Mark Slade Remote Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mark Slade Remote Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 6% 2 6% 4 4 4 4 6% 8% 42%	131195
	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	130324
Transparency	Day	38% 7% 11% 23% 17% 4	134794
	Night	38% 6% 10% 23% 18% 5%	134795
Seeing	Day	19% 12% 27% 24% 14% 5%	16736
	Night	17% 10% 29% 30% 13% 1	26471

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 4 3 5% 6% 47%	5058
	11th to 20th	Day	13% 8% 2 5% 4 3 3 2 3 4 52%	5280
	21st to end	Day	14% 6% 2 7% 4 4 4 3 4 5% 45%	5780
February	1st to 10th	Day	13% 9% 2 7% 4 5% 4 4 5% 7% 41%	1890
	11th to 20th	Day	19% 7% 2 6% 3 4 3 3 5% 7% 41%	1883
	21st to end	Day	17% 8% 3 7% 5% 4 4 3 5% 6% 38%	1739
March	1st to 10th	Day	17% 6% 2 6% 5% 4 4 4 5% 7% 39%	5045
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 4 6% 51%	5057
	21st to end	Day	16% 6% 2 7% 4 4 5% 4 6% 7% 40%	5498
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 4 5% 7% 46%	2400
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 6% 48%	2374
	21st to end	Day	16% 7% 2 5% 4 5% 4 4 6% 6% 41%	2400
May	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 5% 7% 9% 32%	5807
	11th to 20th	Day	21% 7% 2 6% 4 4 5% 4 7% 9% 32%	5952
	21st to end	Day	16% 5% 2 5% 4 4 5% 4 6% 9% 40%	6934
June	1st to 10th	Day	11% 5% 2 4 5% 5% 5% 4 6% 10% 43%	2800
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 6% 9% 42%	2800
	21st to end	Day	8% 4 2 5% 5% 4 5% 4 7% 10% 45%	2800
July	1st to 10th	Day	7% 4 2 6% 4 5% 6% 5% 8% 11% 42%	6360
	11th to 20th	Day	8% 6% 2 7% 6% 6% 7% 6% 11% 13% 28%	5909
	21st to end	Day	8% 5% 2 7% 6% 5% 6% 4 7% 10% 40%	6424
August	1st to 10th	Day	14% 6% 2 7% 4 4 6% 5% 8% 11% 33%	4234
	11th to 20th	Day	10% 5% 2 7% 5% 6% 5% 5% 8% 9% 38%	4019
	21st to end	Day	10% 4 1 5% 4 4 4 3 5% 9% 49%	4312
September	1st to 10th	Day	14% 5% 2 4 4 4 5% 4 5% 7% 47%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 6% 9% 46%	2395
	21st to end	Day	22% 7% 2 6% 4 4 4 4 6% 6% 37%	2089
October	1st to 10th	Day	15% 5% 2 5% 4 5% 3 3 4 6% 48%	2000
	11th to 20th	Day	16% 4 2 5% 3 3 3 3 4 5% 51%	2000
	21st to end	Day	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2200
November	1st to 10th	Day	12% 4 2 4 5% 3 4 3 5% 7% 53%	2020
	11th to 20th	Day	15% 6% 1 4 3 3 3 4 6% 6% 50%	1858
	21st to end	Day	16% 5% 2 5% 3 4 4 3 4 5% 50%	1680
December	1st to 10th	Day	11% 6% 2 5% 4 3 4 4 6% 6% 50%	2680
	11th to 20th	Day	18% 8% 2 5% 3 3 3 2 5% 6% 46%	3070
	21st to end	Day	17% 7% 3 6% 4 4 4 4 5% 7% 40%	4048
January	1st to 10th	Night	12% 6% 2 4 3 4 5% 4 5% 6% 48%	7831
	11th to 20th	Night	14% 7% 2 5% 3 3 3 2 4 4 54%	7680
	21st to end	Night	13% 6% 2 6% 4 3 4 3 4 5% 49%	8401
February	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 6% 6% 42%	2730
	11th to 20th	Night	20% 7% 2 6% 3 3 3 3 4 6% 43%	2621
	21st to end	Night	15% 7% 3 6% 4 4 3 4 5% 6% 43%	2064
March	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 5% 47%	5028
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 6% 7% 50%	5058
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 6% 46%	5433
April	1st to 10th	Night	15% 7% 3 5% 4 3 4 4 5% 6% 44%	2000
	11th to 20th	Night	12% 5% 2 5% 5% 4 3 3 5% 8% 47%	1975
	21st to end	Night	16% 4 2 5% 4 4 4 4 5% 7% 45%	2000
May	1st to 10th	Night	17% 9% 3 8% 5% 5% 4 3 6% 8% 31%	4131
	11th to 20th	Night	24% 7% 2 6% 4 4 4 4 6% 8% 31%	3926
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 3 6% 9% 36%	3920
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 7% 8% 44%	1600
	11th to 20th	Night	12% 6% 2 5% 5% 5% 5% 4 6% 8% 42%	1600
	21st to end	Night	8% 5% 2 5% 4 5% 5% 5% 8% 10% 44%	1600
July	1st to 10th	Night	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 38%	3598
	11th to 20th	Night	10% 8% 4 9% 6% 6% 6% 6% 7% 10% 28%	4044
	21st to end	Night	7% 6% 3 7% 6% 6% 6% 5% 8% 10% 37%	4427
August	1st to 10th	Night	16% 6% 2 7% 5% 5% 6% 4 7% 9% 33%	2915
	11th to 20th	Night	11% 7% 3 9% 5% 6% 5% 4 7% 8% 35%	2997
	21st to end	Night	11% 6% 3 5% 3 4 3 4 5% 8% 47%	3586
September	1st to 10th	Night	14% 6% 2 6% 3 3 4 4 5% 7% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 5% 7% 49%	2001
	21st to end	Night	21% 7% 3 6% 4 3 3 3 5% 6% 38%	2310
October	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 47%	2400
	11th to 20th	Night	16% 6% 1 6% 3 3 4 3 5% 5% 50%	2400
	21st to end	Night	14% 4 2 5% 3 4 4 3 5% 6% 50%	2640
November	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 4 6% 53%	2497
	11th to 20th	Night	14% 7% 2 6% 3 3 3 2 5% 6% 49%	2756
	21st to end	Night	12% 4 2 5% 4 3 4 4 5% 7% 50%	2940
December	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 54%	4720
	11th to 20th	Night	20% 7% 2 6% 3 4 4 3 4 5% 42%	5345
	21st to end	Night	16% 6% 2 5% 4 4 5% 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	39% 4 9% 23% 19% 7%	5059
	11th to 20th	Day	37% 3 5% 19% 24% 12%	5280
	21st to end	Day	41% 4 6% 29% 17% 3	5780
February	1st to 10th	Day	42% 6% 11% 25% 14% 3	1890
	11th to 20th	Day	44% 5% 8% 22% 18% 4	1892
	21st to end	Day	46% 6% 11% 17% 17% 4	1739
March	1st to 10th	Day	43% 5% 8% 19% 16% 9%	5045
	11th to 20th	Day	33% 5% 8% 24% 22% 8%	5057
	21st to end	Day	43% 6% 11% 17% 17% 7%	5498
April	1st to 10th	Day	37% 6% 11% 20% 18% 7%	2400
	11th to 20th	Day	36% 6% 11% 20% 22% 6%	2374
	21st to end	Day	41% 7% 10% 24% 14% 4	2400
May	1st to 10th	Day	44% 10% 14% 18% 12% 1	5841
	11th to 20th	Day	47% 7% 11% 24% 10% 1	5956
	21st to end	Day	38% 9% 13% 21% 17% 2	6949
June	1st to 10th	Day	35% 9% 14% 25% 16% 1	2800
	11th to 20th	Day	37% 8% 15% 23% 16% 2	2800
	21st to end	Day	32% 10% 14% 25% 18% 1	2800
July	1st to 10th	Day	34% 10% 19% 19% 13% 5%	6290
	11th to 20th	Day	40% 12% 20% 21% 7% 1	5839
	21st to end	Day	37% 7% 12% 23% 20% 1	6240
August	1st to 10th	Day	42% 8% 16% 26% 7%	4221
	11th to 20th	Day	38% 9% 13% 25% 15% 1	4058
	21st to end	Day	31% 7% 15% 27% 18% 2	4312
September	1st to 10th	Day	37% 8% 11% 19% 20% 5%	2400
	11th to 20th	Day	34% 6% 9% 27% 20% 3	2395
	21st to end	Day	46% 7% 10% 18% 16% 3	2089
October	1st to 10th	Day	36% 4 7% 18% 30% 5%	3260
	11th to 20th	Day	27% 3 6% 24% 29% 11%	3240
	21st to end	Day	22% 3 10% 28% 25% 13%	3538
November	1st to 10th	Day	32% 4 10% 25% 24% 5%	2020
	11th to 20th	Day	35% 5% 10% 23% 22% 7%	1858
	21st to end	Day	36% 5% 10% 25% 21% 3	1680
December	1st to 10th	Day	34% 5% 10% 26% 18% 6%	2680
	11th to 20th	Day	40% 4 13% 25% 14% 4	3066
	21st to end	Day	42% 7% 12% 23% 15% 1	4048
January	1st to 10th	Night	35% 4 9% 24% 23% 5%	7832
	11th to 20th	Night	35% 3 5% 22% 25% 10%	7680
	21st to end	Night	38% 3 6% 28% 19% 4	8401
February	1st to 10th	Night	41% 6% 10% 24% 17% 2	2730
	11th to 20th	Night	44% 4 8% 21% 18% 6%	2623
	21st to end	Night	41% 5% 10% 20% 17% 6%	2064
March	1st to 10th	Night	38% 5% 7% 23% 19% 8%	5028
	11th to 20th	Night	32% 5% 9% 22% 25% 7%	5058
	21st to end	Night	38% 5% 9% 22% 20% 6%	5433
April	1st to 10th	Night	39% 7% 10% 17% 22% 5%	2000
	11th to 20th	Night	35% 6% 11% 21% 21% 5%	1975
	21st to end	Night	38% 6% 11% 26% 14% 5%	2000
May	1st to 10th	Night	50% 8% 13% 17% 8% 3	4159
	11th to 20th	Night	49% 7% 12% 20% 11% 1	3927
	21st to end	Night	44% 8% 13% 23% 10% 2	3927
June	1st to 10th	Night	34% 8% 14% 26% 15% 2	1600
	11th to 20th	Night	38% 9% 16% 23% 14% 1	1600
	21st to end	Night	32% 11% 16% 20% 19% 3	1600
July	1st to 10th	Night	40% 10% 18% 17% 14% 2	3560
	11th to 20th	Night	49% 10% 18% 16% 6% 1	3994
	21st to end	Night	38% 10% 14% 22% 16%	4297
August	1st to 10th	Night	46% 9% 15% 22% 7% 1	2904
	11th to 20th	Night	44% 8% 13% 22% 14%	3024
	21st to end	Night	34% 7% 12% 28% 17% 1	3586
September	1st to 10th	Night	37% 8% 11% 22% 17% 6%	2000
	11th to 20th	Night	35% 7% 9% 25% 22% 3	2001
	21st to end	Night	47% 7% 10% 18% 15% 3	2310
October	1st to 10th	Night	41% 4 9% 20% 20% 7%	3920
	11th to 20th	Night	25% 4 8% 22% 31% 10%	3899
	21st to end	Night	22% 4 8% 30% 26% 10%	4252
November	1st to 10th	Night	33% 5% 9% 24% 24% 6%	2497
	11th to 20th	Night	37% 4 8% 22% 23% 7%	2756
	21st to end	Night	34% 5% 10% 25% 22% 5%	2940
December	1st to 10th	Night	31% 4 11% 28% 22% 4	4720
	11th to 20th	Night	45% 4 10% 18% 17% 5%	5348
	21st to end	Night	40% 7% 13% 24% 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	26% 21% 30% 19% 4	292
	11th to 20th	Day	27% 30% 25% 16% 3	330
	21st to end	Day	23% 25% 36% 13% 4	363
February	1st to 10th	Day	24% 26% 31% 15% 4	330
	11th to 20th	Day	35% 24% 30% 10% 1	370
	21st to end	Day	28% 18% 31% 19% 3	446
March	1st to 10th	Day	23% 19% 33% 20% 5%	540
	11th to 20th	Day	24% 22% 31% 17% 5% 1	528
	21st to end	Day	24% 18% 34% 17% 6% 1	550
April	1st to 10th	Day	25% 21% 28% 19% 6% 1	500
	11th to 20th	Day	19% 17% 29% 21% 11% 3	492
	21st to end	Day	21% 13% 33% 22% 10% 1	500
May	1st to 10th	Day	19% 16% 33% 19% 10% 3	503
	11th to 20th	Day	21% 9% 30% 24% 12% 3	527
	21st to end	Day	20% 6% 26% 25% 20% 4	573
June	1st to 10th	Day	12% 5% 28% 31% 17% 7%	530
	11th to 20th	Day	12% 6% 26% 29% 20% 6%	530
	21st to end	Day	9% 5% 24% 36% 21% 5%	530
July	1st to 10th	Day	10% 3 17% 33% 26% 10%	525
	11th to 20th	Day	6% 1 16% 37% 27% 13%	524
	21st to end	Day	12% 3 18% 32% 26% 9%	576
August	1st to 10th	Day	13% 4 20% 27% 27% 10%	526
	11th to 20th	Day	13% 3 16% 32% 26% 10%	519
	21st to end	Day	11% 3 19% 29% 23% 14%	583
September	1st to 10th	Day	15% 5% 19% 28% 17% 15%	530
	11th to 20th	Day	15% 3 18% 30% 23% 11%	530
	21st to end	Day	28% 9% 23% 24% 12% 4	530
October	1st to 10th	Day	20% 5% 23% 26% 17% 8%	530
	11th to 20th	Day	18% 14% 24% 26% 16% 2	530
	21st to end	Day	16% 13% 33% 26% 8% 3	583
November	1st to 10th	Day	15% 12% 33% 25% 12% 3	516
	11th to 20th	Day	20% 11% 36% 21% 10% 2	398
	21st to end	Day	24% 15% 32% 19% 10%	220
December	1st to 10th	Day	21% 21% 33% 19% 5% 1	220
	11th to 20th	Day	20% 16% 35% 20% 8%	220
	21st to end	Day	27% 20% 28% 17% 8%	242
January	1st to 10th	Night	19% 18% 39% 20% 3	840
	11th to 20th	Night	21% 16% 34% 26% 4	840
	21st to end	Night	18% 17% 39% 24% 2	924
February	1st to 10th	Night	20% 13% 39% 24% 3	840
	11th to 20th	Night	25% 19% 37% 18% 1	823
	21st to end	Night	21% 17% 39% 21% 2	692
March	1st to 10th	Night	19% 17% 37% 24% 3	840
	11th to 20th	Night	21% 16% 37% 20% 5% 1	829
	21st to end	Night	18% 17% 38% 22% 5%	880
April	1st to 10th	Night	18% 18% 38% 22% 5%	800
	11th to 20th	Night	15% 14% 35% 28% 9%	791
	21st to end	Night	19% 9% 33% 31% 8%	800
May	1st to 10th	Night	20% 9% 32% 28% 11% 1	647
	11th to 20th	Night	20% 5% 29% 32% 14%	550
	21st to end	Night	15% 3 21% 38% 22% 1	433
June	1st to 10th	Night	10% 3 22% 46% 19% 1	400
	11th to 20th	Night	15% 3 19% 42% 20% 1	400
	21st to end	Night	9% 3 23% 45% 19% 2	400
July	1st to 10th	Night	11% 3 19% 36% 29% 3	390
	11th to 20th	Night	8% 2 13% 39% 34% 5%	570
	21st to end	Night	8% 2 16% 40% 30% 3	657
August	1st to 10th	Night	12% 2 12% 41% 30% 3	597
	11th to 20th	Night	13% 3 12% 42% 28% 3	664
	21st to end	Night	12% 3 14% 37% 29% 5%	880
September	1st to 10th	Night	16% 3 16% 33% 29% 4	800
	11th to 20th	Night	13% 3 17% 34% 30% 3	800
	21st to end	Night	25% 4 19% 33% 18% 1	800
October	1st to 10th	Night	20% 4 20% 31% 22% 4	800
	11th to 20th	Night	18% 8% 29% 29% 16% 1	800
	21st to end	Night	17% 8% 33% 29% 12% 1	880
November	1st to 10th	Night	15% 9% 33% 30% 13% 1	820
	11th to 20th	Night	18% 11% 35% 27% 8%	840
	21st to end	Night	15% 11% 35% 30% 9% 1	840
December	1st to 10th	Night	20% 11% 38% 23% 8%	840
	11th to 20th	Night	20% 10% 37% 28% 5%	840
	21st to end	Night	21% 11% 33% 31% 4	924

Page generated at 2026-03-15 02:23:54. Version ac07

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