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 2026-08-30.

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% 3 7% 5% 5% 5% 4 6% 7% 36%	144203
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 37%	138032
Transparency	Day	46% 5% 10% 23% 15% 2	147874
	Night	46% 5% 10% 22% 15% 2	142433
Seeing	Day	21% 13% 27% 21% 13% 5%	17553
	Night	20% 11% 25% 27% 15% 2	27129

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	17% 17% 6% 10% 4 4 4 3 4 5% 26%	5258
	11th to 20th	Day	13% 13% 6% 14% 5% 4 3 2 3 4 33%	5280
	21st to end	Day	20% 12% 5% 9% 7% 6% 5% 3 5% 5% 24%	5780
February	1st to 10th	Day	20% 12% 4 9% 4 4 4 2 5% 5% 31%	1999
	11th to 20th	Day	19% 12% 4 8% 6% 5% 4 3 4 5% 31%	2048
	21st to end	Day	19% 12% 4 8% 4 4 4 3 5% 5% 33%	1730
March	1st to 10th	Day	18% 14% 3 8% 4 3 3 3 4 5% 34%	4666
	11th to 20th	Day	13% 8% 3 6% 4 4 4 4 5% 8% 40%	5090
	21st to end	Day	17% 9% 2 7% 5% 5% 4 4 6% 7% 35%	5586
April	1st to 10th	Day	16% 9% 3 6% 4 5% 3 4 5% 7% 38%	2441
	11th to 20th	Day	17% 8% 3 6% 4 4 4 3 5% 8% 38%	2558
	21st to end	Day	16% 8% 3 7% 5% 4 5% 4 6% 6% 36%	2730
May	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 4 7% 8% 29%	7616
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 6% 7% 36%	7626
	21st to end	Day	18% 8% 3 8% 5% 5% 6% 4 7% 9% 27%	8391
June	1st to 10th	Day	11% 5% 2 7% 5% 4 6% 5% 6% 9% 40%	2730
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 6% 9% 40%	2774
	21st to end	Day	9% 5% 3 6% 6% 6% 6% 6% 7% 9% 39%	2940
July	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 4 7% 10% 38%	8165
	11th to 20th	Day	11% 6% 2 6% 5% 6% 6% 5% 7% 10% 36%	7680
	21st to end	Day	10% 7% 3 8% 7% 6% 6% 5% 7% 10% 31%	8340
August	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 4 8% 11% 42%	4364
	11th to 20th	Day	7% 6% 2 7% 5% 5% 6% 5% 8% 11% 38%	4304
	21st to end	Day	6% 4 2 4 4 4 5% 4 6% 9% 54%	4464
September	1st to 10th	Day	14% 5% 2 5% 4 4 5% 3 5% 7% 47%	2255
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 5% 7% 50%	2200
	21st to end	Day	16% 6% 2 6% 3 4 4 3 5% 6% 45%	2200
October	1st to 10th	Day	12% 4 2 5% 4 4 3 3 5% 7% 53%	2200
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 5% 6% 48%	2021
	21st to end	Day	20% 8% 3 6% 3 3 4 2 4 6% 41%	2087
November	1st to 10th	Day	12% 7% 2 5% 3 4 4 3 4 6% 52%	1852
	11th to 20th	Day	15% 11% 3 7% 4 3 3 3 5% 6% 39%	1890
	21st to end	Day	18% 11% 5% 7% 4 3 3 3 4 6% 37%	1890
December	1st to 10th	Day	19% 11% 3 7% 4 4 4 3 3 4 38%	3020
	11th to 20th	Day	20% 13% 4 8% 5% 5% 4 3 4 4 29%	3463
	21st to end	Day	21% 17% 5% 11% 5% 5% 4 2 4 5% 20%	4565
January	1st to 10th	Night	14% 14% 6% 14% 6% 5% 4 3 4 6% 23%	7635
	11th to 20th	Night	15% 15% 5% 13% 5% 4 4 3 4 5% 27%	7680
	21st to end	Night	18% 11% 4 9% 6% 4 4 3 4 5% 33%	8401
February	1st to 10th	Night	21% 14% 4 8% 4 3 3 3 5% 5% 29%	2619
	11th to 20th	Night	20% 12% 4 9% 6% 4 3 3 3 5% 31%	2458
	21st to end	Night	22% 14% 4 7% 4 3 4 3 4 4 30%	2076
March	1st to 10th	Night	19% 13% 3 7% 4 4 3 3 4 5% 36%	5404
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	5090
	21st to end	Night	16% 10% 3 8% 5% 5% 4 3 5% 6% 33%	5588
April	1st to 10th	Night	15% 8% 2 6% 6% 4 5% 4 5% 7% 38%	2195
	11th to 20th	Night	14% 8% 2 7% 5% 4 4 4 5% 6% 41%	2009
	21st to end	Night	16% 7% 3 6% 4 4 4 4 6% 8% 37%	1890
May	1st to 10th	Night	19% 9% 3 6% 5% 5% 5% 4 6% 7% 31%	5237
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 5% 7% 9% 34%	5234
	21st to end	Night	16% 9% 3 8% 5% 5% 5% 4 6% 8% 32%	5761
June	1st to 10th	Night	7% 5% 2 4 4 4 4 5% 7% 10% 47%	1890
	11th to 20th	Night	8% 4 3 7% 4 4 5% 5% 7% 9% 45%	1835
	21st to end	Night	8% 5% 1 3 5% 5% 5% 5% 8% 10% 45%	1680
July	1st to 10th	Night	8% 7% 2 7% 5% 4 5% 4 6% 8% 45%	4796
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 4 7% 9% 40%	5277
	21st to end	Night	9% 7% 3 8% 5% 5% 6% 5% 7% 10% 35%	5736
August	1st to 10th	Night	7% 6% 3 7% 4 4 5% 4 7% 10% 45%	3005
	11th to 20th	Night	7% 7% 3 8% 4 5% 4 5% 6% 9% 42%	2965
	21st to end	Night	4 3 1 4 3 3 4 3 5% 10% 60%	3628
September	1st to 10th	Night	11% 5% 2 4 4 3 4 3 5% 7% 52%	2125
	11th to 20th	Night	8% 5% 2 5% 3 3 3 3 6% 7% 53%	2200
	21st to end	Night	14% 6% 2 6% 3 4 4 2 4 6% 48%	2200
October	1st to 10th	Night	10% 5% 1 4 3 3 3 3 5% 7% 56%	2200
	11th to 20th	Night	13% 6% 2 5% 3 4 4 4 5% 7% 47%	2377
	21st to end	Night	19% 9% 2 5% 4 3 3 3 4 5% 42%	2750
November	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 49%	2666
	11th to 20th	Night	16% 9% 3 6% 5% 4 4 3 4 6% 41%	2730
	21st to end	Night	20% 11% 4 6% 5% 4 3 3 4 5% 35%	2730
December	1st to 10th	Night	19% 12% 4 7% 4 3 3 2 4 5% 37%	4380
	11th to 20th	Night	20% 13% 6% 8% 4 3 3 3 4 5% 30%	4952
	21st to end	Night	21% 19% 6% 10% 5% 5% 4 3 4 5% 18%	6633

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 11% 18% 7%	5259
	11th to 20th	Day	56% 2 7% 18% 15% 2	5280
	21st to end	Day	63% 2 8% 18% 6% 3	5780
February	1st to 10th	Day	56% 3 8% 21% 10% 2	1999
	11th to 20th	Day	56% 3 8% 19% 11% 2	2058
	21st to end	Day	53% 4 9% 21% 11% 2	1730
March	1st to 10th	Day	53% 3 7% 18% 14% 5%	4666
	11th to 20th	Day	43% 4 9% 27% 16% 2	5090
	21st to end	Day	47% 5% 11% 23% 12% 1	5586
April	1st to 10th	Day	46% 5% 10% 24% 13% 2	2441
	11th to 20th	Day	44% 5% 9% 23% 15% 4	2558
	21st to end	Day	47% 6% 10% 23% 12% 2	2730
May	1st to 10th	Day	50% 7% 11% 20% 11% 1	7651
	11th to 20th	Day	47% 5% 10% 23% 14%	7630
	21st to end	Day	50% 7% 11% 18% 11% 2	8405
June	1st to 10th	Day	39% 8% 10% 24% 18% 1	2730
	11th to 20th	Day	39% 7% 14% 22% 17% 2	2774
	21st to end	Day	39% 8% 13% 22% 15% 3	2940
July	1st to 10th	Day	40% 7% 11% 24% 16% 1	8097
	11th to 20th	Day	40% 8% 12% 26% 13% 1	7611
	21st to end	Day	46% 8% 11% 20% 14% 1	8156
August	1st to 10th	Day	33% 8% 14% 34% 11% 1	4351
	11th to 20th	Day	36% 7% 11% 30% 15% 1	4343
	21st to end	Day	26% 7% 13% 31% 21% 3	4463
September	1st to 10th	Day	37% 6% 11% 22% 19% 6%	2255
	11th to 20th	Day	33% 6% 10% 24% 25% 3	2200
	21st to end	Day	39% 4 8% 24% 21% 3	2200
October	1st to 10th	Day	22% 3 7% 29% 36% 2	3590
	11th to 20th	Day	28% 3 8% 29% 29% 2	3274
	21st to end	Day	34% 3 9% 27% 24% 4	3350
November	1st to 10th	Day	35% 4 8% 28% 21% 3	1852
	11th to 20th	Day	47% 3 7% 22% 18% 3	1890
	21st to end	Day	50% 3 9% 22% 16% 1	1890
December	1st to 10th	Day	52% 2 9% 23% 14%	3020
	11th to 20th	Day	60% 3 7% 20% 9% 1	3460
	21st to end	Day	66% 4 9% 13% 6% 1	4565
January	1st to 10th	Night	62% 4 9% 17% 8%	7636
	11th to 20th	Night	60% 3 7% 15% 13% 3	7680
	21st to end	Night	55% 3 9% 21% 11% 1	8401
February	1st to 10th	Night	58% 3 11% 18% 9% 1	2619
	11th to 20th	Night	57% 3 7% 21% 11% 1	2459
	21st to end	Night	57% 3 10% 16% 11% 3	2076
March	1st to 10th	Night	51% 3 5% 25% 12% 3	5404
	11th to 20th	Night	39% 4 9% 28% 19% 1	5090
	21st to end	Night	51% 5% 7% 23% 13% 1	5588
April	1st to 10th	Night	45% 5% 10% 22% 15% 2	2195
	11th to 20th	Night	42% 5% 10% 19% 18% 5%	2009
	21st to end	Night	44% 5% 11% 23% 14% 3	1890
May	1st to 10th	Night	50% 7% 10% 21% 10% 2	5264
	11th to 20th	Night	44% 8% 14% 21% 12% 1	5235
	21st to end	Night	48% 7% 13% 18% 12% 3	5771
June	1st to 10th	Night	30% 8% 14% 26% 20% 3	1890
	11th to 20th	Night	33% 7% 13% 23% 20% 3	1835
	21st to end	Night	31% 9% 16% 23% 18% 4	1680
July	1st to 10th	Night	37% 7% 11% 25% 17% 2	4757
	11th to 20th	Night	39% 8% 13% 26% 14% 1	5226
	21st to end	Night	42% 7% 13% 23% 14% 2	5606
August	1st to 10th	Night	33% 7% 14% 32% 13% 2	2994
	11th to 20th	Night	36% 8% 10% 29% 17% 1	2992
	21st to end	Night	20% 6% 14% 33% 22% 4	3628
September	1st to 10th	Night	32% 5% 12% 24% 21% 6%	2125
	11th to 20th	Night	29% 5% 11% 25% 27% 3	2200
	21st to end	Night	38% 4 8% 21% 26% 3	2200
October	1st to 10th	Night	22% 3 7% 24% 40% 3	3590
	11th to 20th	Night	26% 3 8% 26% 33% 3	3863
	21st to end	Night	33% 3 7% 21% 28% 7%	4437
November	1st to 10th	Night	37% 3 9% 24% 23% 4	2666
	11th to 20th	Night	45% 3 8% 21% 20% 4	2730
	21st to end	Night	51% 3 9% 22% 13% 2	2730
December	1st to 10th	Night	51% 3 11% 22% 12%	4380
	11th to 20th	Night	56% 3 11% 20% 10% 1	4954
	21st to end	Night	69% 4 8% 14% 5%	6633

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% 20% 33% 13% 4	330
	11th to 20th	Day	38% 24% 30% 7% 1	330
	21st to end	Day	35% 23% 29% 10% 2	363
February	1st to 10th	Day	30% 25% 32% 10% 2	330
	11th to 20th	Day	38% 27% 26% 7% 2	330
	21st to end	Day	34% 16% 41% 7% 2	273
March	1st to 10th	Day	34% 22% 32% 10% 2	364
	11th to 20th	Day	29% 22% 32% 13% 4	540
	21st to end	Day	32% 21% 29% 14% 5%	594
April	1st to 10th	Day	29% 17% 30% 15% 8% 1	540
	11th to 20th	Day	26% 19% 29% 18% 6% 3	532
	21st to end	Day	23% 17% 35% 17% 7% 1	540
May	1st to 10th	Day	24% 19% 31% 16% 8% 2	543
	11th to 20th	Day	19% 14% 31% 22% 9% 5%	567
	21st to end	Day	18% 11% 26% 24% 16% 6%	617
June	1st to 10th	Day	14% 9% 25% 28% 18% 6%	570
	11th to 20th	Day	13% 7% 26% 32% 15% 7%	614
	21st to end	Day	12% 7% 27% 28% 19% 7%	780
July	1st to 10th	Day	14% 6% 20% 22% 22% 16%	735
	11th to 20th	Day	10% 5% 23% 29% 20% 13%	564
	21st to end	Day	12% 4 20% 32% 21% 12%	620
August	1st to 10th	Day	14% 3 20% 29% 22% 12%	566
	11th to 20th	Day	11% 5% 20% 35% 20% 9%	559
	21st to end	Day	9% 4 18% 33% 25% 12%	619
September	1st to 10th	Day	15% 5% 16% 29% 25% 10%	530
	11th to 20th	Day	12% 3 16% 28% 29% 13%	530
	21st to end	Day	20% 8% 21% 23% 21% 7%	530
October	1st to 10th	Day	12% 7% 25% 32% 18% 7%	530
	11th to 20th	Day	17% 15% 30% 21% 14% 3	530
	21st to end	Day	25% 13% 35% 21% 6% 1	470
November	1st to 10th	Day	18% 13% 38% 22% 6% 2	330
	11th to 20th	Day	27% 20% 28% 18% 5% 2	330
	21st to end	Day	30% 16% 37% 13% 4 1	330
December	1st to 10th	Day	32% 17% 35% 11% 5%	330
	11th to 20th	Day	28% 21% 33% 15% 3	330
	21st to end	Day	32% 21% 34% 10% 2	363
January	1st to 10th	Night	26% 22% 31% 18% 4	840
	11th to 20th	Night	32% 20% 31% 15% 2	840
	21st to end	Night	27% 19% 32% 19% 2	924
February	1st to 10th	Night	30% 15% 31% 20% 3	840
	11th to 20th	Night	30% 20% 31% 18% 1	823
	21st to end	Night	33% 16% 33% 17% 1	692
March	1st to 10th	Night	27% 16% 32% 22% 4	840
	11th to 20th	Night	26% 17% 30% 21% 6%	840
	21st to end	Night	25% 15% 31% 24% 6%	924
April	1st to 10th	Night	19% 16% 32% 26% 6%	725
	11th to 20th	Night	18% 13% 32% 24% 10% 1	623
	21st to end	Night	20% 8% 36% 26% 8% 1	630
May	1st to 10th	Night	20% 12% 29% 27% 11% 2	630
	11th to 20th	Night	20% 6% 26% 30% 17% 1	621
	21st to end	Night	13% 4 24% 31% 24% 4	683
June	1st to 10th	Night	10% 2 22% 40% 23% 2	630
	11th to 20th	Night	9% 4 18% 41% 24% 3	630
	21st to end	Night	10% 2 23% 36% 25% 4	630
July	1st to 10th	Night	13% 2 15% 30% 31% 9%	615
	11th to 20th	Night	7% 2 16% 33% 37% 4	612
	21st to end	Night	11% 3 13% 37% 33% 4	690
August	1st to 10th	Night	9% 2 14% 34% 37% 4	627
	11th to 20th	Night	8% 3 14% 37% 33% 5%	624
	21st to end	Night	7% 1 14% 34% 37% 7%	687
September	1st to 10th	Night	13% 3 13% 36% 31% 4	725
	11th to 20th	Night	10% 3 16% 31% 33% 7%	800
	21st to end	Night	17% 6% 19% 32% 24% 2	800
October	1st to 10th	Night	13% 7% 18% 33% 24% 4	800
	11th to 20th	Night	18% 8% 26% 30% 16% 2	800
	21st to end	Night	23% 12% 29% 23% 11% 1	880
November	1st to 10th	Night	17% 10% 27% 28% 15% 2	820
	11th to 20th	Night	22% 15% 29% 25% 9%	840
	21st to end	Night	28% 12% 25% 27% 8% 1	840
December	1st to 10th	Night	29% 15% 29% 23% 4	840
	11th to 20th	Night	27% 14% 30% 23% 5% 1	840
	21st to end	Night	32% 18% 26% 20% 3	924

Page generated at 2026-08-27 04:07:13. Version ac07

Copyright © 2026 Attilla Danko.