CSM Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CSM Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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	6% 5% 2 5% 4 4 4 3 5% 8% 55%	105020
	Night	7% 6% 2 6% 4 4 5% 3 5% 7% 49%	102786
Transparency	Day	25% 7% 11% 17% 22% 18%	104781
	Night	29% 8% 12% 17% 19% 15%	102615
Seeing	Day	9% 12% 29% 32% 16% 4	15455
	Night	11% 7% 17% 32% 27% 6%	24781

By Month

Percentage Cloud Cover SW (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	14% 7% 3 8% 4 4 5% 4 6% 8% 37%	2741
	11th to 20th	Day	13% 9% 4 11% 6% 6% 4 4 4 6% 32%	2860
	21st to end	Day	19% 10% 3 9% 5% 6% 4 3 6% 7% 28%	3146
February	1st to 10th	Day	13% 8% 2 6% 5% 4 4 5% 4 6% 44%	1710
	11th to 20th	Day	11% 8% 4 6% 4 5% 4 4 6% 8% 41%	1741
	21st to end	Day	11% 6% 3 6% 5% 4 3 4 6% 6% 46%	1570
March	1st to 10th	Day	10% 10% 4 9% 7% 6% 7% 5% 7% 8% 28%	3408
	11th to 20th	Day	7% 5% 2 6% 4 4 3 3 6% 8% 51%	3490
	21st to end	Day	10% 7% 3 7% 6% 6% 5% 5% 7% 9% 34%	3872
April	1st to 10th	Day	10% 6% 3 7% 4 4 5% 4 6% 8% 45%	2280
	11th to 20th	Day	8% 5% 2 6% 4 3 3 3 5% 7% 53%	2249
	21st to end	Day	4 5% 2 4 3 3 4 3 6% 10% 56%	2280
May	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 69%	3941
	11th to 20th	Day	3 3 1 3 2 3 2 2 5% 7% 69%	4121
	21st to end	Day	3 8% 2 4 3 3 3 3 5% 8% 58%	4813
June	1st to 10th	Day	5% 6% 2 4 3 3 4 3 6% 9% 56%	2660
	11th to 20th	Day	1 3 1 3 2 2 3 3 5% 8% 69%	2660
	21st to end	Day	2 4 1 3 3 3 3 3 4 9% 65%	2660
July	1st to 10th	Day	2 1 2 2 2 2 2 3 6% 77%	4391
	11th to 20th	Day	1 2 1 3 3 3 4 3 5% 10% 66%	4110
	21st to end	Day	3 1 4 3 3 5% 3 6% 11% 61%	4455
August	1st to 10th	Day	1 3 1 4 3 4 5% 5% 7% 13% 54%	4091
	11th to 20th	Day	2 1 3 2 3 4 2 4 9% 68%	3844
	21st to end	Day	3 2 4 3 3 3 3 5% 10% 64%	4180
September	1st to 10th	Day	1 3 1 4 2 2 3 3 3 8% 70%	2280
	11th to 20th	Day	2 6% 2 5% 3 4 4 3 6% 10% 56%	2280
	21st to end	Day	3 2 1 3 2 2 3 2 4 8% 71%	2009
October	1st to 10th	Day	2 3 1 4 3 2 2 3 4 7% 70%	3180
	11th to 20th	Day	3 3 2 5% 4 5% 4 3 4 7% 60%	3161
	21st to end	Day	4 4 2 5% 4 4 4 4 6% 8% 57%	3448
November	1st to 10th	Day	6% 5% 2 5% 4 4 5% 4 5% 9% 52%	1886
	11th to 20th	Day	7% 7% 2 7% 5% 5% 5% 3 4 8% 47%	1710
	21st to end	Day	9% 5% 2 5% 4 3 4 4 5% 7% 52%	1551
December	1st to 10th	Day	10% 6% 3 7% 5% 4 5% 4 5% 7% 43%	1520
	11th to 20th	Day	18% 10% 4 10% 5% 4 5% 3 6% 7% 30%	1928
	21st to end	Day	15% 10% 3 8% 5% 5% 5% 4 6% 8% 32%	2794
January	1st to 10th	Night	13% 9% 3 7% 4 4 4 4 6% 8% 39%	4207
	11th to 20th	Night	15% 10% 4 10% 6% 6% 5% 4 5% 5% 30%	4110
	21st to end	Night	18% 12% 4 10% 5% 5% 4 3 5% 6% 29%	4521
February	1st to 10th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 46%	2470
	11th to 20th	Night	9% 7% 2 8% 5% 5% 5% 3 6% 8% 42%	2324
	21st to end	Night	10% 7% 2 5% 4 4 4 3 6% 7% 48%	1884
March	1st to 10th	Night	11% 10% 3 9% 6% 5% 5% 5% 6% 7% 33%	3461
	11th to 20th	Night	9% 8% 3 5% 4 4 3 3 4 6% 51%	3490
	21st to end	Night	10% 12% 4 9% 6% 5% 5% 3 5% 6% 35%	3781
April	1st to 10th	Night	7% 8% 3 7% 5% 3 4 4 7% 7% 45%	1900
	11th to 20th	Night	8% 6% 2 5% 4 4 3 3 5% 9% 52%	1879
	21st to end	Night	3 4 1 4 4 3 4 4 5% 10% 57%	1900
May	1st to 10th	Night	4 4 1 5% 4 3 2 2 4 6% 66%	3030
	11th to 20th	Night	2 4 2 4 4 5% 5% 3 6% 8% 58%	2780
	21st to end	Night	2 4 2 5% 5% 4 3 3 4 5% 63%	2762
June	1st to 10th	Night	3 5% 2 6% 3 3 4 4 7% 9% 53%	1520
	11th to 20th	Night	1 2 1 3 2 3 3 3 6% 10% 66%	1520
	21st to end	Night	1 4 2 4 4 3 3 3 5% 8% 63%	1520
July	1st to 10th	Night	3 1 2 2 3 4 2 4 6% 72%	2565
	11th to 20th	Night	1 2 2 4 3 4 9% 5% 6% 9% 56%	2860
	21st to end	Night	5% 3 7% 7% 9% 9% 4 6% 10% 41%	3109
August	1st to 10th	Night	2 4 2 7% 5% 8% 10% 6% 7% 11% 37%	2848
	11th to 20th	Night	4 1 5% 4 6% 5% 3 5% 9% 58%	2999
	21st to end	Night	1 3 2 6% 4 4 6% 5% 6% 8% 55%	3498
September	1st to 10th	Night	1 2 1 3 4 4 5% 3 5% 9% 63%	1900
	11th to 20th	Night	3 5% 2 6% 4 5% 4 3 5% 10% 53%	1900
	21st to end	Night	2 2 2 3 4 3 3 2 3 8% 68%	2166
October	1st to 10th	Night	2 3 1 3 3 3 4 3 5% 6% 68%	3800
	11th to 20th	Night	4 4 2 5% 5% 6% 7% 4 4 6% 53%	3777
	21st to end	Night	5% 4 1 4 3 3 4 4 5% 7% 59%	4121
November	1st to 10th	Night	7% 6% 2 4 4 4 4 3 5% 8% 53%	2290
	11th to 20th	Night	8% 7% 2 6% 5% 4 4 4 6% 8% 47%	2470
	21st to end	Night	7% 5% 2 4 4 4 4 3 5% 8% 54%	2626
December	1st to 10th	Night	11% 8% 3 8% 4 3 4 3 5% 8% 44%	2660
	11th to 20th	Night	22% 10% 3 8% 5% 3 4 3 6% 8% 28%	3276
	21st to end	Night	14% 9% 2 9% 6% 5% 5% 5% 6% 6% 32%	4862

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% 9% 13% 26% 8% 3	2741
	11th to 20th	Day	51% 9% 11% 12% 11% 7%	2860
	21st to end	Day	53% 9% 7% 14% 12% 5%	3146
February	1st to 10th	Day	39% 6% 8% 19% 18% 9%	1710
	11th to 20th	Day	39% 9% 10% 16% 20% 6%	1750
	21st to end	Day	38% 6% 8% 24% 17% 6%	1570
March	1st to 10th	Day	50% 13% 9% 14% 13% 2	3408
	11th to 20th	Day	29% 8% 13% 27% 13% 10%	3490
	21st to end	Day	41% 14% 15% 18% 10% 2	3872
April	1st to 10th	Day	37% 9% 13% 20% 18% 4	2280
	11th to 20th	Day	30% 8% 15% 23% 20% 5%	2252
	21st to end	Day	23% 9% 14% 26% 22% 4	2280
May	1st to 10th	Day	17% 7% 11% 27% 26% 11%	3941
	11th to 20th	Day	15% 7% 9% 24% 34% 11%	4124
	21st to end	Day	22% 11% 13% 24% 27% 3	4814
June	1st to 10th	Day	24% 11% 14% 22% 22% 8%	2660
	11th to 20th	Day	12% 8% 11% 22% 34% 13%	2660
	21st to end	Day	13% 7% 12% 12% 27% 28%	2660
July	1st to 10th	Day	7% 5% 9% 6% 11% 63%	4322
	11th to 20th	Day	9% 6% 10% 6% 14% 54%	4041
	21st to end	Day	8% 4 9% 10% 32% 36%	4315
August	1st to 10th	Day	14% 6% 11% 11% 27% 31%	4076
	11th to 20th	Day	9% 5% 9% 8% 42% 27%	3883
	21st to end	Day	12% 5% 11% 12% 32% 29%	4179
September	1st to 10th	Day	12% 5% 8% 10% 27% 38%	2280
	11th to 20th	Day	21% 8% 11% 20% 25% 15%	2280
	21st to end	Day	13% 5% 7% 20% 33% 22%	2009
October	1st to 10th	Day	15% 3 5% 12% 28% 37%	3180
	11th to 20th	Day	18% 7% 10% 21% 28% 16%	3161
	21st to end	Day	21% 4 11% 24% 26% 14%	3448
November	1st to 10th	Day	29% 7% 11% 23% 19% 11%	1886
	11th to 20th	Day	34% 7% 10% 22% 20% 7%	1710
	21st to end	Day	30% 5% 12% 16% 27% 9%	1551
December	1st to 10th	Day	37% 8% 11% 18% 16% 10%	1520
	11th to 20th	Day	53% 7% 10% 13% 14% 4	1928
	21st to end	Day	48% 8% 11% 18% 12% 3	2794
January	1st to 10th	Night	40% 10% 17% 22% 8% 3	4207
	11th to 20th	Night	51% 10% 12% 10% 9% 7%	4110
	21st to end	Night	56% 7% 8% 11% 11% 6%	4521
February	1st to 10th	Night	37% 6% 10% 18% 18% 11%	2470
	11th to 20th	Night	38% 9% 10% 17% 20% 7%	2326
	21st to end	Night	34% 7% 13% 28% 15% 4	1884
March	1st to 10th	Night	48% 13% 12% 13% 12% 3	3461
	11th to 20th	Night	35% 6% 10% 22% 18% 10%	3490
	21st to end	Night	48% 11% 13% 16% 9% 3	3781
April	1st to 10th	Night	35% 8% 15% 21% 16% 4	1900
	11th to 20th	Night	31% 9% 12% 24% 17% 7%	1879
	21st to end	Night	22% 10% 13% 29% 20% 6%	1900
May	1st to 10th	Night	21% 7% 11% 24% 24% 13%	3030
	11th to 20th	Night	23% 8% 15% 16% 24% 14%	2781
	21st to end	Night	22% 10% 12% 27% 27% 2	2762
June	1st to 10th	Night	24% 12% 14% 20% 22% 8%	1520
	11th to 20th	Night	11% 9% 13% 19% 32% 15%	1520
	21st to end	Night	16% 7% 11% 13% 26% 27%	1520
July	1st to 10th	Night	6% 5% 9% 6% 14% 60%	2525
	11th to 20th	Night	10% 7% 10% 10% 17% 46%	2806
	21st to end	Night	13% 7% 14% 15% 24% 27%	3011
August	1st to 10th	Night	17% 7% 14% 14% 26% 22%	2839
	11th to 20th	Night	12% 6% 13% 9% 37% 24%	3026
	21st to end	Night	13% 8% 15% 11% 28% 25%	3498
September	1st to 10th	Night	12% 8% 9% 11% 23% 37%	1900
	11th to 20th	Night	23% 7% 15% 18% 24% 14%	1900
	21st to end	Night	14% 6% 9% 20% 28% 24%	2166
October	1st to 10th	Night	15% 5% 6% 11% 23% 40%	3800
	11th to 20th	Night	20% 6% 15% 19% 22% 18%	3777
	21st to end	Night	22% 5% 8% 22% 24% 19%	4121
November	1st to 10th	Night	28% 8% 12% 22% 18% 12%	2290
	11th to 20th	Night	31% 8% 12% 23% 17% 8%	2470
	21st to end	Night	28% 7% 11% 20% 22% 13%	2626
December	1st to 10th	Night	38% 8% 9% 17% 17% 12%	2660
	11th to 20th	Night	53% 6% 10% 13% 15% 3	3276
	21st to end	Night	46% 12% 13% 15% 10% 4	4862

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% 12% 28% 19% 14%	304
	11th to 20th	Day	17% 12% 25% 31% 14% 2	370
	21st to end	Day	14% 12% 29% 27% 16% 2	407
February	1st to 10th	Day	15% 16% 26% 24% 16% 2	370
	11th to 20th	Day	17% 18% 30% 24% 9% 1	401
	21st to end	Day	13% 17% 31% 25% 12% 2	382
March	1st to 10th	Day	20% 20% 29% 21% 8% 1	460
	11th to 20th	Day	18% 17% 29% 20% 13% 3	460
	21st to end	Day	14% 21% 32% 25% 7% 1	506
April	1st to 10th	Day	15% 18% 32% 22% 10% 2	460
	11th to 20th	Day	12% 21% 33% 23% 8% 3	452
	21st to end	Day	6% 22% 33% 25% 10% 5%	460
May	1st to 10th	Day	8% 19% 35% 27% 10% 2	463
	11th to 20th	Day	9% 16% 37% 26% 10% 2	488
	21st to end	Day	9% 18% 33% 26% 13% 3	529
June	1st to 10th	Day	7% 13% 35% 30% 11% 3	490
	11th to 20th	Day	3 13% 34% 36% 11% 2	490
	21st to end	Day	2 7% 33% 43% 14% 2	490
July	1st to 10th	Day	5% 4 29% 47% 13% 2	486
	11th to 20th	Day	2 2 25% 50% 18% 2	484
	21st to end	Day	3 1 26% 49% 16% 4	531
August	1st to 10th	Day	5% 6% 28% 45% 14% 3	487
	11th to 20th	Day	3 3 31% 41% 18% 4	477
	21st to end	Day	3 4 26% 43% 19% 5%	539
September	1st to 10th	Day	4 4 23% 40% 22% 7%	490
	11th to 20th	Day	4 13% 26% 36% 15% 6%	490
	21st to end	Day	3 9% 24% 31% 27% 5%	490
October	1st to 10th	Day	3 8% 28% 31% 21% 10%	490
	11th to 20th	Day	5% 10% 25% 28% 25% 7%	490
	21st to end	Day	9% 9% 23% 29% 23% 7%	539
November	1st to 10th	Day	9% 12% 24% 25% 24% 6%	458
	11th to 20th	Day	8% 9% 17% 31% 27% 7%	270
	21st to end	Day	8% 9% 26% 29% 26% 2	194
December	1st to 10th	Day	17% 14% 19% 25% 20% 4	180
	11th to 20th	Day	14% 13% 23% 31% 18% 2	180
	21st to end	Day	19% 14% 29% 25% 14% 1	198
January	1st to 10th	Night	19% 7% 17% 30% 22% 5%	760
	11th to 20th	Night	16% 8% 15% 31% 25% 6%	760
	21st to end	Night	18% 7% 14% 34% 24% 4	836
February	1st to 10th	Night	17% 7% 16% 32% 24% 4	760
	11th to 20th	Night	13% 12% 18% 30% 22% 5%	744
	21st to end	Night	12% 13% 21% 32% 19% 3	628
March	1st to 10th	Night	18% 13% 20% 28% 19% 3	760
	11th to 20th	Night	18% 10% 16% 30% 22% 4	760
	21st to end	Night	15% 11% 24% 29% 18% 2	836
April	1st to 10th	Night	11% 11% 25% 32% 19% 3	760
	11th to 20th	Night	13% 14% 22% 28% 19% 4	751
	21st to end	Night	7% 11% 28% 28% 21% 5%	760
May	1st to 10th	Night	7% 8% 25% 34% 23% 4	675
	11th to 20th	Night	7% 5% 20% 33% 31% 4	534
	21st to end	Night	9% 11% 21% 35% 18% 5%	411
June	1st to 10th	Night	6% 6% 25% 35% 25% 3	380
	11th to 20th	Night	1 6% 21% 44% 25% 3	380
	21st to end	Night	5% 4 14% 39% 34% 4	380
July	1st to 10th	Night	3 3 15% 35% 38% 6%	382
	11th to 20th	Night	4 1 11% 38% 37% 10%	552
	21st to end	Night	6% 3 12% 33% 37% 9%	624
August	1st to 10th	Night	10% 4 15% 33% 32% 7%	567
	11th to 20th	Night	5% 2 17% 36% 33% 7%	673
	21st to end	Night	3 3 14% 36% 36% 8%	836
September	1st to 10th	Night	2 2 12% 32% 40% 11%	760
	11th to 20th	Night	7% 4 17% 33% 32% 7%	760
	21st to end	Night	2 4 15% 29% 40% 11%	760
October	1st to 10th	Night	7% 3 14% 33% 33% 10%	760
	11th to 20th	Night	7% 5% 13% 28% 37% 10%	760
	21st to end	Night	8% 5% 12% 29% 33% 12%	836
November	1st to 10th	Night	10% 5% 14% 28% 34% 9%	760
	11th to 20th	Night	11% 4 12% 34% 32% 8%	760
	21st to end	Night	10% 6% 13% 39% 27% 5%	760
December	1st to 10th	Night	16% 6% 16% 32% 24% 6%	760
	11th to 20th	Night	24% 7% 15% 30% 21% 4	760
	21st to end	Night	20% 11% 18% 29% 20% 2	836

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