Mark's Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mark's Observing Site clear sky chart from 2005-11-06 to 2026-01-02.

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	9% 5% 2 5% 4 3 4 3 5% 7% 53%	127314
	Night	8% 5% 2 5% 3 3 3 3 5% 7% 56%	124352
Transparency	Day	30% 4 11% 27% 23% 6%	131009
	Night	28% 4 10% 27% 24% 7%	128710
Seeing	Day	11% 6% 28% 32% 18% 4	17127
	Night	11% 7% 33% 37% 11% 1	27073

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	10% 7% 2 8% 5% 4 4 3 5% 7% 45%	4086
	11th to 20th	Day	6% 7% 3 9% 5% 4 4 3 5% 6% 49%	4080
	21st to end	Day	8% 9% 3 7% 3 3 4 2 4 5% 52%	4488
February	1st to 10th	Day	7% 10% 3 8% 5% 4 3 3 4 7% 46%	1855
	11th to 20th	Day	6% 6% 3 8% 3 4 4 3 5% 6% 51%	1949
	21st to end	Day	6% 6% 2 5% 3 3 4 3 5% 6% 55%	1650
March	1st to 10th	Day	7% 6% 2 5% 3 3 3 3 4 6% 58%	4710
	11th to 20th	Day	7% 5% 2 4 2 2 3 3 4 7% 60%	4980
	21st to end	Day	8% 5% 2 6% 4 4 4 3 6% 9% 50%	5462
April	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 6% 7% 53%	2210
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 5% 7% 47%	2482
	21st to end	Day	16% 6% 2 5% 4 3 4 4 5% 7% 43%	2600
May	1st to 10th	Day	15% 7% 2 5% 4 4 4 3 4 6% 45%	5862
	11th to 20th	Day	10% 4 1 4 3 3 3 3 4 5% 59%	5862
	21st to end	Day	18% 6% 3 5% 4 3 4 3 5% 6% 42%	6434
June	1st to 10th	Day	5% 4 2 5% 3 4 4 4 6% 8% 56%	2600
	11th to 20th	Day	5% 4 1 4 5% 4 4 4 6% 8% 56%	2600
	21st to end	Day	5% 3 1 3 3 3 5% 4 7% 9% 56%	2600
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 4 6% 8% 52%	5900
	11th to 20th	Day	7% 4 1 4 4 4 5% 4 6% 9% 53%	5900
	21st to end	Day	6% 3 1 5% 4 4 5% 4 6% 9% 53%	6422
August	1st to 10th	Day	6% 6% 2 5% 4 4 4 3 6% 8% 51%	4234
	11th to 20th	Day	8% 4 1 4 3 3 4 4 7% 9% 53%	4171
	21st to end	Day	4 3 1 4 3 3 4 3 6% 9% 59%	4477
September	1st to 10th	Day	10% 5% 2 4 4 3 4 3 4 6% 56%	2215
	11th to 20th	Day	8% 4 2 5% 3 4 3 3 5% 7% 56%	2200
	21st to end	Day	8% 4 2 5% 4 3 4 3 5% 6% 57%	2200
October	1st to 10th	Day	12% 3 1 4 3 2 3 2 3 6% 59%	2175
	11th to 20th	Day	9% 3 1 4 2 2 2 2 4 5% 65%	2000
	21st to end	Day	11% 6% 2 4 3 3 2 2 4 5% 59%	2179
November	1st to 10th	Day	7% 4 2 3 3 3 4 2 5% 6% 62%	1853
	11th to 20th	Day	7% 5% 1 4 4 3 3 2 4 6% 60%	1890
	21st to end	Day	11% 5% 1 3 3 3 2 2 3 5% 62%	1890
December	1st to 10th	Day	4 4 1 4 3 2 2 2 4 6% 68%	3030
	11th to 20th	Day	6% 5% 2 5% 3 2 2 2 3 6% 63%	3481
	21st to end	Day	13% 9% 3 8% 4 4 4 3 5% 7% 41%	4587
January	1st to 10th	Night	11% 7% 3 8% 4 4 4 4 5% 6% 44%	5902
	11th to 20th	Night	4 4 2 5% 5% 4 4 4 5% 5% 58%	5900
	21st to end	Night	11% 8% 3 6% 3 3 3 2 3 4 54%	6490
February	1st to 10th	Night	9% 9% 3 6% 4 4 3 3 4 6% 48%	2562
	11th to 20th	Night	9% 7% 3 6% 4 4 3 3 6% 6% 49%	2338
	21st to end	Night	7% 5% 3 5% 3 4 3 3 4 7% 57%	1980
March	1st to 10th	Night	5% 4 1 3 3 3 3 3 4 6% 65%	5152
	11th to 20th	Night	5% 4 1 4 3 3 4 4 5% 7% 60%	4980
	21st to end	Night	8% 7% 2 6% 4 3 4 3 5% 7% 53%	5467
April	1st to 10th	Night	11% 5% 1 4 4 3 3 3 5% 8% 53%	2185
	11th to 20th	Night	12% 5% 2 5% 3 3 5% 4 5% 7% 50%	1869
	21st to end	Night	15% 6% 2 5% 4 3 4 4 5% 7% 48%	1800
May	1st to 10th	Night	16% 6% 2 4 4 3 3 3 4 6% 49%	4053
	11th to 20th	Night	11% 3 1 3 3 2 3 3 5% 7% 58%	4053
	21st to end	Night	18% 4 1 4 2 3 3 3 5% 7% 51%	4443
June	1st to 10th	Night	6% 3 1 5% 3 3 3 5% 6% 9% 58%	1800
	11th to 20th	Night	5% 3 1 3 4 3 4 5% 8% 9% 56%	1800
	21st to end	Night	4 3 2 3 3 4 4 4 7% 10% 58%	1800
July	1st to 10th	Night	4 3 2 4 4 4 4 4 7% 10% 55%	4078
	11th to 20th	Night	5% 4 1 4 3 4 5% 4 6% 11% 53%	4077
	21st to end	Night	5% 5% 2 5% 4 4 4 4 7% 9% 52%	4449
August	1st to 10th	Night	5% 3 1 5% 4 4 4 3 6% 10% 55%	2926
	11th to 20th	Night	6% 4 2 6% 4 4 5% 4 5% 7% 52%	2888
	21st to end	Night	4 3 2 4 3 2 3 3 6% 9% 62%	3427
September	1st to 10th	Night	7% 3 1 4 3 4 3 3 5% 5% 60%	2185
	11th to 20th	Night	7% 5% 2 5% 4 4 3 4 4 6% 58%	2200
	21st to end	Night	6% 4 2 4 3 3 4 3 5% 6% 61%	2200
October	1st to 10th	Night	9% 4 2 4 3 3 3 2 4 6% 60%	2205
	11th to 20th	Night	9% 2 1 2 3 3 2 2 3 5% 67%	2400
	21st to end	Night	11% 4 1 4 3 2 2 3 3 5% 62%	2659
November	1st to 10th	Night	8% 4 1 4 2 2 2 2 4 6% 66%	2665
	11th to 20th	Night	7% 5% 2 3 3 2 3 2 5% 6% 63%	2730
	21st to end	Night	11% 5% 2 3 2 2 3 2 4 5% 59%	2730
December	1st to 10th	Night	4 4 2 4 3 3 3 2 4 4 67%	4380
	11th to 20th	Night	7% 4 2 4 3 3 3 3 4 6% 62%	4946
	21st to end	Night	13% 8% 3 6% 3 4 3 2 4 6% 46%	6633

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	39% 4 10% 25% 18% 4	4087
	11th to 20th	Day	36% 4 11% 22% 22% 5%	4080
	21st to end	Day	36% 3 9% 24% 23% 4	4488
February	1st to 10th	Day	40% 4 11% 25% 15% 5%	1855
	11th to 20th	Day	34% 5% 13% 26% 17% 6%	1959
	21st to end	Day	29% 4 9% 24% 27% 7%	1650
March	1st to 10th	Day	28% 3 7% 18% 28% 15%	4710
	11th to 20th	Day	24% 4 9% 18% 32% 13%	4980
	21st to end	Day	31% 4 14% 37% 12% 2	5464
April	1st to 10th	Day	31% 4 11% 26% 21% 6%	2210
	11th to 20th	Day	37% 5% 10% 26% 17% 5%	2482
	21st to end	Day	39% 5% 11% 28% 15% 1	2600
May	1st to 10th	Day	40% 6% 10% 22% 17% 5%	5900
	11th to 20th	Day	27% 6% 9% 24% 27% 9%	5866
	21st to end	Day	42% 6% 10% 21% 19% 3	6447
June	1st to 10th	Day	25% 7% 17% 31% 16% 4	2600
	11th to 20th	Day	25% 6% 13% 28% 21% 6%	2600
	21st to end	Day	23% 6% 13% 26% 24% 8%	2600
July	1st to 10th	Day	29% 5% 11% 23% 27% 5%	5836
	11th to 20th	Day	28% 4 12% 31% 22% 3	5831
	21st to end	Day	26% 5% 12% 33% 22% 1	6240
August	1st to 10th	Day	30% 6% 12% 26% 22% 3	4220
	11th to 20th	Day	28% 5% 17% 27% 19% 4	4210
	21st to end	Day	22% 4 14% 33% 20% 6%	4476
September	1st to 10th	Day	29% 5% 9% 26% 23% 10%	2215
	11th to 20th	Day	28% 4 9% 29% 25% 5%	2200
	21st to end	Day	27% 5% 9% 26% 25% 8%	2200
October	1st to 10th	Day	19% 3 5% 22% 42% 10%	3539
	11th to 20th	Day	18% 2 9% 30% 31% 9%	3240
	21st to end	Day	25% 2 8% 25% 32% 9%	3495
November	1st to 10th	Day	24% 3 8% 29% 28% 7%	1853
	11th to 20th	Day	27% 2 8% 28% 27% 7%	1890
	21st to end	Day	27% 2 10% 29% 27% 5%	1890
December	1st to 10th	Day	19% 3 9% 40% 27% 3	3030
	11th to 20th	Day	25% 3 7% 39% 25% 2	3479
	21st to end	Day	43% 4 7% 30% 14% 2	4587
January	1st to 10th	Night	40% 4 9% 25% 18% 3	5902
	11th to 20th	Night	27% 4 11% 25% 27% 6%	5900
	21st to end	Night	35% 3 8% 23% 25% 6%	6490
February	1st to 10th	Night	38% 3 12% 24% 17% 6%	2562
	11th to 20th	Night	35% 4 11% 26% 18% 5%	2338
	21st to end	Night	29% 3 7% 22% 26% 12%	1980
March	1st to 10th	Night	22% 3 6% 24% 31% 14%	5152
	11th to 20th	Night	23% 4 7% 19% 32% 16%	4980
	21st to end	Night	32% 4 15% 34% 12% 2	5467
April	1st to 10th	Night	30% 4 11% 24% 24% 7%	2185
	11th to 20th	Night	33% 5% 12% 24% 21% 4	1869
	21st to end	Night	36% 6% 12% 23% 20% 4	1800
May	1st to 10th	Night	37% 5% 12% 20% 20% 7%	4080
	11th to 20th	Night	25% 6% 12% 27% 19% 11%	4054
	21st to end	Night	34% 6% 14% 25% 19% 3	4452
June	1st to 10th	Night	21% 7% 19% 31% 18% 4	1800
	11th to 20th	Night	23% 6% 15% 32% 21% 4	1800
	21st to end	Night	21% 6% 14% 30% 23% 8%	1800
July	1st to 10th	Night	25% 4 17% 26% 24% 3	4035
	11th to 20th	Night	26% 4 10% 30% 25% 4	4026
	21st to end	Night	28% 5% 12% 35% 19% 1	4320
August	1st to 10th	Night	25% 4 13% 30% 22% 6%	2916
	11th to 20th	Night	31% 6% 14% 26% 20% 4	2915
	21st to end	Night	20% 4 15% 36% 19% 6%	3427
September	1st to 10th	Night	26% 4 12% 23% 24% 11%	2185
	11th to 20th	Night	28% 4 10% 26% 27% 6%	2200
	21st to end	Night	24% 4 10% 23% 31% 9%	2200
October	1st to 10th	Night	18% 3 5% 16% 48% 10%	3608
	11th to 20th	Night	16% 2 8% 26% 32% 15%	3899
	21st to end	Night	21% 2 6% 25% 38% 9%	4283
November	1st to 10th	Night	22% 3 9% 28% 29% 10%	2665
	11th to 20th	Night	23% 2 7% 30% 30% 7%	2730
	21st to end	Night	28% 3 9% 28% 23% 8%	2730
December	1st to 10th	Night	23% 2 9% 40% 22% 3	4380
	11th to 20th	Night	25% 3 6% 36% 24% 7%	4947
	21st to end	Night	40% 4 9% 30% 14% 4	6633

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	12% 7% 39% 28% 12% 2	303
	11th to 20th	Day	14% 11% 40% 26% 8%	300
	21st to end	Day	10% 9% 45% 30% 6% 1	330
February	1st to 10th	Day	13% 12% 46% 24% 4 1	300
	11th to 20th	Day	14% 10% 45% 22% 7% 1	300
	21st to end	Day	8% 13% 49% 24% 4 1	249
March	1st to 10th	Day	11% 10% 43% 29% 6% 1	300
	11th to 20th	Day	17% 11% 38% 23% 8% 2	300
	21st to end	Day	17% 12% 44% 21% 4 1	330
April	1st to 10th	Day	14% 15% 37% 26% 8%	310
	11th to 20th	Day	14% 13% 40% 30% 3	600
	21st to end	Day	20% 12% 37% 21% 8% 2	700
May	1st to 10th	Day	16% 8% 34% 28% 13% 2	703
	11th to 20th	Day	16% 8% 26% 27% 18% 4	725
	21st to end	Day	17% 6% 28% 29% 16% 4	789
June	1st to 10th	Day	6% 4 22% 37% 23% 9%	730
	11th to 20th	Day	6% 3 27% 40% 19% 4	730
	21st to end	Day	5% 4 28% 43% 18% 2	730
July	1st to 10th	Day	8% 1 12% 40% 31% 9%	721
	11th to 20th	Day	5% 2 13% 42% 30% 8%	718
	21st to end	Day	7% 1 13% 35% 33% 11%	794
August	1st to 10th	Day	8% 2 14% 40% 30% 6%	726
	11th to 20th	Day	8% 2 13% 39% 33% 5%	715
	21st to end	Day	6% 3 13% 38% 32% 8%	683
September	1st to 10th	Day	13% 1 10% 34% 34% 8%	345
	11th to 20th	Day	5% 2 18% 43% 25% 7%	330
	21st to end	Day	14% 3 23% 27% 23% 10%	330
October	1st to 10th	Day	12% 5% 28% 28% 18% 8%	330
	11th to 20th	Day	8% 9% 36% 30% 14% 4	330
	21st to end	Day	21% 9% 29% 26% 12% 3	363
November	1st to 10th	Day	11% 10% 32% 32% 12% 3	330
	11th to 20th	Day	12% 10% 39% 23% 12% 3	330
	21st to end	Day	18% 10% 31% 28% 12% 2	330
December	1st to 10th	Day	6% 8% 39% 33% 12% 2	330
	11th to 20th	Day	11% 11% 39% 28% 10% 1	330
	21st to end	Day	16% 10% 40% 25% 9%	363
January	1st to 10th	Night	10% 9% 38% 35% 7%	1006
	11th to 20th	Night	9% 10% 39% 33% 8% 1	1000
	21st to end	Night	10% 8% 43% 34% 6%	1100
February	1st to 10th	Night	12% 10% 42% 31% 4 1	962
	11th to 20th	Night	13% 11% 40% 33% 3	784
	21st to end	Night	9% 11% 44% 31% 4	660
March	1st to 10th	Night	9% 12% 41% 33% 4	708
	11th to 20th	Night	14% 9% 38% 31% 8%	600
	21st to end	Night	17% 9% 42% 27% 5% 1	660
April	1st to 10th	Night	14% 11% 42% 27% 7%	600
	11th to 20th	Night	15% 12% 39% 28% 5%	593
	21st to end	Night	17% 9% 38% 29% 7% 1	600
May	1st to 10th	Night	17% 8% 34% 33% 8%	600
	11th to 20th	Night	15% 6% 27% 37% 13% 1	591
	21st to end	Night	14% 6% 28% 36% 14% 2	650
June	1st to 10th	Night	7% 4 26% 42% 20% 2	600
	11th to 20th	Night	5% 3 33% 45% 14%	600
	21st to end	Night	4 2 36% 45% 13%	600
July	1st to 10th	Night	7% 1 15% 51% 24% 2	585
	11th to 20th	Night	5% 1 19% 50% 24% 1	582
	21st to end	Night	8% 1 16% 47% 27% 1	657
August	1st to 10th	Night	5% 2 18% 50% 24% 1	597
	11th to 20th	Night	8% 2 14% 52% 23% 2	594
	21st to end	Night	5% 1 16% 46% 29% 3	660
September	1st to 10th	Night	8% 1 22% 47% 21% 2	600
	11th to 20th	Night	8% 2 29% 43% 17% 2	600
	21st to end	Night	8% 4 27% 41% 19% 1	600
October	1st to 10th	Night	11% 5% 29% 35% 19% 1	605
	11th to 20th	Night	11% 6% 39% 34% 9% 1	800
	21st to end	Night	14% 7% 35% 33% 10% 1	899
November	1st to 10th	Night	9% 6% 35% 36% 13%	1025
	11th to 20th	Night	9% 10% 38% 35% 9%	1050
	21st to end	Night	14% 9% 30% 37% 10%	1050
December	1st to 10th	Night	10% 8% 36% 37% 8%	1050
	11th to 20th	Night	13% 9% 41% 33% 4	1050
	21st to end	Night	13% 10% 37% 34% 6%	1155

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