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 2024-12-02.

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 3 5% 7% 55%	105544
	Night	7% 7% 2 5% 4 4 4 3 5% 7% 52%	102801
Transparency	Day	28% 8% 11% 20% 20% 14%	105296
	Night	31% 9% 12% 21% 17% 10%	102638
Seeing	Day	10% 13% 23% 29% 19% 6%	15060
	Night	9% 8% 20% 36% 25% 3	24185

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	9% 12% 3 11% 7% 7% 6% 5% 7% 9% 25%	2539
	11th to 20th	Day	17% 12% 5% 10% 5% 4 4 4 6% 7% 25%	2714
	21st to end	Day	10% 8% 3 10% 7% 6% 6% 4 8% 9% 30%	3146
February	1st to 10th	Day	4 7% 3 6% 6% 6% 7% 6% 9% 10% 35%	1710
	11th to 20th	Day	7% 8% 2 7% 6% 5% 5% 5% 8% 9% 38%	1665
	21st to end	Day	4 12% 3 7% 6% 5% 6% 5% 8% 9% 35%	1557
March	1st to 10th	Day	10% 15% 4 9% 6% 6% 6% 4 7% 7% 26%	3344
	11th to 20th	Day	4 7% 2 6% 4 4 4 4 6% 7% 51%	3490
	21st to end	Day	7% 10% 4 9% 6% 7% 6% 5% 7% 9% 29%	3883
April	1st to 10th	Day	7% 8% 3 9% 6% 6% 6% 4 6% 7% 37%	2280
	11th to 20th	Day	9% 9% 3 6% 5% 4 5% 3 6% 9% 42%	2249
	21st to end	Day	5% 8% 2 6% 5% 4 3 4 5% 8% 48%	2337
May	1st to 10th	Day	6% 6% 2 6% 3 2 3 3 4 6% 58%	4156
	11th to 20th	Day	5% 4 1 3 2 2 2 2 4 6% 69%	4377
	21st to end	Day	6% 5% 2 4 3 2 3 3 5% 8% 60%	4813
June	1st to 10th	Day	8% 6% 2 5% 4 4 4 4 6% 8% 51%	2660
	11th to 20th	Day	4 4 1 3 2 3 3 3 5% 6% 67%	2660
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 74%	2660
July	1st to 10th	Day	1 1 1 1 1 1 2 2 3 85%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 3 78%	4420
	21st to end	Day	1 1 1 1 2 3 2 3 4 82%	4528
August	1st to 10th	Day	3 2 1 2 2 2 2 2 4 4 75%	4091
	11th to 20th	Day	1 2 2 1 2 3 2 4 5% 77%	4008
	21st to end	Day	3 1 1 2 2 2 2 2 3 4 79%	4180
September	1st to 10th	Day	1 1 1 1 1 2 2 3 4 83%	2280
	11th to 20th	Day	3 3 1 3 2 3 3 3 4 6% 67%	2280
	21st to end	Day	3 2 1 3 2 3 2 2 4 7% 69%	2001
October	1st to 10th	Day	3 2 1 2 2 1 2 2 4 5% 75%	3180
	11th to 20th	Day	3 3 1 4 4 4 3 3 6% 9% 59%	3161
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 7% 8% 47%	3448
November	1st to 10th	Day	9% 7% 3 8% 6% 6% 5% 4 6% 8% 39%	1812
	11th to 20th	Day	10% 6% 3 9% 5% 5% 6% 5% 7% 11% 31%	1645
	21st to end	Day	6% 8% 2 7% 6% 5% 6% 5% 7% 11% 36%	1600
December	1st to 10th	Day	6% 7% 3 8% 6% 6% 6% 4 7% 10% 36%	1528
	11th to 20th	Day	9% 9% 3 10% 7% 7% 7% 6% 9% 9% 23%	1928
	21st to end	Day	8% 11% 4 12% 8% 7% 7% 6% 8% 9% 19%	2794
January	1st to 10th	Night	10% 13% 4 9% 6% 7% 6% 5% 7% 10% 24%	4406
	11th to 20th	Night	14% 13% 5% 10% 5% 5% 5% 4 6% 8% 26%	4235
	21st to end	Night	15% 12% 4 9% 6% 5% 5% 4 6% 7% 26%	4521
February	1st to 10th	Night	5% 9% 3 7% 6% 4 5% 3 7% 10% 40%	2470
	11th to 20th	Night	7% 9% 3 7% 4 5% 5% 5% 7% 8% 40%	2402
	21st to end	Night	4 10% 2 7% 5% 5% 6% 4 7% 10% 40%	1896
March	1st to 10th	Night	10% 12% 3 9% 6% 5% 5% 5% 8% 9% 30%	3525
	11th to 20th	Night	8% 8% 2 5% 3 4 4 3 4 7% 52%	3490
	21st to end	Night	6% 10% 4 9% 7% 7% 7% 5% 8% 7% 30%	3768
April	1st to 10th	Night	6% 9% 3 7% 5% 5% 5% 5% 6% 7% 41%	1900
	11th to 20th	Night	7% 10% 2 5% 4 4 3 3 6% 7% 50%	1879
	21st to end	Night	10% 8% 2 5% 3 2 4 3 5% 8% 52%	1838
May	1st to 10th	Night	8% 7% 2 5% 2 3 2 2 3 5% 61%	2810
	11th to 20th	Night	5% 3 1 3 2 2 3 2 4 5% 71%	2516
	21st to end	Night	8% 6% 1 4 3 2 3 2 3 5% 60%	2762
June	1st to 10th	Night	7% 6% 2 5% 4 4 3 5% 7% 6% 52%	1520
	11th to 20th	Night	6% 4 1 3 2 2 2 1 3 6% 70%	1520
	21st to end	Night	2 2 1 3 1 2 1 2 3 4 78%	1520
July	1st to 10th	Night	1 1 1 1 1 1 1 2 3 88%	2540
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 3 80%	2540
	21st to end	Night	1 1 1 1 1 2 2 3 88%	3030
August	1st to 10th	Night	2 2 2 1 1 2 2 4 5% 79%	2848
	11th to 20th	Night	1 2 1 1 1 2 2 3 4 84%	2830
	21st to end	Night	8% 2 1 1 1 1 1 1 3 3 78%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 3 5% 86%	1900
	11th to 20th	Night	4 3 1 2 2 2 2 2 4 6% 72%	1900
	21st to end	Night	4 2 1 3 2 3 2 2 4 6% 71%	2176
October	1st to 10th	Night	2 2 2 1 2 2 2 3 4 78%	3800
	11th to 20th	Night	5% 3 1 3 2 3 3 3 5% 6% 65%	3777
	21st to end	Night	6% 5% 1 4 3 4 4 5% 7% 8% 52%	4121
November	1st to 10th	Night	8% 9% 3 6% 5% 5% 5% 4 6% 8% 42%	2486
	11th to 20th	Night	11% 6% 2 5% 5% 5% 6% 6% 7% 10% 36%	2753
	21st to end	Night	6% 8% 3 6% 5% 4 6% 4 6% 10% 43%	2800
December	1st to 10th	Night	6% 8% 1 6% 5% 5% 5% 5% 7% 11% 40%	2686
	11th to 20th	Night	8% 10% 3 9% 7% 7% 6% 5% 8% 10% 27%	3276
	21st to end	Night	10% 11% 4 9% 6% 6% 6% 5% 7% 10% 25%	4862

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	51% 11% 17% 15% 5% 1	2539
	11th to 20th	Day	53% 10% 14% 13% 7% 4	2714
	21st to end	Day	47% 9% 16% 16% 7% 3	3146
February	1st to 10th	Day	39% 13% 15% 19% 11% 3	1710
	11th to 20th	Day	39% 12% 15% 20% 12% 2	1674
	21st to end	Day	44% 11% 13% 18% 10% 3	1557
March	1st to 10th	Day	55% 10% 13% 13% 9%	3344
	11th to 20th	Day	32% 8% 14% 21% 24% 1	3490
	21st to end	Day	51% 13% 13% 18% 4 1	3883
April	1st to 10th	Day	46% 9% 15% 17% 11% 2	2280
	11th to 20th	Day	41% 9% 15% 20% 12% 4	2252
	21st to end	Day	36% 9% 14% 23% 15% 2	2337
May	1st to 10th	Day	27% 7% 12% 27% 24% 3	4156
	11th to 20th	Day	20% 6% 9% 20% 35% 10%	4380
	21st to end	Day	25% 8% 14% 38% 14% 1	4814
June	1st to 10th	Day	31% 9% 14% 28% 16% 2	2660
	11th to 20th	Day	22% 6% 10% 30% 26% 7%	2660
	21st to end	Day	15% 4 6% 25% 33% 17%	2660
July	1st to 10th	Day	7% 2 4 11% 20% 55%	4351
	11th to 20th	Day	11% 4 4 11% 21% 48%	4345
	21st to end	Day	7% 3 4 14% 33% 38%	4385
August	1st to 10th	Day	13% 4 5% 12% 26% 40%	4076
	11th to 20th	Day	9% 4 7% 25% 37% 18%	4047
	21st to end	Day	9% 4 6% 26% 36% 19%	4179
September	1st to 10th	Day	7% 3 5% 16% 31% 38%	2280
	11th to 20th	Day	18% 7% 8% 20% 34% 13%	2280
	21st to end	Day	16% 6% 9% 25% 27% 17%	2001
October	1st to 10th	Day	14% 4 11% 16% 23% 32%	3180
	11th to 20th	Day	20% 7% 13% 36% 19% 5%	3161
	21st to end	Day	26% 13% 16% 20% 18% 7%	3448
November	1st to 10th	Day	40% 8% 13% 20% 15% 4	1812
	11th to 20th	Day	41% 11% 15% 16% 11% 4	1645
	21st to end	Day	36% 10% 14% 18% 16% 6%	1600
December	1st to 10th	Day	38% 10% 15% 16% 17% 3	1528
	11th to 20th	Day	52% 12% 15% 12% 8% 1	1928
	21st to end	Day	51% 13% 17% 13% 6% 1	2794
January	1st to 10th	Night	51% 14% 14% 14% 6% 1	4406
	11th to 20th	Night	47% 15% 13% 13% 8% 4	4235
	21st to end	Night	55% 8% 12% 12% 10% 3	4521
February	1st to 10th	Night	40% 10% 16% 17% 13% 5%	2470
	11th to 20th	Night	42% 12% 13% 19% 11% 3	2404
	21st to end	Night	39% 12% 17% 17% 12% 3	1896
March	1st to 10th	Night	50% 12% 15% 13% 10% 1	3525
	11th to 20th	Night	32% 9% 13% 21% 21% 5%	3490
	21st to end	Night	45% 18% 17% 13% 6% 1	3768
April	1st to 10th	Night	42% 11% 18% 19% 8% 2	1900
	11th to 20th	Night	36% 10% 19% 20% 10% 5%	1879
	21st to end	Night	34% 8% 21% 27% 10% 2	1838
May	1st to 10th	Night	27% 7% 14% 31% 17% 4	2810
	11th to 20th	Night	18% 7% 11% 33% 27% 5%	2516
	21st to end	Night	23% 9% 24% 36% 8%	2762
June	1st to 10th	Night	33% 7% 18% 33% 9% 1	1520
	11th to 20th	Night	19% 6% 13% 38% 21% 4	1520
	21st to end	Night	11% 5% 8% 33% 34% 8%	1520
July	1st to 10th	Night	5% 2 6% 14% 21% 52%	2500
	11th to 20th	Night	9% 4 3 14% 24% 45%	2492
	21st to end	Night	6% 2 4 16% 43% 30%	2935
August	1st to 10th	Night	10% 3 5% 20% 25% 38%	2839
	11th to 20th	Night	6% 3 7% 35% 33% 17%	2857
	21st to end	Night	14% 3 6% 29% 31% 16%	3498
September	1st to 10th	Night	6% 2 6% 17% 32% 37%	1900
	11th to 20th	Night	15% 5% 8% 25% 32% 14%	1900
	21st to end	Night	18% 5% 10% 23% 24% 20%	2176
October	1st to 10th	Night	12% 6% 8% 14% 30% 30%	3800
	11th to 20th	Night	20% 6% 12% 40% 16% 6%	3777
	21st to end	Night	23% 9% 16% 25% 18% 8%	4121
November	1st to 10th	Night	40% 9% 14% 17% 14% 6%	2486
	11th to 20th	Night	40% 11% 16% 16% 13% 4	2753
	21st to end	Night	36% 10% 14% 16% 17% 7%	2800
December	1st to 10th	Night	36% 11% 12% 19% 19% 3	2686
	11th to 20th	Night	51% 11% 10% 15% 12% 1	3276
	21st to end	Night	48% 12% 17% 13% 8% 1	4862

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% 7% 3	180
	11th to 20th	Day	17% 17% 24% 23% 12% 7%	289
	21st to end	Day	15% 13% 22% 28% 17% 6%	407
February	1st to 10th	Day	12% 16% 22% 33% 15% 2	370
	11th to 20th	Day	15% 22% 24% 23% 14% 2	365
	21st to end	Day	14% 18% 24% 30% 13% 1	377
March	1st to 10th	Day	19% 25% 26% 21% 8% 1	460
	11th to 20th	Day	18% 17% 23% 25% 15% 2	460
	21st to end	Day	17% 26% 26% 20% 10% 1	506
April	1st to 10th	Day	16% 21% 22% 28% 12% 2	460
	11th to 20th	Day	20% 19% 24% 21% 12% 4	452
	21st to end	Day	12% 20% 28% 22% 13% 4	460
May	1st to 10th	Day	13% 19% 29% 24% 12% 4	463
	11th to 20th	Day	14% 18% 23% 24% 15% 6%	488
	21st to end	Day	13% 14% 21% 29% 17% 7%	529
June	1st to 10th	Day	11% 15% 22% 29% 17% 7%	490
	11th to 20th	Day	7% 17% 31% 26% 14% 5%	490
	21st to end	Day	3 9% 26% 33% 21% 7%	490
July	1st to 10th	Day	4 7% 23% 36% 21% 9%	486
	11th to 20th	Day	1 6% 26% 35% 22% 10%	484
	21st to end	Day	4 21% 38% 26% 11%	531
August	1st to 10th	Day	2 3 24% 34% 24% 12%	487
	11th to 20th	Day	1 6% 22% 30% 26% 14%	477
	21st to end	Day	2 6% 21% 33% 27% 12%	539
September	1st to 10th	Day	2 5% 17% 38% 29% 9%	490
	11th to 20th	Day	6% 8% 21% 32% 26% 7%	490
	21st to end	Day	5% 11% 20% 32% 27% 5%	490
October	1st to 10th	Day	7% 10% 21% 27% 27% 9%	490
	11th to 20th	Day	10% 11% 15% 27% 31% 7%	490
	21st to end	Day	12% 12% 21% 28% 19% 7%	539
November	1st to 10th	Day	12% 13% 20% 31% 16% 7%	348
	11th to 20th	Day	12% 8% 22% 31% 20% 7%	223
	21st to end	Day	13% 9% 21% 32% 22% 5%	200
December	1st to 10th	Day	14% 10% 25% 28% 12% 11%	182
	11th to 20th	Day	16% 12% 25% 28% 11% 8%	180
	21st to end	Day	16% 12% 20% 29% 21% 2	198
January	1st to 10th	Night	13% 9% 29% 30% 15% 3	760
	11th to 20th	Night	10% 13% 24% 29% 20% 4	760
	21st to end	Night	14% 9% 22% 32% 18% 4	836
February	1st to 10th	Night	11% 10% 24% 38% 16% 2	760
	11th to 20th	Night	12% 9% 25% 35% 17% 2	744
	21st to end	Night	10% 10% 28% 36% 14% 1	628
March	1st to 10th	Night	14% 15% 27% 29% 12% 3	760
	11th to 20th	Night	12% 13% 19% 34% 19% 2	760
	21st to end	Night	11% 16% 29% 30% 13% 1	836
April	1st to 10th	Night	11% 13% 27% 33% 15% 1	760
	11th to 20th	Night	11% 12% 26% 30% 18% 3	751
	21st to end	Night	11% 10% 22% 34% 21% 1	698
May	1st to 10th	Night	7% 6% 24% 38% 23% 1	546
	11th to 20th	Night	9% 5% 22% 34% 28% 1	374
	21st to end	Night	10% 5% 21% 36% 27% 1	411
June	1st to 10th	Night	9% 4 17% 40% 28% 1	380
	11th to 20th	Night	5% 5% 22% 47% 21% 1	380
	21st to end	Night	2 1 13% 49% 34%	380
July	1st to 10th	Night	1 7% 46% 44% 1	370
	11th to 20th	Night	2 7% 49% 40% 2	368
	21st to end	Night	1 3 37% 55% 4	571
August	1st to 10th	Night	2 5% 37% 49% 7%	567
	11th to 20th	Night	2 1 4 42% 43% 8%	569
	21st to end	Night	3 2 11% 39% 41% 4	836
September	1st to 10th	Night	1 1 12% 42% 39% 4	760
	11th to 20th	Night	3 2 13% 39% 36% 7%	760
	21st to end	Night	5% 4 13% 37% 35% 7%	760
October	1st to 10th	Night	4 3 16% 42% 31% 4	760
	11th to 20th	Night	6% 6% 16% 37% 30% 6%	760
	21st to end	Night	8% 5% 19% 36% 23% 8%	836
November	1st to 10th	Night	12% 7% 23% 37% 17% 5%	780
	11th to 20th	Night	13% 9% 24% 34% 16% 4	800
	21st to end	Night	9% 9% 21% 35% 23% 3	800
December	1st to 10th	Night	11% 10% 21% 34% 20% 5%	768
	11th to 20th	Night	15% 11% 23% 32% 15% 4	760
	21st to end	Night	17% 14% 26% 31% 12% 1	836

Page generated at 2024-11-29 07:43:10. Version ac07

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