Robinson Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Robinson Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	7% 6% 2 7% 6% 6% 6% 5% 7% 10% 37%	126230
	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 43%	123485
Transparency	Day	39% 9% 16% 19% 13% 4	129925
	Night	36% 9% 16% 20% 13% 5%	127859
Seeing	Day	9% 7% 20% 26% 22% 17%	16946
	Night	9% 5% 21% 33% 25% 7%	26999

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% 6% 1 4 4 4 4 3 5% 7% 54%	4056
	11th to 20th	Day	18% 9% 3 9% 5% 5% 4 4 5% 6% 31%	4060
	21st to end	Day	14% 6% 2 6% 4 3 4 3 6% 9% 44%	4512
February	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 3 6% 7% 42%	2000
	11th to 20th	Day	7% 5% 3 6% 5% 4 3 4 5% 6% 51%	1948
	21st to end	Day	9% 6% 3 6% 5% 5% 4 4 6% 7% 46%	1793
March	1st to 10th	Day	7% 6% 3 8% 5% 5% 5% 4 6% 9% 42%	4935
	11th to 20th	Day	4 3 1 5% 4 3 3 3 5% 9% 60%	4980
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 7% 9% 44%	5465
April	1st to 10th	Day	6% 4 2 5% 5% 5% 6% 4 7% 8% 47%	2200
	11th to 20th	Day	8% 6% 2 5% 6% 5% 6% 5% 7% 9% 42%	2246
	21st to end	Day	4 5% 2 6% 4 4 6% 5% 8% 8% 49%	2400
May	1st to 10th	Day	4 3 2 6% 5% 6% 6% 5% 8% 11% 44%	5635
	11th to 20th	Day	8% 4 2 6% 5% 5% 6% 5% 6% 9% 43%	5861
	21st to end	Day	4 4 2 6% 5% 6% 6% 5% 8% 12% 42%	6433
June	1st to 10th	Day	12% 8% 3 9% 7% 7% 7% 6% 7% 10% 23%	2600
	11th to 20th	Day	6% 6% 3 9% 9% 9% 9% 6% 8% 10% 26%	2600
	21st to end	Day	9% 7% 3 10% 7% 8% 7% 6% 8% 9% 26%	2600
July	1st to 10th	Day	6% 6% 3 10% 8% 8% 10% 7% 10% 13% 18%	5900
	11th to 20th	Day	4 5% 3 8% 7% 8% 9% 7% 10% 13% 28%	5900
	21st to end	Day	4 5% 2 8% 7% 7% 8% 7% 9% 12% 31%	6424
August	1st to 10th	Day	4 7% 3 10% 7% 8% 8% 7% 9% 12% 25%	4234
	11th to 20th	Day	3 5% 2 8% 7% 8% 8% 6% 9% 12% 32%	4022
	21st to end	Day	7% 6% 3 11% 8% 7% 7% 6% 9% 13% 22%	3955
September	1st to 10th	Day	8% 7% 3 11% 8% 9% 9% 6% 9% 8% 22%	2200
	11th to 20th	Day	11% 8% 3 9% 7% 8% 8% 6% 9% 10% 22%	2200
	21st to end	Day	9% 8% 4 11% 7% 6% 7% 5% 7% 9% 28%	2200
October	1st to 10th	Day	13% 8% 3 10% 6% 6% 6% 5% 6% 8% 29%	2145
	11th to 20th	Day	7% 6% 2 7% 5% 5% 4 5% 7% 8% 45%	1980
	21st to end	Day	8% 5% 2 6% 5% 6% 6% 4 7% 8% 45%	2090
November	1st to 10th	Day	8% 6% 2 6% 4 6% 6% 5% 7% 8% 41%	1957
	11th to 20th	Day	9% 6% 2 7% 5% 6% 6% 4 6% 7% 41%	2000
	21st to end	Day	9% 7% 3 8% 5% 5% 4 4 6% 8% 42%	1930
December	1st to 10th	Day	5% 4 2 5% 4 4 4 3 7% 8% 55%	2930
	11th to 20th	Day	9% 8% 3 7% 5% 5% 6% 5% 7% 8% 36%	3373
	21st to end	Day	10% 7% 3 9% 5% 5% 6% 4 7% 8% 37%	4466
January	1st to 10th	Night	8% 5% 2 5% 3 4 4 3 5% 7% 54%	5889
	11th to 20th	Night	17% 10% 4 8% 5% 4 5% 4 6% 7% 30%	5900
	21st to end	Night	14% 7% 2 6% 4 4 4 4 6% 8% 42%	6426
February	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Night	10% 5% 2 6% 4 4 4 3 6% 7% 50%	2338
	21st to end	Night	8% 4 2 5% 4 3 4 3 5% 7% 54%	1835
March	1st to 10th	Night	11% 7% 2 6% 4 4 4 3 5% 7% 46%	4918
	11th to 20th	Night	5% 3 1 4 3 3 3 4 7% 10% 57%	4980
	21st to end	Night	8% 6% 2 6% 4 4 4 4 6% 7% 48%	5467
April	1st to 10th	Night	7% 5% 1 5% 4 3 4 3 6% 9% 54%	2200
	11th to 20th	Night	6% 4 2 7% 4 4 4 4 7% 7% 51%	2100
	21st to end	Night	5% 4 2 5% 4 4 4 4 5% 8% 56%	2000
May	1st to 10th	Night	4 3 2 6% 5% 4 5% 4 6% 10% 51%	4288
	11th to 20th	Night	7% 6% 2 6% 4 4 4 4 7% 8% 48%	4032
	21st to end	Night	4 4 2 6% 4 4 5% 4 7% 10% 49%	4421
June	1st to 10th	Night	9% 8% 3 8% 6% 5% 5% 6% 9% 8% 34%	1800
	11th to 20th	Night	5% 7% 3 8% 6% 7% 6% 6% 9% 11% 31%	1800
	21st to end	Night	7% 7% 3 9% 6% 5% 5% 5% 9% 11% 33%	1800
July	1st to 10th	Night	7% 7% 3 11% 6% 7% 7% 6% 9% 12% 24%	4058
	11th to 20th	Night	4 5% 2 7% 5% 7% 7% 7% 9% 13% 34%	4057
	21st to end	Night	4 6% 3 8% 6% 7% 7% 6% 7% 13% 33%	4427
August	1st to 10th	Night	4 6% 2 8% 7% 7% 7% 5% 8% 12% 34%	2915
	11th to 20th	Night	3 5% 3 8% 5% 6% 6% 6% 8% 13% 37%	2992
	21st to end	Night	8% 5% 2 8% 6% 6% 7% 6% 9% 12% 32%	3937
September	1st to 10th	Night	8% 6% 3 8% 7% 7% 7% 6% 9% 10% 30%	2200
	11th to 20th	Night	10% 8% 4 10% 6% 5% 5% 5% 8% 9% 29%	2200
	21st to end	Night	9% 7% 3 9% 5% 5% 6% 4 8% 9% 35%	2200
October	1st to 10th	Night	10% 7% 2 7% 5% 6% 5% 5% 6% 9% 38%	2252
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 5% 8% 51%	2377
	21st to end	Night	8% 4 2 5% 4 4 5% 3 6% 8% 50%	2508
November	1st to 10th	Night	9% 5% 2 6% 4 4 5% 5% 6% 7% 46%	2341
	11th to 20th	Night	9% 7% 2 6% 5% 5% 4 3 7% 8% 45%	2400
	21st to end	Night	8% 6% 3 6% 5% 5% 5% 4 5% 8% 45%	2465
December	1st to 10th	Night	5% 3 1 5% 4 4 4 4 6% 8% 58%	4250
	11th to 20th	Night	9% 7% 3 7% 5% 4 5% 3 5% 7% 44%	4822
	21st to end	Night	11% 8% 3 8% 5% 4 5% 4 6% 7% 38%	6490

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	30% 4 9% 17% 28% 12%	4057
	11th to 20th	Day	53% 4 5% 16% 15% 6%	4060
	21st to end	Day	38% 4 7% 27% 17% 7%	4512
February	1st to 10th	Day	40% 6% 10% 15% 21% 8%	2000
	11th to 20th	Day	33% 5% 9% 19% 23% 10%	1958
	21st to end	Day	36% 5% 10% 16% 23% 9%	1793
March	1st to 10th	Day	37% 7% 12% 17% 21% 6%	4935
	11th to 20th	Day	23% 5% 9% 24% 21% 18%	4980
	21st to end	Day	34% 5% 10% 26% 16% 8%	5465
April	1st to 10th	Day	33% 6% 12% 23% 18% 8%	2200
	11th to 20th	Day	36% 8% 10% 17% 21% 8%	2246
	21st to end	Day	30% 6% 12% 21% 22% 9%	2400
May	1st to 10th	Day	28% 10% 21% 24% 14% 4	5667
	11th to 20th	Day	35% 9% 15% 28% 12% 1	5865
	21st to end	Day	31% 9% 18% 29% 13% 1	6447
June	1st to 10th	Day	52% 11% 17% 15% 5%	2600
	11th to 20th	Day	48% 13% 20% 17% 2	2600
	21st to end	Day	51% 13% 20% 12% 3 1	2600
July	1st to 10th	Day	50% 16% 24% 9% 1	5835
	11th to 20th	Day	41% 15% 32% 12%	5831
	21st to end	Day	39% 15% 25% 20% 1	6240
August	1st to 10th	Day	45% 15% 27% 11% 2	4221
	11th to 20th	Day	40% 15% 22% 15% 7%	4061
	21st to end	Day	49% 14% 23% 12% 2	3955
September	1st to 10th	Day	54% 15% 17% 9% 4	2200
	11th to 20th	Day	52% 14% 19% 12% 3	2200
	21st to end	Day	51% 10% 15% 12% 10% 2	2200
October	1st to 10th	Day	58% 9% 14% 9% 7% 4	3496
	11th to 20th	Day	30% 6% 10% 21% 27% 6%	3220
	21st to end	Day	28% 4 9% 21% 30% 7%	3428
November	1st to 10th	Day	38% 6% 11% 17% 20% 8%	1957
	11th to 20th	Day	41% 6% 8% 19% 19% 7%	2000
	21st to end	Day	39% 7% 8% 20% 19% 7%	1930
December	1st to 10th	Day	26% 6% 15% 27% 17% 9%	2930
	11th to 20th	Day	41% 8% 12% 15% 18% 6%	3370
	21st to end	Day	43% 6% 11% 20% 14% 5%	4466
January	1st to 10th	Night	30% 4 8% 20% 26% 12%	5889
	11th to 20th	Night	52% 6% 9% 16% 14% 4	5900
	21st to end	Night	39% 6% 9% 22% 16% 8%	6426
February	1st to 10th	Night	36% 5% 12% 17% 18% 12%	2400
	11th to 20th	Night	33% 6% 11% 20% 20% 10%	2339
	21st to end	Night	29% 5% 14% 20% 24% 8%	1835
March	1st to 10th	Night	37% 6% 11% 20% 18% 7%	4918
	11th to 20th	Night	21% 6% 17% 23% 16% 17%	4980
	21st to end	Night	32% 7% 12% 25% 20% 5%	5467
April	1st to 10th	Night	26% 7% 18% 26% 16% 8%	2200
	11th to 20th	Night	30% 7% 11% 24% 21% 8%	2100
	21st to end	Night	25% 7% 13% 26% 22% 7%	2000
May	1st to 10th	Night	27% 8% 19% 30% 12% 4	4318
	11th to 20th	Night	32% 9% 20% 28% 10% 1	4033
	21st to end	Night	27% 9% 20% 31% 13%	4429
June	1st to 10th	Night	43% 11% 19% 22% 4	1800
	11th to 20th	Night	43% 14% 20% 22% 2	1800
	21st to end	Night	41% 14% 24% 16% 5%	1800
July	1st to 10th	Night	48% 15% 25% 12% 1	4015
	11th to 20th	Night	36% 17% 37% 10% 1	4006
	21st to end	Night	38% 14% 25% 21% 1	4297
August	1st to 10th	Night	39% 14% 33% 11% 2	2904
	11th to 20th	Night	36% 13% 28% 17% 7%	3019
	21st to end	Night	41% 15% 30% 12% 2	3937
September	1st to 10th	Night	46% 17% 20% 13% 4	2200
	11th to 20th	Night	48% 12% 20% 16% 3	2200
	21st to end	Night	43% 10% 18% 18% 9% 2	2200
October	1st to 10th	Night	54% 9% 13% 13% 7% 3	3681
	11th to 20th	Night	31% 7% 9% 25% 24% 4	3876
	21st to end	Night	29% 6% 10% 21% 29% 6%	4120
November	1st to 10th	Night	35% 7% 11% 20% 19% 8%	2341
	11th to 20th	Night	37% 7% 11% 20% 18% 8%	2400
	21st to end	Night	36% 6% 9% 22% 19% 7%	2465
December	1st to 10th	Night	24% 5% 17% 30% 12% 12%	4250
	11th to 20th	Night	38% 7% 14% 19% 14% 9%	4824
	21st to end	Night	42% 7% 13% 21% 12% 5%	6490

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	14% 14% 43% 22% 6% 1	300
	11th to 20th	Day	8% 16% 41% 24% 8% 3	300
	21st to end	Day	16% 18% 42% 19% 5%	330
February	1st to 10th	Day	13% 15% 35% 20% 15% 2	300
	11th to 20th	Day	11% 11% 36% 29% 9% 4	300
	21st to end	Day	10% 14% 29% 29% 18% 1	392
March	1st to 10th	Day	5% 14% 36% 31% 13% 1	500
	11th to 20th	Day	10% 12% 30% 28% 17% 3	500
	21st to end	Day	8% 9% 32% 29% 19% 3	550
April	1st to 10th	Day	8% 12% 32% 28% 16% 4	500
	11th to 20th	Day	8% 9% 31% 29% 18% 6%	492
	21st to end	Day	6% 5% 26% 33% 19% 12%	500
May	1st to 10th	Day	6% 6% 23% 33% 22% 11%	503
	11th to 20th	Day	9% 5% 19% 31% 21% 14%	527
	21st to end	Day	7% 3 12% 25% 32% 21%	573
June	1st to 10th	Day	14% 2 12% 26% 27% 18%	530
	11th to 20th	Day	8% 3 10% 23% 25% 31%	530
	21st to end	Day	7% 1 7% 20% 28% 37%	530
July	1st to 10th	Day	6% 2 4 13% 29% 45%	525
	11th to 20th	Day	5% 1 4 16% 30% 44%	524
	21st to end	Day	7% 2 3 19% 26% 43%	576
August	1st to 10th	Day	3 2 8% 17% 34% 35%	526
	11th to 20th	Day	4 5% 15% 28% 48%	519
	21st to end	Day	10% 2 8% 21% 26% 33%	583
September	1st to 10th	Day	10% 2 8% 19% 27% 33%	530
	11th to 20th	Day	10% 4 10% 28% 24% 25%	530
	21st to end	Day	9% 3 10% 25% 30% 23%	530
October	1st to 10th	Day	14% 5% 21% 31% 22% 7%	530
	11th to 20th	Day	7% 7% 22% 30% 28% 7%	522
	21st to end	Day	8% 5% 26% 35% 22% 5%	539
November	1st to 10th	Day	11% 7% 22% 32% 24% 5%	495
	11th to 20th	Day	12% 8% 26% 34% 16% 4	500
	21st to end	Day	10% 9% 27% 33% 18% 2	430
December	1st to 10th	Day	5% 13% 38% 29% 12% 2	300
	11th to 20th	Day	12% 12% 31% 34% 10% 1	300
	21st to end	Day	8% 14% 32% 34% 9% 2	330
January	1st to 10th	Night	12% 11% 33% 32% 11% 1	800
	11th to 20th	Night	11% 9% 29% 38% 12% 1	800
	21st to end	Night	15% 11% 35% 31% 8%	880
February	1st to 10th	Night	14% 7% 33% 32% 14% 1	800
	11th to 20th	Night	13% 8% 33% 35% 10% 1	783
	21st to end	Night	9% 8% 36% 38% 8%	660
March	1st to 10th	Night	6% 8% 38% 37% 10% 1	800
	11th to 20th	Night	8% 10% 28% 39% 14% 1	800
	21st to end	Night	7% 8% 34% 37% 13% 1	880
April	1st to 10th	Night	7% 8% 33% 39% 13% 1	800
	11th to 20th	Night	7% 8% 31% 38% 16% 1	791
	21st to end	Night	6% 5% 29% 41% 18% 1	800
May	1st to 10th	Night	5% 4 22% 47% 21% 1	715
	11th to 20th	Night	8% 2 16% 41% 29% 4	591
	21st to end	Night	7% 2 9% 37% 40% 4	650
June	1st to 10th	Night	11% 2 9% 33% 39% 7%	600
	11th to 20th	Night	6% 2 7% 25% 39% 20%	600
	21st to end	Night	8% 2 6% 20% 47% 18%	600
July	1st to 10th	Night	8% 2 3 17% 43% 26%	585
	11th to 20th	Night	4 2 4 14% 48% 28%	582
	21st to end	Night	5% 4 16% 51% 24%	657
August	1st to 10th	Night	5% 1 5% 18% 48% 24%	597
	11th to 20th	Night	3 1 3 17% 48% 27%	659
	21st to end	Night	11% 1 7% 23% 39% 19%	880
September	1st to 10th	Night	9% 2 7% 23% 38% 20%	800
	11th to 20th	Night	12% 4 10% 27% 37% 11%	800
	21st to end	Night	12% 2 7% 31% 38% 10%	800
October	1st to 10th	Night	13% 3 16% 36% 26% 6%	800
	11th to 20th	Night	7% 4 17% 34% 33% 4	793
	21st to end	Night	10% 3 18% 39% 27% 3	836
November	1st to 10th	Night	10% 5% 22% 34% 27% 2	780
	11th to 20th	Night	11% 7% 23% 39% 19% 2	800
	21st to end	Night	11% 5% 27% 40% 15% 1	800
December	1st to 10th	Night	6% 4 28% 39% 21% 2	800
	11th to 20th	Night	12% 5% 27% 36% 18% 1	800
	21st to end	Night	11% 6% 29% 37% 16% 1	880

Page generated at 2025-10-16 01:50:26. Version ac07

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