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 2024-01-13.

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	16% 6% 3 5% 5% 3 5% 5% 5% 7% 38%	74710
	Night	17% 6% 2 5% 5% 3 4 4 5% 7% 41%	75005
Transparency	Day	43% 7% 12% 22% 14% 2	74421
	Night	41% 7% 12% 22% 15% 2	74828
Seeing	Day	21% 13% 28% 22% 12% 4	13749
	Night	20% 11% 28% 29% 11% 1	23196

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% 9% 3 5% 6% 3 4 6% 6% 8% 38%	1517
	11th to 20th	Day	16% 10% 4 6% 7% 4 5% 5% 5% 6% 31%	1537
	21st to end	Day	15% 9% 4 7% 6% 4 5% 5% 5% 6% 34%	1782
February	1st to 10th	Day	17% 8% 3 6% 7% 5% 6% 6% 5% 5% 33%	1620
	11th to 20th	Day	15% 8% 4 5% 7% 3 5% 5% 5% 8% 36%	1646
	21st to end	Day	20% 9% 3 6% 5% 4 4 4 4 5% 35%	1480
March	1st to 10th	Day	13% 7% 2 5% 5% 4 5% 4 5% 6% 43%	1857
	11th to 20th	Day	16% 6% 3 7% 6% 4 5% 4 6% 8% 35%	1980
	21st to end	Day	22% 6% 2 5% 6% 3 4 4 5% 5% 37%	2262
April	1st to 10th	Day	16% 8% 3 6% 5% 4 5% 4 5% 8% 37%	2160
	11th to 20th	Day	18% 6% 2 5% 5% 3 4 4 5% 6% 42%	2134
	21st to end	Day	18% 5% 2 5% 5% 4 5% 5% 6% 7% 38%	2160
May	1st to 10th	Day	19% 7% 2 6% 6% 3 5% 5% 5% 6% 36%	2346
	11th to 20th	Day	21% 7% 3 5% 5% 4 4 5% 5% 7% 34%	2478
	21st to end	Day	16% 5% 3 6% 5% 4 5% 6% 5% 8% 39%	2725
June	1st to 10th	Day	13% 6% 3 5% 6% 3 5% 6% 6% 9% 37%	2520
	11th to 20th	Day	16% 5% 2 5% 6% 4 6% 5% 6% 9% 36%	2520
	21st to end	Day	12% 4 2 5% 5% 4 5% 6% 6% 10% 41%	2520
July	1st to 10th	Day	11% 5% 3 5% 7% 5% 7% 6% 7% 9% 37%	2520
	11th to 20th	Day	8% 5% 2 6% 5% 4 6% 6% 7% 10% 42%	2520
	21st to end	Day	11% 6% 3 6% 6% 4 7% 6% 6% 7% 38%	2647
August	1st to 10th	Day	12% 6% 2 5% 6% 4 6% 8% 7% 9% 36%	2327
	11th to 20th	Day	14% 6% 3 5% 5% 3 5% 5% 7% 8% 39%	2192
	21st to end	Day	15% 6% 2 4 5% 3 4 6% 7% 9% 40%	2376
September	1st to 10th	Day	18% 4 2 5% 4 2 5% 5% 5% 6% 43%	2160
	11th to 20th	Day	14% 4 1 4 3 3 4 3 5% 7% 50%	2160
	21st to end	Day	20% 7% 3 4 4 3 4 5% 5% 5% 40%	1922
October	1st to 10th	Day	20% 6% 2 3 4 2 4 4 5% 8% 42%	1800
	11th to 20th	Day	18% 5% 2 5% 5% 3 4 4 5% 6% 43%	1800
	21st to end	Day	22% 5% 2 4 5% 2 4 5% 4 7% 39%	1962
November	1st to 10th	Day	15% 6% 2 4 3 2 3 3 4 7% 50%	1671
	11th to 20th	Day	19% 5% 2 4 5% 3 3 6% 5% 7% 43%	1691
	21st to end	Day	19% 4 3 5% 4 3 4 5% 6% 7% 41%	1520
December	1st to 10th	Day	20% 8% 2 7% 6% 3 5% 5% 5% 6% 33%	1520
	11th to 20th	Day	21% 6% 3 6% 4 3 4 4 5% 7% 37%	1906
	21st to end	Day	21% 8% 3 7% 6% 3 5% 5% 6% 7% 29%	2772
January	1st to 10th	Night	15% 7% 2 4 4 3 4 4 5% 8% 41%	2649
	11th to 20th	Night	19% 8% 3 5% 6% 3 5% 5% 6% 7% 35%	2463
	21st to end	Night	13% 8% 2 6% 6% 3 5% 4 6% 6% 41%	2574
February	1st to 10th	Night	17% 8% 3 6% 5% 3 4 5% 5% 7% 37%	2340
	11th to 20th	Night	16% 7% 3 7% 6% 2 5% 5% 4 7% 38%	2198
	21st to end	Night	18% 8% 3 5% 5% 3 5% 4 5% 6% 38%	1776
March	1st to 10th	Night	16% 5% 2 4 5% 3 4 4 5% 7% 46%	2088
	11th to 20th	Night	18% 8% 2 4 5% 3 4 4 4 4 44%	1979
	21st to end	Night	18% 6% 2 4 6% 3 4 4 5% 7% 41%	2093
April	1st to 10th	Night	18% 6% 3 6% 5% 3 5% 4 6% 7% 37%	1800
	11th to 20th	Night	15% 5% 3 6% 5% 3 4 5% 5% 8% 42%	1775
	21st to end	Night	21% 5% 2 4 4 2 4 4 5% 6% 44%	1800
May	1st to 10th	Night	20% 7% 3 5% 5% 3 4 5% 4 7% 37%	1626
	11th to 20th	Night	19% 5% 2 5% 6% 3 5% 5% 5% 7% 38%	1415
	21st to end	Night	16% 5% 3 5% 5% 4 4 5% 5% 8% 39%	1550
June	1st to 10th	Night	17% 5% 2 5% 4 2 5% 4 5% 7% 43%	1440
	11th to 20th	Night	14% 5% 2 5% 5% 3 5% 5% 6% 9% 41%	1440
	21st to end	Night	12% 4 2 4 5% 3 4 6% 7% 9% 44%	1440
July	1st to 10th	Night	12% 5% 3 6% 7% 3 5% 6% 6% 8% 38%	1440
	11th to 20th	Night	9% 4 2 5% 5% 4 4 6% 7% 11% 45%	1440
	21st to end	Night	12% 5% 2 5% 6% 3 5% 6% 7% 9% 41%	1668
August	1st to 10th	Night	12% 6% 2 5% 5% 3 5% 5% 8% 8% 42%	1611
	11th to 20th	Night	13% 5% 2 7% 7% 3 6% 6% 5% 7% 40%	1689
	21st to end	Night	13% 6% 2 5% 4 4 5% 5% 5% 9% 43%	1980
September	1st to 10th	Night	16% 6% 3 5% 5% 3 3 3 5% 7% 47%	1800
	11th to 20th	Night	14% 5% 2 3 5% 3 3 3 5% 6% 52%	1800
	21st to end	Night	18% 5% 2 5% 4 3 4 4 4 7% 43%	2033
October	1st to 10th	Night	22% 5% 2 4 4 2 3 3 4 7% 43%	2160
	11th to 20th	Night	18% 6% 2 4 4 2 4 4 5% 6% 46%	2160
	21st to end	Night	21% 5% 2 3 4 3 3 4 5% 7% 44%	2394
November	1st to 10th	Night	12% 5% 2 4 4 3 4 4 4 6% 51%	2407
	11th to 20th	Night	16% 7% 3 4 5% 2 4 3 4 6% 45%	2485
	21st to end	Night	15% 5% 3 4 4 3 4 4 5% 9% 42%	2660
December	1st to 10th	Night	20% 7% 3 6% 5% 3 5% 5% 6% 6% 35%	2660
	11th to 20th	Night	19% 7% 3 5% 4 3 5% 5% 6% 6% 38%	3288
	21st to end	Night	19% 8% 4 6% 5% 3 5% 5% 5% 7% 33%	4884

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	42% 7% 14% 24% 11% 2	1518
	11th to 20th	Day	53% 6% 13% 16% 10% 2	1537
	21st to end	Day	48% 6% 12% 21% 11% 2	1782
February	1st to 10th	Day	49% 7% 15% 18% 10% 1	1620
	11th to 20th	Day	47% 5% 12% 23% 11% 2	1655
	21st to end	Day	53% 4 10% 21% 9% 3	1480
March	1st to 10th	Day	41% 5% 13% 26% 12% 2	1860
	11th to 20th	Day	46% 6% 12% 19% 14% 4	1980
	21st to end	Day	48% 5% 9% 18% 16% 3	2262
April	1st to 10th	Day	45% 6% 11% 20% 14% 5%	2160
	11th to 20th	Day	43% 5% 11% 22% 15% 4	2134
	21st to end	Day	42% 7% 11% 20% 15% 5%	2160
May	1st to 10th	Day	47% 7% 10% 18% 14% 4	2346
	11th to 20th	Day	48% 8% 11% 19% 13% 2	2482
	21st to end	Day	42% 9% 13% 21% 13% 2	2725
June	1st to 10th	Day	41% 10% 13% 21% 14% 2	2520
	11th to 20th	Day	43% 9% 13% 22% 10% 2	2520
	21st to end	Day	37% 9% 15% 25% 14% 1	2520
July	1st to 10th	Day	41% 9% 13% 21% 15% 1	2450
	11th to 20th	Day	35% 9% 14% 25% 15% 2	2445
	21st to end	Day	42% 8% 11% 21% 15% 2	2499
August	1st to 10th	Day	40% 10% 14% 21% 14%	2314
	11th to 20th	Day	39% 9% 15% 22% 14% 1	2192
	21st to end	Day	38% 9% 12% 21% 17% 3	2376
September	1st to 10th	Day	41% 7% 10% 22% 16% 3	2160
	11th to 20th	Day	34% 5% 11% 25% 22% 3	2160
	21st to end	Day	44% 7% 8% 18% 21% 2	1922
October	1st to 10th	Day	41% 6% 13% 23% 16% 1	1800
	11th to 20th	Day	41% 6% 11% 24% 17% 1	1800
	21st to end	Day	43% 5% 11% 23% 15% 2	1962
November	1st to 10th	Day	35% 5% 14% 25% 18% 3	1671
	11th to 20th	Day	40% 6% 11% 23% 17% 2	1691
	21st to end	Day	40% 6% 14% 29% 8% 2	1520
December	1st to 10th	Day	51% 7% 15% 18% 9% 1	1520
	11th to 20th	Day	46% 6% 15% 26% 7% 1	1906
	21st to end	Day	51% 8% 15% 18% 7%	2772
January	1st to 10th	Night	42% 5% 14% 23% 14% 3	2649
	11th to 20th	Night	48% 6% 15% 21% 9% 2	2463
	21st to end	Night	42% 6% 12% 24% 15% 2	2574
February	1st to 10th	Night	46% 5% 14% 23% 9% 2	2340
	11th to 20th	Night	46% 5% 11% 19% 16% 2	2200
	21st to end	Night	47% 5% 10% 20% 15% 3	1776
March	1st to 10th	Night	37% 6% 13% 25% 16% 3	2098
	11th to 20th	Night	43% 5% 11% 22% 16% 3	1980
	21st to end	Night	43% 6% 10% 19% 18% 4	2093
April	1st to 10th	Night	45% 5% 12% 21% 14% 3	1800
	11th to 20th	Night	39% 7% 12% 20% 20% 2	1775
	21st to end	Night	41% 7% 10% 21% 16% 5%	1800
May	1st to 10th	Night	46% 7% 10% 20% 13% 3	1626
	11th to 20th	Night	46% 7% 11% 16% 18% 3	1416
	21st to end	Night	42% 10% 13% 18% 14% 3	1551
June	1st to 10th	Night	40% 7% 11% 23% 18% 2	1440
	11th to 20th	Night	38% 10% 14% 23% 14% 2	1440
	21st to end	Night	34% 9% 15% 28% 13% 1	1440
July	1st to 10th	Night	42% 10% 14% 18% 14% 3	1400
	11th to 20th	Night	32% 10% 15% 25% 15% 2	1392
	21st to end	Night	35% 11% 17% 20% 15% 2	1575
August	1st to 10th	Night	37% 9% 15% 23% 16% 1	1600
	11th to 20th	Night	41% 11% 13% 17% 17% 2	1689
	21st to end	Night	36% 11% 13% 19% 19% 3	1980
September	1st to 10th	Night	39% 5% 11% 23% 18% 3	1800
	11th to 20th	Night	34% 6% 9% 24% 24% 3	1800
	21st to end	Night	40% 7% 9% 20% 22% 2	2033
October	1st to 10th	Night	41% 6% 13% 23% 15% 2	2160
	11th to 20th	Night	39% 5% 12% 27% 15% 2	2160
	21st to end	Night	40% 5% 11% 25% 16% 2	2394
November	1st to 10th	Night	34% 6% 13% 25% 19% 4	2407
	11th to 20th	Night	41% 5% 10% 24% 16% 4	2485
	21st to end	Night	39% 5% 14% 27% 13% 2	2660
December	1st to 10th	Night	48% 6% 13% 20% 13%	2660
	11th to 20th	Night	45% 6% 13% 21% 13% 2	3288
	21st to end	Night	47% 9% 14% 22% 8%	4884

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	26% 26% 28% 15% 4 1	180
	11th to 20th	Day	31% 22% 33% 12% 3 1	200
	21st to end	Day	21% 24% 35% 14% 5% 2	264
February	1st to 10th	Day	28% 27% 30% 12% 3 1	240
	11th to 20th	Day	27% 23% 38% 11% 1	313
	21st to end	Day	30% 19% 31% 19% 1	346
March	1st to 10th	Day	22% 22% 33% 21% 2	420
	11th to 20th	Day	23% 25% 31% 15% 5% 1	420
	21st to end	Day	29% 21% 29% 15% 5% 1	462
April	1st to 10th	Day	25% 19% 31% 16% 6% 2	420
	11th to 20th	Day	22% 23% 26% 17% 9% 3	412
	21st to end	Day	27% 16% 33% 17% 5% 3	420
May	1st to 10th	Day	29% 14% 27% 18% 9% 3	423
	11th to 20th	Day	25% 13% 31% 19% 9% 2	447
	21st to end	Day	20% 7% 25% 26% 16% 6%	485
June	1st to 10th	Day	16% 12% 32% 23% 14% 4	450
	11th to 20th	Day	19% 10% 31% 23% 14% 3	450
	21st to end	Day	14% 12% 30% 25% 16% 4	450
July	1st to 10th	Day	14% 4 24% 29% 20% 9%	445
	11th to 20th	Day	8% 4 25% 33% 24% 6%	444
	21st to end	Day	12% 5% 31% 31% 16% 6%	488
August	1st to 10th	Day	12% 4 24% 27% 23% 10%	446
	11th to 20th	Day	16% 5% 19% 32% 21% 8%	439
	21st to end	Day	13% 5% 24% 29% 17% 11%	495
September	1st to 10th	Day	19% 9% 20% 23% 20% 9%	450
	11th to 20th	Day	15% 5% 21% 28% 20% 11%	450
	21st to end	Day	26% 10% 20% 23% 17% 4	450
October	1st to 10th	Day	25% 6% 23% 23% 16% 6%	450
	11th to 20th	Day	21% 14% 26% 24% 13% 2	450
	21st to end	Day	24% 14% 31% 20% 10% 1	486
November	1st to 10th	Day	18% 14% 26% 26% 13% 3	365
	11th to 20th	Day	20% 17% 32% 25% 6%	351
	21st to end	Day	21% 22% 32% 20% 4 1	180
December	1st to 10th	Day	27% 22% 29% 13% 8% 1	180
	11th to 20th	Day	26% 24% 29% 16% 4 1	180
	21st to end	Day	29% 19% 28% 18% 5% 2	198
January	1st to 10th	Night	20% 16% 39% 22% 3	760
	11th to 20th	Night	24% 15% 30% 26% 4	729
	21st to end	Night	16% 19% 36% 27% 2	792
February	1st to 10th	Night	23% 18% 32% 25% 2	720
	11th to 20th	Night	23% 19% 38% 17% 2	703
	21st to end	Night	26% 18% 35% 18% 3	592
March	1st to 10th	Night	22% 18% 32% 25% 4	720
	11th to 20th	Night	25% 16% 32% 21% 5% 1	720
	21st to end	Night	21% 15% 35% 23% 5% 1	792
April	1st to 10th	Night	24% 18% 32% 22% 5%	720
	11th to 20th	Night	16% 14% 37% 24% 8%	711
	21st to end	Night	24% 13% 29% 28% 7%	720
May	1st to 10th	Night	25% 9% 26% 30% 10% 1	546
	11th to 20th	Night	19% 8% 30% 32% 9% 1	354
	21st to end	Night	18% 6% 21% 37% 17% 1	389
June	1st to 10th	Night	19% 5% 19% 35% 21%	360
	11th to 20th	Night	16% 5% 26% 38% 14% 1	360
	21st to end	Night	12% 3 23% 47% 15% 1	360
July	1st to 10th	Night	12% 3 22% 37% 24% 2	350
	11th to 20th	Night	11% 3 17% 41% 28% 1	348
	21st to end	Night	11% 3 16% 43% 25% 2	488
August	1st to 10th	Night	15% 2 11% 40% 28% 3	537
	11th to 20th	Night	15% 3 13% 36% 30% 3	625
	21st to end	Night	15% 3 17% 35% 28% 2	792
September	1st to 10th	Night	17% 3 21% 33% 23% 3	720
	11th to 20th	Night	15% 4 16% 36% 24% 5%	720
	21st to end	Night	20% 4 20% 33% 21% 2	720
October	1st to 10th	Night	25% 6% 21% 28% 17% 4	720
	11th to 20th	Night	21% 9% 24% 32% 13% 1	720
	21st to end	Night	23% 10% 29% 29% 9%	792
November	1st to 10th	Night	16% 8% 30% 32% 13% 1	740
	11th to 20th	Night	19% 16% 28% 28% 8% 1	760
	21st to end	Night	18% 14% 29% 31% 7%	760
December	1st to 10th	Night	26% 14% 32% 23% 6%	760
	11th to 20th	Night	22% 13% 31% 29% 5%	760
	21st to end	Night	24% 12% 32% 29% 3	836

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