Charles Nicholas Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Charles Nicholas Memorial Observatory clear sky chart from 2005-11-06 to 2024-11-28.

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	4 4 1 4 3 3 4 4 6% 7% 60%	104585
	Night	3 3 1 3 2 3 3 3 5% 7% 67%	103653
Transparency	Day	21% 5% 9% 18% 25% 22%	104351
	Night	17% 4 9% 19% 27% 23%	103477
Seeing	Day	12% 16% 27% 26% 16% 4	14830
	Night	9% 15% 26% 29% 17% 3	25036

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	4 1 2 2 2 3 3 4 7% 70%	2858
	11th to 20th	Day	1 1 1 2 2 2 3 2 3 6% 78%	2860
	21st to end	Day	2 4 2 5% 3 3 4 4 6% 9% 58%	3146
February	1st to 10th	Day	1 4 2 3 2 2 3 3 4 6% 70%	1883
	11th to 20th	Day	2 3 1 4 3 3 3 2 4 6% 69%	1849
	21st to end	Day	3 1 2 3 3 2 2 4 6% 74%	1570
March	1st to 10th	Day	1 7% 2 5% 3 3 3 3 4 6% 63%	3273
	11th to 20th	Day	3 10% 2 3 3 3 4 3 5% 6% 57%	3490
	21st to end	Day	1 3 1 3 3 3 3 4 6% 8% 65%	3979
April	1st to 10th	Day	1 3 1 3 3 3 2 2 4 6% 73%	2280
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 6% 70%	2251
	21st to end	Day	2 3 1 2 2 1 2 2 3 6% 76%	2280
May	1st to 10th	Day	2 1 1 2 1 2 1 2 4 5% 79%	3800
	11th to 20th	Day	4 3 1 3 3 4 5% 4 6% 7% 61%	3765
	21st to end	Day	1 1 1 2 2 2 2 2 4 6% 78%	4417
June	1st to 10th	Day	1 3 1 4 4 4 5% 4 8% 10% 57%	2660
	11th to 20th	Day	2 2 1 3 3 2 4 4 6% 7% 66%	2660
	21st to end	Day	8% 4 2 5% 5% 5% 7% 7% 8% 7% 43%	2638
July	1st to 10th	Day	7% 5% 2 7% 6% 6% 7% 6% 9% 10% 35%	4110
	11th to 20th	Day	5% 4 1 6% 5% 5% 7% 6% 7% 8% 45%	4110
	21st to end	Day	9% 5% 2 6% 6% 7% 7% 7% 11% 10% 30%	4404
August	1st to 10th	Day	5% 4 2 5% 4 6% 7% 6% 7% 11% 43%	3784
	11th to 20th	Day	7% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	3726
	21st to end	Day	7% 5% 2 6% 7% 6% 8% 6% 8% 10% 35%	4180
September	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 7% 7% 43%	2280
	11th to 20th	Day	9% 5% 2 5% 4 5% 4 3 5% 7% 51%	2269
	21st to end	Day	6% 3 1 5% 3 3 3 2 4 6% 64%	2090
October	1st to 10th	Day	6% 3 1 3 2 3 3 2 4 4 69%	3214
	11th to 20th	Day	5% 3 1 3 2 2 3 3 4 5% 68%	3150
	21st to end	Day	1 2 2 1 1 1 1 2 3 86%	3438
November	1st to 10th	Day	2 2 1 2 2 2 3 2 3 6% 76%	1959
	11th to 20th	Day	1 2 1 3 1 2 2 2 5% 7% 72%	1807
	21st to end	Day	3 3 1 2 2 2 3 4 5% 7% 67%	1773
December	1st to 10th	Day	3 3 1 4 2 3 3 3 5% 7% 66%	1710
	11th to 20th	Day	1 4 3 6% 2 3 3 4 6% 8% 61%	1991
	21st to end	Day	1 3 1 3 2 2 2 3 6% 6% 72%	2931
January	1st to 10th	Night	1 2 1 3 2 3 3 3 4 7% 72%	4097
	11th to 20th	Night	1 2 1 2 2 2 3 2 4 5% 75%	4110
	21st to end	Night	2 4 2 4 3 3 3 4 7% 10% 59%	4521
February	1st to 10th	Night	3 1 3 2 2 3 3 5% 8% 70%	2295
	11th to 20th	Night	2 2 1 2 2 3 2 3 5% 7% 72%	2218
	21st to end	Night	1 2 1 2 2 1 2 3 4 6% 75%	1884
March	1st to 10th	Night	3 2 3 2 2 3 3 4 7% 71%	3592
	11th to 20th	Night	3 14% 3 4 2 2 3 2 3 5% 58%	3490
	21st to end	Night	1 1 2 2 2 2 3 6% 9% 71%	3653
April	1st to 10th	Night	1 2 1 2 2 2 3 3 5% 8% 71%	1900
	11th to 20th	Night	2 2 1 1 1 2 3 2 4 7% 74%	1877
	21st to end	Night	1 3 1 1 2 2 3 2 4 6% 76%	1900
May	1st to 10th	Night	2 2 1 1 1 1 2 1 4 5% 80%	3170
	11th to 20th	Night	3 3 1 2 2 2 3 2 4 5% 73%	3140
	21st to end	Night	2 1 1 1 1 2 2 3 4 83%	3146
June	1st to 10th	Night	2 4 1 5% 4 5% 4 3 7% 9% 57%	1520
	11th to 20th	Night	3 2 1 2 2 2 3 3 5% 6% 72%	1520
	21st to end	Night	6% 4 2 4 4 5% 6% 4 6% 8% 49%	1533
July	1st to 10th	Night	8% 5% 2 4 3 4 5% 5% 8% 11% 46%	2860
	11th to 20th	Night	3 3 1 4 4 5% 6% 6% 8% 10% 53%	2860
	21st to end	Night	8% 6% 2 4 4 4 5% 5% 10% 15% 38%	3159
August	1st to 10th	Night	3 2 1 3 4 4 5% 4 9% 15% 50%	3155
	11th to 20th	Night	4 3 1 3 3 3 4 4 6% 12% 58%	3113
	21st to end	Night	5% 3 1 4 4 4 4 5% 9% 14% 47%	3487
September	1st to 10th	Night	8% 4 1 4 3 3 4 4 7% 9% 52%	1900
	11th to 20th	Night	8% 3 1 3 2 2 3 3 6% 7% 61%	1909
	21st to end	Night	5% 3 1 3 2 2 3 2 4 7% 68%	2090
October	1st to 10th	Night	3 2 2 2 2 2 2 3 5% 78%	3751
	11th to 20th	Night	2 4 1 3 2 2 3 3 4 4 72%	3778
	21st to end	Night	2 1 1 2 1 1 2 1 2 4 83%	4120
November	1st to 10th	Night	3 3 1 3 2 2 2 2 3 5% 74%	2340
	11th to 20th	Night	1 2 1 2 2 2 3 3 5% 7% 73%	2570
	21st to end	Night	2 5% 1 3 3 3 3 3 4 6% 67%	2562
December	1st to 10th	Night	3 3 1 3 1 2 3 3 4 7% 69%	2470
	11th to 20th	Night	1 3 1 3 3 3 4 4 6% 7% 65%	3200
	21st to end	Night	1 3 1 4 2 2 2 3 5% 5% 72%	4763

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	15% 3 4 21% 31% 26%	2859
	11th to 20th	Day	10% 2 7% 21% 37% 23%	2860
	21st to end	Day	22% 2 12% 20% 29% 15%	3146
February	1st to 10th	Day	16% 4 5% 18% 30% 27%	1883
	11th to 20th	Day	18% 4 5% 18% 27% 28%	1859
	21st to end	Day	15% 1 6% 12% 29% 37%	1570
March	1st to 10th	Day	22% 4 6% 12% 22% 35%	3273
	11th to 20th	Day	22% 9% 6% 14% 32% 16%	3490
	21st to end	Day	17% 4 7% 22% 31% 20%	3979
April	1st to 10th	Day	14% 3 5% 17% 36% 26%	2280
	11th to 20th	Day	16% 2 4 14% 30% 34%	2254
	21st to end	Day	13% 2 5% 18% 34% 28%	2280
May	1st to 10th	Day	9% 1 4 12% 25% 49%	3800
	11th to 20th	Day	18% 9% 6% 14% 24% 29%	3768
	21st to end	Day	9% 3 4 12% 17% 55%	4417
June	1st to 10th	Day	19% 6% 11% 19% 25% 19%	2660
	11th to 20th	Day	15% 4 5% 16% 23% 36%	2660
	21st to end	Day	32% 8% 12% 16% 18% 14%	2638
July	1st to 10th	Day	38% 11% 17% 15% 12% 7%	4046
	11th to 20th	Day	32% 8% 13% 21% 23% 4	4041
	21st to end	Day	39% 9% 19% 23% 9% 1	4265
August	1st to 10th	Day	30% 8% 12% 32% 15% 2	3772
	11th to 20th	Day	33% 7% 13% 21% 24% 2	3762
	21st to end	Day	36% 10% 18% 25% 9% 1	4179
September	1st to 10th	Day	36% 6% 13% 17% 18% 10%	2280
	11th to 20th	Day	33% 5% 8% 18% 26% 10%	2269
	21st to end	Day	22% 3 7% 23% 31% 14%	2090
October	1st to 10th	Day	19% 2 7% 11% 31% 30%	3214
	11th to 20th	Day	18% 3 8% 18% 30% 22%	3150
	21st to end	Day	8% 1 3 14% 40% 34%	3438
November	1st to 10th	Day	13% 2 4 22% 33% 25%	1959
	11th to 20th	Day	14% 1 7% 21% 31% 26%	1807
	21st to end	Day	17% 3 8% 18% 26% 28%	1773
December	1st to 10th	Day	18% 3 7% 24% 32% 17%	1710
	11th to 20th	Day	20% 3 10% 18% 30% 19%	1989
	21st to end	Day	13% 2 8% 16% 30% 32%	2931
January	1st to 10th	Night	15% 3 5% 19% 31% 26%	4097
	11th to 20th	Night	13% 3 8% 17% 35% 24%	4110
	21st to end	Night	20% 2 12% 21% 29% 15%	4521
February	1st to 10th	Night	14% 4 8% 18% 27% 29%	2295
	11th to 20th	Night	14% 4 6% 18% 29% 28%	2218
	21st to end	Night	11% 1 9% 16% 28% 35%	1884
March	1st to 10th	Night	14% 3 6% 14% 29% 34%	3592
	11th to 20th	Night	26% 7% 6% 14% 31% 16%	3490
	21st to end	Night	12% 2 7% 26% 34% 19%	3653
April	1st to 10th	Night	11% 3 6% 18% 36% 26%	1900
	11th to 20th	Night	12% 2 7% 17% 30% 32%	1877
	21st to end	Night	11% 4 5% 19% 35% 25%	1900
May	1st to 10th	Night	8% 4 3 9% 23% 52%	3170
	11th to 20th	Night	14% 3 7% 18% 26% 32%	3141
	21st to end	Night	8% 2 4 10% 20% 56%	3147
June	1st to 10th	Night	21% 6% 10% 19% 23% 21%	1520
	11th to 20th	Night	14% 2 5% 17% 22% 40%	1520
	21st to end	Night	31% 5% 15% 17% 14% 17%	1533
July	1st to 10th	Night	29% 7% 20% 21% 15% 7%	2815
	11th to 20th	Night	23% 7% 15% 30% 21% 4	2806
	21st to end	Night	31% 7% 24% 27% 11% 1	3060
August	1st to 10th	Night	21% 6% 19% 35% 17% 2	3144
	11th to 20th	Night	20% 5% 16% 30% 27% 2	3143
	21st to end	Night	26% 8% 23% 27% 15% 1	3487
September	1st to 10th	Night	29% 6% 16% 18% 19% 13%	1900
	11th to 20th	Night	23% 6% 12% 20% 27% 12%	1909
	21st to end	Night	19% 3 10% 22% 33% 13%	2090
October	1st to 10th	Night	13% 2 6% 11% 34% 33%	3751
	11th to 20th	Night	17% 3 8% 14% 33% 25%	3778
	21st to end	Night	9% 2 3 14% 37% 35%	4120
November	1st to 10th	Night	14% 2 4 19% 31% 29%	2340
	11th to 20th	Night	12% 4 5% 21% 31% 28%	2570
	21st to end	Night	19% 3 7% 18% 29% 24%	2562
December	1st to 10th	Night	16% 3 9% 21% 28% 23%	2470
	11th to 20th	Night	19% 2 9% 22% 26% 22%	3201
	21st to end	Night	15% 2 7% 19% 28% 29%	4763

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	16% 28% 34% 18% 4	270
	11th to 20th	Day	21% 21% 32% 18% 7% 1	270
	21st to end	Day	11% 26% 31% 22% 10% 1	297
February	1st to 10th	Day	16% 25% 30% 22% 6% 1	270
	11th to 20th	Day	22% 26% 33% 15% 3 1	270
	21st to end	Day	32% 28% 25% 10% 4	225
March	1st to 10th	Day	20% 29% 35% 11% 4	348
	11th to 20th	Day	22% 25% 33% 13% 6% 1	460
	21st to end	Day	25% 31% 33% 8% 3 1	506
April	1st to 10th	Day	14% 32% 37% 13% 3 1	460
	11th to 20th	Day	17% 33% 33% 15% 3	452
	21st to end	Day	16% 30% 35% 16% 3	460
May	1st to 10th	Day	8% 30% 40% 19% 3	463
	11th to 20th	Day	9% 21% 38% 22% 8% 2	488
	21st to end	Day	7% 18% 43% 22% 9% 1	642
June	1st to 10th	Day	3 8% 32% 35% 19% 3	680
	11th to 20th	Day	3 9% 32% 38% 15% 3	680
	21st to end	Day	8% 4 19% 34% 28% 7%	658
July	1st to 10th	Day	7% 1 14% 36% 33% 10%	486
	11th to 20th	Day	4 1 8% 39% 35% 12%	484
	21st to end	Day	5% 1 8% 31% 40% 15%	531
August	1st to 10th	Day	5% 2 8% 32% 39% 16%	487
	11th to 20th	Day	9% 2 12% 34% 32% 11%	477
	21st to end	Day	8% 3 9% 37% 33% 11%	539
September	1st to 10th	Day	12% 3 17% 38% 23% 7%	490
	11th to 20th	Day	5% 4 19% 45% 22% 5%	479
	21st to end	Day	6% 7% 30% 32% 21% 4	300
October	1st to 10th	Day	11% 10% 27% 33% 16% 4	300
	11th to 20th	Day	12% 11% 34% 23% 17% 4	300
	21st to end	Day	13% 16% 29% 25% 11% 6%	330
November	1st to 10th	Day	17% 22% 32% 17% 10% 1	300
	11th to 20th	Day	14% 16% 31% 20% 15% 4	300
	21st to end	Day	19% 26% 27% 16% 12% 1	291
December	1st to 10th	Day	20% 19% 31% 23% 6% 1	270
	11th to 20th	Day	17% 26% 32% 21% 5%	270
	21st to end	Day	22% 27% 32% 14% 5%	297
January	1st to 10th	Night	9% 22% 32% 23% 11% 3	760
	11th to 20th	Night	11% 23% 30% 24% 9% 3	760
	21st to end	Night	12% 22% 28% 26% 10% 2	836
February	1st to 10th	Night	9% 23% 35% 21% 8% 3	760
	11th to 20th	Night	12% 22% 41% 19% 5% 1	744
	21st to end	Night	17% 30% 35% 14% 3 1	628
March	1st to 10th	Night	14% 27% 29% 23% 6% 1	760
	11th to 20th	Night	13% 20% 30% 27% 9% 1	760
	21st to end	Night	12% 25% 38% 19% 6%	738
April	1st to 10th	Night	11% 23% 42% 19% 5%	570
	11th to 20th	Night	11% 22% 38% 23% 5%	563
	21st to end	Night	9% 24% 34% 27% 6%	570
May	1st to 10th	Night	5% 20% 43% 28% 4	570
	11th to 20th	Night	7% 13% 32% 35% 11% 2	561
	21st to end	Night	7% 13% 38% 31% 11%	617
June	1st to 10th	Night	3 5% 29% 44% 18% 1	570
	11th to 20th	Night	4 5% 31% 41% 17% 2	570
	21st to end	Night	6% 3 17% 42% 29% 2	570
July	1st to 10th	Night	8% 2 6% 40% 41% 3	555
	11th to 20th	Night	4 1 5% 35% 46% 8%	552
	21st to end	Night	6% 2 3 28% 50% 11%	624
August	1st to 10th	Night	6% 1 3 30% 47% 13%	567
	11th to 20th	Night	5% 2 5% 32% 43% 13%	564
	21st to end	Night	5% 1 6% 29% 50% 9%	627
September	1st to 10th	Night	7% 3 7% 37% 38% 7%	570
	11th to 20th	Night	9% 1 12% 43% 30% 4	570
	21st to end	Night	7% 4 16% 43% 27% 3	570
October	1st to 10th	Night	7% 8% 23% 39% 20% 3	724
	11th to 20th	Night	8% 10% 28% 35% 16% 3	760
	21st to end	Night	9% 13% 27% 28% 16% 7%	836
November	1st to 10th	Night	9% 14% 30% 28% 14% 5%	780
	11th to 20th	Night	7% 14% 29% 34% 13% 3	970
	21st to end	Night	12% 20% 29% 23% 13% 3	986
December	1st to 10th	Night	13% 20% 28% 27% 10% 2	950
	11th to 20th	Night	12% 25% 31% 23% 8% 1	950
	21st to end	Night	15% 25% 31% 22% 5% 2	974

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