Mark's place
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mark's place clear sky chart from 2005-11-06 to 2025-09-19.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 38%	126666
	Night	12% 9% 4 8% 5% 4 4 3 5% 6% 39%	122651
Transparency	Day	43% 6% 12% 24% 13% 1	130149
	Night	44% 5% 11% 23% 15% 2	127241
Seeing	Day	16% 11% 29% 25% 14% 5%	17130
	Night	17% 9% 31% 30% 11% 1	26900

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	8% 10% 8% 16% 7% 7% 5% 3 4 5% 27%	3617
	11th to 20th	Day	7% 10% 6% 12% 5% 4 4 7% 7% 7% 30%	3620
	21st to end	Day	11% 16% 6% 9% 3 3 3 3 4 6% 35%	4277
February	1st to 10th	Day	12% 13% 6% 9% 5% 5% 4 4 5% 6% 31%	1800
	11th to 20th	Day	13% 12% 4 7% 5% 4 5% 3 5% 7% 36%	1858
	21st to end	Day	12% 14% 4 10% 6% 4 3 3 5% 6% 34%	1650
March	1st to 10th	Day	15% 8% 3 6% 4 3 3 2 4 6% 46%	4480
	11th to 20th	Day	12% 6% 2 8% 4 3 3 3 6% 8% 45%	4887
	21st to end	Day	21% 13% 4 9% 5% 4 4 3 4 5% 29%	5845
April	1st to 10th	Day	18% 12% 4 7% 5% 5% 5% 4 5% 5% 31%	2400
	11th to 20th	Day	16% 11% 3 7% 5% 4 4 3 5% 6% 34%	2374
	21st to end	Day	18% 10% 3 7% 5% 5% 4 3 6% 6% 32%	2430
May	1st to 10th	Day	14% 7% 3 7% 4 4 4 3 6% 7% 39%	6130
	11th to 20th	Day	13% 6% 3 6% 5% 4 4 4 6% 8% 41%	6319
	21st to end	Day	14% 6% 2 6% 5% 5% 5% 4 7% 9% 38%	6935
June	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 4 7% 8% 43%	2800
	11th to 20th	Day	12% 5% 2 6% 5% 4 4 4 6% 9% 42%	2800
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 7% 8% 38%	2800
July	1st to 10th	Day	8% 6% 3 7% 5% 5% 6% 5% 8% 9% 38%	6360
	11th to 20th	Day	7% 6% 2 6% 4 4 4 4 6% 9% 46%	6360
	21st to end	Day	10% 6% 3 7% 6% 5% 6% 5% 6% 9% 38%	6924
August	1st to 10th	Day	9% 5% 3 6% 5% 5% 6% 4 7% 9% 40%	4410
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 6% 8% 46%	3975
	21st to end	Day	9% 6% 2 6% 4 4 4 4 6% 10% 46%	4312
September	1st to 10th	Day	12% 5% 2 7% 4 5% 4 4 6% 7% 44%	2400
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 8% 51%	2336
	21st to end	Day	12% 8% 2 6% 5% 4 4 3 5% 7% 44%	2090
October	1st to 10th	Day	12% 7% 2 7% 4 3 3 3 5% 6% 48%	1906
	11th to 20th	Day	18% 9% 2 7% 4 4 3 4 5% 6% 38%	1900
	21st to end	Day	18% 9% 3 7% 4 3 3 3 4 5% 40%	1944
November	1st to 10th	Day	11% 9% 2 8% 5% 5% 4 4 5% 5% 44%	1762
	11th to 20th	Day	15% 12% 3 8% 5% 5% 4 3 5% 6% 34%	1765
	21st to end	Day	14% 12% 4 10% 5% 3 4 3 4 6% 36%	1600
December	1st to 10th	Day	8% 10% 4 10% 4 4 5% 4 6% 7% 40%	2610
	11th to 20th	Day	13% 14% 5% 11% 6% 5% 6% 4 6% 7% 24%	3008
	21st to end	Day	18% 23% 9% 11% 5% 4 4 4 5% 4 14%	3982
January	1st to 10th	Night	8% 11% 6% 15% 7% 4 4 3 4 5% 32%	6348
	11th to 20th	Night	10% 13% 6% 12% 6% 5% 5% 5% 8% 6% 24%	6360
	21st to end	Night	12% 14% 6% 11% 4 4 4 3 5% 5% 32%	6697
February	1st to 10th	Night	12% 12% 5% 9% 6% 6% 4 4 5% 5% 33%	2600
	11th to 20th	Night	10% 11% 4 8% 6% 5% 4 4 4 6% 39%	2427
	21st to end	Night	15% 12% 4 8% 4 4 2 3 5% 6% 36%	1980
March	1st to 10th	Night	16% 9% 4 7% 4 4 4 3 4 4 42%	5373
	11th to 20th	Night	9% 7% 2 6% 4 3 4 3 6% 8% 48%	5103
	21st to end	Night	22% 13% 4 11% 5% 4 4 3 4 5% 26%	5083
April	1st to 10th	Night	17% 9% 3 6% 5% 4 4 3 5% 6% 39%	2000
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 4 7% 37%	1975
	21st to end	Night	16% 8% 3 6% 5% 5% 5% 4 5% 8% 37%	1970
May	1st to 10th	Night	16% 7% 3 6% 4 4 4 3 5% 7% 40%	3774
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 6% 7% 47%	3596
	21st to end	Night	14% 7% 2 7% 4 4 4 4 6% 8% 40%	3942
June	1st to 10th	Night	9% 4 2 5% 5% 4 5% 4 7% 10% 45%	1600
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 7% 9% 44%	1600
	21st to end	Night	9% 6% 3 6% 4 4 3 4 6% 7% 48%	1600
July	1st to 10th	Night	12% 6% 3 7% 4 4 5% 4 6% 9% 42%	3618
	11th to 20th	Night	5% 5% 1 6% 4 4 4 4 6% 9% 52%	3617
	21st to end	Night	7% 5% 3 6% 4 4 5% 4 6% 9% 49%	3947
August	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 7% 50%	2746
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 5% 7% 50%	3076
	21st to end	Night	10% 5% 2 6% 4 4 4 4 5% 7% 50%	3586
September	1st to 10th	Night	7% 5% 2 5% 3 3 3 3 6% 7% 56%	2000
	11th to 20th	Night	10% 5% 1 5% 3 3 5% 4 5% 7% 52%	2000
	21st to end	Night	9% 5% 2 5% 3 4 5% 3 5% 7% 51%	2090
October	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 6% 52%	2270
	11th to 20th	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	2280
	21st to end	Night	17% 9% 3 7% 4 4 4 3 5% 7% 37%	2652
November	1st to 10th	Night	12% 8% 3 6% 4 3 3 3 5% 5% 48%	2536
	11th to 20th	Night	14% 11% 3 9% 5% 4 4 3 5% 5% 35%	2630
	21st to end	Night	17% 10% 3 8% 4 4 3 3 4 6% 38%	2800
December	1st to 10th	Night	8% 8% 3 8% 5% 5% 5% 4 6% 7% 42%	4580
	11th to 20th	Night	15% 16% 5% 9% 5% 5% 5% 4 4 5% 27%	5199
	21st to end	Night	19% 19% 6% 12% 6% 5% 4 3 4 4 17%	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	61% 4 10% 18% 7%	3618
	11th to 20th	Day	50% 6% 12% 11% 19% 2	3620
	21st to end	Day	51% 4 14% 19% 11%	4277
February	1st to 10th	Day	54% 5% 12% 18% 11% 1	1800
	11th to 20th	Day	48% 5% 11% 24% 9% 2	1867
	21st to end	Day	52% 5% 10% 20% 10% 2	1650
March	1st to 10th	Day	40% 5% 11% 29% 11% 4	4480
	11th to 20th	Day	37% 5% 13% 31% 12% 2	4887
	21st to end	Day	58% 6% 10% 15% 9% 2	5847
April	1st to 10th	Day	53% 6% 10% 20% 11%	2400
	11th to 20th	Day	50% 6% 10% 20% 11% 2	2374
	21st to end	Day	51% 7% 10% 19% 12% 1	2430
May	1st to 10th	Day	42% 6% 9% 24% 18% 1	6165
	11th to 20th	Day	40% 8% 10% 29% 14% 1	6323
	21st to end	Day	40% 8% 11% 27% 12% 2	6949
June	1st to 10th	Day	36% 7% 15% 26% 15% 1	2800
	11th to 20th	Day	37% 9% 14% 23% 17% 1	2800
	21st to end	Day	41% 8% 14% 23% 13% 2	2800
July	1st to 10th	Day	38% 8% 14% 29% 10% 1	6291
	11th to 20th	Day	34% 7% 13% 31% 13% 1	6285
	21st to end	Day	41% 8% 14% 21% 15% 1	6728
August	1st to 10th	Day	38% 8% 15% 26% 12% 1	4395
	11th to 20th	Day	35% 6% 14% 30% 14% 1	4014
	21st to end	Day	33% 5% 13% 31% 15% 1	4311
September	1st to 10th	Day	37% 8% 10% 25% 18% 1	2400
	11th to 20th	Day	30% 6% 13% 33% 17% 2	2336
	21st to end	Day	39% 5% 10% 28% 17% 1	2090
October	1st to 10th	Day	23% 4 6% 39% 27% 1	3166
	11th to 20th	Day	34% 4 9% 26% 24% 2	3140
	21st to end	Day	41% 4 10% 27% 14% 3	3182
November	1st to 10th	Day	42% 5% 13% 25% 13% 2	1762
	11th to 20th	Day	51% 6% 12% 20% 10% 1	1765
	21st to end	Day	50% 5% 15% 22% 7% 1	1600
December	1st to 10th	Day	43% 4 19% 26% 7% 1	2610
	11th to 20th	Day	59% 5% 11% 16% 7% 2	3005
	21st to end	Day	73% 5% 7% 9% 5%	3982
January	1st to 10th	Night	54% 5% 10% 21% 10% 1	6348
	11th to 20th	Night	58% 5% 9% 11% 16% 1	6360
	21st to end	Night	55% 5% 11% 18% 11%	6697
February	1st to 10th	Night	53% 5% 12% 17% 12% 2	2600
	11th to 20th	Night	47% 5% 11% 23% 14% 1	2428
	21st to end	Night	49% 4 13% 20% 11% 3	1980
March	1st to 10th	Night	46% 4 10% 22% 11% 7%	5373
	11th to 20th	Night	33% 4 12% 34% 14% 3	5103
	21st to end	Night	61% 4 8% 16% 10% 1	5083
April	1st to 10th	Night	45% 6% 11% 22% 14% 2	2000
	11th to 20th	Night	48% 4 7% 19% 18% 3	1975
	21st to end	Night	44% 8% 12% 22% 12% 2	1970
May	1st to 10th	Night	43% 6% 11% 18% 21% 1	3801
	11th to 20th	Night	35% 8% 14% 22% 18% 2	3597
	21st to end	Night	38% 8% 11% 25% 15% 2	3950
June	1st to 10th	Night	32% 8% 14% 27% 19% 1	1600
	11th to 20th	Night	35% 6% 12% 24% 21% 3	1600
	21st to end	Night	34% 7% 15% 22% 18% 4	1600
July	1st to 10th	Night	39% 7% 14% 25% 14% 2	3580
	11th to 20th	Night	29% 7% 13% 31% 19% 2	3572
	21st to end	Night	32% 7% 17% 23% 18% 3	3832
August	1st to 10th	Night	31% 7% 13% 29% 19% 1	2737
	11th to 20th	Night	32% 7% 12% 31% 17% 1	3103
	21st to end	Night	33% 5% 12% 29% 18% 4	3586
September	1st to 10th	Night	27% 7% 12% 28% 23% 2	2000
	11th to 20th	Night	31% 6% 12% 32% 18% 2	2000
	21st to end	Night	32% 6% 10% 30% 20% 2	2090
October	1st to 10th	Night	24% 4 6% 37% 28% 2	3790
	11th to 20th	Night	32% 4 10% 27% 25% 3	3779
	21st to end	Night	43% 4 9% 26% 15% 2	4364
November	1st to 10th	Night	39% 5% 11% 26% 17% 2	2536
	11th to 20th	Night	50% 5% 13% 19% 12% 2	2630
	21st to end	Night	49% 4 13% 23% 10% 1	2800
December	1st to 10th	Night	41% 5% 15% 28% 10% 1	4580
	11th to 20th	Night	59% 5% 11% 16% 9% 1	5201
	21st to end	Night	70% 5% 7% 12% 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	17% 14% 32% 27% 8% 2	300
	11th to 20th	Day	18% 18% 40% 18% 5% 1	300
	21st to end	Day	21% 16% 36% 22% 5%	330
February	1st to 10th	Day	16% 22% 37% 20% 5% 1	300
	11th to 20th	Day	22% 20% 35% 19% 4	300
	21st to end	Day	23% 19% 43% 14% 1	249
March	1st to 10th	Day	17% 19% 42% 17% 4	300
	11th to 20th	Day	21% 22% 35% 14% 5% 2	300
	21st to end	Day	26% 15% 32% 19% 6% 1	493
April	1st to 10th	Day	26% 16% 32% 21% 5%	500
	11th to 20th	Day	27% 16% 27% 18% 11% 2	492
	21st to end	Day	23% 12% 31% 25% 8% 2	500
May	1st to 10th	Day	20% 10% 33% 25% 10% 2	615
	11th to 20th	Day	19% 12% 29% 23% 12% 5%	725
	21st to end	Day	16% 10% 27% 25% 15% 7%	789
June	1st to 10th	Day	11% 7% 29% 29% 19% 5%	730
	11th to 20th	Day	12% 7% 32% 30% 15% 4	730
	21st to end	Day	12% 8% 29% 30% 15% 4	730
July	1st to 10th	Day	8% 4 20% 28% 27% 12%	721
	11th to 20th	Day	11% 6% 25% 28% 22% 8%	718
	21st to end	Day	10% 3 25% 32% 21% 8%	794
August	1st to 10th	Day	8% 3 20% 36% 23% 10%	726
	11th to 20th	Day	15% 5% 19% 27% 22% 12%	597
	21st to end	Day	10% 7% 19% 32% 22% 9%	583
September	1st to 10th	Day	13% 8% 22% 29% 19% 9%	530
	11th to 20th	Day	11% 3 20% 29% 26% 10%	522
	21st to end	Day	15% 10% 27% 26% 17% 5%	490
October	1st to 10th	Day	16% 11% 28% 27% 14% 3	306
	11th to 20th	Day	23% 16% 34% 18% 9%	300
	21st to end	Day	27% 16% 29% 22% 5% 2	330
November	1st to 10th	Day	18% 14% 39% 19% 8% 2	300
	11th to 20th	Day	19% 19% 38% 17% 8%	300
	21st to end	Day	22% 14% 38% 21% 4	300
December	1st to 10th	Day	17% 18% 40% 17% 7% 1	300
	11th to 20th	Day	19% 14% 39% 20% 8%	300
	21st to end	Day	25% 18% 37% 17% 3	330
January	1st to 10th	Night	16% 15% 38% 24% 7% 1	1000
	11th to 20th	Night	19% 14% 38% 24% 5% 1	1000
	21st to end	Night	21% 12% 36% 26% 5%	1100
February	1st to 10th	Night	18% 13% 39% 24% 6%	1000
	11th to 20th	Night	17% 15% 39% 25% 4	873
	21st to end	Night	22% 13% 41% 20% 4	660
March	1st to 10th	Night	22% 13% 37% 22% 5%	800
	11th to 20th	Night	17% 11% 35% 29% 8%	658
	21st to end	Night	24% 10% 34% 25% 5% 1	660
April	1st to 10th	Night	20% 15% 36% 25% 5%	600
	11th to 20th	Night	22% 15% 30% 27% 7%	593
	21st to end	Night	19% 9% 34% 29% 8% 1	600
May	1st to 10th	Night	17% 8% 36% 30% 8% 1	600
	11th to 20th	Night	18% 7% 31% 33% 11% 1	591
	21st to end	Night	14% 3 29% 37% 15% 1	650
June	1st to 10th	Night	10% 5% 25% 46% 14% 1	600
	11th to 20th	Night	14% 5% 30% 38% 13%	600
	21st to end	Night	11% 4 27% 41% 17% 1	600
July	1st to 10th	Night	9% 2 14% 39% 32% 4	585
	11th to 20th	Night	8% 3 22% 42% 23% 3	582
	21st to end	Night	7% 1 18% 46% 24% 2	657
August	1st to 10th	Night	6% 3 15% 44% 29% 3	597
	11th to 20th	Night	12% 2 13% 39% 28% 5%	594
	21st to end	Night	10% 2 15% 44% 26% 3	660
September	1st to 10th	Night	9% 3 22% 39% 23% 5%	600
	11th to 20th	Night	11% 4 20% 34% 26% 4	614
	21st to end	Night	11% 7% 26% 36% 18% 2	760
October	1st to 10th	Night	16% 6% 26% 33% 18% 1	760
	11th to 20th	Night	18% 12% 30% 30% 9%	760
	21st to end	Night	21% 11% 33% 27% 7%	836
November	1st to 10th	Night	17% 9% 38% 26% 9% 1	780
	11th to 20th	Night	19% 15% 37% 23% 6%	830
	21st to end	Night	22% 13% 36% 23% 7%	1000
December	1st to 10th	Night	20% 12% 38% 25% 5%	1000
	11th to 20th	Night	23% 11% 36% 24% 5% 1	1000
	21st to end	Night	28% 13% 34% 23% 3	1100

Page generated at 2025-09-16 00:49:11. Version ac07

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