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 2025-01-07.

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 9% 5% 5% 5% 4 6% 7% 30%	105589
	Night	14% 11% 4 9% 5% 5% 5% 4 5% 6% 32%	103428
Transparency	Day	50% 7% 13% 19% 10% 1	109180
	Night	49% 7% 12% 19% 11% 1	107914
Seeing	Day	18% 12% 29% 23% 13% 5%	15076
	Night	19% 11% 30% 28% 11% 1	24201

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	15% 18% 8% 13% 6% 5% 4 3 4 5% 20%	2670
	11th to 20th	Day	12% 19% 8% 20% 7% 5% 5% 4 5% 6% 9%	2840
	21st to end	Day	21% 21% 7% 13% 6% 6% 5% 3 4 4 11%	3124
February	1st to 10th	Day	15% 18% 6% 12% 9% 7% 6% 3 5% 5% 15%	1710
	11th to 20th	Day	12% 15% 8% 12% 8% 5% 5% 4 6% 6% 18%	1663
	21st to end	Day	16% 15% 6% 12% 6% 5% 5% 4 4 6% 22%	1535
March	1st to 10th	Day	19% 15% 6% 9% 6% 4 3 3 4 5% 26%	3385
	11th to 20th	Day	12% 14% 5% 9% 6% 5% 5% 4 6% 7% 28%	3480
	21st to end	Day	16% 11% 4 10% 6% 5% 4 3 5% 5% 31%	3815
April	1st to 10th	Day	16% 15% 4 9% 5% 4 5% 4 6% 7% 27%	2183
	11th to 20th	Day	18% 11% 3 7% 5% 4 3 3 5% 7% 34%	2399
	21st to end	Day	20% 10% 4 8% 5% 4 4 3 4 7% 32%	2470
May	1st to 10th	Day	18% 8% 3 7% 5% 4 5% 4 6% 7% 33%	4108
	11th to 20th	Day	15% 10% 3 8% 5% 4 5% 4 6% 8% 31%	4173
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 7% 45%	4837
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 8% 36%	2660
	11th to 20th	Day	10% 6% 3 7% 4 4 6% 5% 7% 9% 38%	2790
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 10% 38%	2850
July	1st to 10th	Day	10% 5% 3 7% 5% 5% 6% 5% 7% 10% 38%	4760
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 10% 40%	4243
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 5% 7% 9% 41%	4466
August	1st to 10th	Day	15% 7% 2 6% 5% 5% 5% 4 7% 9% 36%	4104
	11th to 20th	Day	9% 7% 3 7% 5% 5% 7% 5% 7% 10% 35%	4044
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	4443
September	1st to 10th	Day	11% 6% 2 6% 5% 6% 5% 4 7% 9% 38%	2264
	11th to 20th	Day	10% 7% 2 6% 4 4 5% 4 6% 8% 43%	2090
	21st to end	Day	15% 8% 2 8% 5% 5% 5% 4 7% 8% 34%	2090
October	1st to 10th	Day	16% 9% 3 8% 4 4 4 3 5% 8% 34%	1918
	11th to 20th	Day	20% 10% 3 9% 5% 4 4 4 5% 8% 29%	1900
	21st to end	Day	21% 12% 5% 9% 5% 5% 5% 4 5% 7% 21%	2076
November	1st to 10th	Day	13% 11% 4 8% 6% 5% 5% 5% 5% 7% 31%	1753
	11th to 20th	Day	14% 15% 5% 9% 6% 5% 4 4 7% 8% 24%	1600
	21st to end	Day	17% 17% 7% 11% 5% 5% 5% 3 5% 6% 17%	1600
December	1st to 10th	Day	17% 18% 7% 14% 8% 7% 6% 5% 5% 4 11%	2600
	11th to 20th	Day	21% 18% 6% 12% 5% 4 5% 4 5% 5% 15%	2986
	21st to end	Day	26% 20% 7% 13% 6% 5% 4 3 4 4 10%	3960
January	1st to 10th	Night	16% 16% 7% 15% 7% 5% 5% 4 5% 5% 15%	4404
	11th to 20th	Night	13% 15% 8% 15% 9% 8% 6% 5% 5% 5% 11%	4120
	21st to end	Night	21% 15% 10% 13% 6% 5% 4 3 4 5% 16%	4532
February	1st to 10th	Night	14% 16% 6% 11% 8% 6% 5% 3 5% 6% 20%	2470
	11th to 20th	Night	13% 13% 6% 11% 7% 5% 6% 4 5% 7% 23%	2403
	21st to end	Night	16% 13% 4 12% 8% 5% 5% 4 5% 5% 23%	1918
March	1st to 10th	Night	15% 10% 4 8% 5% 5% 5% 4 4 6% 34%	3453
	11th to 20th	Night	11% 9% 4 9% 5% 5% 5% 4 6% 8% 35%	3479
	21st to end	Night	15% 10% 3 8% 4 4 4 3 5% 5% 39%	3817
April	1st to 10th	Night	18% 11% 3 8% 5% 5% 5% 3 5% 6% 32%	1995
	11th to 20th	Night	17% 8% 4 7% 5% 4 4 3 5% 7% 36%	1745
	21st to end	Night	13% 10% 3 7% 6% 4 5% 4 6% 8% 34%	1710
May	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 7% 40%	2833
	11th to 20th	Night	12% 9% 3 8% 4 5% 5% 4 5% 7% 36%	2717
	21st to end	Night	10% 4 2 5% 3 3 4 3 5% 9% 51%	2738
June	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 7% 7% 44%	1520
	11th to 20th	Night	8% 5% 2 6% 5% 5% 5% 4 8% 10% 42%	1383
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 6% 7% 46%	1330
July	1st to 10th	Night	5% 5% 2 5% 4 3 4 4 6% 10% 53%	2210
	11th to 20th	Night	9% 5% 1 5% 3 3 4 4 5% 8% 52%	2696
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 7% 8% 48%	3087
August	1st to 10th	Night	11% 6% 2 6% 3 5% 5% 4 6% 7% 46%	2825
	11th to 20th	Night	10% 7% 2 5% 4 4 4 4 6% 8% 45%	2785
	21st to end	Night	8% 6% 2 6% 5% 4 5% 4 6% 9% 47%	3179
September	1st to 10th	Night	10% 6% 2 7% 4 4 4 3 5% 6% 48%	1913
	11th to 20th	Night	9% 5% 1 4 3 3 4 3 5% 8% 54%	2090
	21st to end	Night	16% 7% 2 7% 4 4 4 3 5% 6% 40%	2090
October	1st to 10th	Night	14% 8% 3 6% 4 4 4 4 5% 6% 41%	2261
	11th to 20th	Night	18% 8% 3 7% 6% 5% 4 4 6% 7% 34%	2280
	21st to end	Night	21% 8% 3 8% 5% 4 5% 4 6% 6% 28%	2519
November	1st to 10th	Night	13% 10% 3 7% 5% 5% 4 4 7% 7% 35%	2542
	11th to 20th	Night	15% 13% 4 10% 6% 5% 5% 4 6% 7% 24%	2800
	21st to end	Night	18% 14% 5% 10% 6% 5% 5% 3 5% 7% 22%	2800
December	1st to 10th	Night	12% 18% 6% 14% 9% 7% 6% 5% 4 5% 14%	4580
	11th to 20th	Night	13% 20% 7% 12% 6% 5% 4 4 4 5% 20%	5208
	21st to end	Night	23% 19% 7% 11% 6% 5% 4 3 4 5% 15%	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	66% 7% 11% 9% 6%	2671
	11th to 20th	Day	73% 7% 10% 6% 3	2840
	21st to end	Day	76% 8% 7% 6% 3	3124
February	1st to 10th	Day	67% 8% 10% 11% 3	1710
	11th to 20th	Day	63% 7% 10% 13% 6% 1	1672
	21st to end	Day	63% 6% 10% 13% 7% 1	1535
March	1st to 10th	Day	60% 6% 10% 19% 5% 1	3388
	11th to 20th	Day	53% 8% 15% 17% 7% 1	3480
	21st to end	Day	52% 7% 13% 16% 11% 2	3815
April	1st to 10th	Day	54% 7% 12% 15% 10%	2183
	11th to 20th	Day	49% 6% 12% 20% 12% 1	2399
	21st to end	Day	53% 5% 10% 18% 11% 2	2470
May	1st to 10th	Day	47% 8% 13% 19% 11% 3	4120
	11th to 20th	Day	47% 11% 15% 20% 6% 1	4177
	21st to end	Day	35% 8% 13% 19% 20% 4	4837
June	1st to 10th	Day	42% 8% 13% 22% 13% 1	2660
	11th to 20th	Day	39% 9% 13% 22% 16% 1	2790
	21st to end	Day	40% 8% 13% 25% 14%	2850
July	1st to 10th	Day	39% 6% 15% 24% 14% 2	4686
	11th to 20th	Day	35% 6% 13% 33% 12% 1	4169
	21st to end	Day	38% 6% 15% 22% 18% 1	4324
August	1st to 10th	Day	42% 7% 14% 22% 14% 1	4091
	11th to 20th	Day	41% 7% 17% 25% 9%	4083
	21st to end	Day	39% 6% 13% 31% 11% 1	4443
September	1st to 10th	Day	40% 8% 15% 24% 12% 1	2264
	11th to 20th	Day	36% 7% 14% 27% 15%	2090
	21st to end	Day	45% 8% 14% 24% 9% 1	2090
October	1st to 10th	Day	41% 5% 13% 23% 18% 1	3178
	11th to 20th	Day	41% 4 14% 23% 18%	3140
	21st to end	Day	52% 5% 16% 20% 7%	3402
November	1st to 10th	Day	50% 8% 13% 20% 7% 1	1753
	11th to 20th	Day	56% 9% 15% 12% 8% 1	1600
	21st to end	Day	64% 9% 12% 12% 3	1600
December	1st to 10th	Day	73% 6% 11% 6% 4	2600
	11th to 20th	Day	70% 6% 8% 10% 7%	2986
	21st to end	Day	77% 6% 6% 7% 4	3960
January	1st to 10th	Night	65% 11% 10% 9% 4	4404
	11th to 20th	Night	70% 11% 10% 7% 2	4120
	21st to end	Night	71% 7% 9% 10% 3 1	4532
February	1st to 10th	Night	63% 7% 9% 15% 5%	2470
	11th to 20th	Night	57% 8% 13% 15% 7%	2405
	21st to end	Night	58% 8% 11% 15% 7%	1918
March	1st to 10th	Night	50% 7% 14% 22% 6% 1	3463
	11th to 20th	Night	43% 8% 18% 19% 11% 1	3480
	21st to end	Night	45% 6% 11% 22% 15% 2	3817
April	1st to 10th	Night	51% 5% 11% 18% 13% 1	1995
	11th to 20th	Night	47% 6% 10% 19% 16% 2	1745
	21st to end	Night	46% 7% 10% 19% 15% 3	1710
May	1st to 10th	Night	42% 7% 12% 19% 16% 4	2840
	11th to 20th	Night	43% 10% 15% 22% 8% 2	2718
	21st to end	Night	29% 8% 14% 23% 23% 3	2739
June	1st to 10th	Night	37% 7% 12% 26% 16% 2	1520
	11th to 20th	Night	33% 9% 14% 25% 17% 2	1383
	21st to end	Night	35% 8% 15% 22% 19% 1	1330
July	1st to 10th	Night	27% 5% 14% 28% 20% 6%	2175
	11th to 20th	Night	30% 4 15% 31% 18% 1	2648
	21st to end	Night	32% 6% 15% 26% 17% 3	2988
August	1st to 10th	Night	37% 6% 14% 26% 17% 2	2814
	11th to 20th	Night	35% 6% 18% 27% 13% 1	2812
	21st to end	Night	33% 5% 13% 32% 16% 1	3179
September	1st to 10th	Night	36% 6% 13% 27% 16% 2	1913
	11th to 20th	Night	28% 7% 13% 31% 19% 1	2090
	21st to end	Night	43% 7% 12% 26% 12%	2090
October	1st to 10th	Night	34% 5% 8% 29% 24% 1	3768
	11th to 20th	Night	42% 5% 14% 23% 15%	3779
	21st to end	Night	42% 4 15% 26% 12%	4143
November	1st to 10th	Night	44% 8% 14% 21% 12% 1	2542
	11th to 20th	Night	54% 9% 14% 15% 7% 1	2800
	21st to end	Night	60% 7% 16% 12% 4	2800
December	1st to 10th	Night	69% 6% 13% 9% 3	4580
	11th to 20th	Night	64% 5% 10% 11% 9% 1	5208
	21st to end	Night	72% 6% 8% 9% 5%	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	22% 23% 31% 17% 5% 1	223
	11th to 20th	Day	25% 21% 37% 13% 3 1	270
	21st to end	Day	26% 17% 34% 19% 3 1	297
February	1st to 10th	Day	25% 19% 40% 12% 4	270
	11th to 20th	Day	24% 24% 38% 12% 1	270
	21st to end	Day	29% 29% 30% 11% 1	225
March	1st to 10th	Day	24% 16% 38% 17% 5%	424
	11th to 20th	Day	22% 16% 37% 19% 5% 1	460
	21st to end	Day	27% 15% 31% 20% 6% 1	506
April	1st to 10th	Day	20% 20% 32% 20% 6% 2	460
	11th to 20th	Day	29% 15% 26% 17% 10% 3	452
	21st to end	Day	26% 13% 31% 21% 8% 2	460
May	1st to 10th	Day	22% 12% 26% 25% 11% 4	463
	11th to 20th	Day	15% 10% 31% 22% 15% 6%	487
	21st to end	Day	14% 6% 27% 28% 18% 7%	529
June	1st to 10th	Day	14% 12% 30% 27% 13% 4	490
	11th to 20th	Day	12% 9% 29% 31% 15% 4	620
	21st to end	Day	15% 8% 31% 31% 11% 3	680
July	1st to 10th	Day	9% 6% 22% 29% 25% 8%	671
	11th to 20th	Day	12% 5% 26% 26% 20% 11%	493
	21st to end	Day	8% 4 23% 34% 22% 8%	532
August	1st to 10th	Day	11% 5% 19% 28% 23% 13%	486
	11th to 20th	Day	10% 5% 22% 26% 22% 15%	479
	21st to end	Day	11% 6% 23% 29% 20% 12%	539
September	1st to 10th	Day	13% 7% 23% 26% 18% 13%	490
	11th to 20th	Day	9% 5% 19% 24% 27% 16%	490
	21st to end	Day	17% 9% 26% 25% 15% 8%	490
October	1st to 10th	Day	20% 11% 26% 22% 14% 7%	490
	11th to 20th	Day	19% 17% 33% 20% 8% 3	490
	21st to end	Day	27% 14% 32% 20% 6% 1	525
November	1st to 10th	Day	22% 24% 28% 18% 6% 2	295
	11th to 20th	Day	24% 21% 34% 14% 7% 2	200
	21st to end	Day	23% 21% 36% 18% 3	200
December	1st to 10th	Day	31% 20% 35% 9% 6% 1	200
	11th to 20th	Day	27% 24% 33% 13% 4 1	200
	21st to end	Day	29% 21% 35% 10% 4	220
January	1st to 10th	Night	21% 20% 37% 16% 6% 1	785
	11th to 20th	Night	27% 17% 33% 20% 3	760
	21st to end	Night	19% 12% 39% 25% 6%	836
February	1st to 10th	Night	23% 16% 37% 22% 2	760
	11th to 20th	Night	21% 16% 36% 23% 5%	743
	21st to end	Night	23% 18% 34% 22% 3	628
March	1st to 10th	Night	18% 14% 38% 24% 6%	760
	11th to 20th	Night	20% 15% 36% 22% 6% 1	760
	21st to end	Night	21% 13% 33% 24% 9%	836
April	1st to 10th	Night	21% 15% 33% 26% 5%	760
	11th to 20th	Night	20% 14% 31% 25% 9% 1	621
	21st to end	Night	18% 10% 32% 32% 8%	570
May	1st to 10th	Night	18% 6% 31% 29% 14% 1	570
	11th to 20th	Night	16% 5% 28% 36% 13% 1	505
	21st to end	Night	11% 4 29% 38% 17% 1	411
June	1st to 10th	Night	16% 3 29% 39% 12% 1	380
	11th to 20th	Night	11% 5% 28% 43% 12%	380
	21st to end	Night	13% 4 27% 39% 16%	380
July	1st to 10th	Night	5% 2 22% 48% 20% 2	370
	11th to 20th	Night	8% 4 23% 39% 24% 3	479
	21st to end	Night	9% 3 19% 46% 20% 2	624
August	1st to 10th	Night	9% 2 18% 43% 27% 2	567
	11th to 20th	Night	9% 1 15% 40% 30% 4	564
	21st to end	Night	10% 3 21% 37% 26% 3	656
September	1st to 10th	Night	12% 5% 22% 34% 23% 4	760
	11th to 20th	Night	12% 3 18% 33% 29% 5%	760
	21st to end	Night	18% 7% 21% 31% 18% 5%	760
October	1st to 10th	Night	18% 7% 26% 28% 19% 2	760
	11th to 20th	Night	20% 13% 30% 26% 10% 1	760
	21st to end	Night	26% 10% 32% 28% 5%	836
November	1st to 10th	Night	19% 12% 29% 29% 10% 1	780
	11th to 20th	Night	22% 17% 32% 21% 7% 1	800
	21st to end	Night	25% 14% 34% 22% 6%	800
December	1st to 10th	Night	26% 18% 31% 20% 4	800
	11th to 20th	Night	23% 21% 35% 17% 4	800
	21st to end	Night	28% 15% 35% 19% 3	880

Page generated at 2025-01-04 03:24:03. Version ac07

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