Bright Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bright Skies Observatory clear sky chart from 2005-11-06 to 2024-11-06.

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 3 3 4 6% 69%	104977
	Night	5% 4 1 3 2 2 3 3 4 6% 66%	102846
Transparency	Day	17% 4 7% 17% 27% 29%	104740
	Night	19% 4 9% 17% 25% 26%	102673
Seeing	Day	7% 7% 22% 34% 23% 7%	15624
	Night	7% 3 11% 35% 36% 9%	25444

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	6% 3 1 3 2 3 2 2 4 6% 67%	2539
	11th to 20th	Day	8% 4 2 4 3 2 3 3 6% 9% 56%	2825
	21st to end	Day	13% 5% 2 4 3 3 3 3 4 7% 54%	3441
February	1st to 10th	Day	9% 4 2 3 3 2 2 3 4 5% 62%	1900
	11th to 20th	Day	8% 3 1 3 3 2 4 3 4 6% 61%	1850
	21st to end	Day	8% 5% 2 4 4 4 3 3 5% 7% 57%	1570
March	1st to 10th	Day	9% 7% 2 6% 4 4 4 4 6% 7% 49%	3138
	11th to 20th	Day	4 3 1 3 2 2 3 3 5% 8% 64%	3290
	21st to end	Day	9% 4 1 4 4 3 4 4 6% 9% 51%	4165
April	1st to 10th	Day	5% 4 1 4 3 3 3 4 4 7% 62%	2280
	11th to 20th	Day	4 3 2 3 2 2 4 3 4 6% 67%	2249
	21st to end	Day	4 4 1 3 2 3 2 4 4 6% 67%	2280
May	1st to 10th	Day	2 3 1 3 2 2 2 3 4 6% 73%	3861
	11th to 20th	Day	2 7% 3 4 3 2 3 3 4 7% 63%	4071
	21st to end	Day	3 6% 2 4 2 3 2 3 4 6% 67%	4477
June	1st to 10th	Day	4 4 2 4 2 3 3 2 5% 8% 64%	2532
	11th to 20th	Day	2 4 1 2 2 2 2 2 3 6% 73%	2660
	21st to end	Day	1 2 1 2 1 2 2 2 4 5% 77%	2660
July	1st to 10th	Day	1 1 2 1 1 1 2 3 3 84%	4420
	11th to 20th	Day	1 1 1 2 2 2 3 3 4 4 77%	4406
	21st to end	Day	1 1 1 1 1 1 2 2 3 4 84%	4133
August	1st to 10th	Day	1 1 1 2 1 1 2 1 3 4 83%	3783
	11th to 20th	Day	1 1 1 1 1 1 2 3 88%	3724
	21st to end	Day	1 1 1 2 1 2 3 4 85%	4180
September	1st to 10th	Day	2 2 1 2 1 2 2 2 3 4 79%	2280
	11th to 20th	Day	1 2 2 1 2 2 2 3 5% 79%	2157
	21st to end	Day	2 2 1 2 2 2 2 2 4 4 79%	2090
October	1st to 10th	Day	2 2 1 1 2 2 2 2 3 4 81%	3490
	11th to 20th	Day	2 2 1 2 2 3 2 2 4 6% 75%	3223
	21st to end	Day	2 2 1 3 2 3 3 3 5% 6% 69%	3404
November	1st to 10th	Day	3 3 1 2 3 3 3 3 4 5% 72%	1719
	11th to 20th	Day	4 3 1 3 3 2 2 3 4 5% 70%	1710
	21st to end	Day	4 3 2 4 4 3 3 3 4 8% 63%	1710
December	1st to 10th	Day	7% 3 1 5% 3 2 4 4 5% 7% 59%	1710
	11th to 20th	Day	16% 5% 2 6% 4 4 3 4 5% 7% 45%	2169
	21st to end	Day	13% 5% 2 6% 6% 4 5% 3 5% 6% 46%	2881
January	1st to 10th	Night	7% 4 1 3 2 2 2 3 3 5% 67%	4406
	11th to 20th	Night	7% 4 2 6% 3 3 3 3 5% 8% 56%	4141
	21st to end	Night	14% 5% 2 4 2 2 3 3 5% 6% 54%	4229
February	1st to 10th	Night	8% 5% 2 5% 3 3 2 2 3 5% 63%	2280
	11th to 20th	Night	8% 4 1 4 3 3 3 2 4 5% 63%	2218
	21st to end	Night	10% 5% 2 4 4 3 3 3 4 6% 57%	1884
March	1st to 10th	Night	8% 7% 2 5% 3 3 4 3 5% 9% 50%	3736
	11th to 20th	Night	4 4 1 3 3 3 3 4 6% 7% 62%	3686
	21st to end	Night	9% 4 1 4 3 3 3 3 5% 8% 57%	3488
April	1st to 10th	Night	5% 5% 1 4 3 3 2 3 4 6% 63%	1900
	11th to 20th	Night	4 4 1 2 3 3 3 3 4 6% 67%	1879
	21st to end	Night	4 4 1 3 3 3 3 3 5% 8% 65%	1900
May	1st to 10th	Night	3 5% 2 3 3 2 3 2 4 6% 67%	3116
	11th to 20th	Night	2 5% 3 4 4 4 4 3 6% 9% 56%	2834
	21st to end	Night	2 6% 1 4 2 2 2 2 4 6% 69%	3110
June	1st to 10th	Night	2 4 1 4 3 3 4 4 4 8% 62%	1642
	11th to 20th	Night	2 4 1 2 2 1 3 2 4 7% 72%	1520
	21st to end	Night	1 2 1 3 2 2 3 3 4 5% 74%	1520
July	1st to 10th	Night	1 2 1 1 2 2 3 5% 81%	2540
	11th to 20th	Night	1 1 2 2 2 3 3 6% 7% 75%	2540
	21st to end	Night	1 1 1 2 1 1 2 3 6% 80%	3421
August	1st to 10th	Night	2 1 2 1 2 2 2 4 5% 80%	3166
	11th to 20th	Night	1 1 1 1 1 1 2 4 5% 82%	3126
	21st to end	Night	1 1 1 1 2 2 2 4 5% 80%	3498
September	1st to 10th	Night	3 2 1 3 2 1 2 2 3 5% 75%	1900
	11th to 20th	Night	2 2 2 2 1 2 2 3 4 79%	2023
	21st to end	Night	1 1 1 2 2 2 2 3 4 6% 78%	2090
October	1st to 10th	Night	2 1 1 2 2 2 2 1 2 3 82%	3490
	11th to 20th	Night	2 4 2 3 2 3 4 3 4 7% 64%	3710
	21st to end	Night	2 2 1 2 2 3 3 2 3 4 76%	4151
November	1st to 10th	Night	3 3 1 2 2 2 2 3 4 6% 71%	2482
	11th to 20th	Night	4 2 1 3 2 1 2 3 4 6% 72%	2470
	21st to end	Night	4 3 1 3 3 4 4 3 6% 6% 62%	2470
December	1st to 10th	Night	7% 3 2 5% 3 2 3 2 5% 7% 60%	2470
	11th to 20th	Night	16% 7% 1 5% 4 3 3 3 5% 7% 48%	3041
	21st to end	Night	12% 6% 2 5% 4 4 4 4 6% 6% 47%	4769

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	19% 3 5% 23% 32% 18%	2539
	11th to 20th	Day	25% 6% 12% 20% 19% 17%	2825
	21st to end	Day	31% 4 6% 19% 24% 17%	3441
February	1st to 10th	Day	24% 3 6% 13% 25% 28%	1900
	11th to 20th	Day	23% 4 6% 16% 27% 25%	1860
	21st to end	Day	25% 5% 8% 18% 23% 20%	1570
March	1st to 10th	Day	30% 7% 10% 21% 16% 16%	3138
	11th to 20th	Day	17% 5% 13% 32% 17% 17%	3290
	21st to end	Day	27% 7% 14% 27% 16% 9%	4165
April	1st to 10th	Day	22% 5% 12% 23% 24% 14%	2280
	11th to 20th	Day	17% 5% 7% 22% 29% 19%	2252
	21st to end	Day	17% 4 8% 25% 33% 13%	2280
May	1st to 10th	Day	14% 4 9% 18% 38% 18%	3861
	11th to 20th	Day	21% 3 11% 22% 34% 9%	4074
	21st to end	Day	19% 3 12% 27% 33% 6%	4477
June	1st to 10th	Day	20% 4 11% 21% 31% 13%	2532
	11th to 20th	Day	14% 3 7% 14% 33% 28%	2660
	21st to end	Day	13% 2 6% 11% 32% 37%	2660
July	1st to 10th	Day	8% 2 3 8% 14% 66%	4351
	11th to 20th	Day	10% 5% 4 11% 27% 42%	4331
	21st to end	Day	7% 2 5% 17% 31% 38%	4003
August	1st to 10th	Day	8% 2 3 11% 40% 36%	3770
	11th to 20th	Day	5% 1 3 10% 32% 49%	3760
	21st to end	Day	5% 3 3 9% 30% 50%	4179
September	1st to 10th	Day	11% 2 4 10% 32% 41%	2280
	11th to 20th	Day	11% 2 4 13% 34% 36%	2157
	21st to end	Day	11% 3 3 16% 34% 33%	2090
October	1st to 10th	Day	10% 2 3 11% 19% 55%	3490
	11th to 20th	Day	11% 4 4 15% 35% 31%	3223
	21st to end	Day	15% 4 3 10% 27% 41%	3404
November	1st to 10th	Day	16% 2 6% 16% 25% 35%	1719
	11th to 20th	Day	18% 3 6% 14% 21% 38%	1710
	21st to end	Day	21% 3 7% 12% 24% 32%	1710
December	1st to 10th	Day	25% 4 6% 17% 20% 28%	1710
	11th to 20th	Day	38% 4 8% 13% 20% 17%	2168
	21st to end	Day	37% 4 9% 22% 15% 13%	2881
January	1st to 10th	Night	22% 3 7% 14% 33% 21%	4406
	11th to 20th	Night	26% 6% 10% 23% 18% 18%	4141
	21st to end	Night	31% 4 9% 15% 24% 18%	4229
February	1st to 10th	Night	26% 2 8% 13% 23% 28%	2280
	11th to 20th	Night	24% 3 7% 16% 23% 26%	2219
	21st to end	Night	30% 3 7% 16% 22% 21%	1884
March	1st to 10th	Night	29% 7% 10% 20% 20% 15%	3736
	11th to 20th	Night	20% 5% 15% 28% 16% 16%	3686
	21st to end	Night	24% 7% 15% 25% 20% 9%	3488
April	1st to 10th	Night	24% 5% 11% 24% 20% 16%	1900
	11th to 20th	Night	19% 4 10% 23% 25% 19%	1879
	21st to end	Night	19% 4 13% 23% 28% 12%	1900
May	1st to 10th	Night	17% 5% 13% 16% 31% 18%	3116
	11th to 20th	Night	19% 6% 19% 22% 26% 8%	2835
	21st to end	Night	15% 4 19% 31% 25% 5%	3110
June	1st to 10th	Night	19% 7% 16% 23% 23% 12%	1642
	11th to 20th	Night	13% 4 12% 16% 33% 22%	1520
	21st to end	Night	15% 2 10% 12% 29% 32%	1520
July	1st to 10th	Night	7% 3 3 10% 16% 60%	2500
	11th to 20th	Night	9% 6% 7% 11% 31% 36%	2492
	21st to end	Night	9% 2 8% 23% 27% 32%	3314
August	1st to 10th	Night	9% 2 4 17% 38% 29%	3156
	11th to 20th	Night	7% 3 5% 13% 34% 39%	3156
	21st to end	Night	7% 5% 5% 12% 35% 37%	3498
September	1st to 10th	Night	14% 2 6% 10% 27% 40%	1900
	11th to 20th	Night	11% 2 7% 13% 31% 35%	2023
	21st to end	Night	10% 3 6% 15% 32% 33%	2090
October	1st to 10th	Night	10% 2 4 10% 19% 54%	3490
	11th to 20th	Night	17% 8% 5% 14% 27% 29%	3710
	21st to end	Night	13% 3 4 9% 29% 43%	4151
November	1st to 10th	Night	16% 3 9% 16% 22% 35%	2482
	11th to 20th	Night	17% 2 7% 13% 20% 40%	2470
	21st to end	Night	22% 3 7% 16% 20% 32%	2470
December	1st to 10th	Night	25% 3 7% 15% 19% 30%	2470
	11th to 20th	Night	38% 3 7% 13% 21% 17%	3041
	21st to end	Night	35% 5% 10% 20% 16% 13%	4769

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	17% 3 27% 33% 16% 3	180
	11th to 20th	Day	11% 8% 32% 30% 15% 3	348
	21st to end	Day	12% 12% 29% 32% 13% 2	492
February	1st to 10th	Day	12% 10% 30% 29% 16% 3	460
	11th to 20th	Day	14% 12% 33% 28% 10% 2	455
	21st to end	Day	11% 14% 36% 29% 10% 1	382
March	1st to 10th	Day	14% 14% 34% 26% 11% 1	460
	11th to 20th	Day	13% 15% 29% 30% 12% 1	460
	21st to end	Day	11% 14% 36% 26% 11% 2	506
April	1st to 10th	Day	12% 9% 32% 27% 15% 5%	460
	11th to 20th	Day	8% 13% 36% 29% 10% 4	452
	21st to end	Day	5% 12% 31% 29% 15% 8%	460
May	1st to 10th	Day	7% 12% 28% 34% 14% 4	463
	11th to 20th	Day	7% 7% 28% 34% 20% 4	488
	21st to end	Day	6% 6% 29% 36% 18% 5%	529
June	1st to 10th	Day	4 4 19% 37% 28% 7%	490
	11th to 20th	Day	4 5% 18% 39% 25% 9%	490
	21st to end	Day	5% 2 16% 39% 28% 11%	490
July	1st to 10th	Day	3 1 6% 40% 38% 12%	486
	11th to 20th	Day	1 1 4 34% 43% 17%	484
	21st to end	Day	2 5% 37% 40% 16%	531
August	1st to 10th	Day	3 1 9% 39% 32% 15%	487
	11th to 20th	Day	3 1 7% 36% 40% 14%	477
	21st to end	Day	2 1 9% 41% 34% 14%	539
September	1st to 10th	Day	4 1 14% 42% 27% 12%	490
	11th to 20th	Day	3 3 20% 39% 26% 7%	490
	21st to end	Day	2 3 23% 37% 25% 10%	490
October	1st to 10th	Day	3 6% 21% 37% 25% 8%	490
	11th to 20th	Day	4 5% 24% 40% 20% 7%	490
	21st to end	Day	5% 8% 23% 31% 24% 8%	519
November	1st to 10th	Day	5% 6% 18% 36% 26% 8%	285
	11th to 20th	Day	6% 6% 19% 38% 24% 7%	270
	21st to end	Day	6% 6% 23% 30% 29% 6%	270
December	1st to 10th	Day	10% 7% 19% 37% 25% 3	270
	11th to 20th	Day	8% 7% 17% 34% 26% 7%	270
	21st to end	Day	20% 9% 22% 37% 11% 1	221
January	1st to 10th	Night	12% 4 12% 43% 23% 6%	760
	11th to 20th	Night	9% 3 16% 41% 25% 5%	760
	21st to end	Night	13% 4 17% 39% 23% 4	836
February	1st to 10th	Night	10% 4 15% 37% 27% 7%	760
	11th to 20th	Night	10% 4 15% 39% 25% 7%	744
	21st to end	Night	12% 6% 16% 39% 22% 4	628
March	1st to 10th	Night	11% 5% 15% 45% 21% 3	760
	11th to 20th	Night	10% 3 14% 40% 26% 6%	760
	21st to end	Night	9% 5% 17% 42% 22% 4	836
April	1st to 10th	Night	8% 4 14% 42% 27% 5%	760
	11th to 20th	Night	6% 5% 15% 41% 30% 4	751
	21st to end	Night	5% 7% 15% 38% 28% 7%	760
May	1st to 10th	Night	7% 3 15% 40% 31% 5%	722
	11th to 20th	Night	4 3 15% 39% 35% 4	561
	21st to end	Night	5% 2 16% 41% 33% 3	617
June	1st to 10th	Night	3 2 11% 38% 40% 5%	502
	11th to 20th	Night	3 1 4 35% 50% 7%	380
	21st to end	Night	3 1 2 31% 57% 7%	380
July	1st to 10th	Night	2 2 15% 71% 11%	370
	11th to 20th	Night	1 11% 73% 14%	368
	21st to end	Night	3 1 20% 57% 19%	809
August	1st to 10th	Night	3 1 2 23% 55% 16%	756
	11th to 20th	Night	2 1 1 17% 60% 18%	752
	21st to end	Night	2 1 20% 58% 18%	836
September	1st to 10th	Night	6% 1 3 22% 51% 17%	760
	11th to 20th	Night	3 1 3 30% 51% 13%	760
	21st to end	Night	2 1 5% 33% 48% 11%	760
October	1st to 10th	Night	4 1 7% 38% 43% 7%	760
	11th to 20th	Night	5% 3 8% 34% 38% 11%	760
	21st to end	Night	5% 3 10% 35% 34% 14%	836
November	1st to 10th	Night	4 3 12% 36% 34% 11%	764
	11th to 20th	Night	5% 2 10% 39% 32% 12%	760
	21st to end	Night	6% 3 11% 42% 29% 8%	760
December	1st to 10th	Night	9% 4 13% 38% 29% 6%	760
	11th to 20th	Night	14% 5% 13% 40% 24% 5%	760
	21st to end	Night	12% 6% 16% 42% 19% 6%	836

Page generated at 2024-11-03 10:18:59. Version ac07

Copyright © 2024 Attilla Danko.