LJR Observatory
Clear Sky Chart
History

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

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	5% 5% 2 5% 4 4 4 4 5% 7% 57%	140134
	Night	6% 6% 2 5% 4 4 4 3 5% 7% 53%	138419
Transparency	Day	26% 8% 12% 22% 20% 12%	134988
	Night	28% 9% 13% 23% 18% 9%	134570
Seeing	Day	10% 13% 23% 29% 19% 7%	16587
	Night	9% 8% 20% 36% 24% 3	25815

By Month

Percentage Cloud Cover SW (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% 9% 3 9% 7% 7% 7% 6% 8% 9% 27%	4722
	11th to 20th	Day	9% 7% 3 6% 4 3 3 3 5% 7% 50%	5082
	21st to end	Day	6% 4 2 5% 4 5% 5% 5% 8% 9% 46%	5843
February	1st to 10th	Day	5% 7% 3 6% 6% 6% 7% 6% 9% 9% 35%	1890
	11th to 20th	Day	7% 8% 2 7% 7% 5% 5% 5% 8% 10% 36%	1845
	21st to end	Day	5% 11% 3 7% 6% 5% 6% 5% 7% 9% 36%	1717
March	1st to 10th	Day	9% 13% 3 9% 5% 5% 5% 4 6% 7% 34%	4836
	11th to 20th	Day	6% 8% 2 6% 5% 5% 5% 4 7% 8% 43%	5000
	21st to end	Day	6% 9% 3 8% 6% 7% 6% 5% 7% 9% 34%	5570
April	1st to 10th	Day	7% 8% 3 8% 6% 6% 6% 4 6% 8% 38%	2400
	11th to 20th	Day	8% 9% 2 6% 5% 4 5% 3 5% 9% 44%	2369
	21st to end	Day	6% 8% 2 6% 5% 4 3 4 5% 8% 49%	2460
May	1st to 10th	Day	6% 5% 2 5% 2 2 2 2 4 6% 64%	5939
	11th to 20th	Day	4 5% 1 4 2 3 3 3 5% 7% 63%	6277
	21st to end	Day	5% 3 1 3 2 2 3 2 4 7% 68%	6902
June	1st to 10th	Day	7% 5% 2 4 4 4 4 4 5% 8% 53%	2800
	11th to 20th	Day	4 4 1 3 2 3 3 3 5% 6% 66%	2800
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 74%	2800
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 4 82%	6319
	11th to 20th	Day	1 1 2 1 2 2 2 3 3 83%	6320
	21st to end	Day	1 1 2 2 2 4 3 5% 5% 74%	6501
August	1st to 10th	Day	2 2 1 2 2 2 3 3 4 5% 75%	5777
	11th to 20th	Day	2 2 2 2 2 3 2 3 4 79%	5765
	21st to end	Day	2 2 1 2 2 2 3 3 4 5% 76%	5984
September	1st to 10th	Day	1 1 2 1 1 2 2 3 5% 82%	2400
	11th to 20th	Day	3 3 1 3 2 3 3 3 4 6% 67%	2400
	21st to end	Day	3 2 1 3 2 3 3 2 4 7% 69%	2106
October	1st to 10th	Day	3 2 1 3 2 1 3 2 4 5% 74%	3280
	11th to 20th	Day	3 3 1 4 4 4 3 3 6% 9% 59%	3261
	21st to end	Day	8% 4 2 6% 4 5% 6% 4 7% 8% 47%	3558
November	1st to 10th	Day	9% 7% 3 8% 6% 5% 5% 5% 5% 8% 39%	1904
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 7% 12% 32%	1727
	21st to end	Day	6% 8% 2 7% 6% 5% 5% 5% 7% 11% 38%	1680
December	1st to 10th	Day	4 4 2 6% 5% 5% 6% 5% 7% 10% 46%	2700
	11th to 20th	Day	9% 8% 3 8% 7% 7% 8% 8% 10% 10% 22%	3108
	21st to end	Day	11% 10% 3 12% 8% 8% 8% 6% 8% 9% 17%	4092
January	1st to 10th	Night	8% 11% 3 9% 7% 7% 8% 5% 7% 9% 26%	8172
	11th to 20th	Night	8% 7% 3 6% 3 4 4 3 4 7% 52%	7879
	21st to end	Night	8% 7% 2 6% 5% 6% 6% 4 6% 7% 43%	8371
February	1st to 10th	Night	6% 9% 3 7% 6% 4 5% 3 7% 10% 40%	2730
	11th to 20th	Night	7% 9% 3 7% 5% 6% 5% 5% 7% 8% 38%	2662
	21st to end	Night	5% 9% 2 7% 5% 5% 6% 4 7% 10% 40%	2088
March	1st to 10th	Night	11% 11% 3 8% 5% 5% 5% 4 7% 8% 35%	5098
	11th to 20th	Night	7% 8% 2 5% 4 4 4 4 6% 9% 47%	5000
	21st to end	Night	6% 10% 3 8% 7% 7% 7% 5% 7% 7% 31%	5403
April	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 5% 6% 7% 41%	2000
	11th to 20th	Night	6% 9% 2 5% 4 4 3 3 5% 7% 51%	1979
	21st to end	Night	10% 8% 2 5% 3 2 4 3 5% 8% 52%	1935
May	1st to 10th	Night	8% 5% 2 4 2 2 2 2 3 5% 67%	4036
	11th to 20th	Night	6% 6% 2 4 2 2 3 2 4 5% 63%	3616
	21st to end	Night	6% 5% 1 3 2 2 2 2 3 5% 68%	3972
June	1st to 10th	Night	7% 5% 1 5% 3 4 3 5% 7% 6% 54%	1600
	11th to 20th	Night	6% 4 1 3 2 2 2 1 3 6% 71%	1600
	21st to end	Night	2 2 1 3 1 2 1 2 3 4 78%	1600
July	1st to 10th	Night	1 1 2 2 2 2 1 2 3 84%	3640
	11th to 20th	Night	2 1 1 2 1 2 2 1 2 2 84%	3640
	21st to end	Night	1 1 1 2 2 2 2 3 3 5% 79%	4351
August	1st to 10th	Night	1 2 2 1 1 2 1 4 5% 79%	4035
	11th to 20th	Night	2 2 1 2 1 1 2 2 2 4 82%	4070
	21st to end	Night	6% 2 1 2 2 2 2 2 3 3 76%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 2 3 5% 84%	2000
	11th to 20th	Night	4 3 1 2 3 2 2 2 4 6% 72%	2000
	21st to end	Night	4 2 1 3 2 3 2 2 4 6% 71%	2290
October	1st to 10th	Night	3 2 1 2 2 2 2 2 3 4 77%	3920
	11th to 20th	Night	5% 3 1 3 2 4 3 3 5% 6% 64%	3897
	21st to end	Night	6% 5% 1 4 3 4 4 5% 7% 8% 52%	4253
November	1st to 10th	Night	8% 8% 3 7% 5% 5% 5% 4 6% 8% 42%	2614
	11th to 20th	Night	11% 6% 2 6% 5% 5% 6% 6% 7% 10% 36%	2891
	21st to end	Night	6% 8% 2 6% 5% 4 5% 4 6% 10% 44%	2940
December	1st to 10th	Night	5% 6% 1 4 4 4 4 4 5% 10% 52%	4700
	11th to 20th	Night	7% 8% 3 9% 7% 7% 8% 6% 9% 11% 27%	5316
	21st to end	Night	12% 10% 3 9% 6% 6% 6% 5% 7% 10% 27%	7106

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	42% 14% 20% 20% 4	4723
	11th to 20th	Day	29% 6% 11% 27% 23% 5%	5082
	21st to end	Day	30% 9% 14% 20% 19% 8%	5843
February	1st to 10th	Day	39% 13% 15% 19% 11% 3	1890
	11th to 20th	Day	39% 12% 16% 20% 11% 2	1854
	21st to end	Day	43% 11% 14% 17% 11% 3	1717
March	1st to 10th	Day	48% 9% 12% 21% 10% 1	4836
	11th to 20th	Day	35% 10% 17% 20% 18% 1	5000
	21st to end	Day	44% 12% 14% 22% 7% 1	5570
April	1st to 10th	Day	45% 9% 16% 17% 11% 2	2400
	11th to 20th	Day	39% 9% 14% 20% 14% 4	2372
	21st to end	Day	35% 9% 14% 24% 15% 2	2460
May	1st to 10th	Day	22% 5% 10% 28% 30% 4	5939
	11th to 20th	Day	21% 8% 13% 23% 28% 7%	6280
	21st to end	Day	18% 7% 12% 44% 18% 1	6904
June	1st to 10th	Day	30% 9% 14% 28% 18% 2	2800
	11th to 20th	Day	21% 6% 10% 30% 26% 7%	2800
	21st to end	Day	15% 4 7% 26% 33% 16%	2800
July	1st to 10th	Day	8% 3 5% 19% 23% 42%	6251
	11th to 20th	Day	8% 3 3 11% 26% 48%	6245
	21st to end	Day	11% 6% 8% 23% 27% 26%	6313
August	1st to 10th	Day	13% 4 5% 13% 26% 39%	4206
	11th to 20th	Day	9% 4 7% 24% 38% 18%	4176
	21st to end	Day	9% 4 6% 26% 36% 19%	4311
September	1st to 10th	Day	7% 3 6% 17% 30% 37%	2400
	11th to 20th	Day	18% 7% 8% 21% 33% 13%	2400
	21st to end	Day	16% 5% 9% 24% 27% 18%	2106
October	1st to 10th	Day	14% 4 11% 16% 23% 31%	3280
	11th to 20th	Day	20% 7% 13% 36% 18% 5%	3261
	21st to end	Day	26% 13% 16% 20% 17% 7%	3558
November	1st to 10th	Day	39% 9% 15% 20% 14% 4	1904
	11th to 20th	Day	40% 12% 17% 16% 11% 4	1727
	21st to end	Day	34% 10% 16% 19% 15% 6%	1680
December	1st to 10th	Day	26% 10% 14% 29% 19% 3	2700
	11th to 20th	Day	44% 16% 21% 12% 7% 1	3108
	21st to end	Day	51% 14% 17% 12% 5% 1	4092
January	1st to 10th	Night	43% 16% 19% 17% 4 1	8172
	11th to 20th	Night	27% 10% 10% 26% 24% 4	7879
	21st to end	Night	37% 7% 12% 18% 18% 8%	8371
February	1st to 10th	Night	40% 10% 15% 17% 13% 4	2730
	11th to 20th	Night	41% 13% 14% 19% 11% 3	2664
	21st to end	Night	38% 12% 18% 17% 13% 3	2088
March	1st to 10th	Night	45% 11% 13% 19% 9% 1	5098
	11th to 20th	Night	29% 12% 17% 22% 17% 3	5000
	21st to end	Night	42% 16% 18% 18% 6%	5403
April	1st to 10th	Night	41% 11% 18% 19% 8% 2	2000
	11th to 20th	Night	35% 10% 19% 20% 12% 5%	1979
	21st to end	Night	34% 7% 20% 27% 10% 2	1935
May	1st to 10th	Night	22% 6% 11% 29% 28% 3	4033
	11th to 20th	Night	24% 7% 13% 31% 21% 3	3616
	21st to end	Night	18% 7% 19% 44% 12%	3972
June	1st to 10th	Night	31% 7% 18% 33% 10% 1	1600
	11th to 20th	Night	18% 6% 13% 38% 21% 4	1600
	21st to end	Night	11% 5% 9% 33% 34% 8%	1600
July	1st to 10th	Night	7% 3 6% 22% 24% 38%	3600
	11th to 20th	Night	7% 4 3 13% 33% 41%	3592
	21st to end	Night	10% 4 8% 26% 31% 21%	4227
August	1st to 10th	Night	10% 3 5% 21% 24% 37%	2929
	11th to 20th	Night	6% 3 7% 34% 34% 17%	2947
	21st to end	Night	14% 3 6% 29% 31% 16%	3608
September	1st to 10th	Night	7% 3 6% 18% 31% 35%	2000
	11th to 20th	Night	16% 5% 9% 25% 32% 14%	2000
	21st to end	Night	17% 5% 10% 24% 24% 20%	2290
October	1st to 10th	Night	13% 6% 9% 14% 30% 29%	3920
	11th to 20th	Night	20% 7% 12% 40% 15% 6%	3897
	21st to end	Night	23% 9% 16% 26% 18% 7%	4253
November	1st to 10th	Night	39% 9% 15% 18% 13% 6%	2614
	11th to 20th	Night	39% 12% 16% 16% 12% 4	2891
	21st to end	Night	34% 10% 15% 17% 17% 7%	2940
December	1st to 10th	Night	24% 10% 13% 30% 21% 2	4700
	11th to 20th	Night	43% 16% 16% 16% 8% 1	5316
	21st to end	Night	44% 14% 20% 14% 7% 1	7106

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	18% 18% 30% 24% 9% 2	220
	11th to 20th	Day	14% 14% 21% 23% 18% 9%	341
	21st to end	Day	13% 12% 22% 29% 18% 6%	473
February	1st to 10th	Day	14% 15% 21% 32% 15% 3	430
	11th to 20th	Day	14% 22% 26% 23% 13% 1	425
	21st to end	Day	15% 17% 24% 30% 13% 1	441
March	1st to 10th	Day	18% 24% 26% 22% 9% 1	509
	11th to 20th	Day	19% 17% 24% 24% 14% 2	500
	21st to end	Day	17% 26% 25% 19% 11% 1	550
April	1st to 10th	Day	16% 20% 23% 29% 11% 2	500
	11th to 20th	Day	18% 18% 23% 23% 13% 4	492
	21st to end	Day	12% 19% 27% 24% 14% 5%	500
May	1st to 10th	Day	13% 18% 28% 25% 12% 4	503
	11th to 20th	Day	13% 19% 24% 24% 15% 5%	528
	21st to end	Day	12% 13% 20% 30% 17% 7%	573
June	1st to 10th	Day	10% 14% 22% 29% 18% 7%	530
	11th to 20th	Day	7% 17% 31% 26% 15% 5%	530
	21st to end	Day	3 9% 25% 33% 21% 8%	530
July	1st to 10th	Day	4 7% 23% 35% 21% 10%	526
	11th to 20th	Day	1 6% 25% 35% 23% 11%	524
	21st to end	Day	4 20% 37% 26% 12%	575
August	1st to 10th	Day	2 3 23% 33% 25% 14%	527
	11th to 20th	Day	2 6% 21% 30% 26% 15%	517
	21st to end	Day	2 5% 20% 33% 28% 13%	583
September	1st to 10th	Day	2 5% 17% 38% 28% 10%	530
	11th to 20th	Day	5% 8% 20% 31% 28% 8%	530
	21st to end	Day	5% 11% 19% 31% 27% 7%	530
October	1st to 10th	Day	7% 11% 21% 26% 26% 9%	530
	11th to 20th	Day	10% 11% 15% 27% 30% 8%	530
	21st to end	Day	12% 11% 20% 29% 20% 8%	583
November	1st to 10th	Day	11% 14% 20% 30% 18% 7%	380
	11th to 20th	Day	12% 8% 23% 29% 21% 7%	245
	21st to end	Day	11% 8% 19% 33% 23% 6%	220
December	1st to 10th	Day	12% 8% 22% 31% 16% 10%	220
	11th to 20th	Day	15% 13% 25% 28% 10% 8%	220
	21st to end	Day	17% 14% 23% 26% 17% 3	242
January	1st to 10th	Night	13% 10% 29% 31% 14% 3	840
	11th to 20th	Night	9% 12% 23% 30% 22% 4	840
	21st to end	Night	12% 9% 23% 33% 19% 4	924
February	1st to 10th	Night	12% 9% 24% 37% 16% 2	840
	11th to 20th	Night	12% 9% 27% 36% 15% 2	824
	21st to end	Night	10% 12% 28% 36% 14% 1	692
March	1st to 10th	Night	14% 14% 28% 29% 12% 2	808
	11th to 20th	Night	12% 14% 20% 34% 18% 2	800
	21st to end	Night	11% 16% 28% 30% 13% 1	880
April	1st to 10th	Night	10% 13% 27% 34% 15% 1	800
	11th to 20th	Night	10% 12% 26% 31% 18% 3	791
	21st to end	Night	11% 10% 22% 35% 21% 1	735
May	1st to 10th	Night	7% 6% 24% 38% 23% 1	575
	11th to 20th	Night	10% 5% 23% 34% 27% 1	394
	21st to end	Night	10% 5% 20% 37% 27% 1	433
June	1st to 10th	Night	9% 4 17% 42% 28% 1	400
	11th to 20th	Night	5% 5% 21% 48% 21% 1	400
	21st to end	Night	2 2 12% 50% 34%	400
July	1st to 10th	Night	1 1 7% 48% 43% 1	390
	11th to 20th	Night	2 7% 49% 40% 2	388
	21st to end	Night	1 3 36% 56% 4	601
August	1st to 10th	Night	2 5% 37% 49% 7%	597
	11th to 20th	Night	3 1 4 42% 43% 8%	599
	21st to end	Night	3 2 11% 40% 41% 4	880
September	1st to 10th	Night	1 2 12% 43% 39% 4	800
	11th to 20th	Night	4 2 13% 39% 36% 7%	800
	21st to end	Night	5% 4 13% 38% 35% 6%	800
October	1st to 10th	Night	4 4 16% 42% 30% 4	800
	11th to 20th	Night	6% 5% 17% 38% 29% 5%	800
	21st to end	Night	8% 5% 19% 36% 24% 8%	880
November	1st to 10th	Night	12% 7% 22% 36% 17% 5%	820
	11th to 20th	Night	14% 9% 24% 33% 16% 4	840
	21st to end	Night	9% 9% 21% 35% 23% 3	840
December	1st to 10th	Night	10% 9% 21% 35% 21% 5%	840
	11th to 20th	Night	15% 12% 23% 31% 15% 4	840
	21st to end	Night	17% 15% 26% 30% 11% 2	924

Page generated at 2026-02-27 22:40:21. Version ac07

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