The Looking Glass Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Looking Glass Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	13% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	109321
	Night	12% 8% 3 7% 5% 4 4 4 5% 7% 41%	110028
Transparency	Day	42% 8% 11% 21% 15% 3	113091
	Night	40% 7% 11% 21% 16% 4	114396
Seeing	Day	18% 14% 25% 22% 15% 7%	15176
	Night	16% 12% 23% 28% 18% 3	25319

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	14% 13% 6% 11% 4 4 4 3 5% 5% 30%	4056
	11th to 20th	Day	14% 8% 6% 14% 4 3 3 2 3 4 37%	4060
	21st to end	Day	18% 11% 5% 6% 4 3 3 2 4 6% 37%	4556
February	1st to 10th	Day	16% 12% 4 8% 5% 5% 4 3 5% 6% 34%	2000
	11th to 20th	Day	20% 10% 5% 8% 5% 4 4 3 5% 5% 31%	1948
	21st to end	Day	19% 10% 3 9% 4 4 4 3 5% 5% 35%	1650
March	1st to 10th	Day	21% 12% 4 8% 4 4 4 3 5% 6% 29%	3245
	11th to 20th	Day	14% 8% 3 6% 3 4 4 3 4 5% 46%	3590
	21st to end	Day	14% 9% 3 6% 5% 4 4 4 5% 6% 39%	3881
April	1st to 10th	Day	12% 8% 2 6% 6% 4 5% 4 5% 7% 41%	2228
	11th to 20th	Day	13% 8% 3 7% 4 4 4 3 4 7% 43%	2254
	21st to end	Day	13% 7% 2 7% 5% 4 5% 4 6% 7% 40%	2443
May	1st to 10th	Day	17% 9% 3 8% 5% 5% 4 4 6% 7% 32%	4082
	11th to 20th	Day	16% 10% 3 8% 5% 5% 5% 3 6% 6% 34%	4081
	21st to end	Day	10% 7% 3 8% 5% 5% 5% 4 7% 9% 38%	4476
June	1st to 10th	Day	10% 7% 3 7% 5% 6% 6% 6% 7% 9% 35%	2470
	11th to 20th	Day	5% 5% 2 7% 6% 5% 7% 5% 7% 10% 41%	2470
	21st to end	Day	7% 5% 2 6% 6% 6% 5% 6% 8% 11% 37%	2470
July	1st to 10th	Day	6% 4 2 7% 6% 7% 7% 6% 9% 11% 34%	4120
	11th to 20th	Day	9% 6% 3 8% 7% 6% 6% 6% 7% 9% 34%	4120
	21st to end	Day	15% 11% 5% 12% 7% 7% 6% 5% 7% 7% 18%	4466
August	1st to 10th	Day	6% 5% 2 7% 5% 6% 6% 6% 8% 10% 40%	4104
	11th to 20th	Day	7% 5% 2 6% 6% 6% 6% 6% 10% 11% 35%	3835
	21st to end	Day	6% 4 2 6% 5% 5% 5% 5% 7% 8% 47%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2149
	11th to 20th	Day	12% 8% 3 7% 4 5% 5% 4 6% 7% 39%	2090
	21st to end	Day	14% 6% 2 5% 4 5% 4 4 6% 7% 43%	2090
October	1st to 10th	Day	10% 5% 2 6% 3 4 4 3 5% 7% 52%	2090
	11th to 20th	Day	12% 6% 2 5% 3 3 3 3 5% 7% 51%	1986
	21st to end	Day	14% 8% 2 4 3 3 3 4 5% 7% 47%	2000
November	1st to 10th	Day	10% 6% 2 4 3 3 4 3 6% 6% 52%	1762
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 5% 6% 46%	1800
	21st to end	Day	14% 9% 3 6% 5% 4 4 3 4 6% 42%	1800
December	1st to 10th	Day	12% 10% 3 9% 5% 4 4 3 3 4 42%	2930
	11th to 20th	Day	18% 14% 6% 9% 5% 3 5% 3 5% 5% 27%	3373
	21st to end	Day	15% 13% 5% 9% 5% 4 4 4 5% 8% 29%	4466
January	1st to 10th	Night	13% 12% 5% 12% 5% 4 3 3 4 5% 34%	5889
	11th to 20th	Night	17% 9% 4 8% 5% 5% 3 2 4 5% 38%	5900
	21st to end	Night	15% 11% 4 7% 5% 4 3 2 4 5% 40%	6398
February	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 4 6% 6% 34%	2400
	11th to 20th	Night	22% 9% 4 9% 5% 4 4 2 5% 5% 32%	2338
	21st to end	Night	19% 9% 4 8% 4 5% 4 4 5% 7% 32%	1980
March	1st to 10th	Night	20% 13% 3 6% 5% 4 4 3 5% 6% 30%	3826
	11th to 20th	Night	11% 7% 2 6% 4 4 4 3 5% 6% 47%	3590
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 3 5% 6% 40%	3890
April	1st to 10th	Night	11% 6% 2 5% 4 4 4 5% 6% 7% 45%	1967
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 5% 7% 47%	1875
	21st to end	Night	11% 6% 2 7% 4 5% 5% 3 5% 8% 45%	1734
May	1st to 10th	Night	12% 8% 3 8% 6% 5% 4 4 6% 9% 36%	2813
	11th to 20th	Night	14% 9% 3 7% 4 4 4 4 6% 7% 38%	2812
	21st to end	Night	5% 5% 2 7% 5% 5% 5% 4 8% 9% 42%	3079
June	1st to 10th	Night	6% 6% 2 7% 5% 6% 6% 5% 7% 8% 43%	1710
	11th to 20th	Night	3 3 1 5% 4 5% 5% 4 7% 12% 52%	1710
	21st to end	Night	5% 4 3 6% 5% 5% 5% 6% 7% 11% 43%	1710
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 9% 11% 40%	2840
	11th to 20th	Night	8% 5% 2 5% 4 3 5% 5% 6% 11% 47%	2840
	21st to end	Night	11% 10% 4 12% 7% 7% 6% 5% 6% 8% 24%	3087
August	1st to 10th	Night	5% 4 2 6% 4 5% 5% 4 7% 9% 48%	2825
	11th to 20th	Night	6% 5% 2 6% 4 5% 4 4 7% 10% 47%	2976
	21st to end	Night	5% 3 1 4 3 3 3 3 6% 9% 59%	3476
September	1st to 10th	Night	10% 6% 3 7% 4 5% 5% 3 6% 8% 44%	2010
	11th to 20th	Night	11% 6% 2 7% 4 3 4 4 5% 8% 46%	2090
	21st to end	Night	11% 5% 3 5% 3 4 3 3 4 6% 51%	2090
October	1st to 10th	Night	10% 4 2 4 3 2 3 3 4 6% 58%	2090
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 6% 52%	2192
	21st to end	Night	12% 8% 2 6% 3 3 3 3 4 6% 50%	2593
November	1st to 10th	Night	11% 5% 2 5% 4 4 3 3 5% 6% 51%	2536
	11th to 20th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 46%	2600
	21st to end	Night	12% 10% 3 7% 4 3 3 3 5% 7% 42%	2600
December	1st to 10th	Night	14% 12% 4 9% 5% 4 3 3 4 4 37%	4250
	11th to 20th	Night	17% 14% 6% 10% 4 3 3 3 4 4 33%	4822
	21st to end	Night	13% 10% 6% 9% 5% 6% 5% 4 6% 7% 28%	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	53% 5% 9% 18% 13% 1	4057
	11th to 20th	Day	50% 5% 5% 16% 22% 2	4060
	21st to end	Day	48% 5% 8% 23% 15% 2	4556
February	1st to 10th	Day	52% 5% 9% 16% 16% 2	2000
	11th to 20th	Day	55% 6% 7% 17% 12% 3	1958
	21st to end	Day	50% 5% 8% 15% 16% 5%	1650
March	1st to 10th	Day	55% 6% 8% 14% 11% 6%	3245
	11th to 20th	Day	40% 5% 9% 25% 16% 6%	3590
	21st to end	Day	45% 7% 9% 25% 14% 1	3881
April	1st to 10th	Day	41% 7% 11% 21% 16% 3	2228
	11th to 20th	Day	41% 6% 8% 18% 24% 3	2254
	21st to end	Day	41% 8% 12% 25% 13% 1	2443
May	1st to 10th	Day	48% 10% 10% 22% 10% 1	4120
	11th to 20th	Day	47% 10% 12% 23% 8%	4085
	21st to end	Day	39% 11% 13% 20% 16% 2	4489
June	1st to 10th	Day	42% 11% 14% 19% 12% 2	2470
	11th to 20th	Day	35% 11% 17% 20% 15% 2	2470
	21st to end	Day	35% 12% 17% 18% 13% 4	2470
July	1st to 10th	Day	37% 15% 19% 18% 9% 1	4055
	11th to 20th	Day	42% 11% 16% 23% 6% 2	4051
	21st to end	Day	62% 10% 11% 12% 5% 1	4324
August	1st to 10th	Day	34% 12% 17% 27% 9%	4091
	11th to 20th	Day	33% 12% 19% 22% 13% 1	3874
	21st to end	Day	30% 10% 14% 33% 11% 2	4179
September	1st to 10th	Day	42% 9% 12% 17% 16% 5%	2149
	11th to 20th	Day	41% 8% 13% 20% 16% 2	2090
	21st to end	Day	38% 7% 11% 21% 18% 5%	2090
October	1st to 10th	Day	24% 6% 9% 19% 32% 10%	3480
	11th to 20th	Day	23% 4 9% 23% 30% 11%	3278
	21st to end	Day	27% 6% 11% 25% 21% 10%	3276
November	1st to 10th	Day	32% 5% 9% 23% 26% 5%	1762
	11th to 20th	Day	39% 5% 7% 22% 23% 6%	1800
	21st to end	Day	43% 5% 8% 21% 20% 3	1800
December	1st to 10th	Day	45% 6% 5% 20% 22% 2	2930
	11th to 20th	Day	58% 5% 8% 16% 11% 2	3370
	21st to end	Day	51% 6% 14% 19% 9% 1	4466
January	1st to 10th	Night	48% 7% 10% 19% 15% 1	5889
	11th to 20th	Night	48% 6% 6% 17% 19% 5%	5900
	21st to end	Night	47% 4 7% 17% 20% 4	6398
February	1st to 10th	Night	49% 7% 10% 17% 15% 2	2400
	11th to 20th	Night	55% 5% 7% 17% 12% 4	2339
	21st to end	Night	52% 6% 7% 16% 15% 4	1980
March	1st to 10th	Night	52% 8% 11% 11% 14% 4	3826
	11th to 20th	Night	36% 6% 8% 26% 15% 9%	3590
	21st to end	Night	43% 7% 11% 23% 15% 1	3890
April	1st to 10th	Night	36% 7% 12% 24% 19% 3	1967
	11th to 20th	Night	37% 6% 10% 20% 22% 6%	1875
	21st to end	Night	38% 7% 12% 28% 13% 1	1734
May	1st to 10th	Night	43% 11% 12% 24% 8% 1	2840
	11th to 20th	Night	42% 10% 15% 20% 13% 1	2813
	21st to end	Night	32% 13% 15% 23% 16% 1	3087
June	1st to 10th	Night	34% 11% 15% 24% 13% 2	1710
	11th to 20th	Night	24% 10% 22% 28% 15% 2	1710
	21st to end	Night	32% 12% 16% 24% 13% 4	1710
July	1st to 10th	Night	32% 13% 23% 22% 8% 1	2795
	11th to 20th	Night	30% 9% 21% 29% 8% 2	2786
	21st to end	Night	54% 13% 14% 14% 6% 1	2988
August	1st to 10th	Night	28% 10% 16% 32% 12% 1	2814
	11th to 20th	Night	30% 10% 22% 25% 12% 1	3003
	21st to end	Night	22% 7% 14% 42% 14% 1	3476
September	1st to 10th	Night	37% 8% 14% 19% 17% 6%	2010
	11th to 20th	Night	36% 8% 13% 20% 21% 2	2090
	21st to end	Night	34% 7% 10% 22% 23% 4	2090
October	1st to 10th	Night	22% 8% 10% 20% 30% 11%	3480
	11th to 20th	Night	22% 4 7% 25% 27% 15%	3639
	21st to end	Night	24% 4 9% 24% 27% 11%	4267
November	1st to 10th	Night	34% 5% 8% 21% 24% 7%	2536
	11th to 20th	Night	37% 5% 7% 20% 23% 7%	2600
	21st to end	Night	43% 4 7% 23% 19% 4	2600
December	1st to 10th	Night	50% 6% 6% 16% 21% 2	4250
	11th to 20th	Night	54% 5% 7% 19% 14% 2	4824
	21st to end	Night	50% 9% 15% 17% 9%	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	28% 30% 29% 12% 1	300
	11th to 20th	Day	35% 29% 23% 11% 2 1	300
	21st to end	Day	25% 30% 34% 9% 2	330
February	1st to 10th	Day	30% 27% 30% 12% 1	300
	11th to 20th	Day	43% 26% 26% 4 1 1	300
	21st to end	Day	39% 25% 29% 6% 1	249
March	1st to 10th	Day	30% 23% 33% 12% 1	312
	11th to 20th	Day	26% 27% 28% 15% 4	500
	21st to end	Day	25% 24% 30% 16% 5%	530
April	1st to 10th	Day	26% 20% 25% 20% 9% 1	460
	11th to 20th	Day	20% 19% 32% 18% 9% 1	452
	21st to end	Day	21% 15% 33% 18% 12% 1	460
May	1st to 10th	Day	19% 17% 33% 18% 9% 4	463
	11th to 20th	Day	15% 12% 26% 26% 13% 7%	487
	21st to end	Day	14% 9% 22% 28% 16% 11%	529
June	1st to 10th	Day	12% 9% 23% 28% 17% 12%	490
	11th to 20th	Day	5% 5% 18% 31% 28% 13%	490
	21st to end	Day	8% 5% 24% 30% 21% 12%	490
July	1st to 10th	Day	9% 4 16% 28% 24% 19%	485
	11th to 20th	Day	9% 7% 15% 26% 25% 18%	484
	21st to end	Day	12% 7% 13% 25% 29% 14%	532
August	1st to 10th	Day	13% 4 19% 27% 22% 15%	486
	11th to 20th	Day	10% 4 19% 32% 25% 10%	479
	21st to end	Day	11% 5% 10% 26% 31% 18%	539
September	1st to 10th	Day	10% 5% 15% 27% 30% 12%	490
	11th to 20th	Day	14% 4 14% 28% 28% 11%	490
	21st to end	Day	18% 5% 18% 28% 24% 8%	490
October	1st to 10th	Day	11% 9% 22% 27% 23% 8%	490
	11th to 20th	Day	16% 14% 28% 23% 17% 3	490
	21st to end	Day	19% 13% 30% 24% 12% 2	449
November	1st to 10th	Day	16% 14% 37% 24% 7% 2	300
	11th to 20th	Day	20% 18% 39% 20% 3	300
	21st to end	Day	19% 21% 36% 17% 6% 1	300
December	1st to 10th	Day	30% 21% 33% 12% 3	300
	11th to 20th	Day	27% 23% 28% 16% 5%	300
	21st to end	Day	32% 22% 32% 12% 3 1	330
January	1st to 10th	Night	22% 24% 30% 20% 4	800
	11th to 20th	Night	27% 23% 28% 19% 4	800
	21st to end	Night	21% 19% 38% 21% 2	880
February	1st to 10th	Night	22% 17% 35% 23% 3	800
	11th to 20th	Night	29% 21% 32% 18% 1	783
	21st to end	Night	27% 23% 30% 19% 1	660
March	1st to 10th	Night	21% 17% 33% 25% 4	800
	11th to 20th	Night	20% 20% 31% 23% 5% 1	800
	21st to end	Night	23% 15% 29% 25% 8%	863
April	1st to 10th	Night	16% 18% 28% 27% 10%	637
	11th to 20th	Night	17% 12% 30% 29% 12% 1	563
	21st to end	Night	13% 10% 32% 34% 10% 1	570
May	1st to 10th	Night	13% 10% 28% 31% 16% 2	570
	11th to 20th	Night	11% 9% 20% 35% 21% 4	561
	21st to end	Night	7% 4 18% 34% 32% 6%	617
June	1st to 10th	Night	7% 6% 15% 38% 29% 5%	570
	11th to 20th	Night	3 2 15% 36% 37% 7%	570
	21st to end	Night	6% 3 12% 39% 32% 8%	570
July	1st to 10th	Night	11% 2 7% 31% 40% 9%	555
	11th to 20th	Night	7% 4 8% 27% 44% 10%	552
	21st to end	Night	8% 2 9% 31% 41% 9%	624
August	1st to 10th	Night	8% 3 8% 29% 44% 8%	567
	11th to 20th	Night	7% 3 11% 34% 40% 5%	564
	21st to end	Night	10% 2 7% 28% 41% 12%	627
September	1st to 10th	Night	9% 3 12% 32% 37% 8%	680
	11th to 20th	Night	13% 3 12% 32% 34% 7%	760
	21st to end	Night	14% 4 17% 30% 31% 4	760
October	1st to 10th	Night	12% 7% 18% 27% 30% 7%	760
	11th to 20th	Night	15% 9% 24% 31% 18% 2	760
	21st to end	Night	14% 12% 26% 31% 15% 2	836
November	1st to 10th	Night	13% 12% 21% 35% 17% 3	780
	11th to 20th	Night	14% 18% 29% 27% 11% 1	800
	21st to end	Night	19% 17% 26% 28% 10%	800
December	1st to 10th	Night	23% 20% 28% 24% 5%	800
	11th to 20th	Night	21% 17% 28% 27% 6% 1	800
	21st to end	Night	26% 17% 26% 25% 5%	880

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