Morning Calm Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morning Calm Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	13% 6% 2 6% 4 4 5% 4 6% 8% 42%	148747
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	142315
Transparency	Day	39% 6% 13% 22% 16% 4	152350
	Night	38% 5% 12% 22% 18% 4	146783
Seeing	Day	18% 12% 26% 25% 14% 5%	17419
	Night	17% 8% 27% 33% 14% 2	27040

By Month

Percentage Cloud Cover SE (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% 6% 2 7% 4 4 4 3 4 5% 47%	5158
	11th to 20th	Day	12% 7% 2 6% 4 3 3 3 3 3 52%	5280
	21st to end	Day	13% 7% 3 7% 4 4 4 3 4 5% 45%	5780
February	1st to 10th	Day	12% 8% 3 6% 5% 5% 4 4 5% 6% 43%	1890
	11th to 20th	Day	18% 8% 2 6% 3 3 3 4 5% 7% 41%	1850
	21st to end	Day	16% 9% 2 7% 4 4 4 3 6% 5% 40%	1772
March	1st to 10th	Day	17% 6% 2 6% 4 4 5% 3 5% 6% 43%	5045
	11th to 20th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 51%	5090
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 6% 6% 41%	5586
April	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 5% 7% 47%	2504
	11th to 20th	Day	13% 6% 1 6% 4 3 4 3 5% 6% 49%	2494
	21st to end	Day	16% 7% 2 6% 4 5% 4 3 5% 6% 42%	2536
May	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 7% 9% 35%	7616
	11th to 20th	Day	18% 5% 2 5% 4 4 5% 4 6% 9% 37%	7626
	21st to end	Day	21% 5% 2 6% 4 4 4 4 5% 7% 38%	9043
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 5% 7% 9% 43%	2940
	11th to 20th	Day	11% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	2940
	21st to end	Day	9% 4 2 4 4 6% 4 5% 8% 10% 44%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 11% 41%	8162
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 6% 9% 13% 30%	7680
	21st to end	Day	8% 6% 2 7% 6% 6% 6% 5% 8% 10% 38%	8340
August	1st to 10th	Day	13% 6% 2 7% 5% 7% 6% 5% 8% 11% 29%	6014
	11th to 20th	Day	8% 5% 2 7% 6% 6% 7% 5% 8% 11% 34%	5811
	21st to end	Day	8% 3 1 5% 4 4 5% 5% 6% 11% 47%	6080
September	1st to 10th	Day	12% 5% 2 6% 4 3 4 3 5% 7% 48%	2520
	11th to 20th	Day	13% 5% 2 6% 3 3 4 3 6% 8% 47%	2388
	21st to end	Day	23% 7% 3 5% 4 4 4 4 5% 5% 36%	2111
October	1st to 10th	Day	14% 5% 2 5% 4 4 3 3 4 6% 48%	2000
	11th to 20th	Day	15% 4 1 5% 4 4 4 3 4 5% 52%	2000
	21st to end	Day	14% 5% 2 4 3 3 4 2 4 6% 53%	2200
November	1st to 10th	Day	11% 5% 1 4 4 3 3 3 5% 6% 54%	2057
	11th to 20th	Day	14% 5% 2 4 4 3 3 4 5% 6% 51%	1816
	21st to end	Day	15% 4 2 4 4 3 4 4 4 6% 51%	1680
December	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 5% 6% 51%	2680
	11th to 20th	Day	18% 7% 2 4 3 3 2 2 4 6% 48%	3070
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 7% 39%	4048
January	1st to 10th	Night	11% 6% 2 5% 3 4 5% 4 5% 6% 50%	7717
	11th to 20th	Night	13% 7% 2 4 2 3 3 2 4 5% 55%	7680
	21st to end	Night	13% 7% 2 5% 3 3 3 2 4 5% 51%	8401
February	1st to 10th	Night	14% 7% 3 6% 4 4 5% 4 6% 6% 43%	2730
	11th to 20th	Night	19% 8% 2 6% 3 4 3 3 5% 5% 43%	2654
	21st to end	Night	14% 8% 3 6% 4 3 4 3 5% 5% 45%	2029
March	1st to 10th	Night	16% 6% 2 5% 4 3 3 3 4 4 49%	5028
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 7% 53%	5090
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 5% 6% 46%	5588
April	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 6% 45%	2116
	11th to 20th	Night	12% 5% 2 5% 4 3 4 3 5% 8% 49%	2075
	21st to end	Night	15% 5% 2 5% 3 3 4 4 6% 7% 45%	2100
May	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 4 5% 8% 35%	5239
	11th to 20th	Night	19% 5% 2 5% 4 4 4 4 6% 9% 37%	5234
	21st to end	Night	22% 7% 2 7% 4 4 4 3 5% 7% 35%	5108
June	1st to 10th	Night	11% 4 2 5% 5% 4 5% 4 6% 10% 46%	1680
	11th to 20th	Night	11% 6% 3 5% 6% 5% 5% 4 7% 8% 42%	1680
	21st to end	Night	10% 4 2 5% 4 5% 5% 4 8% 10% 43%	1680
July	1st to 10th	Night	11% 6% 2 7% 5% 5% 6% 4 7% 9% 39%	4790
	11th to 20th	Night	11% 8% 3 9% 6% 6% 6% 5% 7% 9% 29%	5277
	21st to end	Night	9% 6% 2 8% 6% 5% 6% 5% 7% 9% 38%	5736
August	1st to 10th	Night	16% 7% 3 8% 5% 5% 5% 5% 7% 10% 29%	4135
	11th to 20th	Night	10% 8% 3 9% 7% 6% 5% 5% 7% 9% 31%	4188
	21st to end	Night	11% 6% 2 7% 5% 4 5% 4 6% 8% 44%	5045
September	1st to 10th	Night	15% 5% 2 4 3 4 4 3 6% 8% 47%	2100
	11th to 20th	Night	13% 6% 2 6% 5% 3 3 3 5% 7% 47%	2075
	21st to end	Night	24% 6% 2 5% 4 4 3 3 4 6% 38%	2288
October	1st to 10th	Night	19% 6% 2 5% 3 4 3 3 3 6% 47%	2400
	11th to 20th	Night	15% 5% 2 5% 3 3 4 2 5% 6% 51%	2400
	21st to end	Night	15% 4 2 4 3 3 4 3 5% 6% 52%	2640
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 5% 53%	2461
	11th to 20th	Night	15% 6% 2 5% 3 3 4 3 5% 6% 49%	2796
	21st to end	Night	13% 5% 1 5% 4 4 4 4 4 6% 50%	2940
December	1st to 10th	Night	10% 5% 2 4 4 3 4 3 5% 7% 55%	4720
	11th to 20th	Night	19% 7% 2 5% 3 3 4 3 4 6% 44%	5345
	21st to end	Night	17% 6% 2 6% 3 3 4 4 5% 7% 42%	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	37% 6% 9% 22% 19% 7%	5159
	11th to 20th	Day	35% 4 6% 19% 22% 14%	5280
	21st to end	Day	40% 5% 7% 27% 17% 4	5780
February	1st to 10th	Day	40% 5% 12% 24% 15% 3	1890
	11th to 20th	Day	43% 5% 9% 21% 18% 4	1859
	21st to end	Day	44% 6% 12% 18% 16% 5%	1772
March	1st to 10th	Day	40% 6% 8% 19% 17% 10%	5045
	11th to 20th	Day	32% 5% 8% 26% 20% 8%	5090
	21st to end	Day	43% 5% 11% 17% 17% 7%	5586
April	1st to 10th	Day	36% 6% 13% 20% 18% 7%	2504
	11th to 20th	Day	35% 6% 11% 21% 21% 6%	2494
	21st to end	Day	43% 6% 10% 23% 14% 4	2536
May	1st to 10th	Day	42% 8% 13% 21% 14% 2	7651
	11th to 20th	Day	43% 7% 12% 23% 14% 1	7630
	21st to end	Day	44% 7% 12% 17% 18% 3	9059
June	1st to 10th	Day	35% 9% 14% 23% 18% 1	2940
	11th to 20th	Day	37% 9% 15% 23% 13% 2	2940
	21st to end	Day	32% 10% 16% 24% 16% 1	2940
July	1st to 10th	Day	38% 7% 19% 22% 12% 3	8093
	11th to 20th	Day	42% 8% 18% 24% 8% 1	7611
	21st to end	Day	39% 6% 14% 23% 18% 1	8156
August	1st to 10th	Day	47% 7% 20% 20% 5%	6001
	11th to 20th	Day	41% 7% 19% 22% 11%	5850
	21st to end	Day	32% 5% 20% 27% 15% 1	6080
September	1st to 10th	Day	36% 7% 14% 19% 20% 5%	2520
	11th to 20th	Day	35% 6% 10% 26% 19% 3	2388
	21st to end	Day	48% 6% 10% 18% 15% 2	2111
October	1st to 10th	Day	35% 4 7% 19% 29% 5%	3260
	11th to 20th	Day	25% 4 7% 26% 27% 10%	3240
	21st to end	Day	22% 3 10% 29% 24% 12%	3538
November	1st to 10th	Day	31% 5% 10% 25% 24% 5%	2057
	11th to 20th	Day	34% 5% 12% 21% 22% 7%	1816
	21st to end	Day	35% 4 12% 25% 21% 3	1680
December	1st to 10th	Day	35% 4 10% 26% 18% 6%	2680
	11th to 20th	Day	41% 2 14% 25% 13% 5%	3066
	21st to end	Day	43% 7% 14% 21% 15% 1	4048
January	1st to 10th	Night	33% 5% 9% 24% 24% 6%	7718
	11th to 20th	Night	33% 3 6% 21% 25% 12%	7680
	21st to end	Night	36% 4 7% 28% 21% 5%	8401
February	1st to 10th	Night	41% 5% 12% 23% 16% 3	2730
	11th to 20th	Night	43% 4 9% 20% 18% 6%	2656
	21st to end	Night	40% 6% 11% 20% 17% 6%	2029
March	1st to 10th	Night	37% 6% 8% 21% 19% 9%	5028
	11th to 20th	Night	30% 5% 10% 22% 26% 7%	5090
	21st to end	Night	37% 6% 11% 20% 20% 6%	5588
April	1st to 10th	Night	39% 5% 12% 18% 21% 5%	2116
	11th to 20th	Night	34% 6% 12% 23% 20% 5%	2075
	21st to end	Night	37% 7% 11% 27% 14% 5%	2100
May	1st to 10th	Night	47% 6% 13% 19% 10% 4	5266
	11th to 20th	Night	43% 6% 14% 25% 12% 1	5235
	21st to end	Night	49% 6% 12% 18% 12% 3	5115
June	1st to 10th	Night	32% 9% 14% 26% 16% 2	1680
	11th to 20th	Night	38% 9% 16% 22% 14% 1	1680
	21st to end	Night	34% 10% 16% 20% 19% 3	1680
July	1st to 10th	Night	42% 8% 16% 18% 13% 2	4751
	11th to 20th	Night	51% 7% 16% 18% 8% 1	5226
	21st to end	Night	41% 6% 16% 20% 16%	5606
August	1st to 10th	Night	50% 8% 17% 19% 5% 1	4124
	11th to 20th	Night	48% 7% 16% 18% 11%	4215
	21st to end	Night	38% 5% 16% 27% 13% 1	5045
September	1st to 10th	Night	35% 7% 12% 24% 16% 6%	2100
	11th to 20th	Night	37% 5% 11% 25% 19% 3	2075
	21st to end	Night	47% 6% 10% 18% 16% 3	2288
October	1st to 10th	Night	42% 3 9% 19% 20% 6%	3920
	11th to 20th	Night	25% 4 10% 22% 30% 10%	3899
	21st to end	Night	22% 4 9% 31% 27% 8%	4252
November	1st to 10th	Night	34% 5% 9% 23% 24% 7%	2461
	11th to 20th	Night	36% 4 8% 22% 22% 7%	2796
	21st to end	Night	35% 4 11% 24% 22% 5%	2940
December	1st to 10th	Night	30% 4 11% 29% 22% 4	4720
	11th to 20th	Night	43% 3 12% 19% 18% 6%	5348
	21st to end	Night	39% 6% 14% 23% 16% 1	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	24% 24% 32% 16% 4	311
	11th to 20th	Day	25% 26% 29% 16% 3	330
	21st to end	Day	22% 24% 37% 13% 2 1	363
February	1st to 10th	Day	21% 26% 36% 15% 2	330
	11th to 20th	Day	35% 24% 32% 7% 1	337
	21st to end	Day	27% 15% 36% 19% 3	446
March	1st to 10th	Day	22% 18% 33% 22% 5%	540
	11th to 20th	Day	24% 21% 29% 20% 6% 1	540
	21st to end	Day	23% 18% 36% 17% 6% 1	594
April	1st to 10th	Day	21% 24% 24% 20% 9% 2	540
	11th to 20th	Day	19% 17% 27% 22% 12% 3	532
	21st to end	Day	22% 12% 33% 22% 10% 2	540
May	1st to 10th	Day	22% 15% 29% 22% 9% 3	543
	11th to 20th	Day	21% 10% 28% 26% 12% 4	567
	21st to end	Day	22% 6% 26% 26% 15% 6%	617
June	1st to 10th	Day	12% 6% 28% 28% 19% 7%	570
	11th to 20th	Day	13% 9% 25% 31% 14% 7%	570
	21st to end	Day	10% 6% 23% 35% 19% 7%	570
July	1st to 10th	Day	12% 3 14% 33% 28% 11%	565
	11th to 20th	Day	5% 3 16% 33% 29% 13%	564
	21st to end	Day	10% 5% 17% 32% 25% 11%	620
August	1st to 10th	Day	11% 4 20% 27% 25% 12%	566
	11th to 20th	Day	11% 4 19% 30% 25% 11%	559
	21st to end	Day	8% 3 20% 33% 23% 13%	627
September	1st to 10th	Day	13% 5% 19% 30% 19% 14%	570
	11th to 20th	Day	17% 4 18% 26% 23% 12%	542
	21st to end	Day	30% 8% 19% 25% 15% 4	530
October	1st to 10th	Day	19% 5% 20% 32% 17% 8%	530
	11th to 20th	Day	15% 14% 26% 26% 15% 3	530
	21st to end	Day	17% 13% 32% 25% 11% 3	583
November	1st to 10th	Day	14% 12% 34% 27% 10% 3	535
	11th to 20th	Day	19% 12% 33% 25% 10% 1	356
	21st to end	Day	22% 16% 36% 21% 5%	220
December	1st to 10th	Day	20% 15% 40% 19% 5%	220
	11th to 20th	Day	24% 13% 36% 20% 6% 1	220
	21st to end	Day	23% 19% 34% 19% 6%	242
January	1st to 10th	Night	18% 13% 39% 26% 4	840
	11th to 20th	Night	19% 12% 35% 30% 5%	840
	21st to end	Night	18% 10% 39% 31% 2	924
February	1st to 10th	Night	17% 9% 40% 31% 3	840
	11th to 20th	Night	24% 15% 37% 22% 1	823
	21st to end	Night	19% 12% 41% 26% 3	692
March	1st to 10th	Night	17% 14% 37% 28% 4	840
	11th to 20th	Night	20% 14% 34% 26% 6% 1	840
	21st to end	Night	17% 13% 38% 25% 6%	924
April	1st to 10th	Night	18% 15% 34% 27% 6%	840
	11th to 20th	Night	15% 11% 34% 30% 9% 1	831
	21st to end	Night	18% 9% 30% 34% 9% 1	840
May	1st to 10th	Night	21% 7% 26% 29% 16% 1	632
	11th to 20th	Night	19% 4 26% 33% 17% 1	621
	21st to end	Night	17% 2 19% 38% 20% 2	455
June	1st to 10th	Night	11% 2 18% 44% 23% 1	420
	11th to 20th	Night	15% 2 17% 41% 24% 1	420
	21st to end	Night	9% 3 15% 46% 24% 3	420
July	1st to 10th	Night	12% 2 12% 35% 34% 4	443
	11th to 20th	Night	6% 2 8% 37% 41% 6%	612
	21st to end	Night	9% 1 12% 40% 33% 5%	690
August	1st to 10th	Night	13% 2 10% 41% 30% 4	627
	11th to 20th	Night	11% 3 12% 38% 32% 4	665
	21st to end	Night	13% 2 13% 37% 31% 4	924
September	1st to 10th	Night	16% 2 13% 37% 28% 5%	840
	11th to 20th	Night	14% 3 13% 36% 29% 5%	813
	21st to end	Night	28% 3 18% 31% 19% 2	800
October	1st to 10th	Night	20% 5% 17% 32% 22% 4	800
	11th to 20th	Night	15% 7% 27% 33% 16% 1	800
	21st to end	Night	17% 6% 27% 37% 11% 2	880
November	1st to 10th	Night	14% 6% 30% 36% 12% 2	820
	11th to 20th	Night	17% 9% 31% 34% 8% 1	840
	21st to end	Night	15% 8% 34% 32% 11% 1	840
December	1st to 10th	Night	17% 9% 35% 30% 8%	840
	11th to 20th	Night	17% 6% 35% 36% 5%	840
	21st to end	Night	19% 10% 30% 37% 4	924

Page generated at 2026-09-11 04:11:56. Version ac07

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