CAS DS-2
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAS DS-2 clear sky chart from 2005-11-06 to 2025-12-03.

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	17% 7% 2 6% 4 4 5% 4 6% 7% 38%	127168
	Night	17% 7% 2 6% 4 4 4 3 5% 7% 41%	123534
Transparency	Day	42% 7% 12% 22% 14% 2	130742
	Night	41% 6% 12% 23% 16% 3	128047
Seeing	Day	21% 14% 28% 22% 11% 4	16259
	Night	20% 11% 27% 29% 11% 1	25590

By Month

Percentage Cloud Cover NE (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% 8% 3 6% 5% 4 5% 4 6% 6% 41%	3597
	11th to 20th	Day	13% 11% 5% 11% 6% 5% 5% 3 5% 6% 31%	3775
	21st to end	Day	17% 9% 3 7% 4 5% 4 4 4 5% 36%	4466
February	1st to 10th	Day	16% 8% 3 8% 6% 6% 5% 5% 5% 6% 34%	1800
	11th to 20th	Day	15% 9% 3 7% 5% 4 5% 3 5% 7% 36%	1835
	21st to end	Day	18% 9% 3 7% 6% 4 5% 3 4 6% 35%	1650
March	1st to 10th	Day	20% 5% 2 5% 4 4 4 3 5% 5% 42%	4648
	11th to 20th	Day	12% 7% 3 6% 5% 4 5% 3 5% 7% 43%	4980
	21st to end	Day	19% 7% 2 5% 4 4 4 4 5% 5% 42%	5688
April	1st to 10th	Day	17% 9% 3 7% 5% 5% 5% 3 5% 8% 35%	2400
	11th to 20th	Day	18% 7% 2 6% 4 5% 4 3 5% 5% 41%	2374
	21st to end	Day	17% 6% 2 6% 4 5% 5% 4 6% 7% 39%	2400
May	1st to 10th	Day	28% 6% 2 6% 4 4 4 4 5% 8% 30%	5922
	11th to 20th	Day	26% 8% 3 6% 4 4 4 3 5% 7% 30%	6318
	21st to end	Day	20% 5% 2 6% 4 4 5% 4 6% 8% 36%	6949
June	1st to 10th	Day	13% 6% 3 6% 5% 4 5% 5% 7% 9% 38%	2800
	11th to 20th	Day	16% 5% 2 6% 5% 5% 6% 4 6% 9% 37%	2800
	21st to end	Day	12% 4 1 6% 5% 5% 5% 5% 7% 9% 42%	2800
July	1st to 10th	Day	8% 5% 3 7% 5% 6% 6% 4 7% 10% 39%	6360
	11th to 20th	Day	13% 7% 2 7% 5% 6% 6% 5% 8% 10% 32%	6360
	21st to end	Day	12% 7% 3 6% 4 5% 5% 4 6% 8% 40%	6664
August	1st to 10th	Day	21% 5% 2 6% 5% 5% 5% 5% 6% 8% 32%	4234
	11th to 20th	Day	16% 5% 2 5% 4 4 5% 4 7% 8% 40%	3959
	21st to end	Day	16% 5% 2 4 3 3 3 3 5% 8% 48%	4312
September	1st to 10th	Day	17% 5% 2 6% 3 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	13% 5% 1 5% 3 4 4 3 5% 7% 50%	2400
	21st to end	Day	22% 7% 3 5% 4 4 4 3 5% 5% 37%	2135
October	1st to 10th	Day	19% 6% 2 4 4 3 4 3 5% 8% 44%	2000
	11th to 20th	Day	17% 5% 2 5% 4 4 4 3 5% 7% 44%	2000
	21st to end	Day	20% 5% 2 4 4 3 4 4 5% 7% 42%	2180
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 8% 51%	1851
	11th to 20th	Day	17% 6% 2 5% 5% 4 4 5% 5% 7% 42%	1869
	21st to end	Day	18% 6% 2 5% 5% 4 4 3 6% 7% 40%	1680
December	1st to 10th	Day	16% 8% 2 6% 4 4 4 4 5% 7% 38%	2616
	11th to 20th	Day	23% 8% 3 5% 3 3 3 3 5% 6% 37%	2986
	21st to end	Day	20% 8% 3 6% 5% 4 5% 4 6% 8% 33%	3960
January	1st to 10th	Night	13% 7% 2 6% 4 5% 5% 3 5% 7% 43%	6349
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 4 6% 8% 40%	6105
	21st to end	Night	17% 7% 2 8% 4 3 4 4 6% 6% 40%	6490
February	1st to 10th	Night	16% 9% 3 6% 4 4 5% 4 5% 7% 38%	2600
	11th to 20th	Night	16% 8% 3 7% 5% 4 5% 4 4 6% 38%	2449
	21st to end	Night	18% 8% 2 6% 5% 4 4 3 6% 7% 37%	1980
March	1st to 10th	Night	19% 8% 2 6% 4 4 3 3 4 6% 41%	5190
	11th to 20th	Night	12% 7% 3 5% 4 4 3 3 5% 6% 50%	4979
	21st to end	Night	16% 8% 2 6% 4 4 4 3 4 5% 43%	5243
April	1st to 10th	Night	19% 7% 2 6% 5% 4 4 4 7% 7% 36%	2000
	11th to 20th	Night	16% 6% 2 7% 4 4 4 4 6% 7% 40%	1975
	21st to end	Night	20% 5% 2 5% 3 3 4 4 5% 6% 45%	2000
May	1st to 10th	Night	30% 6% 2 5% 4 4 4 4 5% 7% 28%	4059
	11th to 20th	Night	25% 8% 2 6% 4 3 3 4 5% 7% 32%	3575
	21st to end	Night	23% 6% 2 5% 3 3 4 3 5% 7% 37%	3926
June	1st to 10th	Night	17% 6% 2 6% 4 3 4 3 6% 7% 42%	1600
	11th to 20th	Night	15% 5% 2 6% 4 4 5% 4 6% 8% 41%	1600
	21st to end	Night	12% 4 2 5% 4 4 4 4 8% 9% 44%	1600
July	1st to 10th	Night	12% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	3600
	11th to 20th	Night	16% 6% 2 8% 5% 4 6% 5% 7% 9% 32%	3600
	21st to end	Night	14% 5% 2 5% 4 3 4 4 6% 8% 44%	4171
August	1st to 10th	Night	22% 6% 2 6% 4 4 4 3 6% 7% 35%	2915
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 3 4 6% 42%	3055
	21st to end	Night	14% 5% 1 4 3 4 5% 4 5% 8% 48%	3586
September	1st to 10th	Night	15% 7% 2 5% 4 3 3 2 5% 7% 48%	2000
	11th to 20th	Night	13% 5% 2 5% 4 3 4 3 5% 6% 52%	2000
	21st to end	Night	21% 6% 2 5% 4 3 4 3 5% 7% 40%	2260
October	1st to 10th	Night	22% 5% 2 4 4 3 3 3 4 7% 45%	2400
	11th to 20th	Night	17% 6% 2 4 3 3 4 3 5% 6% 47%	2400
	21st to end	Night	19% 5% 2 4 3 3 3 3 5% 7% 46%	2660
November	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 6% 51%	2667
	11th to 20th	Night	16% 8% 2 5% 4 4 4 3 4 6% 44%	2747
	21st to end	Night	15% 6% 2 5% 4 5% 4 4 5% 8% 41%	2940
December	1st to 10th	Night	14% 9% 3 6% 4 3 4 4 5% 6% 41%	4609
	11th to 20th	Night	23% 7% 2 5% 3 3 4 3 5% 5% 39%	5208
	21st to end	Night	19% 10% 3 6% 4 4 4 4 5% 6% 35%	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	38% 7% 12% 24% 17% 2	3598
	11th to 20th	Day	51% 5% 11% 17% 12% 4	3807
	21st to end	Day	47% 8% 11% 21% 12% 1	4466
February	1st to 10th	Day	48% 7% 14% 20% 11% 1	1800
	11th to 20th	Day	47% 5% 11% 23% 12% 2	1844
	21st to end	Day	51% 5% 11% 22% 9% 3	1650
March	1st to 10th	Day	41% 6% 11% 24% 12% 6%	4651
	11th to 20th	Day	38% 7% 14% 26% 10% 5%	4980
	21st to end	Day	42% 6% 10% 16% 21% 4	5688
April	1st to 10th	Day	46% 6% 11% 20% 13% 5%	2400
	11th to 20th	Day	44% 6% 11% 21% 14% 4	2374
	21st to end	Day	41% 7% 11% 22% 15% 5%	2400
May	1st to 10th	Day	51% 8% 13% 17% 10% 2	5931
	11th to 20th	Day	51% 8% 10% 20% 10% 1	6322
	21st to end	Day	42% 8% 13% 23% 13% 1	6949
June	1st to 10th	Day	40% 9% 13% 22% 15% 2	2800
	11th to 20th	Day	42% 9% 14% 22% 11% 2	2800
	21st to end	Day	37% 8% 15% 25% 14% 1	2800
July	1st to 10th	Day	39% 11% 12% 22% 15% 1	6290
	11th to 20th	Day	44% 10% 15% 21% 10% 1	6285
	21st to end	Day	40% 7% 11% 21% 21% 1	6473
August	1st to 10th	Day	47% 9% 12% 22% 10%	4221
	11th to 20th	Day	38% 8% 12% 29% 11% 1	3998
	21st to end	Day	34% 6% 11% 29% 18% 2	4312
September	1st to 10th	Day	39% 7% 10% 22% 19% 3	2400
	11th to 20th	Day	33% 5% 11% 26% 22% 3	2400
	21st to end	Day	48% 6% 8% 17% 19% 2	2135
October	1st to 10th	Day	37% 5% 8% 22% 23% 5%	3260
	11th to 20th	Day	29% 5% 11% 28% 21% 5%	3240
	21st to end	Day	27% 4 14% 26% 26% 3	3506
November	1st to 10th	Day	34% 4 14% 25% 19% 3	1851
	11th to 20th	Day	39% 6% 11% 24% 17% 2	1869
	21st to end	Day	42% 6% 14% 28% 8% 2	1680
December	1st to 10th	Day	44% 5% 16% 24% 9% 2	2616
	11th to 20th	Day	47% 5% 13% 27% 7% 1	2986
	21st to end	Day	46% 7% 16% 19% 11% 1	3960
January	1st to 10th	Night	37% 6% 13% 24% 17% 3	6349
	11th to 20th	Night	39% 7% 15% 21% 14% 5%	6148
	21st to end	Night	42% 6% 12% 22% 16% 2	6490
February	1st to 10th	Night	45% 6% 14% 24% 9% 2	2600
	11th to 20th	Night	46% 5% 11% 20% 16% 2	2451
	21st to end	Night	46% 5% 10% 20% 16% 3	1980
March	1st to 10th	Night	44% 6% 10% 21% 12% 6%	5200
	11th to 20th	Night	34% 6% 16% 27% 12% 5%	4980
	21st to end	Night	42% 5% 8% 17% 23% 4	5243
April	1st to 10th	Night	46% 6% 11% 21% 14% 3	2000
	11th to 20th	Night	41% 7% 12% 19% 19% 2	1975
	21st to end	Night	39% 7% 10% 22% 17% 5%	2000
May	1st to 10th	Night	54% 7% 9% 17% 10% 2	4066
	11th to 20th	Night	50% 9% 12% 16% 12% 1	3576
	21st to end	Night	46% 8% 10% 22% 12% 3	3927
June	1st to 10th	Night	40% 7% 10% 23% 18% 2	1600
	11th to 20th	Night	38% 9% 14% 24% 14% 1	1600
	21st to end	Night	34% 8% 15% 28% 14% 2	1600
July	1st to 10th	Night	41% 9% 13% 19% 15% 3	3560
	11th to 20th	Night	45% 10% 13% 20% 11% 1	3552
	21st to end	Night	35% 8% 13% 22% 21% 1	4048
August	1st to 10th	Night	47% 8% 16% 19% 10% 1	2904
	11th to 20th	Night	42% 7% 11% 23% 16% 1	3082
	21st to end	Night	33% 8% 10% 30% 18% 1	3586
September	1st to 10th	Night	38% 5% 11% 24% 20% 3	2000
	11th to 20th	Night	33% 6% 9% 25% 24% 3	2000
	21st to end	Night	44% 7% 9% 18% 20% 2	2260
October	1st to 10th	Night	40% 5% 10% 21% 22% 2	3920
	11th to 20th	Night	29% 5% 9% 29% 23% 5%	3899
	21st to end	Night	27% 4 13% 31% 23% 2	4284
November	1st to 10th	Night	33% 6% 13% 25% 19% 4	2667
	11th to 20th	Night	41% 5% 10% 24% 17% 4	2747
	21st to end	Night	40% 5% 14% 27% 13% 1	2940
December	1st to 10th	Night	42% 5% 14% 28% 10% 1	4609
	11th to 20th	Night	46% 5% 13% 23% 11% 2	5208
	21st to end	Night	47% 7% 11% 20% 14% 1	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	30% 26% 27% 14% 4 1	200
	11th to 20th	Day	33% 21% 33% 10% 2	245
	21st to end	Day	22% 25% 33% 14% 4 1	330
February	1st to 10th	Day	25% 26% 31% 13% 4 1	300
	11th to 20th	Day	30% 24% 34% 11% 1	382
	21st to end	Day	28% 20% 30% 20% 2	414
March	1st to 10th	Day	25% 22% 31% 20% 3	500
	11th to 20th	Day	24% 24% 32% 14% 5% 1	500
	21st to end	Day	28% 22% 30% 15% 5% 1	550
April	1st to 10th	Day	28% 19% 30% 15% 6% 2	500
	11th to 20th	Day	25% 24% 25% 16% 8% 2	492
	21st to end	Day	25% 17% 30% 19% 5% 3	500
May	1st to 10th	Day	29% 14% 28% 18% 8% 3	503
	11th to 20th	Day	25% 14% 30% 20% 9% 2	527
	21st to end	Day	20% 8% 26% 25% 16% 5%	573
June	1st to 10th	Day	15% 13% 31% 22% 15% 4	530
	11th to 20th	Day	20% 9% 30% 24% 14% 3	530
	21st to end	Day	15% 11% 29% 26% 15% 4	530
July	1st to 10th	Day	13% 5% 25% 31% 18% 8%	525
	11th to 20th	Day	9% 5% 26% 32% 23% 6%	524
	21st to end	Day	12% 5% 31% 31% 16% 5%	576
August	1st to 10th	Day	13% 5% 24% 28% 21% 10%	526
	11th to 20th	Day	16% 5% 20% 32% 21% 7%	519
	21st to end	Day	13% 6% 25% 29% 17% 11%	583
September	1st to 10th	Day	18% 9% 20% 24% 21% 8%	530
	11th to 20th	Day	14% 5% 21% 30% 20% 11%	530
	21st to end	Day	29% 10% 21% 23% 15% 3	530
October	1st to 10th	Day	23% 8% 22% 24% 16% 7%	530
	11th to 20th	Day	21% 15% 26% 22% 13% 3	530
	21st to end	Day	22% 14% 30% 22% 11% 1	572
November	1st to 10th	Day	18% 16% 27% 25% 11% 2	425
	11th to 20th	Day	21% 18% 32% 23% 6%	409
	21st to end	Day	26% 22% 30% 17% 4 1	220
December	1st to 10th	Day	28% 21% 30% 13% 7%	204
	11th to 20th	Day	28% 22% 31% 16% 4 1	200
	21st to end	Day	28% 18% 30% 19% 5% 1	220
January	1st to 10th	Night	22% 16% 39% 21% 3	800
	11th to 20th	Night	25% 17% 30% 25% 4	800
	21st to end	Night	17% 18% 37% 26% 2	880
February	1st to 10th	Night	23% 17% 32% 26% 2	800
	11th to 20th	Night	24% 19% 38% 17% 1	783
	21st to end	Night	25% 18% 35% 20% 3	660
March	1st to 10th	Night	23% 17% 32% 24% 4	800
	11th to 20th	Night	24% 17% 32% 22% 5% 1	800
	21st to end	Night	21% 17% 34% 23% 5%	880
April	1st to 10th	Night	25% 18% 32% 22% 4	800
	11th to 20th	Night	18% 15% 36% 23% 7%	791
	21st to end	Night	23% 12% 28% 29% 8%	800
May	1st to 10th	Night	26% 9% 26% 30% 9%	606
	11th to 20th	Night	20% 8% 29% 32% 10% 1	394
	21st to end	Night	19% 7% 22% 35% 16% 1	433
June	1st to 10th	Night	19% 5% 21% 35% 21%	400
	11th to 20th	Night	17% 5% 25% 39% 14% 1	400
	21st to end	Night	12% 3 23% 46% 15% 1	400
July	1st to 10th	Night	12% 4 21% 39% 23% 2	390
	11th to 20th	Night	11% 3 18% 39% 28% 1	388
	21st to end	Night	12% 3 16% 43% 25% 1	543
August	1st to 10th	Night	16% 2 11% 41% 27% 3	597
	11th to 20th	Night	15% 2 14% 36% 31% 2	696
	21st to end	Night	15% 3 17% 35% 28% 2	880
September	1st to 10th	Night	16% 4 21% 34% 23% 2	800
	11th to 20th	Night	14% 4 15% 38% 25% 4	800
	21st to end	Night	23% 4 19% 32% 20% 2	800
October	1st to 10th	Night	24% 6% 20% 27% 19% 4	800
	11th to 20th	Night	21% 9% 25% 31% 13% 1	800
	21st to end	Night	21% 10% 29% 30% 9%	880
November	1st to 10th	Night	17% 9% 30% 32% 12%	820
	11th to 20th	Night	20% 17% 28% 28% 7% 1	840
	21st to end	Night	19% 16% 29% 30% 6%	840
December	1st to 10th	Night	25% 15% 31% 23% 5%	809
	11th to 20th	Night	23% 13% 31% 28% 5%	800
	21st to end	Night	24% 12% 31% 29% 4	880

Page generated at 2025-11-30 01:00:53. Version ac07

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