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 2025-05-11.

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 4 4 6% 7% 43%	113849
	Night	14% 6% 2 5% 4 4 4 3 5% 7% 46%	115098
Transparency	Day	37% 7% 12% 22% 17% 5%	117495
	Night	36% 6% 11% 23% 19% 5%	119591
Seeing	Day	18% 11% 26% 25% 14% 5%	15603
	Night	17% 7% 27% 33% 14% 2	25385

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	15% 5% 2 6% 4 4 3 3 4 4 49%	3982
	11th to 20th	Day	12% 7% 3 6% 4 4 4 3 4 4 50%	4060
	21st to end	Day	14% 8% 3 6% 4 4 4 3 4 5% 46%	4466
February	1st to 10th	Day	13% 9% 3 6% 5% 5% 4 4 5% 6% 42%	1800
	11th to 20th	Day	18% 8% 2 6% 3 3 3 4 5% 7% 40%	1759
	21st to end	Day	16% 8% 2 7% 4 4 4 3 6% 5% 41%	1690
March	1st to 10th	Day	17% 5% 2 6% 4 4 5% 3 5% 6% 43%	4935
	11th to 20th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 52%	4980
	21st to end	Day	16% 7% 2 6% 4 4 5% 4 5% 6% 41%	5465
April	1st to 10th	Day	12% 7% 2 5% 4 4 5% 4 5% 6% 46%	2385
	11th to 20th	Day	13% 6% 2 6% 5% 3 4 2 5% 6% 49%	2374
	21st to end	Day	16% 7% 2 6% 4 5% 5% 3 5% 6% 42%	2415
May	1st to 10th	Day	19% 7% 2 8% 5% 5% 4 4 7% 8% 31%	4212
	11th to 20th	Day	23% 6% 3 5% 4 4 4 4 5% 8% 33%	4081
	21st to end	Day	11% 4 2 5% 4 4 5% 5% 7% 9% 43%	4821
June	1st to 10th	Day	11% 5% 2 5% 5% 4 5% 5% 7% 8% 43%	2660
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 7% 10% 40%	2660
	21st to end	Day	8% 4 2 5% 4 6% 4 5% 8% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 6% 8% 11% 41%	4378
	11th to 20th	Day	8% 5% 2 6% 5% 6% 5% 6% 8% 12% 37%	4120
	21st to end	Day	11% 8% 2 8% 6% 6% 5% 5% 7% 9% 34%	4466
August	1st to 10th	Day	14% 5% 2 6% 5% 6% 6% 5% 8% 11% 33%	4104
	11th to 20th	Day	8% 5% 2 6% 6% 5% 6% 5% 7% 10% 39%	3948
	21st to end	Day	10% 4 2 5% 4 4 4 4 5% 9% 49%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 46%	2280
	11th to 20th	Day	13% 5% 2 6% 3 3 4 3 6% 8% 48%	2235
	21st to end	Day	22% 7% 3 5% 4 4 4 4 5% 5% 37%	2006
October	1st to 10th	Day	15% 5% 2 5% 4 4 4 3 5% 6% 47%	1900
	11th to 20th	Day	16% 4 1 5% 4 4 3 3 4 5% 52%	1900
	21st to end	Day	14% 5% 2 4 3 3 4 2 4 5% 53%	2090
November	1st to 10th	Day	12% 5% 1 4 4 3 3 3 4 6% 54%	1957
	11th to 20th	Day	14% 5% 2 4 4 3 3 4 5% 6% 50%	1730
	21st to end	Day	15% 4 2 4 4 3 4 4 4 6% 51%	1600
December	1st to 10th	Day	11% 5% 2 5% 4 4 3 3 5% 6% 52%	2600
	11th to 20th	Day	19% 7% 2 4 3 3 2 2 4 6% 47%	2990
	21st to end	Day	19% 7% 2 6% 4 4 4 3 5% 7% 39%	3960
January	1st to 10th	Night	12% 6% 1 4 3 3 4 3 4 5% 55%	5958
	11th to 20th	Night	13% 7% 2 4 2 3 3 2 4 5% 55%	5900
	21st to end	Night	14% 8% 2 5% 3 3 3 2 3 5% 51%	6490
February	1st to 10th	Night	14% 7% 3 6% 4 4 4 4 6% 6% 42%	2600
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 5% 5% 43%	2525
	21st to end	Night	15% 7% 3 6% 4 3 4 3 5% 5% 45%	1935
March	1st to 10th	Night	16% 6% 2 5% 4 3 3 3 5% 4 49%	4918
	11th to 20th	Night	10% 6% 2 4 3 3 3 3 5% 7% 53%	4980
	21st to end	Night	11% 7% 2 6% 4 4 5% 4 5% 6% 46%	5467
April	1st to 10th	Night	14% 7% 2 5% 4 3 4 4 5% 7% 44%	2015
	11th to 20th	Night	12% 5% 2 5% 4 3 5% 2 5% 7% 49%	1975
	21st to end	Night	16% 5% 2 5% 3 3 4 4 6% 7% 45%	2000
May	1st to 10th	Night	21% 7% 3 7% 4 4 5% 4 5% 9% 32%	2905
	11th to 20th	Night	25% 7% 2 5% 4 5% 4 4 6% 6% 33%	2818
	21st to end	Night	12% 5% 2 5% 4 4 4 4 6% 9% 45%	2732
June	1st to 10th	Night	11% 4 2 5% 4 4 4 4 6% 10% 46%	1520
	11th to 20th	Night	11% 6% 3 5% 6% 5% 5% 4 6% 8% 43%	1520
	21st to end	Night	9% 4 2 5% 4 5% 5% 4 8% 10% 44%	1520
July	1st to 10th	Night	7% 4 2 6% 5% 6% 7% 5% 7% 10% 41%	2576
	11th to 20th	Night	7% 6% 2 7% 6% 5% 6% 4 7% 11% 38%	2840
	21st to end	Night	9% 7% 2 8% 6% 5% 5% 5% 8% 9% 34%	3087
August	1st to 10th	Night	16% 6% 2 7% 5% 5% 6% 4 7% 10% 33%	2825
	11th to 20th	Night	10% 7% 3 9% 6% 5% 5% 4 5% 8% 37%	2848
	21st to end	Night	12% 6% 2 6% 4 3 4 3 5% 8% 49%	3476
September	1st to 10th	Night	15% 5% 2 4 3 4 4 3 6% 8% 45%	1900
	11th to 20th	Night	12% 6% 2 6% 5% 3 3 4 5% 6% 48%	1942
	21st to end	Night	23% 6% 2 5% 4 4 3 3 4 6% 40%	2173
October	1st to 10th	Night	19% 5% 2 5% 3 4 4 3 3 6% 46%	2280
	11th to 20th	Night	15% 5% 1 5% 3 3 4 2 5% 6% 51%	2280
	21st to end	Night	15% 4 2 4 3 3 4 3 5% 6% 52%	2508
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 3 5% 5% 53%	2341
	11th to 20th	Night	15% 6% 2 4 3 3 4 3 5% 6% 49%	2663
	21st to end	Night	14% 5% 1 5% 4 4 4 4 4 6% 51%	2800
December	1st to 10th	Night	10% 5% 2 4 4 3 4 3 5% 6% 55%	4580
	11th to 20th	Night	20% 7% 2 5% 3 3 4 3 4 6% 43%	5205
	21st to end	Night	17% 5% 2 6% 3 3 4 4 5% 7% 42%	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	35% 7% 6% 23% 20% 9%	3983
	11th to 20th	Day	37% 5% 7% 20% 19% 13%	4060
	21st to end	Day	40% 5% 8% 25% 18% 4	4466
February	1st to 10th	Day	42% 5% 12% 24% 14% 3	1800
	11th to 20th	Day	43% 6% 9% 21% 18% 4	1768
	21st to end	Day	44% 6% 12% 17% 17% 5%	1690
March	1st to 10th	Day	40% 6% 8% 19% 17% 10%	4935
	11th to 20th	Day	32% 6% 8% 26% 21% 8%	4980
	21st to end	Day	43% 6% 11% 17% 17% 7%	5465
April	1st to 10th	Day	37% 6% 12% 20% 17% 8%	2385
	11th to 20th	Day	36% 6% 11% 20% 21% 6%	2374
	21st to end	Day	43% 7% 10% 22% 14% 4	2415
May	1st to 10th	Day	46% 11% 12% 18% 10% 2	4250
	11th to 20th	Day	46% 11% 12% 20% 10% 1	4085
	21st to end	Day	32% 11% 17% 20% 18% 2	4835
June	1st to 10th	Day	35% 10% 15% 24% 16% 1	2660
	11th to 20th	Day	37% 10% 15% 23% 13% 2	2660
	21st to end	Day	32% 11% 16% 24% 16% 1	2660
July	1st to 10th	Day	35% 8% 22% 19% 10% 5%	4309
	11th to 20th	Day	36% 8% 20% 26% 9% 1	4051
	21st to end	Day	44% 8% 11% 21% 14% 1	4324
August	1st to 10th	Day	43% 8% 19% 24% 6%	4091
	11th to 20th	Day	37% 8% 16% 25% 14% 1	3987
	21st to end	Day	33% 6% 17% 29% 14% 2	4180
September	1st to 10th	Day	38% 7% 13% 17% 20% 5%	2280
	11th to 20th	Day	34% 7% 10% 27% 19% 4	2235
	21st to end	Day	47% 7% 10% 18% 16% 3	2006
October	1st to 10th	Day	36% 4 7% 18% 29% 5%	3160
	11th to 20th	Day	25% 4 8% 26% 28% 10%	3140
	21st to end	Day	21% 3 11% 30% 24% 12%	3428
November	1st to 10th	Day	31% 5% 9% 26% 24% 5%	1957
	11th to 20th	Day	34% 5% 12% 20% 22% 7%	1730
	21st to end	Day	35% 4 12% 25% 22% 3	1600
December	1st to 10th	Day	34% 5% 10% 26% 19% 7%	2600
	11th to 20th	Day	41% 2 14% 25% 13% 5%	2986
	21st to end	Day	43% 7% 14% 21% 15% 1	3960
January	1st to 10th	Night	29% 5% 7% 23% 28% 7%	5958
	11th to 20th	Night	32% 4 7% 23% 23% 11%	5900
	21st to end	Night	37% 4 8% 27% 20% 4	6490
February	1st to 10th	Night	41% 6% 12% 23% 16% 2	2600
	11th to 20th	Night	43% 4 9% 20% 18% 7%	2527
	21st to end	Night	40% 6% 10% 20% 17% 6%	1935
March	1st to 10th	Night	36% 6% 8% 22% 19% 9%	4918
	11th to 20th	Night	30% 5% 10% 22% 26% 7%	4980
	21st to end	Night	37% 6% 10% 20% 20% 6%	5467
April	1st to 10th	Night	40% 6% 11% 18% 21% 5%	2015
	11th to 20th	Night	35% 6% 12% 22% 20% 6%	1975
	21st to end	Night	37% 7% 11% 26% 14% 5%	2000
May	1st to 10th	Night	49% 9% 12% 18% 9% 3	2932
	11th to 20th	Night	47% 10% 12% 20% 9% 1	2819
	21st to end	Night	33% 10% 17% 25% 12% 3	2739
June	1st to 10th	Night	31% 10% 15% 27% 15% 2	1520
	11th to 20th	Night	37% 10% 15% 23% 13% 2	1520
	21st to end	Night	33% 10% 18% 19% 18% 3	1520
July	1st to 10th	Night	37% 10% 22% 15% 13% 3	2535
	11th to 20th	Night	39% 8% 21% 22% 8% 1	2786
	21st to end	Night	43% 9% 16% 19% 12%	2988
August	1st to 10th	Night	45% 8% 17% 23% 6% 1	2814
	11th to 20th	Night	44% 7% 12% 22% 14% 1	2875
	21st to end	Night	34% 6% 14% 30% 14% 1	3476
September	1st to 10th	Night	38% 7% 12% 21% 16% 6%	1900
	11th to 20th	Night	36% 6% 11% 25% 19% 3	1942
	21st to end	Night	45% 6% 10% 19% 16% 3	2173
October	1st to 10th	Night	42% 3 9% 19% 20% 6%	3800
	11th to 20th	Night	24% 4 10% 22% 30% 10%	3779
	21st to end	Night	21% 4 9% 32% 26% 8%	4120
November	1st to 10th	Night	33% 5% 9% 23% 23% 7%	2341
	11th to 20th	Night	36% 4 8% 22% 22% 7%	2663
	21st to end	Night	34% 4 11% 24% 22% 5%	2800
December	1st to 10th	Night	30% 4 11% 29% 23% 4	4580
	11th to 20th	Night	43% 3 12% 18% 17% 6%	5208
	21st to end	Night	39% 7% 14% 23% 16% 1	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	24% 24% 31% 16% 4	283
	11th to 20th	Day	27% 24% 29% 17% 3	300
	21st to end	Day	22% 23% 38% 13% 3 1	330
February	1st to 10th	Day	22% 25% 36% 15% 2	300
	11th to 20th	Day	36% 24% 32% 7% 1	306
	21st to end	Day	27% 15% 35% 20% 3	414
March	1st to 10th	Day	23% 18% 32% 22% 5%	500
	11th to 20th	Day	23% 21% 29% 20% 6% 2	500
	21st to end	Day	23% 17% 35% 17% 6% 1	550
April	1st to 10th	Day	22% 25% 24% 20% 8% 2	500
	11th to 20th	Day	19% 17% 27% 22% 11% 3	492
	21st to end	Day	23% 11% 33% 21% 10% 2	500
May	1st to 10th	Day	21% 14% 29% 22% 10% 4	503
	11th to 20th	Day	21% 9% 28% 26% 12% 4	487
	21st to end	Day	19% 5% 26% 27% 16% 7%	529
June	1st to 10th	Day	13% 6% 27% 28% 20% 7%	490
	11th to 20th	Day	12% 8% 25% 32% 15% 8%	490
	21st to end	Day	9% 6% 25% 35% 18% 7%	490
July	1st to 10th	Day	13% 3 14% 33% 27% 11%	485
	11th to 20th	Day	5% 4 17% 32% 29% 13%	484
	21st to end	Day	11% 4 17% 33% 25% 10%	532
August	1st to 10th	Day	10% 4 20% 27% 25% 13%	486
	11th to 20th	Day	11% 3 19% 29% 25% 12%	479
	21st to end	Day	9% 3 17% 32% 24% 15%	539
September	1st to 10th	Day	15% 5% 17% 28% 20% 15%	490
	11th to 20th	Day	17% 3 19% 25% 23% 12%	490
	21st to end	Day	29% 8% 19% 24% 15% 4	490
October	1st to 10th	Day	20% 5% 20% 31% 17% 6%	490
	11th to 20th	Day	14% 14% 26% 27% 16% 3	490
	21st to end	Day	17% 11% 32% 26% 12% 3	539
November	1st to 10th	Day	15% 10% 33% 28% 11% 3	495
	11th to 20th	Day	19% 12% 31% 26% 11% 2	330
	21st to end	Day	20% 16% 39% 21% 5%	200
December	1st to 10th	Day	19% 16% 40% 21% 6% 1	200
	11th to 20th	Day	25% 11% 37% 20% 7% 1	200
	21st to end	Day	23% 18% 33% 20% 6%	220
January	1st to 10th	Night	18% 14% 39% 25% 4	800
	11th to 20th	Night	19% 11% 35% 31% 5%	800
	21st to end	Night	18% 9% 39% 31% 2	880
February	1st to 10th	Night	17% 9% 40% 30% 4	800
	11th to 20th	Night	25% 15% 37% 22% 2	783
	21st to end	Night	19% 12% 41% 26% 3	660
March	1st to 10th	Night	17% 14% 37% 29% 4	800
	11th to 20th	Night	20% 14% 34% 26% 6% 1	800
	21st to end	Night	18% 13% 38% 25% 6%	880
April	1st to 10th	Night	19% 15% 33% 27% 6%	800
	11th to 20th	Night	15% 12% 34% 29% 9% 1	791
	21st to end	Night	18% 9% 30% 34% 9% 1	800
May	1st to 10th	Night	20% 7% 26% 30% 16% 1	602
	11th to 20th	Night	19% 4 25% 33% 18% 1	563
	21st to end	Night	16% 1 19% 39% 22% 3	411
June	1st to 10th	Night	11% 3 18% 44% 23% 2	380
	11th to 20th	Night	14% 2 15% 41% 26% 2	380
	21st to end	Night	9% 3 14% 47% 24% 2	380
July	1st to 10th	Night	13% 2 13% 35% 33% 5%	399
	11th to 20th	Night	6% 2 9% 37% 42% 5%	552
	21st to end	Night	9% 1 13% 39% 33% 5%	624
August	1st to 10th	Night	13% 1 10% 42% 30% 4	567
	11th to 20th	Night	12% 2 12% 38% 31% 4	601
	21st to end	Night	14% 2 12% 35% 33% 5%	836
September	1st to 10th	Night	17% 2 13% 34% 28% 5%	760
	11th to 20th	Night	14% 3 14% 35% 29% 5%	760
	21st to end	Night	28% 2 18% 30% 20% 3	760
October	1st to 10th	Night	20% 5% 17% 32% 21% 4	760
	11th to 20th	Night	15% 7% 27% 33% 16% 1	760
	21st to end	Night	17% 6% 26% 37% 12% 2	836
November	1st to 10th	Night	14% 6% 29% 36% 13% 2	780
	11th to 20th	Night	17% 8% 31% 35% 8% 1	800
	21st to end	Night	15% 8% 33% 32% 12% 1	800
December	1st to 10th	Night	17% 9% 35% 30% 9%	800
	11th to 20th	Night	17% 7% 34% 36% 6% 1	800
	21st to end	Night	19% 9% 29% 38% 4	880

Page generated at 2025-05-08 09:33:56. Version ac07

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