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 2024-12-06.

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% 70%	104990
	Night	5% 3 1 3 2 2 3 2 5% 6% 66%	103471
Transparency	Day	16% 3 8% 18% 28% 26%	104747
	Night	17% 4 7% 17% 27% 28%	103305
Seeing	Day	8% 10% 26% 34% 18% 4	15715
	Night	6% 5% 18% 34% 29% 8%	25399

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	7% 4 2 5% 3 2 3 3 5% 6% 62%	2681
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 8% 50%	2860
	21st to end	Day	11% 6% 2 5% 4 5% 4 4 5% 8% 46%	3146
February	1st to 10th	Day	12% 6% 1 4 3 2 3 3 4 5% 58%	1855
	11th to 20th	Day	8% 5% 1 4 3 4 4 4 5% 8% 55%	1850
	21st to end	Day	10% 5% 2 4 3 4 4 3 4 5% 54%	1570
March	1st to 10th	Day	13% 7% 2 6% 4 4 3 4 5% 8% 45%	3161
	11th to 20th	Day	5% 3 1 3 2 2 2 2 4 7% 68%	3490
	21st to end	Day	8% 7% 2 7% 4 4 5% 4 6% 7% 45%	4039
April	1st to 10th	Day	7% 4 1 4 3 4 3 4 6% 7% 57%	2280
	11th to 20th	Day	5% 4 1 4 3 3 2 3 5% 6% 64%	2249
	21st to end	Day	5% 3 1 2 2 2 3 3 5% 6% 66%	2280
May	1st to 10th	Day	3 2 1 2 2 2 2 2 3 5% 77%	3800
	11th to 20th	Day	1 1 2 2 2 2 3 6% 10% 70%	3933
	21st to end	Day	1 2 1 2 1 2 2 2 5% 6% 75%	4661
June	1st to 10th	Day	3 2 1 3 2 3 3 2 5% 9% 68%	2660
	11th to 20th	Day	1 2 1 1 1 1 2 2 3 7% 78%	2660
	21st to end	Day	1 1 2 2 2 2 3 5% 81%	2660
July	1st to 10th	Day	1 1 1 1 1 1 3 5% 86%	4420
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 79%	4157
	21st to end	Day	1 1 1 1 1 1 2 1 3 6% 82%	4455
August	1st to 10th	Day	1 1 1 1 2 2 2 3 5% 83%	3788
	11th to 20th	Day	1 1 1 1 1 1 1 4 89%	3724
	21st to end	Day	1 1 1 1 1 1 2 4 87%	4180
September	1st to 10th	Day	2 1 1 2 1 2 1 1 4 6% 79%	2280
	11th to 20th	Day	1 1 2 1 1 2 1 4 5% 81%	2244
	21st to end	Day	1 1 1 1 2 2 1 3 4 83%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 80%	3281
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 80%	3161
	21st to end	Day	3 2 1 2 2 3 3 3 3 5% 72%	3448
November	1st to 10th	Day	4 3 1 3 2 2 3 2 3 5% 70%	1854
	11th to 20th	Day	4 3 1 3 2 2 3 2 5% 5% 69%	1800
	21st to end	Day	5% 4 1 5% 3 4 3 3 5% 7% 59%	1800
December	1st to 10th	Day	8% 5% 2 5% 4 3 3 3 5% 6% 58%	1751
	11th to 20th	Day	20% 8% 2 5% 4 3 4 3 5% 6% 42%	1928
	21st to end	Day	14% 7% 2 6% 4 4 3 3 6% 7% 46%	2794
January	1st to 10th	Night	8% 5% 2 5% 3 3 3 3 4 5% 59%	4251
	11th to 20th	Night	8% 6% 3 6% 4 4 4 4 5% 8% 48%	4110
	21st to end	Night	13% 8% 2 5% 4 3 4 3 5% 8% 46%	4521
February	1st to 10th	Night	9% 6% 2 4 3 2 3 3 4 6% 57%	2322
	11th to 20th	Night	8% 5% 1 4 3 2 3 2 6% 8% 58%	2218
	21st to end	Night	9% 6% 2 5% 4 3 3 3 4 4 56%	1884
March	1st to 10th	Night	11% 8% 3 4 4 4 4 4 7% 8% 44%	3709
	11th to 20th	Night	5% 3 1 4 3 3 3 3 5% 7% 62%	3490
	21st to end	Night	10% 6% 2 4 3 3 4 3 6% 7% 52%	3612
April	1st to 10th	Night	6% 4 2 3 3 3 3 3 5% 7% 60%	1900
	11th to 20th	Night	5% 3 1 2 3 2 3 3 6% 6% 66%	1879
	21st to end	Night	4 3 1 2 3 3 3 3 5% 8% 66%	1900
May	1st to 10th	Night	2 2 1 2 1 2 2 2 3 5% 79%	3180
	11th to 20th	Night	1 1 1 2 2 3 4 3 9% 8% 65%	2974
	21st to end	Night	1 2 1 2 1 1 2 2 9% 7% 72%	2901
June	1st to 10th	Night	1 2 1 2 2 3 3 4 7% 8% 69%	1520
	11th to 20th	Night	1 2 1 1 1 1 2 1 3 8% 78%	1520
	21st to end	Night	1 1 1 1 1 1 2 4 7% 81%	1520
July	1st to 10th	Night	1 1 1 1 1 3 5% 87%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 4 7% 80%	2793
	21st to end	Night	1 1 1 1 1 1 1 4 7% 82%	3109
August	1st to 10th	Night	1 1 1 1 1 1 2 5% 84%	3156
	11th to 20th	Night	1 1 1 1 1 1 2 3 89%	3126
	21st to end	Night	1 1 1 1 1 3 6% 86%	3498
September	1st to 10th	Night	1 1 1 2 1 2 2 3 6% 81%	1900
	11th to 20th	Night	1 2 2 1 1 2 2 4 6% 78%	1934
	21st to end	Night	1 1 1 1 1 2 4 4 83%	2090
October	1st to 10th	Night	2 1 1 2 1 1 2 2 3 4 82%	3695
	11th to 20th	Night	2 2 1 2 1 1 2 3 6% 7% 74%	3777
	21st to end	Night	3 2 1 3 2 2 2 1 3 6% 75%	4121
November	1st to 10th	Night	3 3 1 3 3 3 2 2 4 7% 69%	2442
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 6% 68%	2600
	21st to end	Night	5% 4 2 4 4 3 3 3 5% 7% 59%	2600
December	1st to 10th	Night	7% 5% 1 4 3 3 3 3 6% 6% 59%	2541
	11th to 20th	Night	16% 8% 3 5% 4 3 4 3 5% 6% 41%	3276
	21st to end	Night	11% 6% 3 6% 5% 5% 5% 4 5% 7% 43%	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	22% 4 7% 31% 29% 8%	2681
	11th to 20th	Day	29% 6% 12% 18% 22% 12%	2860
	21st to end	Day	34% 5% 8% 25% 18% 10%	3146
February	1st to 10th	Day	29% 2 6% 19% 23% 20%	1855
	11th to 20th	Day	25% 4 9% 22% 27% 14%	1860
	21st to end	Day	30% 3 9% 21% 27% 10%	1570
March	1st to 10th	Day	34% 6% 10% 25% 18% 8%	3161
	11th to 20th	Day	17% 3 16% 31% 20% 13%	3490
	21st to end	Day	31% 7% 14% 27% 16% 5%	4039
April	1st to 10th	Day	25% 5% 13% 22% 25% 10%	2280
	11th to 20th	Day	20% 4 9% 29% 28% 10%	2252
	21st to end	Day	16% 5% 10% 27% 33% 9%	2280
May	1st to 10th	Day	12% 3 9% 19% 40% 17%	3800
	11th to 20th	Day	8% 6% 17% 23% 36% 9%	3936
	21st to end	Day	9% 4 13% 35% 35% 5%	4661
June	1st to 10th	Day	14% 4 12% 25% 31% 14%	2660
	11th to 20th	Day	8% 3 7% 15% 37% 29%	2660
	21st to end	Day	8% 2 6% 9% 33% 42%	2660
July	1st to 10th	Day	4 1 4 7% 16% 68%	4351
	11th to 20th	Day	11% 3 4 10% 22% 51%	4086
	21st to end	Day	7% 2 5% 15% 32% 39%	4315
August	1st to 10th	Day	7% 2 4 12% 39% 36%	3774
	11th to 20th	Day	3 1 4 7% 34% 50%	3760
	21st to end	Day	5% 1 4 9% 33% 48%	4179
September	1st to 10th	Day	8% 2 7% 10% 28% 46%	2280
	11th to 20th	Day	7% 3 6% 15% 37% 33%	2244
	21st to end	Day	8% 2 4 15% 38% 33%	2090
October	1st to 10th	Day	10% 2 4 11% 24% 48%	3281
	11th to 20th	Day	11% 2 5% 20% 37% 26%	3161
	21st to end	Day	16% 2 4 11% 37% 30%	3448
November	1st to 10th	Day	17% 3 5% 16% 32% 27%	1854
	11th to 20th	Day	18% 3 8% 17% 28% 27%	1800
	21st to end	Day	24% 4 7% 15% 24% 26%	1800
December	1st to 10th	Day	28% 3 7% 16% 25% 21%	1751
	11th to 20th	Day	42% 5% 8% 16% 19% 11%	1928
	21st to end	Day	36% 6% 9% 25% 15% 9%	2794
January	1st to 10th	Night	25% 5% 6% 22% 30% 12%	4251
	11th to 20th	Night	31% 8% 8% 18% 21% 14%	4110
	21st to end	Night	32% 8% 6% 19% 23% 12%	4521
February	1st to 10th	Night	28% 4 6% 17% 24% 22%	2322
	11th to 20th	Night	24% 4 7% 19% 29% 18%	2219
	21st to end	Night	30% 4 7% 19% 26% 15%	1884
March	1st to 10th	Night	31% 6% 9% 24% 21% 8%	3709
	11th to 20th	Night	19% 5% 11% 27% 24% 14%	3490
	21st to end	Night	27% 7% 15% 24% 21% 7%	3612
April	1st to 10th	Night	23% 4 12% 25% 24% 11%	1900
	11th to 20th	Night	17% 4 11% 25% 30% 13%	1879
	21st to end	Night	14% 4 12% 24% 33% 12%	1900
May	1st to 10th	Night	9% 3 8% 21% 34% 25%	3180
	11th to 20th	Night	9% 4 16% 19% 31% 21%	2975
	21st to end	Night	7% 7% 11% 25% 41% 9%	2901
June	1st to 10th	Night	10% 7% 15% 21% 30% 18%	1520
	11th to 20th	Night	8% 3 7% 12% 34% 36%	1520
	21st to end	Night	5% 1 7% 9% 27% 50%	1520
July	1st to 10th	Night	4 1 4 4 15% 72%	2500
	11th to 20th	Night	6% 2 6% 10% 18% 58%	2742
	21st to end	Night	6% 1 4 11% 30% 48%	3011
August	1st to 10th	Night	6% 1 3 10% 32% 47%	3147
	11th to 20th	Night	3 1 3 7% 24% 62%	3156
	21st to end	Night	3 1 2 7% 26% 60%	3498
September	1st to 10th	Night	6% 3 6% 8% 25% 52%	1900
	11th to 20th	Night	8% 4 4 10% 37% 39%	1934
	21st to end	Night	6% 3 4 12% 39% 37%	2090
October	1st to 10th	Night	8% 1 3 10% 22% 56%	3695
	11th to 20th	Night	9% 2 5% 15% 33% 36%	3777
	21st to end	Night	12% 2 3 10% 38% 35%	4121
November	1st to 10th	Night	16% 3 5% 16% 32% 27%	2442
	11th to 20th	Night	17% 3 7% 15% 27% 31%	2600
	21st to end	Night	23% 4 8% 18% 21% 26%	2600
December	1st to 10th	Night	25% 4 7% 16% 23% 24%	2541
	11th to 20th	Night	42% 4 8% 16% 18% 12%	3276
	21st to end	Night	38% 5% 9% 21% 18% 9%	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	20% 16% 30% 23% 12% 1	270
	11th to 20th	Day	18% 15% 25% 28% 14% 1	370
	21st to end	Day	14% 15% 34% 25% 10% 2	407
February	1st to 10th	Day	15% 14% 26% 23% 18% 4	439
	11th to 20th	Day	18% 16% 29% 25% 10% 2	455
	21st to end	Day	15% 19% 31% 25% 8% 2	382
March	1st to 10th	Day	20% 20% 33% 20% 7%	460
	11th to 20th	Day	15% 18% 35% 24% 7% 1	460
	21st to end	Day	17% 23% 32% 21% 6% 1	506
April	1st to 10th	Day	11% 19% 33% 22% 12% 2	460
	11th to 20th	Day	12% 18% 35% 24% 8% 3	452
	21st to end	Day	9% 17% 34% 24% 13% 3	460
May	1st to 10th	Day	13% 14% 33% 27% 10% 3	463
	11th to 20th	Day	5% 13% 35% 33% 13% 2	488
	21st to end	Day	10% 13% 31% 30% 13% 2	529
June	1st to 10th	Day	5% 9% 27% 34% 22% 3	490
	11th to 20th	Day	4 7% 28% 37% 18% 6%	490
	21st to end	Day	2 4 23% 40% 24% 7%	490
July	1st to 10th	Day	1 1 17% 45% 28% 8%	486
	11th to 20th	Day	2 1 11% 46% 30% 10%	484
	21st to end	Day	2 12% 47% 26% 13%	531
August	1st to 10th	Day	2 2 19% 45% 25% 6%	487
	11th to 20th	Day	2 1 13% 45% 30% 9%	477
	21st to end	Day	2 2 14% 47% 28% 7%	539
September	1st to 10th	Day	3 3 18% 45% 22% 8%	490
	11th to 20th	Day	3 7% 29% 43% 15% 4	490
	21st to end	Day	1 8% 22% 40% 26% 4	490
October	1st to 10th	Day	3 10% 24% 35% 25% 3	490
	11th to 20th	Day	3 7% 27% 39% 19% 4	490
	21st to end	Day	6% 8% 27% 35% 20% 4	539
November	1st to 10th	Day	7% 11% 23% 35% 21% 4	390
	11th to 20th	Day	8% 9% 24% 39% 16% 5%	300
	21st to end	Day	9% 10% 21% 38% 18% 4	300
December	1st to 10th	Day	9% 12% 28% 31% 17% 4	283
	11th to 20th	Day	11% 17% 21% 29% 17% 6%	180
	21st to end	Day	20% 15% 31% 21% 10% 3	198
January	1st to 10th	Night	13% 7% 22% 30% 19% 10%	760
	11th to 20th	Night	8% 7% 23% 34% 20% 9%	760
	21st to end	Night	13% 8% 23% 35% 16% 6%	836
February	1st to 10th	Night	13% 7% 21% 32% 22% 6%	760
	11th to 20th	Night	10% 10% 24% 32% 19% 6%	744
	21st to end	Night	12% 9% 28% 31% 17% 3	628
March	1st to 10th	Night	13% 9% 24% 33% 18% 3	760
	11th to 20th	Night	10% 6% 25% 33% 22% 4	760
	21st to end	Night	10% 11% 26% 33% 17% 3	836
April	1st to 10th	Night	8% 8% 30% 34% 18% 3	760
	11th to 20th	Night	8% 9% 27% 32% 21% 4	751
	21st to end	Night	6% 8% 28% 30% 21% 8%	760
May	1st to 10th	Night	5% 7% 21% 36% 25% 6%	760
	11th to 20th	Night	4 6% 21% 35% 31% 4	649
	21st to end	Night	3 9% 22% 37% 27% 3	499
June	1st to 10th	Night	1 4 13% 38% 39% 4	380
	11th to 20th	Night	2 2 9% 39% 41% 6%	380
	21st to end	Night	2 1 9% 33% 51% 4	380
July	1st to 10th	Night	1 1 7% 27% 51% 14%	370
	11th to 20th	Night	2 2 20% 57% 19%	514
	21st to end	Night	1 2 21% 57% 19%	624
August	1st to 10th	Night	2 1 7% 34% 44% 12%	746
	11th to 20th	Night	1 1 4 28% 54% 13%	752
	21st to end	Night	1 4 31% 49% 15%	836
September	1st to 10th	Night	2 1 6% 32% 46% 13%	760
	11th to 20th	Night	2 2 12% 39% 36% 9%	760
	21st to end	Night	1 2 11% 37% 39% 10%	760
October	1st to 10th	Night	3 3 13% 39% 35% 6%	760
	11th to 20th	Night	4 4 14% 40% 31% 8%	760
	21st to end	Night	3 4 18% 36% 28% 10%	836
November	1st to 10th	Night	5% 4 18% 37% 26% 9%	780
	11th to 20th	Night	5% 5% 18% 38% 28% 7%	800
	21st to end	Night	7% 5% 20% 39% 22% 7%	800
December	1st to 10th	Night	10% 5% 19% 35% 24% 7%	782
	11th to 20th	Night	12% 6% 22% 37% 18% 5%	760
	21st to end	Night	13% 10% 23% 34% 16% 5%	836

Page generated at 2024-12-03 03:45:12. Version ac07

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