Byrne Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Byrne Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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	4 3 1 3 2 2 2 2 4 6% 71%	139363
	Night	5% 3 1 3 2 2 3 3 5% 7% 66%	139629
Transparency	Day	16% 3 7% 18% 30% 26%	134443
	Night	18% 3 7% 16% 27% 29%	135524
Seeing	Day	8% 10% 26% 33% 18% 5%	17383
	Night	6% 5% 18% 34% 29% 7%	27138

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% 4 2 5% 3 2 3 3 4 5% 61%	5004
	11th to 20th	Day	5% 3 1 3 2 2 3 3 4 6% 69%	5340
	21st to end	Day	7% 5% 2 4 4 4 4 3 5% 8% 54%	5843
February	1st to 10th	Day	11% 6% 1 4 3 2 3 4 4 6% 57%	2051
	11th to 20th	Day	9% 5% 1 5% 3 4 4 4 5% 8% 53%	2050
	21st to end	Day	9% 5% 2 4 3 3 4 3 4 6% 55%	1730
March	1st to 10th	Day	13% 6% 2 5% 3 4 3 3 5% 7% 49%	4655
	11th to 20th	Day	9% 5% 2 5% 3 2 3 2 3 6% 60%	5078
	21st to end	Day	8% 6% 2 6% 4 4 4 4 7% 9% 47%	5791
April	1st to 10th	Day	6% 4 1 4 3 3 3 4 6% 7% 57%	2400
	11th to 20th	Day	5% 4 1 4 3 3 2 3 5% 6% 64%	2369
	21st to end	Day	5% 3 1 3 3 3 3 3 5% 6% 66%	2400
May	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 77%	5440
	11th to 20th	Day	1 1 1 2 2 2 2 3 5% 9% 72%	5638
	21st to end	Day	1 1 1 2 1 1 2 2 4 7% 76%	6685
June	1st to 10th	Day	3 2 1 3 2 3 3 3 5% 9% 68%	2800
	11th to 20th	Day	1 2 1 1 1 1 2 2 3 7% 79%	2800
	21st to end	Day	1 1 2 2 2 2 3 5% 81%	2800
July	1st to 10th	Day	1 1 1 1 1 2 4 89%	6319
	11th to 20th	Day	1 1 2 2 1 2 1 3 4 83%	5953
	21st to end	Day	1 1 1 1 1 1 2 1 4 6% 82%	6402
August	1st to 10th	Day	1 1 1 2 2 2 3 5% 82%	5351
	11th to 20th	Day	1 1 1 1 1 3 91%	5364
	21st to end	Day	1 1 1 1 1 1 2 4 87%	5984
September	1st to 10th	Day	2 1 1 2 1 2 1 2 4 6% 79%	2400
	11th to 20th	Day	2 1 2 1 1 2 1 4 5% 80%	2362
	21st to end	Day	1 2 2 2 2 2 2 3 4 81%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 80%	3384
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 79%	3261
	21st to end	Day	3 2 1 2 2 3 3 3 3 5% 72%	3558
November	1st to 10th	Day	4 3 1 3 2 2 3 2 3 5% 71%	1949
	11th to 20th	Day	6% 3 1 3 2 2 3 2 5% 5% 67%	1890
	21st to end	Day	5% 4 1 5% 3 4 3 3 5% 7% 60%	1890
December	1st to 10th	Day	5% 3 2 5% 3 3 3 3 5% 6% 62%	3022
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 4 6% 51%	3108
	21st to end	Day	12% 5% 2 5% 4 3 3 3 7% 9% 48%	4092
January	1st to 10th	Night	9% 6% 2 5% 3 3 2 2 4 5% 58%	7887
	11th to 20th	Night	4 3 2 3 2 2 3 3 5% 6% 66%	7650
	21st to end	Night	9% 6% 2 5% 3 3 4 3 5% 7% 51%	8371
February	1st to 10th	Night	9% 7% 2 4 3 2 3 3 4 6% 57%	2566
	11th to 20th	Night	9% 5% 1 4 3 3 3 2 6% 8% 56%	2458
	21st to end	Night	8% 6% 2 5% 4 3 3 3 4 5% 57%	2076
March	1st to 10th	Night	9% 8% 3 5% 4 4 4 4 6% 8% 45%	5455
	11th to 20th	Night	7% 5% 2 5% 3 3 3 3 5% 6% 57%	5077
	21st to end	Night	9% 6% 2 5% 3 3 4 3 6% 8% 51%	5182
April	1st to 10th	Night	6% 4 2 3 3 3 3 3 5% 7% 61%	2000
	11th to 20th	Night	5% 3 1 2 3 2 3 3 6% 6% 66%	1979
	21st to end	Night	4 3 1 2 3 3 3 3 5% 9% 65%	2000
May	1st to 10th	Night	1 1 1 2 2 2 3 2 6% 7% 72%	4560
	11th to 20th	Night	1 1 2 2 3 3 3 9% 9% 67%	4270
	21st to end	Night	1 1 2 1 1 1 1 11% 9% 71%	4167
June	1st to 10th	Night	1 2 1 2 2 3 3 4 7% 8% 69%	1600
	11th to 20th	Night	1 2 1 1 1 1 2 1 3 8% 78%	1600
	21st to end	Night	1 1 1 1 1 1 2 4 7% 82%	1600
July	1st to 10th	Night	1 1 1 1 1 2 4 90%	3640
	11th to 20th	Night	1 1 1 2 2 6% 8% 78%	4005
	21st to end	Night	1 1 1 1 2 2 10% 9% 72%	4472
August	1st to 10th	Night	1 1 1 1 1 1 3 6% 84%	4476
	11th to 20th	Night	1 1 1 1 4 5% 88%	4506
	21st to end	Night	1 1 1 1 1 3 6% 86%	5015
September	1st to 10th	Night	1 1 1 2 1 2 2 3 6% 81%	2000
	11th to 20th	Night	2 2 2 1 1 2 2 4 6% 77%	2036
	21st to end	Night	1 2 1 1 1 2 2 4 4 81%	2200
October	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 83%	3812
	11th to 20th	Night	2 2 1 2 1 1 2 2 6% 6% 74%	3897
	21st to end	Night	3 2 1 3 2 2 2 1 4 6% 76%	4253
November	1st to 10th	Night	3 2 1 3 3 3 2 2 4 6% 70%	2567
	11th to 20th	Night	4 3 1 4 3 3 3 3 5% 6% 66%	2730
	21st to end	Night	5% 4 2 4 3 3 3 3 5% 7% 60%	2730
December	1st to 10th	Night	4 3 1 3 3 3 3 3 6% 6% 64%	4370
	11th to 20th	Night	10% 6% 2 4 3 3 5% 4 6% 7% 50%	5316
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 8% 11% 40%	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	25% 3 6% 23% 29% 15%	5005
	11th to 20th	Day	17% 4 7% 11% 26% 36%	5340
	21st to end	Day	27% 4 6% 19% 26% 19%	5843
February	1st to 10th	Day	29% 2 6% 19% 24% 20%	2051
	11th to 20th	Day	27% 4 8% 22% 25% 13%	2060
	21st to end	Day	28% 3 8% 22% 27% 12%	1730
March	1st to 10th	Day	32% 4 9% 20% 25% 9%	4655
	11th to 20th	Day	27% 2 13% 24% 24% 10%	5078
	21st to end	Day	28% 6% 13% 29% 20% 4	5791
April	1st to 10th	Day	24% 5% 13% 23% 26% 10%	2400
	11th to 20th	Day	20% 4 9% 29% 28% 10%	2372
	21st to end	Day	17% 6% 10% 27% 31% 9%	2400
May	1st to 10th	Day	10% 4 8% 29% 35% 14%	5440
	11th to 20th	Day	9% 5% 14% 22% 39% 12%	5641
	21st to end	Day	8% 3 10% 33% 37% 9%	6686
June	1st to 10th	Day	14% 4 12% 26% 31% 14%	2800
	11th to 20th	Day	8% 3 7% 15% 37% 30%	2800
	21st to end	Day	7% 2 6% 9% 34% 43%	2800
July	1st to 10th	Day	3 1 3 6% 22% 65%	6251
	11th to 20th	Day	7% 2 3 7% 30% 50%	5882
	21st to end	Day	5% 2 5% 17% 39% 31%	6217
August	1st to 10th	Day	7% 2 4 12% 40% 36%	3894
	11th to 20th	Day	3 1 4 7% 35% 50%	3880
	21st to end	Day	5% 1 4 9% 33% 48%	4311
September	1st to 10th	Day	7% 2 7% 10% 29% 44%	2400
	11th to 20th	Day	8% 3 6% 14% 38% 31%	2362
	21st to end	Day	9% 2 4 15% 37% 33%	2200
October	1st to 10th	Day	10% 2 5% 11% 25% 48%	3384
	11th to 20th	Day	11% 2 5% 19% 38% 26%	3261
	21st to end	Day	15% 2 4 11% 37% 31%	3558
November	1st to 10th	Day	16% 3 5% 16% 33% 27%	1949
	11th to 20th	Day	20% 3 8% 16% 27% 26%	1890
	21st to end	Day	23% 4 7% 15% 24% 26%	1890
December	1st to 10th	Day	24% 2 5% 14% 31% 25%	3022
	11th to 20th	Day	34% 3 7% 18% 23% 14%	3108
	21st to end	Day	31% 5% 10% 27% 20% 8%	4092
January	1st to 10th	Night	28% 4 6% 17% 28% 17%	7887
	11th to 20th	Night	18% 4 5% 11% 21% 42%	7650
	21st to end	Night	28% 5% 5% 15% 25% 21%	8371
February	1st to 10th	Night	28% 3 6% 16% 25% 21%	2566
	11th to 20th	Night	25% 4 7% 20% 27% 16%	2459
	21st to end	Night	28% 4 7% 20% 26% 17%	2076
March	1st to 10th	Night	31% 5% 9% 20% 25% 9%	5455
	11th to 20th	Night	26% 5% 11% 23% 24% 12%	5077
	21st to end	Night	27% 6% 13% 24% 24% 5%	5182
April	1st to 10th	Night	22% 4 12% 25% 25% 11%	2000
	11th to 20th	Night	17% 4 11% 25% 30% 13%	1979
	21st to end	Night	15% 4 12% 25% 32% 12%	2000
May	1st to 10th	Night	9% 5% 12% 22% 32% 20%	4557
	11th to 20th	Night	8% 3 12% 23% 32% 21%	4271
	21st to end	Night	5% 5% 8% 23% 41% 18%	4167
June	1st to 10th	Night	9% 7% 15% 21% 31% 18%	1600
	11th to 20th	Night	7% 3 7% 12% 34% 37%	1600
	21st to end	Night	5% 1 6% 9% 27% 51%	1600
July	1st to 10th	Night	3 1 3 5% 15% 74%	3600
	11th to 20th	Night	4 2 5% 7% 24% 59%	3954
	21st to end	Night	5% 2 5% 14% 32% 43%	4345
August	1st to 10th	Night	6% 1 3 10% 33% 47%	3247
	11th to 20th	Night	3 1 3 6% 25% 62%	3256
	21st to end	Night	3 1 2 7% 26% 60%	3608
September	1st to 10th	Night	6% 3 6% 8% 26% 50%	2000
	11th to 20th	Night	8% 3 4 10% 37% 38%	2036
	21st to end	Night	7% 2 5% 12% 38% 36%	2200
October	1st to 10th	Night	7% 1 3 10% 23% 55%	3812
	11th to 20th	Night	9% 2 4 14% 33% 37%	3897
	21st to end	Night	12% 2 3 10% 38% 35%	4253
November	1st to 10th	Night	15% 3 5% 16% 33% 27%	2567
	11th to 20th	Night	19% 3 7% 15% 26% 30%	2730
	21st to end	Night	23% 4 8% 18% 21% 26%	2730
December	1st to 10th	Night	20% 3 5% 14% 28% 29%	4370
	11th to 20th	Night	33% 3 9% 17% 23% 15%	5316
	21st to end	Night	34% 5% 11% 23% 21% 7%	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	19% 16% 28% 23% 13% 2	320
	11th to 20th	Day	15% 14% 24% 29% 15% 3	430
	21st to end	Day	13% 14% 34% 25% 11% 3	473
February	1st to 10th	Day	14% 14% 26% 25% 17% 3	515
	11th to 20th	Day	19% 16% 29% 25% 9% 2	535
	21st to end	Day	13% 17% 33% 26% 10% 2	446
March	1st to 10th	Day	19% 20% 34% 20% 6% 1	540
	11th to 20th	Day	16% 17% 34% 24% 8% 2	529
	21st to end	Day	17% 22% 32% 22% 6% 1	550
April	1st to 10th	Day	11% 19% 32% 23% 12% 3	500
	11th to 20th	Day	11% 18% 35% 25% 9% 3	492
	21st to end	Day	9% 17% 33% 25% 14% 3	500
May	1st to 10th	Day	13% 14% 32% 27% 11% 3	503
	11th to 20th	Day	5% 14% 35% 31% 13% 2	528
	21st to end	Day	9% 13% 32% 31% 13% 2	573
June	1st to 10th	Day	4 9% 27% 35% 22% 4	530
	11th to 20th	Day	4 8% 28% 37% 18% 5%	530
	21st to end	Day	2 4 23% 39% 25% 7%	530
July	1st to 10th	Day	1 1 17% 45% 28% 8%	526
	11th to 20th	Day	2 1 11% 44% 30% 12%	524
	21st to end	Day	2 1 13% 46% 26% 13%	575
August	1st to 10th	Day	2 2 19% 44% 26% 7%	527
	11th to 20th	Day	2 2 13% 45% 29% 9%	517
	21st to end	Day	2 2 14% 45% 29% 8%	583
September	1st to 10th	Day	3 3 19% 44% 22% 9%	530
	11th to 20th	Day	4 7% 28% 41% 15% 5%	530
	21st to end	Day	1 9% 22% 39% 24% 4	530
October	1st to 10th	Day	3 10% 24% 35% 25% 4	530
	11th to 20th	Day	4 8% 27% 38% 19% 5%	530
	21st to end	Day	6% 8% 27% 35% 20% 5%	583
November	1st to 10th	Day	6% 10% 22% 34% 22% 5%	425
	11th to 20th	Day	11% 10% 22% 37% 15% 5%	330
	21st to end	Day	8% 9% 21% 37% 21% 4	330
December	1st to 10th	Day	8% 10% 25% 30% 22% 5%	327
	11th to 20th	Day	10% 15% 19% 30% 20% 6%	220
	21st to end	Day	21% 15% 29% 23% 10% 3	242
January	1st to 10th	Night	13% 6% 23% 31% 17% 9%	840
	11th to 20th	Night	7% 6% 23% 36% 20% 8%	840
	21st to end	Night	12% 8% 22% 37% 16% 5%	924
February	1st to 10th	Night	12% 7% 20% 34% 21% 5%	840
	11th to 20th	Night	11% 10% 25% 31% 17% 5%	824
	21st to end	Night	11% 9% 27% 35% 16% 2	692
March	1st to 10th	Night	13% 10% 25% 33% 17% 2	840
	11th to 20th	Night	10% 7% 24% 34% 21% 4	828
	21st to end	Night	9% 11% 26% 34% 17% 3	880
April	1st to 10th	Night	7% 8% 30% 34% 18% 3	800
	11th to 20th	Night	7% 8% 27% 33% 21% 4	791
	21st to end	Night	6% 8% 27% 30% 21% 8%	800
May	1st to 10th	Night	5% 7% 21% 36% 25% 6%	800
	11th to 20th	Night	4 6% 22% 35% 29% 4	683
	21st to end	Night	3 9% 21% 38% 27% 2	525
June	1st to 10th	Night	1 4 13% 38% 40% 5%	400
	11th to 20th	Night	2 2 10% 39% 41% 6%	400
	21st to end	Night	2 1 10% 33% 50% 5%	400
July	1st to 10th	Night	1 1 6% 27% 52% 13%	390
	11th to 20th	Night	1 2 20% 56% 21%	542
	21st to end	Night	1 3 21% 56% 18%	657
August	1st to 10th	Night	2 1 8% 34% 44% 12%	786
	11th to 20th	Night	1 1 4 28% 54% 13%	792
	21st to end	Night	1 4 31% 50% 14%	880
September	1st to 10th	Night	2 2 6% 33% 45% 12%	800
	11th to 20th	Night	3 2 13% 38% 36% 9%	800
	21st to end	Night	2 2 11% 37% 38% 9%	800
October	1st to 10th	Night	3 3 13% 40% 35% 6%	800
	11th to 20th	Night	4 4 15% 40% 30% 7%	800
	21st to end	Night	3 3 18% 37% 29% 10%	880
November	1st to 10th	Night	5% 4 17% 38% 27% 9%	820
	11th to 20th	Night	6% 5% 18% 38% 27% 7%	840
	21st to end	Night	7% 5% 20% 40% 22% 7%	840
December	1st to 10th	Night	9% 5% 19% 37% 24% 6%	840
	11th to 20th	Night	11% 6% 21% 38% 19% 5%	840
	21st to end	Night	13% 9% 23% 34% 15% 5%	924

Page generated at 2026-03-14 23:23:54. Version ac07

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