 24% 4	978
	11th to 20th	Night	20% 13% 36% 26% 4	783
	21st to end	Night	22% 12% 39% 23% 4	660
March	1st to 10th	Night	21% 13% 36% 24% 6%	800
	11th to 20th	Night	17% 11% 37% 28% 7% 1	750
	21st to end	Night	24% 12% 34% 23% 6%	660
April	1st to 10th	Night	19% 14% 36% 26% 6%	600
	11th to 20th	Night	22% 12% 33% 26% 7%	593
	21st to end	Night	20% 11% 34% 28% 7%	600
May	1st to 10th	Night	19% 8% 34% 31% 7% 1	600
	11th to 20th	Night	17% 8% 28% 34% 12% 1	591
	21st to end	Night	13% 6% 26% 38% 17%	650
June	1st to 10th	Night	11% 5% 25% 44% 15%	600
	11th to 20th	Night	11% 4 28% 46% 12% 1	600
	21st to end	Night	11% 4 27% 40% 17% 1	600
July	1st to 10th	Night	8% 3 16% 42% 28% 3	585
	11th to 20th	Night	8% 2 21% 39% 27% 3	582
	21st to end	Night	5% 2 16% 46% 29% 2	657
August	1st to 10th	Night	5% 3 13% 47% 29% 3	597
	11th to 20th	Night	6% 3 12% 42% 34% 3	594
	21st to end	Night	9% 1 16% 40% 32% 2	660
September	1st to 10th	Night	11% 2 19% 37% 29% 3	600
	11th to 20th	Night	8% 4 17% 37% 30% 4	688
	21st to end	Night	11% 6% 24% 36% 21% 2	800
October	1st to 10th	Night	12% 6% 26% 36% 19% 2	800
	11th to 20th	Night	20% 11% 28% 31% 10%	793
	21st to end	Night	24% 11% 31% 25% 8%	836
November	1st to 10th	Night	17% 9% 38% 25% 10% 1	780
	11th to 20th	Night	19% 15% 33% 27% 7%	800
	21st to end	Night	21% 12% 36% 25% 6%	883
December	1st to 10th	Night	22% 11% 37% 25% 5%	1000
	11th to 20th	Night	23% 11% 34% 27% 5%	1000
	21st to end	Night	28% 14% 32% 23% 3	1100

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