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 2024-10-24.

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 7% 5% 5% 5% 4 6% 7% 37%	100827
	Night	12% 8% 3 7% 5% 4 4 4 6% 7% 41%	97892
Transparency	Day	43% 8% 12% 21% 14% 2	100639
	Night	41% 8% 12% 21% 15% 3	97749
Seeing	Day	18% 14% 24% 22% 15% 7%	14694
	Night	16% 12% 23% 28% 18% 3	24701

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% 14% 5% 10% 5% 4 4 4 5% 6% 28%	2836
	11th to 20th	Day	18% 10% 5% 14% 5% 4 4 3 4 4 30%	2840
	21st to end	Day	20% 14% 6% 6% 4 3 4 2 4 5% 31%	3186
February	1st to 10th	Day	17% 11% 4 8% 5% 4 4 3 5% 6% 33%	1900
	11th to 20th	Day	20% 10% 5% 8% 5% 4 4 3 5% 5% 32%	1848
	21st to end	Day	20% 10% 3 9% 5% 4 4 3 4 5% 34%	1570
March	1st to 10th	Day	21% 12% 4 8% 4 4 4 3 5% 6% 29%	3144
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 4 5% 46%	3480
	21st to end	Day	15% 9% 3 6% 5% 4 5% 4 5% 6% 38%	3815
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 3 5% 7% 47%	1925
November	1st to 10th	Day	10% 6% 2 5% 3 3 4 3 5% 6% 54%	1672
	11th to 20th	Day	13% 6% 3 6% 4 3 4 3 5% 6% 47%	1710
	21st to end	Day	15% 9% 3 6% 5% 4 3 3 4 6% 43%	1710
December	1st to 10th	Day	18% 12% 4 9% 5% 4 4 3 3 5% 34%	1710
	11th to 20th	Day	14% 12% 4 7% 5% 4 6% 4 6% 6% 33%	2153
	21st to end	Day	16% 14% 4 7% 4 4 4 3 5% 8% 30%	3124
January	1st to 10th	Night	15% 13% 5% 9% 5% 4 4 3 5% 6% 31%	4109
	11th to 20th	Night	18% 11% 4 8% 5% 5% 3 3 4 4 34%	4120
	21st to end	Night	18% 14% 4 8% 4 4 4 2 4 5% 32%	4468
February	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 4 6% 6% 34%	2280
	11th to 20th	Night	21% 9% 4 8% 5% 4 4 3 5% 5% 33%	2218
	21st to end	Night	20% 9% 4 8% 4 5% 4 4 4 6% 30%	1884
March	1st to 10th	Night	20% 13% 3 6% 5% 4 4 4 5% 6% 30%	3707
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 5% 6% 47%	3480
	21st to end	Night	14% 8% 3 7% 5% 5% 5% 3 5% 6% 39%	3817
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 2 4 6% 50%	2493
November	1st to 10th	Night	10% 5% 2 5% 4 4 3 3 5% 5% 53%	2406
	11th to 20th	Night	11% 7% 2 6% 4 4 4 4 5% 7% 47%	2470
	21st to end	Night	13% 10% 3 7% 4 3 3 3 4 7% 43%	2470
December	1st to 10th	Night	17% 12% 4 8% 5% 4 3 4 5% 5% 32%	2470
	11th to 20th	Night	14% 11% 5% 10% 5% 3 4 3 5% 5% 36%	3042
	21st to end	Night	14% 11% 6% 9% 6% 6% 6% 5% 6% 7% 25%	4532

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% 10% 17% 13% 2	2837
	11th to 20th	Day	57% 6% 6% 15% 16% 1	2840
	21st to end	Day	54% 5% 8% 20% 12% 1	3186
February	1st to 10th	Day	53% 5% 8% 16% 16% 2	1900
	11th to 20th	Day	54% 5% 7% 18% 13% 3	1858
	21st to end	Day	52% 5% 9% 14% 15% 5%	1570
March	1st to 10th	Day	55% 6% 8% 14% 11% 6%	3144
	11th to 20th	Day	40% 5% 9% 26% 16% 5%	3480
	21st to end	Day	45% 7% 9% 24% 13% 1	3815
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	32% 6% 10% 20% 25% 6%	2090
	11th to 20th	Day	32% 6% 10% 20% 24% 7%	1986
	21st to end	Day	36% 6% 9% 24% 21% 5%	1925
November	1st to 10th	Day	31% 5% 9% 23% 27% 5%	1672
	11th to 20th	Day	38% 5% 7% 22% 22% 6%	1710
	21st to end	Day	42% 5% 8% 20% 21% 3	1710
December	1st to 10th	Day	54% 5% 6% 20% 13% 2	1710
	11th to 20th	Day	49% 7% 9% 17% 16% 2	2150
	21st to end	Day	51% 5% 12% 19% 10% 2	3124
January	1st to 10th	Night	49% 8% 10% 18% 14% 2	4109
	11th to 20th	Night	52% 5% 7% 14% 17% 5%	4120
	21st to end	Night	55% 4 8% 14% 18% 1	4468
February	1st to 10th	Night	49% 6% 10% 18% 15% 2	2280
	11th to 20th	Night	53% 5% 7% 18% 13% 5%	2219
	21st to end	Night	54% 6% 7% 14% 14% 4	1884
March	1st to 10th	Night	52% 8% 11% 11% 14% 3	3707
	11th to 20th	Night	36% 6% 8% 26% 14% 9%	3480
	21st to end	Night	43% 7% 11% 23% 15% 1	3817
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	28% 5% 12% 21% 27% 8%	2090
	11th to 20th	Night	34% 6% 9% 21% 22% 8%	2192
	21st to end	Night	37% 4 6% 22% 24% 8%	2493
November	1st to 10th	Night	34% 5% 8% 21% 25% 8%	2406
	11th to 20th	Night	36% 5% 7% 20% 23% 8%	2470
	21st to end	Night	42% 4 7% 23% 19% 4	2470
December	1st to 10th	Night	53% 6% 8% 18% 12% 3	2470
	11th to 20th	Night	48% 6% 8% 17% 18% 3	3044
	21st to end	Night	53% 9% 13% 14% 11% 1	4532

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% 29% 30% 12% 1	270
	11th to 20th	Day	35% 29% 22% 11% 2 1	270
	21st to end	Day	25% 31% 33% 8% 2	297
February	1st to 10th	Day	31% 25% 30% 11% 1	270
	11th to 20th	Day	41% 27% 26% 4 1 1	270
	21st to end	Day	42% 25% 28% 4 1	225
March	1st to 10th	Day	30% 23% 34% 12% 1	281
	11th to 20th	Day	25% 27% 27% 16% 4	460
	21st to end	Day	25% 23% 31% 16% 5%	506
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	21% 13% 29% 22% 14% 2	422
November	1st to 10th	Day	16% 13% 36% 26% 7% 2	270
	11th to 20th	Day	19% 17% 40% 21% 3	270
	21st to end	Day	18% 21% 37% 17% 6% 1	270
December	1st to 10th	Day	31% 21% 31% 12% 4	270
	11th to 20th	Day	25% 24% 27% 17% 6%	270
	21st to end	Day	31% 23% 33% 11% 2 1	297
January	1st to 10th	Night	21% 24% 31% 20% 4	760
	11th to 20th	Night	27% 23% 27% 18% 4	760
	21st to end	Night	21% 19% 37% 21% 2	836
February	1st to 10th	Night	22% 17% 35% 23% 3	760
	11th to 20th	Night	28% 20% 32% 18% 1	743
	21st to end	Night	28% 24% 29% 18% 1	628
March	1st to 10th	Night	21% 17% 33% 25% 4	760
	11th to 20th	Night	20% 20% 31% 23% 5% 1	760
	21st to end	Night	23% 15% 29% 25% 8%	836
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	15% 12% 26% 31% 14% 2	805
November	1st to 10th	Night	12% 11% 21% 36% 17% 3	740
	11th to 20th	Night	14% 18% 29% 27% 11% 1	760
	21st to end	Night	18% 17% 25% 29% 10%	760
December	1st to 10th	Night	24% 20% 27% 24% 6%	760
	11th to 20th	Night	20% 17% 28% 28% 6% 1	760
	21st to end	Night	27% 17% 27% 25% 4	836

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