ARGUS Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ARGUS Dark Sky Site clear sky chart from 2005-11-06 to 2025-08-05.

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	14% 9% 4 8% 5% 4 5% 4 6% 7% 35%	125401
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 35%	122832
Transparency	Day	47% 6% 11% 22% 13% 1	128983
	Night	48% 5% 11% 21% 14% 2	127323
Seeing	Day	17% 11% 28% 23% 16% 6%	16672
	Night	19% 9% 29% 29% 13% 1	26473

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	13% 14% 9% 17% 6% 5% 4 4 5% 5% 20%	3617
	11th to 20th	Day	13% 16% 9% 13% 7% 7% 6% 5% 4 4 16%	3620
	21st to end	Day	24% 18% 7% 12% 4 4 3 3 3 3 20%	4112
February	1st to 10th	Day	17% 15% 5% 11% 7% 5% 4 3 5% 5% 22%	1953
	11th to 20th	Day	17% 12% 4 12% 6% 5% 4 3 5% 5% 27%	1948
	21st to end	Day	16% 15% 5% 10% 5% 3 4 4 5% 5% 28%	1650
March	1st to 10th	Day	19% 15% 7% 9% 5% 3 3 2 3 4 30%	4480
	11th to 20th	Day	11% 11% 3 6% 4 3 4 3 6% 8% 41%	4835
	21st to end	Day	17% 14% 5% 10% 6% 5% 4 4 5% 7% 25%	5680
April	1st to 10th	Day	17% 12% 4 8% 5% 4 4 4 5% 6% 31%	2400
	11th to 20th	Day	17% 10% 3 7% 4 4 5% 3 5% 7% 35%	2379
	21st to end	Day	19% 8% 3 7% 5% 4 4 3 5% 8% 33%	2600
May	1st to 10th	Day	19% 9% 3 8% 5% 5% 5% 4 6% 8% 29%	5862
	11th to 20th	Day	17% 7% 2 7% 5% 5% 5% 4 6% 8% 35%	5861
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 5% 7% 9% 37%	6656
June	1st to 10th	Day	10% 5% 3 7% 5% 5% 5% 5% 6% 7% 42%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 5% 7% 8% 43%	2800
	21st to end	Day	10% 6% 2 6% 5% 6% 6% 5% 7% 9% 40%	2800
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 7% 10% 41%	6360
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 5% 7% 12% 40%	6360
	21st to end	Day	10% 7% 3 7% 5% 5% 5% 5% 7% 8% 38%	6773
August	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 7% 10% 44%	4156
	11th to 20th	Day	9% 4 2 6% 5% 4 5% 4 7% 8% 46%	3953
	21st to end	Day	7% 4 2 6% 4 4 4 4 6% 8% 50%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 3 6% 6% 46%	2242
	11th to 20th	Day	11% 4 2 5% 4 4 5% 4 7% 8% 47%	2090
	21st to end	Day	14% 6% 2 5% 4 4 4 3 6% 8% 44%	2090
October	1st to 10th	Day	12% 6% 2 6% 4 4 4 4 6% 6% 46%	2090
	11th to 20th	Day	15% 6% 3 7% 4 5% 4 4 6% 6% 41%	1918
	21st to end	Day	19% 10% 3 8% 5% 4 4 4 5% 7% 31%	1881
November	1st to 10th	Day	10% 7% 2 5% 4 4 4 4 5% 7% 48%	1762
	11th to 20th	Day	17% 12% 4 8% 5% 5% 3 3 5% 6% 32%	1800
	21st to end	Day	19% 13% 4 9% 5% 3 4 3 4 5% 31%	1800
December	1st to 10th	Day	13% 13% 3 9% 5% 4 3 3 4 5% 38%	2903
	11th to 20th	Day	16% 16% 7% 11% 6% 5% 4 3 4 6% 23%	3008
	21st to end	Day	28% 19% 7% 11% 6% 4 5% 3 4 4 10%	3982
January	1st to 10th	Night	16% 16% 8% 14% 5% 5% 4 3 4 5% 22%	6348
	11th to 20th	Night	13% 21% 8% 15% 7% 5% 5% 3 3 3 18%	6360
	21st to end	Night	21% 14% 7% 8% 5% 5% 4 3 3 4 26%	6844
February	1st to 10th	Night	19% 15% 6% 10% 5% 4 3 3 4 5% 27%	2445
	11th to 20th	Night	13% 14% 4 9% 7% 6% 4 3 5% 5% 30%	2338
	21st to end	Night	18% 13% 4 9% 5% 4 3 2 4 6% 31%	1980
March	1st to 10th	Night	17% 12% 4 7% 4 4 4 2 4 5% 37%	5373
	11th to 20th	Night	12% 7% 3 5% 4 5% 5% 4 6% 7% 42%	5123
	21st to end	Night	18% 9% 3 8% 5% 4 4 4 5% 6% 34%	5293
April	1st to 10th	Night	14% 10% 2 7% 5% 5% 4 4 5% 7% 36%	2000
	11th to 20th	Night	17% 8% 2 7% 4 4 5% 3 5% 6% 38%	1970
	21st to end	Night	18% 9% 3 7% 3 4 4 4 6% 7% 36%	1800
May	1st to 10th	Night	20% 10% 3 8% 6% 4 4 4 5% 6% 29%	4033
	11th to 20th	Night	16% 8% 3 6% 4 4 4 3 5% 9% 39%	4032
	21st to end	Night	13% 7% 2 7% 4 4 5% 4 6% 8% 40%	4204
June	1st to 10th	Night	9% 5% 3 6% 4 4 5% 4 6% 7% 48%	1600
	11th to 20th	Night	8% 6% 3 6% 4 4 5% 4 6% 10% 46%	1600
	21st to end	Night	9% 5% 1 6% 5% 4 4 4 6% 10% 46%	1600
July	1st to 10th	Night	8% 5% 2 5% 5% 4 5% 5% 6% 10% 46%	3618
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 7% 10% 48%	3617
	21st to end	Night	10% 6% 3 8% 5% 5% 5% 4 6% 9% 40%	4077
August	1st to 10th	Night	11% 5% 2 6% 3 4 5% 4 6% 9% 46%	2861
	11th to 20th	Night	7% 7% 2 5% 3 4 5% 4 6% 8% 48%	2871
	21st to end	Night	8% 5% 2 6% 4 4 4 4 5% 8% 52%	3476
September	1st to 10th	Night	11% 4 2 6% 4 3 4 3 5% 7% 49%	1918
	11th to 20th	Night	9% 3 2 4 3 3 4 4 5% 7% 58%	2090
	21st to end	Night	12% 6% 2 5% 4 4 4 3 5% 6% 48%	2090
October	1st to 10th	Night	10% 5% 2 6% 3 5% 4 3 5% 7% 51%	2090
	11th to 20th	Night	17% 8% 2 7% 4 3 3 3 4 5% 45%	2260
	21st to end	Night	20% 8% 2 6% 5% 4 4 4 5% 6% 38%	2717
November	1st to 10th	Night	12% 9% 3 6% 4 3 3 4 5% 6% 46%	2536
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 37%	2600
	21st to end	Night	18% 12% 4 9% 5% 5% 4 4 3 6% 29%	2600
December	1st to 10th	Night	13% 10% 4 7% 5% 4 4 4 5% 7% 36%	4273
	11th to 20th	Night	18% 15% 7% 12% 5% 5% 4 4 4 5% 22%	5199
	21st to end	Night	25% 20% 7% 11% 5% 4 5% 3 4 4 12%	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	67% 4 7% 17% 5%	3618
	11th to 20th	Day	72% 3 10% 6% 7% 2	3620
	21st to end	Day	71% 3 8% 12% 6%	4112
February	1st to 10th	Day	65% 4 9% 12% 8% 1	1953
	11th to 20th	Day	59% 4 9% 20% 7% 1	1958
	21st to end	Day	57% 5% 12% 17% 8% 1	1650
March	1st to 10th	Day	60% 3 6% 21% 10%	4480
	11th to 20th	Day	41% 4 10% 34% 9% 1	4835
	21st to end	Day	58% 5% 14% 16% 5% 1	5682
April	1st to 10th	Day	52% 7% 11% 18% 12%	2400
	11th to 20th	Day	49% 6% 10% 22% 11% 3	2379
	21st to end	Day	49% 5% 11% 21% 10% 3	2600
May	1st to 10th	Day	52% 7% 12% 17% 11%	5897
	11th to 20th	Day	46% 7% 12% 22% 12% 1	5865
	21st to end	Day	41% 7% 11% 26% 14% 1	6670
June	1st to 10th	Day	39% 7% 11% 24% 16% 3	2800
	11th to 20th	Day	36% 7% 11% 24% 19% 3	2800
	21st to end	Day	38% 9% 15% 19% 16% 3	2800
July	1st to 10th	Day	37% 6% 11% 27% 17% 2	6291
	11th to 20th	Day	35% 8% 16% 26% 15% 1	6285
	21st to end	Day	41% 7% 13% 21% 16% 2	6579
August	1st to 10th	Day	32% 8% 18% 26% 14% 1	4143
	11th to 20th	Day	32% 8% 12% 31% 16% 1	3992
	21st to end	Day	30% 6% 14% 27% 19% 3	4179
September	1st to 10th	Day	36% 7% 12% 23% 18% 4	2242
	11th to 20th	Day	32% 8% 10% 26% 22% 2	2090
	21st to end	Day	38% 6% 10% 23% 20% 2	2090
October	1st to 10th	Day	26% 5% 11% 32% 25% 1	3480
	11th to 20th	Day	35% 5% 10% 24% 26% 1	3160
	21st to end	Day	44% 3 9% 26% 16% 1	3081
November	1st to 10th	Day	36% 6% 13% 27% 17% 2	1762
	11th to 20th	Day	54% 4 11% 19% 11% 2	1800
	21st to end	Day	56% 4 11% 20% 8% 1	1800
December	1st to 10th	Day	50% 3 14% 23% 9%	2903
	11th to 20th	Day	64% 4 11% 17% 3	3005
	21st to end	Day	78% 4 7% 8% 3	3982
January	1st to 10th	Night	65% 4 8% 16% 7%	6348
	11th to 20th	Night	73% 3 7% 9% 8% 1	6360
	21st to end	Night	63% 3 10% 15% 8% 1	6844
February	1st to 10th	Night	61% 4 12% 15% 8% 1	2445
	11th to 20th	Night	56% 4 10% 18% 11% 1	2339
	21st to end	Night	56% 4 11% 17% 9% 2	1980
March	1st to 10th	Night	50% 4 8% 26% 10% 1	5373
	11th to 20th	Night	39% 5% 12% 27% 14% 2	5123
	21st to end	Night	50% 5% 11% 25% 8% 1	5293
April	1st to 10th	Night	46% 7% 11% 22% 14% 1	2000
	11th to 20th	Night	45% 5% 10% 21% 15% 4	1970
	21st to end	Night	46% 7% 10% 22% 12% 3	1800
May	1st to 10th	Night	55% 7% 8% 16% 12% 3	4060
	11th to 20th	Night	42% 8% 12% 25% 14% 1	4033
	21st to end	Night	40% 8% 12% 23% 15% 2	4212
June	1st to 10th	Night	33% 6% 10% 26% 21% 3	1600
	11th to 20th	Night	34% 7% 12% 24% 20% 3	1600
	21st to end	Night	33% 8% 16% 22% 16% 5%	1600
July	1st to 10th	Night	33% 8% 11% 27% 17% 5%	3580
	11th to 20th	Night	31% 7% 16% 25% 19% 1	3572
	21st to end	Night	39% 7% 15% 21% 16% 3	3959
August	1st to 10th	Night	33% 8% 15% 28% 15% 1	2850
	11th to 20th	Night	32% 8% 12% 30% 16% 1	2898
	21st to end	Night	31% 6% 12% 28% 21% 3	3476
September	1st to 10th	Night	33% 7% 12% 22% 20% 6%	1918
	11th to 20th	Night	26% 5% 10% 29% 27% 3	2090
	21st to end	Night	36% 7% 10% 23% 22% 3	2090
October	1st to 10th	Night	23% 3 7% 32% 33% 1	3480
	11th to 20th	Night	32% 4 9% 23% 30% 1	3757
	21st to end	Night	35% 5% 8% 29% 22% 2	4467
November	1st to 10th	Night	39% 5% 10% 25% 20% 2	2536
	11th to 20th	Night	47% 5% 10% 21% 15% 3	2600
	21st to end	Night	57% 5% 10% 21% 8%	2600
December	1st to 10th	Night	46% 5% 14% 26% 7% 1	4273
	11th to 20th	Night	64% 4 11% 13% 7% 1	5201
	21st to end	Night	75% 4 10% 8% 3	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	23% 19% 37% 16% 5% 1	300
	11th to 20th	Day	24% 20% 38% 13% 5%	300
	21st to end	Day	29% 16% 32% 18% 5%	330
February	1st to 10th	Day	25% 19% 36% 16% 5%	300
	11th to 20th	Day	26% 20% 35% 15% 4 1	300
	21st to end	Day	26% 21% 39% 12% 3	249
March	1st to 10th	Day	21% 21% 39% 15% 3	300
	11th to 20th	Day	25% 14% 35% 20% 6% 1	433
	21st to end	Day	30% 16% 30% 18% 6% 1	550
April	1st to 10th	Day	23% 15% 34% 18% 9% 1	500
	11th to 20th	Day	21% 20% 28% 20% 9% 3	492
	21st to end	Day	23% 16% 31% 20% 9% 2	500
May	1st to 10th	Day	22% 13% 32% 22% 9% 3	503
	11th to 20th	Day	21% 8% 28% 23% 13% 6%	527
	21st to end	Day	12% 6% 30% 26% 17% 9%	666
June	1st to 10th	Day	11% 9% 29% 26% 20% 5%	730
	11th to 20th	Day	12% 6% 26% 29% 21% 6%	730
	21st to end	Day	12% 6% 28% 28% 19% 7%	730
July	1st to 10th	Day	12% 3 18% 28% 25% 14%	721
	11th to 20th	Day	8% 3 22% 30% 25% 13%	718
	21st to end	Day	10% 3 17% 31% 27% 10%	731
August	1st to 10th	Day	7% 3 15% 30% 31% 14%	502
	11th to 20th	Day	10% 3 20% 29% 23% 14%	479
	21st to end	Day	8% 4 14% 28% 34% 13%	539
September	1st to 10th	Day	13% 5% 17% 28% 27% 10%	490
	11th to 20th	Day	10% 2 16% 24% 31% 17%	490
	21st to end	Day	17% 9% 20% 24% 22% 7%	490
October	1st to 10th	Day	12% 6% 24% 32% 18% 8%	490
	11th to 20th	Day	16% 19% 29% 24% 10% 2	422
	21st to end	Day	25% 15% 37% 19% 3 1	330
November	1st to 10th	Day	12% 16% 33% 25% 12% 3	300
	11th to 20th	Day	24% 17% 33% 17% 9% 1	300
	21st to end	Day	27% 15% 37% 18% 3	300
December	1st to 10th	Day	22% 20% 37% 13% 8%	300
	11th to 20th	Day	22% 16% 41% 15% 6%	300
	21st to end	Day	32% 17% 32% 16% 2	330
January	1st to 10th	Night	23% 16% 35% 21% 5%	1000
	11th to 20th	Night	25% 15% 37% 20% 4	1000
	21st to end	Night	22% 16% 36% 22% 4	1039
February	1st to 10th	Night	28% 11% 35% 24% 3	800
	11th to 20th	Night	21% 14% 38% 25% 3	783
	21st to end	Night	26% 14% 38% 18% 3	660
March	1st to 10th	Night	21% 15% 34% 25% 6%	800
	11th to 20th	Night	19% 13% 37% 25% 6% 1	800
	21st to end	Night	25% 11% 33% 26% 5%	809
April	1st to 10th	Night	19% 12% 36% 27% 6% 1	600
	11th to 20th	Night	21% 11% 33% 26% 8% 1	593
	21st to end	Night	22% 10% 35% 25% 8% 1	600
May	1st to 10th	Night	22% 7% 31% 31% 9%	600
	11th to 20th	Night	20% 5% 27% 31% 16% 2	591
	21st to end	Night	11% 3 24% 44% 18% 1	650
June	1st to 10th	Night	9% 3 26% 43% 17% 1	600
	11th to 20th	Night	11% 4 24% 41% 20% 1	600
	21st to end	Night	10% 3 23% 39% 25% 1	600
July	1st to 10th	Night	10% 2 15% 37% 31% 4	585
	11th to 20th	Night	7% 3 16% 40% 31% 4	582
	21st to end	Night	8% 3 14% 42% 32% 2	657
August	1st to 10th	Night	7% 2 13% 43% 31% 4	579
	11th to 20th	Night	8% 2 16% 38% 31% 4	564
	21st to end	Night	8% 2 17% 33% 37% 5%	627
September	1st to 10th	Night	13% 3 15% 37% 29% 4	588
	11th to 20th	Night	11% 3 14% 36% 33% 4	760
	21st to end	Night	14% 5% 22% 34% 22% 2	760
October	1st to 10th	Night	12% 6% 24% 35% 20% 4	760
	11th to 20th	Night	21% 12% 28% 28% 11% 1	760
	21st to end	Night	24% 11% 32% 25% 7%	836
November	1st to 10th	Night	16% 11% 34% 27% 10% 1	780
	11th to 20th	Night	20% 15% 34% 25% 7%	800
	21st to end	Night	25% 14% 31% 22% 8%	800
December	1st to 10th	Night	25% 13% 36% 21% 5%	810
	11th to 20th	Night	25% 14% 32% 24% 6%	1000
	21st to end	Night	32% 13% 33% 20% 2	1100

Page generated at 2025-08-02 01:32:28. Version ac07

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