Robinson Lunar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robinson Lunar Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 7% 3 6% 4 4 4 3 5% 7% 46%	131373
	Night	11% 7% 2 6% 4 3 4 3 5% 6% 48%	131080
Transparency	Day	37% 4 12% 27% 18% 3	134985
	Night	36% 4 11% 27% 19% 3	135545
Seeing	Day	15% 9% 29% 28% 16% 4	17551
	Night	14% 8% 32% 33% 12% 1	27549

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% 9% 3 9% 4 4 3 3 4 5% 41%	5289
	11th to 20th	Day	6% 7% 4 9% 7% 6% 5% 3 4 5% 43%	5310
	21st to end	Day	13% 13% 4 7% 4 4 3 2 3 5% 40%	5812
February	1st to 10th	Day	14% 10% 4 8% 5% 3 3 2 4 7% 39%	1890
	11th to 20th	Day	12% 10% 3 7% 4 5% 3 3 4 6% 41%	1844
	21st to end	Day	10% 9% 3 6% 4 3 4 4 6% 6% 46%	1788
March	1st to 10th	Day	14% 9% 2 5% 3 2 2 2 3 5% 52%	5046
	11th to 20th	Day	10% 6% 2 4 3 3 3 3 4 6% 55%	5090
	21st to end	Day	13% 10% 4 7% 5% 5% 5% 4 6% 6% 36%	5583
April	1st to 10th	Day	16% 7% 3 6% 5% 3 3 2 5% 6% 45%	2415
	11th to 20th	Day	14% 9% 2 6% 5% 4 4 4 5% 6% 41%	2446
	21st to end	Day	19% 9% 2 6% 5% 5% 5% 3 4 6% 36%	2560
May	1st to 10th	Day	16% 7% 3 6% 4 4 4 3 4 6% 45%	5862
	11th to 20th	Day	15% 5% 2 6% 4 4 4 3 5% 7% 46%	5862
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 41%	6520
June	1st to 10th	Day	7% 4 2 5% 4 5% 5% 4 7% 10% 49%	2800
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 48%	2800
	21st to end	Day	7% 3 1 5% 4 5% 5% 4 6% 8% 52%	2764
July	1st to 10th	Day	9% 5% 2 6% 4 4 4 4 7% 9% 46%	5900
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 47%	5900
	21st to end	Day	7% 5% 2 6% 4 4 6% 5% 6% 10% 45%	6422
August	1st to 10th	Day	5% 3 2 5% 4 4 5% 3 6% 9% 53%	4234
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 6% 8% 48%	4171
	21st to end	Day	6% 4 2 5% 3 4 4 3 5% 8% 56%	4422
September	1st to 10th	Day	13% 4 1 5% 3 3 4 3 5% 7% 53%	2400
	11th to 20th	Day	9% 5% 3 5% 4 4 5% 4 5% 8% 48%	2200
	21st to end	Day	9% 4 2 5% 4 3 3 3 5% 6% 55%	2196
October	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 5% 6% 54%	2000
	11th to 20th	Day	12% 6% 2 4 3 3 2 3 4 4 57%	2000
	21st to end	Day	16% 7% 3 5% 3 3 3 2 4 5% 48%	2200
November	1st to 10th	Day	10% 6% 2 5% 3 4 3 3 5% 6% 55%	2038
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 4 6% 52%	1829
	21st to end	Day	16% 9% 3 6% 3 3 2 2 3 6% 47%	1680
December	1st to 10th	Day	6% 6% 2 6% 5% 4 3 2 5% 6% 55%	2690
	11th to 20th	Day	11% 7% 3 7% 4 4 3 3 4 5% 49%	3088
	21st to end	Day	19% 17% 6% 11% 4 3 4 3 4 5% 25%	4322
January	1st to 10th	Night	12% 10% 4 13% 5% 4 3 3 3 4 39%	7634
	11th to 20th	Night	7% 8% 3 8% 4 5% 4 3 4 5% 49%	7680
	21st to end	Night	14% 10% 4 9% 4 4 4 3 4 5% 39%	8402
February	1st to 10th	Night	11% 11% 3 7% 6% 4 4 3 4 7% 40%	2730
	11th to 20th	Night	12% 9% 3 7% 4 4 4 3 4 6% 43%	2663
	21st to end	Night	12% 6% 3 6% 5% 3 4 3 5% 5% 48%	2035
March	1st to 10th	Night	15% 7% 3 5% 3 3 3 3 3 4 51%	5029
	11th to 20th	Night	9% 7% 1 4 3 2 3 3 5% 7% 57%	5090
	21st to end	Night	15% 8% 3 6% 4 4 4 4 5% 8% 40%	5588
April	1st to 10th	Night	12% 6% 2 5% 4 4 4 3 5% 6% 48%	2205
	11th to 20th	Night	14% 7% 2 6% 4 3 4 2 5% 6% 47%	2035
	21st to end	Night	17% 7% 2 6% 4 4 4 4 4 6% 41%	1835
May	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 5% 7% 44%	4053
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 4 7% 51%	4053
	21st to end	Night	17% 5% 2 5% 3 3 3 3 4 6% 50%	4352
June	1st to 10th	Night	6% 3 2 4 4 3 5% 4 6% 10% 55%	1600
	11th to 20th	Night	6% 2 2 6% 4 4 5% 5% 6% 9% 52%	1600
	21st to end	Night	5% 3 1 4 2 3 4 4 6% 8% 61%	1628
July	1st to 10th	Night	8% 5% 2 5% 4 3 4 3 5% 8% 51%	4078
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 6% 10% 54%	4077
	21st to end	Night	8% 4 2 5% 4 4 5% 4 8% 11% 46%	4449
August	1st to 10th	Night	2 1 1 3 3 4 4 5% 6% 10% 60%	2926
	11th to 20th	Night	10% 6% 2 5% 4 3 4 3 4 8% 52%	2888
	21st to end	Night	4 3 1 5% 4 3 4 3 6% 7% 59%	3477
September	1st to 10th	Night	10% 5% 2 4 3 3 3 3 4 5% 57%	2000
	11th to 20th	Night	8% 4 2 4 3 4 4 3 5% 7% 55%	2195
	21st to end	Night	6% 4 2 5% 3 4 3 4 4 6% 59%	2200
October	1st to 10th	Night	11% 4 1 4 3 3 3 3 5% 4 59%	2383
	11th to 20th	Night	14% 4 2 4 2 3 3 2 4 6% 57%	2400
	21st to end	Night	15% 7% 3 5% 3 3 3 3 3 4 51%	2640
November	1st to 10th	Night	9% 5% 2 4 3 3 3 3 4 6% 61%	2476
	11th to 20th	Night	11% 6% 2 4 4 3 3 3 5% 6% 55%	2788
	21st to end	Night	15% 8% 2 5% 3 2 3 3 4 5% 51%	2940
December	1st to 10th	Night	7% 6% 2 6% 3 3 4 3 5% 5% 56%	4720
	11th to 20th	Night	12% 7% 3 7% 3 3 4 3 5% 6% 47%	5339
	21st to end	Night	19% 15% 5% 9% 4 4 3 3 3 4 31%	6892

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	46% 4 10% 28% 11% 1	5290
	11th to 20th	Day	44% 3 11% 23% 14% 5%	5310
	21st to end	Day	49% 3 11% 19% 16% 2	5812
February	1st to 10th	Day	46% 3 13% 24% 11% 2	1890
	11th to 20th	Day	45% 3 12% 23% 15% 2	1853
	21st to end	Day	37% 4 11% 26% 19% 4	1788
March	1st to 10th	Day	37% 3 9% 21% 23% 7%	5046
	11th to 20th	Day	31% 4 9% 21% 29% 6%	5090
	21st to end	Day	47% 6% 14% 26% 7% 1	5585
April	1st to 10th	Day	41% 3 12% 23% 18% 2	2415
	11th to 20th	Day	43% 4 13% 23% 16% 1	2446
	21st to end	Day	49% 4 11% 24% 11% 1	2560
May	1st to 10th	Day	41% 6% 10% 26% 16% 1	5900
	11th to 20th	Day	38% 5% 10% 24% 20% 3	5866
	21st to end	Day	42% 5% 9% 29% 12% 1	6534
June	1st to 10th	Day	29% 6% 16% 30% 16% 2	2800
	11th to 20th	Day	31% 7% 14% 30% 17% 1	2800
	21st to end	Day	28% 6% 15% 30% 17% 4	2764
July	1st to 10th	Day	34% 6% 13% 27% 19% 2	5836
	11th to 20th	Day	32% 6% 13% 29% 17% 2	5831
	21st to end	Day	31% 7% 14% 35% 11% 2	6240
August	1st to 10th	Day	28% 5% 14% 29% 22% 3	4220
	11th to 20th	Day	35% 5% 17% 26% 17% 1	4210
	21st to end	Day	27% 5% 15% 32% 15% 6%	4421
September	1st to 10th	Day	32% 4 11% 27% 19% 6%	2400
	11th to 20th	Day	34% 5% 10% 27% 21% 3	2200
	21st to end	Day	30% 4 9% 30% 24% 3	2196
October	1st to 10th	Day	19% 2 7% 23% 42% 6%	3260
	11th to 20th	Day	21% 2 4 28% 36% 9%	3240
	21st to end	Day	33% 3 11% 25% 25% 3	3538
November	1st to 10th	Day	30% 4 12% 31% 19% 5%	2038
	11th to 20th	Day	34% 3 9% 29% 21% 3	1829
	21st to end	Day	41% 2 9% 27% 20% 1	1680
December	1st to 10th	Day	31% 3 13% 36% 17% 1	2690
	11th to 20th	Day	39% 3 12% 34% 12% 1	3085
	21st to end	Day	62% 3 11% 16% 6% 1	4322
January	1st to 10th	Night	50% 3 12% 23% 11% 1	7635
	11th to 20th	Night	38% 3 10% 27% 19% 3	7680
	21st to end	Night	48% 3 12% 18% 15% 4	8402
February	1st to 10th	Night	46% 3 11% 22% 15% 3	2730
	11th to 20th	Night	43% 3 12% 23% 16% 3	2664
	21st to end	Night	37% 3 8% 23% 22% 6%	2035
March	1st to 10th	Night	37% 3 7% 23% 26% 4	5029
	11th to 20th	Night	28% 3 7% 23% 32% 8%	5090
	21st to end	Night	43% 6% 15% 25% 9% 2	5588
April	1st to 10th	Night	36% 4 11% 24% 21% 4	2205
	11th to 20th	Night	39% 4 9% 27% 17% 4	2035
	21st to end	Night	44% 5% 11% 24% 15% 1	1835
May	1st to 10th	Night	39% 6% 11% 25% 17% 1	4080
	11th to 20th	Night	35% 5% 10% 25% 19% 6%	4054
	21st to end	Night	35% 5% 10% 30% 19% 1	4360
June	1st to 10th	Night	24% 7% 17% 33% 16% 3	1600
	11th to 20th	Night	26% 7% 15% 32% 19% 2	1600
	21st to end	Night	20% 5% 14% 36% 22% 4	1628
July	1st to 10th	Night	32% 5% 12% 25% 23% 3	4035
	11th to 20th	Night	26% 5% 15% 32% 20% 2	4026
	21st to end	Night	30% 5% 16% 36% 12% 1	4320
August	1st to 10th	Night	18% 6% 20% 29% 23% 5%	2916
	11th to 20th	Night	32% 4 13% 30% 20% 2	2915
	21st to end	Night	25% 4 12% 37% 17% 5%	3477
September	1st to 10th	Night	30% 5% 8% 28% 21% 8%	2000
	11th to 20th	Night	29% 4 11% 28% 25% 4	2195
	21st to end	Night	27% 4 7% 27% 31% 4	2200
October	1st to 10th	Night	19% 2 6% 19% 47% 6%	3903
	11th to 20th	Night	21% 2 5% 28% 31% 13%	3899
	21st to end	Night	35% 3 6% 29% 25% 2	4252
November	1st to 10th	Night	27% 3 11% 29% 24% 6%	2476
	11th to 20th	Night	31% 3 10% 28% 23% 5%	2788
	21st to end	Night	37% 2 9% 30% 19% 3	2940
December	1st to 10th	Night	30% 4 13% 34% 18% 1	4720
	11th to 20th	Night	38% 3 13% 28% 15% 2	5341
	21st to end	Night	58% 4 9% 22% 7% 1	6892

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	22% 12% 38% 22% 5%	330
	11th to 20th	Day	21% 19% 38% 18% 2	330
	21st to end	Day	18% 12% 42% 25% 3	363
February	1st to 10th	Day	20% 19% 40% 18% 2 1	330
	11th to 20th	Day	22% 18% 40% 15% 6%	330
	21st to end	Day	16% 23% 43% 13% 5%	273
March	1st to 10th	Day	21% 15% 43% 17% 3	330
	11th to 20th	Day	28% 13% 35% 20% 4 1	330
	21st to end	Day	30% 19% 34% 13% 4 1	363
April	1st to 10th	Day	27% 13% 35% 18% 6%	435
	11th to 20th	Day	20% 14% 37% 21% 7% 1	516
	21st to end	Day	24% 13% 34% 20% 9%	660
May	1st to 10th	Day	20% 11% 35% 25% 8% 1	703
	11th to 20th	Day	21% 7% 27% 28% 14% 3	725
	21st to end	Day	18% 7% 29% 25% 17% 4	789
June	1st to 10th	Day	7% 4 22% 35% 25% 8%	730
	11th to 20th	Day	9% 3 26% 36% 20% 6%	730
	21st to end	Day	8% 5% 34% 36% 16% 2	730
July	1st to 10th	Day	9% 3 16% 32% 28% 12%	721
	11th to 20th	Day	6% 1 19% 38% 28% 8%	718
	21st to end	Day	9% 2 15% 35% 31% 8%	794
August	1st to 10th	Day	7% 1 20% 39% 28% 5%	726
	11th to 20th	Day	10% 3 15% 37% 28% 7%	715
	21st to end	Day	9% 2 14% 33% 33% 10%	654
September	1st to 10th	Day	14% 2 15% 37% 25% 7%	530
	11th to 20th	Day	6% 2 21% 42% 23% 6%	330
	21st to end	Day	14% 6% 23% 35% 16% 6%	330
October	1st to 10th	Day	15% 5% 28% 27% 18% 7%	330
	11th to 20th	Day	12% 15% 38% 26% 8% 1	330
	21st to end	Day	22% 15% 36% 19% 7% 1	363
November	1st to 10th	Day	16% 13% 38% 24% 5% 4	330
	11th to 20th	Day	16% 17% 33% 23% 8% 2	330
	21st to end	Day	25% 13% 33% 21% 8%	330
December	1st to 10th	Day	9% 15% 42% 25% 6% 2	330
	11th to 20th	Day	20% 15% 37% 21% 8% 1	330
	21st to end	Day	21% 20% 37% 19% 3	363
January	1st to 10th	Night	13% 12% 38% 30% 7%	1050
	11th to 20th	Night	16% 13% 39% 26% 5% 1	1050
	21st to end	Night	16% 12% 40% 29% 4	1155
February	1st to 10th	Night	19% 11% 40% 26% 3	1050
	11th to 20th	Night	17% 13% 39% 27% 4	1029
	21st to end	Night	17% 11% 43% 24% 5%	651
March	1st to 10th	Night	17% 12% 42% 23% 5%	630
	11th to 20th	Night	21% 9% 36% 28% 6%	630
	21st to end	Night	25% 11% 37% 23% 4	693
April	1st to 10th	Night	15% 13% 40% 27% 5%	630
	11th to 20th	Night	20% 12% 34% 29% 5%	611
	21st to end	Night	21% 7% 37% 29% 6% 1	600
May	1st to 10th	Night	17% 6% 33% 35% 8% 1	600
	11th to 20th	Night	18% 4 28% 35% 14% 1	591
	21st to end	Night	12% 5% 28% 36% 16% 2	650
June	1st to 10th	Night	6% 3 22% 44% 24% 1	600
	11th to 20th	Night	6% 3 26% 45% 19% 2	600
	21st to end	Night	6% 3 32% 44% 15% 1	600
July	1st to 10th	Night	8% 13% 44% 31% 3	585
	11th to 20th	Night	4 1 15% 47% 30% 3	582
	21st to end	Night	7% 1 12% 47% 29% 3	657
August	1st to 10th	Night	5% 1 14% 49% 29% 2	597
	11th to 20th	Night	8% 2 12% 45% 31% 2	594
	21st to end	Night	6% 1 12% 45% 32% 2	660
September	1st to 10th	Night	13% 2 17% 42% 24% 3	600
	11th to 20th	Night	10% 2 20% 41% 25% 3	600
	21st to end	Night	9% 4 18% 49% 18% 2	600
October	1st to 10th	Night	12% 4 29% 32% 21% 1	783
	11th to 20th	Night	16% 7% 39% 30% 8%	800
	21st to end	Night	20% 10% 33% 28% 9%	880
November	1st to 10th	Night	12% 9% 34% 33% 11% 1	836
	11th to 20th	Night	14% 13% 37% 29% 7%	1050
	21st to end	Night	19% 11% 34% 30% 6%	1050
December	1st to 10th	Night	15% 11% 36% 31% 6% 1	1050
	11th to 20th	Night	19% 8% 40% 27% 5%	1050
	21st to end	Night	20% 13% 35% 28% 5%	1155

Page generated at 2026-04-14 01:44:27. Version ac07

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