CAS Starfield Observatory Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAS Starfield Observatory Dark Site clear sky chart from 2005-11-06 to 2025-04-26.

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	15% 9% 3 7% 5% 4 4 4 5% 7% 37%	114450
	Night	15% 10% 3 7% 4 4 4 3 5% 7% 37%	114187
Transparency	Day	45% 5% 10% 23% 15% 2	118165
	Night	45% 5% 9% 22% 16% 2	118611
Seeing	Day	21% 13% 27% 22% 12% 5%	15683
	Night	20% 11% 25% 27% 15% 2	25445

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	15% 19% 7% 10% 4 4 4 3 4 5% 26%	4056
	11th to 20th	Day	15% 14% 7% 13% 5% 4 3 2 3 4 31%	4060
	21st to end	Day	22% 12% 6% 8% 6% 5% 4 2 5% 5% 27%	4466
February	1st to 10th	Day	20% 12% 4 9% 4 4 4 3 5% 5% 31%	1904
	11th to 20th	Day	19% 12% 4 8% 6% 5% 4 3 4 5% 31%	1948
	21st to end	Day	19% 12% 4 8% 4 4 4 3 5% 5% 33%	1650
March	1st to 10th	Day	18% 14% 3 8% 4 3 3 3 4 5% 34%	4564
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 6% 8% 40%	4980
	21st to end	Day	17% 9% 3 7% 5% 5% 4 4 5% 7% 35%	5465
April	1st to 10th	Day	17% 10% 3 6% 4 5% 3 4 6% 7% 37%	2325
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 5% 8% 38%	2435
	21st to end	Day	17% 8% 3 7% 5% 4 5% 3 6% 6% 36%	2535
May	1st to 10th	Day	21% 10% 3 7% 5% 4 4 4 6% 7% 29%	4082
	11th to 20th	Day	20% 8% 3 6% 4 4 4 4 5% 7% 35%	4081
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	4476
June	1st to 10th	Day	12% 5% 2 7% 5% 4 6% 5% 6% 9% 40%	2470
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 9% 42%	2510
	21st to end	Day	8% 6% 3 6% 6% 6% 6% 5% 6% 8% 39%	2660
July	1st to 10th	Day	12% 7% 3 6% 4 4 5% 4 6% 10% 39%	4381
	11th to 20th	Day	8% 4 2 5% 5% 5% 6% 5% 7% 10% 43%	4120
	21st to end	Day	13% 10% 4 9% 6% 6% 6% 5% 7% 8% 27%	4466
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 8% 11% 43%	4104
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 4 8% 11% 39%	4044
	21st to end	Day	6% 4 2 4 4 4 5% 4 6% 9% 53%	4222
September	1st to 10th	Day	15% 5% 2 5% 4 4 5% 4 4 7% 46%	2142
	11th to 20th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 49%	2090
	21st to end	Day	16% 6% 2 6% 3 3 4 3 5% 6% 45%	2090
October	1st to 10th	Day	12% 4 2 5% 4 4 3 3 5% 7% 52%	2090
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 5% 6% 47%	1920
	21st to end	Day	20% 8% 3 5% 3 3 4 3 4 6% 41%	1983
November	1st to 10th	Day	12% 7% 1 5% 3 3 3 3 4 6% 52%	1762
	11th to 20th	Day	15% 11% 4 7% 4 3 3 3 5% 6% 39%	1800
	21st to end	Day	18% 10% 5% 7% 4 3 3 3 4 6% 38%	1800
December	1st to 10th	Day	20% 11% 3 7% 4 4 4 3 3 4 39%	2930
	11th to 20th	Day	20% 13% 4 8% 5% 5% 4 3 4 4 28%	3373
	21st to end	Day	21% 17% 5% 10% 5% 5% 4 2 5% 5% 21%	4466
January	1st to 10th	Night	15% 16% 7% 16% 6% 5% 4 4 4 5% 19%	5889
	11th to 20th	Night	17% 16% 6% 12% 5% 4 4 3 4 4 25%	5900
	21st to end	Night	21% 11% 4 7% 5% 4 4 3 4 5% 34%	6490
February	1st to 10th	Night	22% 14% 4 8% 4 3 3 3 5% 5% 29%	2495
	11th to 20th	Night	20% 12% 4 9% 6% 4 3 3 3 5% 31%	2338
	21st to end	Night	23% 13% 4 7% 4 3 4 3 4 4 30%	1980
March	1st to 10th	Night	18% 13% 3 7% 4 4 3 3 4 5% 36%	5286
	11th to 20th	Night	13% 7% 2 6% 4 4 4 3 5% 7% 44%	4980
	21st to end	Night	16% 10% 3 8% 5% 5% 4 3 5% 6% 33%	5467
April	1st to 10th	Night	15% 8% 2 6% 6% 4 5% 4 5% 7% 37%	2090
	11th to 20th	Night	14% 8% 2 6% 5% 4 4 4 5% 6% 42%	1912
	21st to end	Night	16% 8% 3 6% 4 4 4 3 7% 8% 37%	1761
May	1st to 10th	Night	20% 11% 3 8% 5% 4 4 4 6% 6% 27%	2813
	11th to 20th	Night	20% 6% 2 5% 3 3 5% 4 6% 9% 37%	2812
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 10% 45%	3079
June	1st to 10th	Night	7% 5% 2 4 4 5% 4 5% 8% 10% 47%	1710
	11th to 20th	Night	7% 4 2 7% 3 4 5% 5% 8% 9% 46%	1661
	21st to end	Night	7% 5% 1 3 5% 5% 5% 5% 8% 10% 46%	1520
July	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 6% 8% 47%	2579
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 7% 9% 49%	2840
	21st to end	Night	11% 9% 3 8% 5% 4 5% 4 6% 9% 35%	3087
August	1st to 10th	Night	8% 6% 3 6% 3 3 4 4 7% 10% 46%	2825
	11th to 20th	Night	7% 7% 3 8% 4 5% 4 5% 6% 9% 43%	2785
	21st to end	Night	4 3 1 4 3 3 4 3 5% 10% 59%	3431
September	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 7% 51%	2019
	11th to 20th	Night	8% 6% 2 5% 3 3 3 3 6% 7% 53%	2090
	21st to end	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	2090
October	1st to 10th	Night	10% 5% 1 5% 3 3 3 3 5% 8% 55%	2090
	11th to 20th	Night	14% 6% 2 5% 3 4 4 4 5% 7% 46%	2258
	21st to end	Night	18% 9% 2 6% 4 3 3 3 4 6% 42%	2612
November	1st to 10th	Night	15% 6% 2 5% 4 4 3 3 5% 6% 49%	2536
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 4 6% 40%	2600
	21st to end	Night	19% 11% 4 6% 5% 4 3 3 4 5% 36%	2600
December	1st to 10th	Night	20% 11% 4 6% 4 3 3 2 4 5% 37%	4250
	11th to 20th	Night	20% 13% 6% 8% 4 3 3 3 4 5% 30%	4822
	21st to end	Night	21% 19% 6% 10% 5% 5% 4 3 4 5% 18%	6490

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% 3 9% 19% 8%	4057
	11th to 20th	Day	59% 2 5% 18% 13% 2	4060
	21st to end	Day	61% 2 8% 20% 6% 3	4466
February	1st to 10th	Day	57% 3 8% 20% 10% 2	1904
	11th to 20th	Day	57% 3 8% 19% 11% 2	1958
	21st to end	Day	54% 4 9% 20% 11% 2	1650
March	1st to 10th	Day	53% 3 6% 18% 14% 5%	4564
	11th to 20th	Day	42% 4 9% 27% 16% 2	4980
	21st to end	Day	48% 5% 11% 23% 12% 1	5465
April	1st to 10th	Day	47% 5% 9% 24% 13% 2	2325
	11th to 20th	Day	45% 5% 8% 23% 15% 4	2435
	21st to end	Day	47% 6% 11% 22% 12% 2	2535
May	1st to 10th	Day	52% 7% 11% 20% 9% 1	4120
	11th to 20th	Day	48% 7% 11% 22% 13% 1	4085
	21st to end	Day	40% 9% 13% 19% 16% 2	4489
June	1st to 10th	Day	40% 8% 10% 25% 17% 1	2470
	11th to 20th	Day	38% 7% 14% 22% 18% 2	2510
	21st to end	Day	39% 8% 13% 21% 15% 4	2660
July	1st to 10th	Day	41% 6% 10% 22% 19% 1	4313
	11th to 20th	Day	33% 6% 11% 31% 16% 1	4051
	21st to end	Day	53% 7% 11% 16% 12% 2	4324
August	1st to 10th	Day	33% 8% 13% 34% 11% 1	4091
	11th to 20th	Day	36% 6% 11% 31% 15% 1	4083
	21st to end	Day	27% 7% 13% 30% 20% 3	4221
September	1st to 10th	Day	37% 6% 11% 21% 19% 6%	2142
	11th to 20th	Day	34% 5% 10% 23% 24% 3	2090
	21st to end	Day	39% 4 9% 25% 21% 2	2090
October	1st to 10th	Day	22% 4 7% 29% 35% 2	3480
	11th to 20th	Day	28% 3 9% 29% 29% 2	3173
	21st to end	Day	33% 3 9% 28% 24% 4	3246
November	1st to 10th	Day	34% 4 8% 29% 21% 3	1762
	11th to 20th	Day	46% 3 7% 21% 19% 3	1800
	21st to end	Day	49% 3 9% 21% 16% 1	1800
December	1st to 10th	Day	51% 2 9% 23% 15%	2930
	11th to 20th	Day	60% 4 7% 19% 9% 1	3370
	21st to end	Day	66% 4 9% 14% 6% 1	4466
January	1st to 10th	Night	66% 4 7% 14% 8%	5889
	11th to 20th	Night	62% 3 8% 14% 10% 4	5900
	21st to end	Night	54% 3 7% 22% 12% 2	6490
February	1st to 10th	Night	59% 3 11% 18% 8% 1	2495
	11th to 20th	Night	58% 3 7% 20% 11% 1	2339
	21st to end	Night	57% 3 10% 16% 11% 3	1980
March	1st to 10th	Night	51% 3 5% 26% 12% 3	5286
	11th to 20th	Night	39% 4 9% 28% 19% 1	4980
	21st to end	Night	51% 5% 7% 23% 13% 1	5467
April	1st to 10th	Night	45% 5% 10% 22% 16% 2	2090
	11th to 20th	Night	42% 5% 9% 20% 18% 6%	1912
	21st to end	Night	45% 5% 11% 23% 14% 3	1761
May	1st to 10th	Night	54% 9% 9% 19% 7% 2	2840
	11th to 20th	Night	43% 8% 14% 24% 10% 1	2813
	21st to end	Night	32% 8% 13% 25% 18% 4	3087
June	1st to 10th	Night	30% 8% 14% 27% 18% 4	1710
	11th to 20th	Night	30% 8% 14% 23% 21% 3	1661
	21st to end	Night	31% 9% 15% 23% 18% 4	1520
July	1st to 10th	Night	35% 7% 12% 22% 18% 5%	2538
	11th to 20th	Night	30% 7% 12% 31% 18% 2	2786
	21st to end	Night	45% 6% 12% 21% 13% 3	2988
August	1st to 10th	Night	33% 6% 14% 32% 13% 2	2814
	11th to 20th	Night	36% 8% 9% 29% 18% 1	2812
	21st to end	Night	20% 6% 15% 33% 22% 4	3431
September	1st to 10th	Night	33% 5% 12% 24% 20% 7%	2019
	11th to 20th	Night	29% 5% 11% 24% 27% 4	2090
	21st to end	Night	37% 4 8% 22% 25% 3	2090
October	1st to 10th	Night	23% 3 7% 24% 40% 3	3480
	11th to 20th	Night	26% 3 8% 26% 33% 3	3744
	21st to end	Night	33% 3 7% 22% 28% 7%	4299
November	1st to 10th	Night	37% 3 9% 24% 24% 4	2536
	11th to 20th	Night	46% 3 8% 20% 20% 4	2600
	21st to end	Night	50% 3 9% 22% 13% 2	2600
December	1st to 10th	Night	51% 3 11% 23% 13%	4250
	11th to 20th	Night	56% 3 11% 19% 10% 1	4824
	21st to end	Night	69% 4 8% 14% 5%	6490

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	30% 21% 32% 13% 3	300
	11th to 20th	Day	37% 24% 30% 7% 1	300
	21st to end	Day	34% 23% 30% 11% 2	330
February	1st to 10th	Day	31% 26% 32% 10% 1	300
	11th to 20th	Day	38% 27% 25% 7% 2	300
	21st to end	Day	36% 17% 40% 6% 2	249
March	1st to 10th	Day	33% 21% 33% 11% 2	332
	11th to 20th	Day	28% 22% 32% 13% 4	500
	21st to end	Day	33% 21% 27% 14% 5%	550
April	1st to 10th	Day	30% 17% 28% 16% 8% 1	500
	11th to 20th	Day	26% 19% 28% 19% 6% 3	492
	21st to end	Day	25% 16% 34% 16% 7% 2	480
May	1st to 10th	Day	24% 17% 31% 17% 8% 3	463
	11th to 20th	Day	21% 12% 30% 23% 9% 5%	487
	21st to end	Day	14% 10% 26% 25% 18% 7%	529
June	1st to 10th	Day	14% 9% 24% 29% 18% 7%	490
	11th to 20th	Day	12% 6% 24% 34% 16% 8%	530
	21st to end	Day	11% 7% 29% 27% 18% 7%	680
July	1st to 10th	Day	15% 6% 21% 21% 20% 17%	639
	11th to 20th	Day	10% 5% 24% 28% 20% 13%	484
	21st to end	Day	12% 3 20% 32% 20% 12%	532
August	1st to 10th	Day	14% 3 17% 30% 22% 12%	486
	11th to 20th	Day	11% 5% 19% 36% 19% 10%	479
	21st to end	Day	10% 4 16% 32% 25% 13%	539
September	1st to 10th	Day	15% 4 16% 30% 25% 10%	490
	11th to 20th	Day	13% 3 17% 28% 28% 12%	490
	21st to end	Day	21% 9% 21% 23% 20% 7%	490
October	1st to 10th	Day	12% 7% 26% 32% 18% 6%	490
	11th to 20th	Day	17% 16% 29% 21% 14% 3	490
	21st to end	Day	24% 11% 36% 22% 6% 1	432
November	1st to 10th	Day	18% 13% 37% 23% 7% 3	300
	11th to 20th	Day	26% 20% 28% 20% 5% 2	300
	21st to end	Day	29% 16% 37% 14% 4 1	300
December	1st to 10th	Day	32% 18% 34% 11% 5%	300
	11th to 20th	Day	28% 20% 33% 16% 4	300
	21st to end	Day	32% 20% 34% 11% 3	330
January	1st to 10th	Night	26% 22% 30% 18% 5%	800
	11th to 20th	Night	32% 19% 31% 16% 2	800
	21st to end	Night	28% 18% 31% 20% 2	880
February	1st to 10th	Night	31% 15% 31% 20% 3	800
	11th to 20th	Night	30% 20% 31% 18% 1	783
	21st to end	Night	33% 16% 33% 17% 1	660
March	1st to 10th	Night	27% 15% 32% 23% 4	800
	11th to 20th	Night	26% 17% 30% 21% 6%	800
	21st to end	Night	25% 15% 31% 24% 6%	880
April	1st to 10th	Night	20% 16% 32% 27% 6%	690
	11th to 20th	Night	19% 13% 32% 25% 10% 1	593
	21st to end	Night	21% 8% 37% 25% 9% 1	587
May	1st to 10th	Night	20% 12% 29% 26% 11% 2	570
	11th to 20th	Night	20% 5% 26% 30% 18% 1	561
	21st to end	Night	11% 4 23% 32% 26% 5%	617
June	1st to 10th	Night	10% 2 21% 41% 24% 2	570
	11th to 20th	Night	8% 4 17% 42% 26% 3	570
	21st to end	Night	10% 2 24% 36% 24% 4	570
July	1st to 10th	Night	14% 2 16% 29% 30% 10%	555
	11th to 20th	Night	6% 3 15% 34% 38% 4	552
	21st to end	Night	11% 2 14% 37% 33% 4	624
August	1st to 10th	Night	10% 1 11% 34% 39% 4	567
	11th to 20th	Night	9% 2 15% 37% 32% 5%	564
	21st to end	Night	7% 1 14% 32% 38% 7%	627
September	1st to 10th	Night	13% 3 13% 34% 32% 4	689
	11th to 20th	Night	10% 4 17% 31% 33% 6%	760
	21st to end	Night	16% 6% 19% 32% 24% 2	760
October	1st to 10th	Night	13% 7% 19% 33% 23% 4	760
	11th to 20th	Night	19% 8% 26% 29% 16% 1	760
	21st to end	Night	23% 12% 29% 24% 11% 1	836
November	1st to 10th	Night	18% 10% 26% 28% 16% 2	780
	11th to 20th	Night	22% 14% 29% 25% 10% 1	800
	21st to end	Night	27% 12% 26% 27% 8% 1	800
December	1st to 10th	Night	30% 15% 28% 23% 4	800
	11th to 20th	Night	27% 14% 29% 23% 5% 1	800
	21st to end	Night	32% 17% 26% 21% 4	880

Page generated at 2025-04-23 09:25:04. Version ac07

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