CMOR
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CMOR clear sky chart from 2005-11-06 to 2026-05-02.

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% 11% 4 10% 6% 5% 5% 4 6% 7% 30%	132646
	Night	13% 11% 4 10% 6% 5% 5% 4 5% 6% 31%	129814
Transparency	Day	52% 6% 12% 19% 11% 1	136227
	Night	51% 6% 12% 19% 12% 1	134309
Seeing	Day	18% 13% 29% 23% 12% 5%	16787
	Night	19% 11% 30% 28% 11% 1	25906

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	14% 15% 8% 20% 8% 6% 5% 3 4 4 15%	4878
	11th to 20th	Day	9% 20% 9% 22% 10% 8% 4 3 4 4 6%	5246
	21st to end	Day	14% 15% 6% 17% 9% 8% 6% 4 4 5% 12%	5780
February	1st to 10th	Day	14% 18% 6% 13% 8% 7% 6% 3 5% 5% 16%	1890
	11th to 20th	Day	13% 14% 7% 13% 9% 5% 6% 4 5% 6% 18%	1843
	21st to end	Day	15% 15% 6% 12% 6% 5% 5% 4 4 6% 21%	1692
March	1st to 10th	Day	17% 12% 5% 13% 7% 4 3 3 4 5% 26%	4965
	11th to 20th	Day	11% 12% 5% 10% 5% 5% 5% 4 6% 8% 29%	5090
	21st to end	Day	17% 12% 5% 11% 6% 5% 4 3 4 4 28%	5586
April	1st to 10th	Day	16% 15% 4 9% 4 4 4 4 5% 6% 28%	2413
	11th to 20th	Day	17% 11% 4 7% 5% 4 3 3 5% 7% 33%	2655
	21st to end	Day	19% 10% 3 8% 5% 4 4 3 4 6% 33%	2730
May	1st to 10th	Day	19% 9% 3 7% 5% 4 4 3 5% 6% 33%	5901
	11th to 20th	Day	16% 10% 3 8% 5% 4 5% 4 6% 7% 32%	5995
	21st to end	Day	17% 7% 2 6% 4 4 5% 4 5% 7% 38%	6949
June	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 8% 37%	2800
	11th to 20th	Day	10% 6% 3 7% 4 4 6% 5% 7% 9% 37%	2937
	21st to end	Day	12% 5% 2 6% 5% 5% 5% 4 6% 10% 38%	3000
July	1st to 10th	Day	9% 5% 2 7% 4 5% 5% 5% 7% 10% 40%	6819
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6079
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	6424
August	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 36%	4234
	11th to 20th	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 35%	4174
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	4583
September	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 4 7% 9% 39%	2383
	11th to 20th	Day	10% 7% 2 5% 4 4 5% 4 6% 8% 45%	2200
	21st to end	Day	15% 8% 2 8% 5% 5% 4 4 7% 8% 34%	2200
October	1st to 10th	Day	16% 9% 3 8% 4 4 4 3 5% 8% 36%	2019
	11th to 20th	Day	20% 9% 3 9% 4 4 4 4 5% 8% 29%	2000
	21st to end	Day	21% 12% 5% 9% 5% 5% 5% 4 5% 7% 22%	2185
November	1st to 10th	Day	13% 12% 4 8% 6% 5% 5% 4 5% 7% 31%	1842
	11th to 20th	Day	14% 15% 5% 9% 6% 5% 4 4 7% 8% 23%	1680
	21st to end	Day	17% 18% 7% 11% 6% 6% 5% 3 5% 6% 17%	1680
December	1st to 10th	Day	17% 18% 7% 14% 8% 7% 6% 4 5% 4 11%	2680
	11th to 20th	Day	21% 18% 7% 12% 5% 4 5% 4 5% 5% 15%	3066
	21st to end	Day	25% 20% 7% 13% 6% 5% 4 3 4 4 10%	4048
January	1st to 10th	Night	14% 13% 6% 18% 7% 6% 6% 4 5% 5% 15%	8012
	11th to 20th	Night	9% 16% 9% 18% 10% 8% 6% 5% 5% 5% 10%	7639
	21st to end	Night	13% 12% 8% 13% 7% 7% 6% 4 6% 6% 18%	8401
February	1st to 10th	Night	13% 16% 6% 11% 8% 6% 5% 3 5% 6% 21%	2730
	11th to 20th	Night	13% 13% 6% 11% 7% 5% 6% 4 6% 7% 23%	2663
	21st to end	Night	14% 13% 4 12% 8% 5% 5% 4 6% 5% 23%	2112
March	1st to 10th	Night	13% 9% 4 9% 7% 6% 5% 4 5% 7% 32%	5093
	11th to 20th	Night	9% 8% 4 9% 5% 6% 5% 4 7% 9% 34%	5089
	21st to end	Night	17% 10% 4 8% 4 4 4 3 5% 5% 37%	5588
April	1st to 10th	Night	18% 10% 3 9% 5% 5% 5% 3 5% 7% 31%	2205
	11th to 20th	Night	16% 8% 4 8% 5% 5% 4 3 5% 7% 35%	1931
	21st to end	Night	13% 10% 2 7% 6% 4 5% 4 6% 8% 35%	1890
May	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 6% 37%	4062
	11th to 20th	Night	14% 9% 4 8% 5% 5% 5% 4 4 6% 36%	3895
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 8% 46%	3926
June	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 7% 7% 45%	1600
	11th to 20th	Night	8% 5% 3 6% 5% 5% 5% 4 8% 9% 43%	1456
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 7% 46%	1400
July	1st to 10th	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 49%	3160
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 6% 9% 49%	3847
	21st to end	Night	7% 5% 2 6% 4 4 4 4 6% 7% 50%	4428
August	1st to 10th	Night	11% 6% 2 6% 3 5% 5% 4 6% 7% 46%	2915
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 6% 8% 45%	2875
	21st to end	Night	7% 5% 2 6% 5% 4 5% 4 6% 8% 48%	3280
September	1st to 10th	Night	9% 6% 2 7% 4 4 4 3 6% 6% 48%	2014
	11th to 20th	Night	9% 5% 1 4 3 3 4 3 5% 8% 55%	2200
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 6% 40%	2200
October	1st to 10th	Night	14% 8% 3 6% 4 4 4 3 5% 6% 43%	2380
	11th to 20th	Night	18% 9% 3 7% 6% 5% 4 3 6% 6% 34%	2400
	21st to end	Night	21% 8% 3 8% 5% 4 5% 4 6% 6% 29%	2652
November	1st to 10th	Night	13% 10% 3 7% 5% 5% 4 4 7% 7% 35%	2673
	11th to 20th	Night	15% 13% 4 11% 5% 6% 6% 4 6% 7% 24%	2940
	21st to end	Night	18% 14% 5% 11% 6% 5% 5% 3 5% 7% 21%	2940
December	1st to 10th	Night	12% 18% 6% 14% 9% 7% 6% 5% 4 5% 14%	4720
	11th to 20th	Night	13% 20% 6% 12% 6% 5% 4 4 4 5% 20%	5348
	21st to end	Night	23% 19% 7% 11% 6% 5% 4 3 4 5% 15%	7150

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	71% 4 8% 10% 7%	4879
	11th to 20th	Day	81% 4 6% 5% 3	5280
	21st to end	Day	73% 5% 9% 7% 6%	5780
February	1st to 10th	Day	67% 7% 9% 12% 4	1890
	11th to 20th	Day	64% 7% 10% 14% 5% 1	1852
	21st to end	Day	64% 6% 10% 13% 6% 1	1692
March	1st to 10th	Day	60% 4 13% 16% 6% 1	4968
	11th to 20th	Day	52% 5% 17% 18% 6% 2	5090
	21st to end	Day	57% 5% 11% 17% 9% 1	5586
April	1st to 10th	Day	55% 7% 12% 16% 10% 1	2413
	11th to 20th	Day	50% 5% 11% 21% 11% 1	2655
	21st to end	Day	53% 5% 10% 19% 11% 2	2730
May	1st to 10th	Day	50% 6% 12% 17% 10% 5%	5910
	11th to 20th	Day	48% 8% 11% 19% 12% 2	5999
	21st to end	Day	44% 6% 11% 19% 17% 4	6949
June	1st to 10th	Day	42% 7% 13% 22% 14% 1	2800
	11th to 20th	Day	40% 8% 13% 22% 15% 1	2937
	21st to end	Day	40% 7% 13% 25% 14%	3000
July	1st to 10th	Day	39% 4 13% 25% 17% 2	6746
	11th to 20th	Day	35% 4 13% 32% 15% 1	6005
	21st to end	Day	34% 5% 14% 27% 19% 1	6240
August	1st to 10th	Day	42% 7% 14% 22% 14% 1	4221
	11th to 20th	Day	41% 7% 17% 26% 9%	4213
	21st to end	Day	39% 5% 13% 32% 10% 1	4583
September	1st to 10th	Day	40% 8% 14% 25% 12% 1	2383
	11th to 20th	Day	35% 6% 14% 29% 15%	2200
	21st to end	Day	45% 7% 13% 24% 9% 1	2200
October	1st to 10th	Day	40% 5% 12% 24% 18% 1	3279
	11th to 20th	Day	41% 4 13% 23% 18%	3240
	21st to end	Day	52% 5% 15% 20% 8%	3511
November	1st to 10th	Day	50% 7% 13% 21% 7% 1	1842
	11th to 20th	Day	56% 8% 14% 13% 8% 1	1680
	21st to end	Day	64% 9% 12% 12% 4	1680
December	1st to 10th	Day	73% 6% 11% 6% 4	2680
	11th to 20th	Day	70% 6% 8% 10% 7%	3066
	21st to end	Day	77% 5% 6% 7% 4	4048
January	1st to 10th	Night	66% 7% 8% 13% 5%	8012
	11th to 20th	Night	73% 6% 7% 8% 6%	7680
	21st to end	Night	66% 5% 8% 12% 10%	8401
February	1st to 10th	Night	63% 7% 9% 16% 5% 1	2730
	11th to 20th	Night	57% 7% 13% 16% 6%	2665
	21st to end	Night	59% 8% 11% 15% 7%	2112
March	1st to 10th	Night	51% 6% 14% 19% 9% 2	5103
	11th to 20th	Night	43% 6% 19% 21% 10% 1	5090
	21st to end	Night	49% 4 10% 19% 16% 2	5588
April	1st to 10th	Night	52% 5% 11% 17% 13% 1	2205
	11th to 20th	Night	48% 5% 10% 20% 16% 2	1931
	21st to end	Night	45% 7% 11% 20% 15% 3	1890
May	1st to 10th	Night	47% 5% 13% 15% 13% 8%	4069
	11th to 20th	Night	46% 8% 13% 21% 10% 3	3896
	21st to end	Night	37% 6% 13% 24% 18% 2	3927
June	1st to 10th	Night	36% 6% 12% 26% 16% 3	1600
	11th to 20th	Night	34% 8% 14% 26% 16% 2	1456
	21st to end	Night	35% 7% 15% 22% 20% 1	1400
July	1st to 10th	Night	31% 4 15% 26% 19% 4	3125
	11th to 20th	Night	33% 4 12% 29% 19% 3	3799
	21st to end	Night	32% 5% 12% 26% 22% 3	4297
August	1st to 10th	Night	36% 5% 14% 25% 17% 2	2904
	11th to 20th	Night	35% 5% 17% 28% 13% 1	2902
	21st to end	Night	32% 5% 13% 33% 16% 1	3280
September	1st to 10th	Night	35% 6% 13% 28% 15% 2	2014
	11th to 20th	Night	27% 7% 14% 32% 19% 1	2200
	21st to end	Night	43% 7% 12% 26% 13%	2200
October	1st to 10th	Night	33% 4 7% 30% 24% 1	3887
	11th to 20th	Night	42% 4 14% 23% 16% 1	3899
	21st to end	Night	42% 4 15% 26% 12%	4276
November	1st to 10th	Night	44% 7% 14% 22% 12% 1	2673
	11th to 20th	Night	54% 8% 14% 15% 7% 1	2940
	21st to end	Night	62% 6% 16% 12% 4	2940
December	1st to 10th	Night	68% 6% 13% 10% 3	4720
	11th to 20th	Night	64% 5% 10% 11% 9% 1	5348
	21st to end	Night	72% 6% 8% 9% 5%	7150

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	23% 23% 34% 15% 4 1	259
	11th to 20th	Day	25% 24% 35% 13% 2 1	330
	21st to end	Day	25% 20% 34% 18% 2 1	363
February	1st to 10th	Day	24% 19% 38% 14% 5%	330
	11th to 20th	Day	25% 23% 38% 12% 2	330
	21st to end	Day	26% 27% 33% 13% 1	273
March	1st to 10th	Day	23% 16% 38% 17% 5%	500
	11th to 20th	Day	23% 19% 34% 18% 5% 1	540
	21st to end	Day	28% 15% 32% 19% 5% 1	594
April	1st to 10th	Day	23% 21% 30% 19% 5% 1	540
	11th to 20th	Day	28% 16% 26% 17% 10% 3	532
	21st to end	Day	26% 12% 32% 21% 7% 2	540
May	1st to 10th	Day	23% 12% 28% 24% 10% 4	507
	11th to 20th	Day	16% 10% 31% 22% 15% 5%	527
	21st to end	Day	15% 7% 28% 27% 17% 6%	573
June	1st to 10th	Day	14% 12% 31% 28% 12% 4	530
	11th to 20th	Day	12% 9% 30% 31% 15% 4	667
	21st to end	Day	14% 8% 31% 32% 12% 3	730
July	1st to 10th	Day	10% 7% 22% 29% 25% 7%	721
	11th to 20th	Day	12% 5% 26% 26% 21% 11%	533
	21st to end	Day	8% 4 23% 34% 23% 8%	576
August	1st to 10th	Day	10% 5% 18% 29% 24% 13%	526
	11th to 20th	Day	10% 5% 22% 26% 22% 15%	519
	21st to end	Day	11% 6% 22% 29% 21% 11%	583
September	1st to 10th	Day	13% 6% 23% 25% 18% 15%	530
	11th to 20th	Day	9% 5% 18% 24% 28% 17%	530
	21st to end	Day	18% 9% 25% 25% 15% 8%	530
October	1st to 10th	Day	19% 10% 25% 24% 15% 7%	530
	11th to 20th	Day	18% 16% 34% 21% 8% 2	530
	21st to end	Day	26% 14% 33% 21% 6% 1	568
November	1st to 10th	Day	23% 23% 28% 18% 6% 2	324
	11th to 20th	Day	23% 22% 32% 15% 7% 1	220
	21st to end	Day	22% 24% 34% 18% 2	220
December	1st to 10th	Day	30% 20% 34% 10% 5%	220
	11th to 20th	Day	26% 25% 32% 12% 4	220
	21st to end	Day	30% 21% 36% 10% 3	242
January	1st to 10th	Night	20% 20% 37% 17% 5% 1	840
	11th to 20th	Night	25% 20% 33% 19% 3	840
	21st to end	Night	19% 14% 40% 23% 5%	924
February	1st to 10th	Night	23% 16% 37% 23% 2	840
	11th to 20th	Night	21% 16% 36% 22% 4	823
	21st to end	Night	22% 19% 34% 22% 3	692
March	1st to 10th	Night	18% 15% 38% 24% 5%	840
	11th to 20th	Night	21% 15% 36% 22% 6%	840
	21st to end	Night	21% 13% 33% 24% 8%	924
April	1st to 10th	Night	22% 15% 33% 25% 4	840
	11th to 20th	Night	20% 14% 30% 26% 9% 1	687
	21st to end	Night	18% 10% 32% 33% 7%	630
May	1st to 10th	Night	18% 6% 31% 30% 13% 1	603
	11th to 20th	Night	16% 5% 28% 36% 14% 1	532
	21st to end	Night	12% 4 30% 37% 16% 1	433
June	1st to 10th	Night	16% 3 30% 39% 12% 1	400
	11th to 20th	Night	11% 5% 29% 43% 12%	400
	21st to end	Night	13% 5% 27% 40% 16%	400
July	1st to 10th	Night	6% 2 22% 49% 20% 2	390
	11th to 20th	Night	8% 3 23% 39% 24% 3	505
	21st to end	Night	10% 3 19% 45% 21% 2	657
August	1st to 10th	Night	8% 2 17% 43% 28% 2	597
	11th to 20th	Night	9% 2 15% 41% 29% 4	594
	21st to end	Night	9% 3 21% 38% 27% 3	691
September	1st to 10th	Night	11% 5% 22% 34% 24% 4	800
	11th to 20th	Night	11% 3 17% 34% 30% 5%	800
	21st to end	Night	18% 6% 21% 31% 18% 5%	800
October	1st to 10th	Night	18% 7% 25% 29% 20% 2	800
	11th to 20th	Night	20% 12% 31% 26% 10% 1	800
	21st to end	Night	26% 10% 32% 28% 5%	880
November	1st to 10th	Night	19% 13% 29% 29% 9% 1	820
	11th to 20th	Night	21% 17% 32% 22% 7% 1	840
	21st to end	Night	25% 14% 33% 22% 5%	840
December	1st to 10th	Night	26% 18% 31% 21% 4	840
	11th to 20th	Night	23% 21% 36% 16% 4	840
	21st to end	Night	29% 15% 35% 19% 3	924

Page generated at 2026-04-29 02:21:43. Version ac07

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