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 2024-01-16.

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% 3 1 3 3 2 3 3 4 6% 67%	74957
	Night	5% 3 1 3 3 2 3 3 4 6% 65%	74852
Transparency	Day	19% 3 7% 17% 27% 26%	74683
	Night	21% 3 9% 17% 24% 26%	74648
Seeing	Day	7% 7% 22% 35% 23% 7%	14459
	Night	7% 3 10% 35% 36% 9%	24378

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% 4 2 4 4 3 3 4 4 7% 57%	1519
	11th to 20th	Day	7% 4 2 4 3 2 3 4 4 6% 62%	1644
	21st to end	Day	10% 5% 2 4 3 2 2 3 4 5% 60%	1951
February	1st to 10th	Day	8% 4 2 3 3 2 2 3 4 6% 64%	1800
	11th to 20th	Day	8% 3 1 3 3 2 3 4 4 7% 63%	1750
	21st to end	Day	7% 5% 2 3 4 3 3 3 4 6% 59%	1480
March	1st to 10th	Day	7% 5% 2 5% 4 3 4 5% 4 6% 54%	1800
	11th to 20th	Day	6% 4 2 3 4 2 4 4 6% 9% 57%	1858
	21st to end	Day	6% 3 2 3 4 2 4 5% 5% 8% 59%	2376
April	1st to 10th	Day	6% 4 1 3 4 2 4 4 4 7% 61%	2160
	11th to 20th	Day	3 3 2 3 2 2 3 4 4 6% 68%	2129
	21st to end	Day	4 3 1 3 2 2 3 4 4 7% 68%	2160
May	1st to 10th	Day	4 4 2 3 3 2 3 5% 5% 7% 63%	2195
	11th to 20th	Day	3 4 2 4 3 2 3 4 4 7% 65%	2301
	21st to end	Day	4 4 1 3 3 3 3 4 5% 6% 65%	2530
June	1st to 10th	Day	4 4 2 4 3 2 3 3 4 8% 65%	2398
	11th to 20th	Day	3 4 1 2 2 1 2 3 3 6% 73%	2520
	21st to end	Day	2 2 1 2 2 1 3 3 3 5% 77%	2520
July	1st to 10th	Day	1 3 1 2 2 2 3 3 4 5% 74%	2520
	11th to 20th	Day	1 2 1 2 2 1 2 3 4 5% 76%	2519
	21st to end	Day	1 1 1 2 2 2 3 3 5% 6% 73%	2364
August	1st to 10th	Day	2 2 1 2 2 1 2 3 4 4 78%	2149
	11th to 20th	Day	1 1 1 1 1 1 3 4 5% 82%	2122
	21st to end	Day	1 1 1 2 2 2 2 3 4 5% 80%	2376
September	1st to 10th	Day	3 2 1 2 2 1 2 2 4 4 78%	2160
	11th to 20th	Day	1 2 2 2 1 2 2 3 4 80%	2044
	21st to end	Day	2 2 1 1 2 1 2 2 4 4 79%	1980
October	1st to 10th	Day	3 2 1 2 3 2 2 3 4 5% 71%	1980
	11th to 20th	Day	3 3 1 2 2 2 2 3 3 4 76%	1836
	21st to end	Day	3 2 1 2 3 1 2 4 4 5% 73%	1962
November	1st to 10th	Day	3 2 1 2 3 2 3 3 4 5% 72%	1674
	11th to 20th	Day	4 3 1 3 3 2 2 4 4 5% 70%	1710
	21st to end	Day	4 3 1 3 4 2 3 4 4 8% 63%	1710
December	1st to 10th	Day	8% 3 2 4 3 2 3 4 5% 8% 59%	1710
	11th to 20th	Day	16% 5% 2 5% 5% 3 4 4 5% 8% 45%	2169
	21st to end	Day	13% 5% 2 5% 7% 3 4 4 5% 6% 46%	2881
January	1st to 10th	Night	10% 3 2 3 3 2 3 4 4 7% 59%	2646
	11th to 20th	Night	6% 3 1 3 3 1 3 3 4 8% 64%	2424
	21st to end	Night	9% 4 2 3 3 1 2 3 4 6% 63%	2399
February	1st to 10th	Night	6% 4 2 4 3 2 2 2 3 5% 65%	2160
	11th to 20th	Night	8% 3 1 3 4 2 3 3 4 5% 64%	2098
	21st to end	Night	10% 4 2 3 4 2 3 3 4 6% 59%	1776
March	1st to 10th	Night	7% 5% 2 4 3 3 4 4 5% 8% 54%	2160
	11th to 20th	Night	7% 4 2 2 4 3 3 5% 6% 7% 56%	2098
	21st to end	Night	7% 3 2 3 3 2 5% 4 5% 7% 60%	1980
April	1st to 10th	Night	6% 4 2 4 4 2 3 3 4 6% 62%	1800
	11th to 20th	Night	4 4 2 2 3 1 4 4 4 7% 67%	1779
	21st to end	Night	4 3 1 3 3 2 3 4 5% 8% 65%	1800
May	1st to 10th	Night	4 6% 2 3 3 2 3 5% 5% 8% 60%	1764
	11th to 20th	Night	3 5% 2 3 3 2 4 4 4 8% 63%	1594
	21st to end	Night	4 4 2 3 2 2 2 2 4 6% 69%	1746
June	1st to 10th	Night	2 4 1 3 3 2 4 4 4 8% 63%	1556
	11th to 20th	Night	2 4 1 2 2 1 2 3 4 7% 73%	1440
	21st to end	Night	1 2 1 3 2 1 2 3 4 6% 74%	1440
July	1st to 10th	Night	1 2 1 3 2 2 2 3 3 7% 75%	1440
	11th to 20th	Night	1 1 2 3 2 3 4 5% 7% 72%	1440
	21st to end	Night	3 2 1 2 3 2 3 4 5% 8% 69%	1948
August	1st to 10th	Night	3 2 1 2 2 1 3 3 4 6% 74%	1790
	11th to 20th	Night	2 2 1 2 3 1 2 4 4 7% 74%	1780
	21st to end	Night	1 1 1 2 2 1 2 3 3 6% 78%	1980
September	1st to 10th	Night	3 2 1 3 3 1 2 3 3 6% 75%	1800
	11th to 20th	Night	2 1 1 2 1 2 3 3 4 80%	1916
	21st to end	Night	1 1 1 2 2 1 2 3 4 6% 78%	1980
October	1st to 10th	Night	3 2 1 3 3 2 3 3 3 5% 71%	1980
	11th to 20th	Night	4 2 1 2 3 1 2 4 3 5% 72%	2119
	21st to end	Night	3 3 1 2 2 1 3 3 3 4 74%	2394
November	1st to 10th	Night	3 3 1 2 3 2 2 3 4 6% 71%	2405
	11th to 20th	Night	4 2 1 3 2 1 3 3 3 6% 72%	2470
	21st to end	Night	4 3 2 3 4 3 4 4 5% 6% 63%	2470
December	1st to 10th	Night	7% 3 2 5% 3 2 3 3 5% 7% 60%	2470
	11th to 20th	Night	16% 6% 2 4 4 2 3 3 4 7% 48%	3041
	21st to end	Night	12% 6% 2 4 4 3 5% 5% 5% 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	28% 3 6% 17% 23% 22%	1519
	11th to 20th	Day	23% 4 7% 13% 26% 27%	1644
	21st to end	Day	27% 4 7% 17% 17% 29%	1951
February	1st to 10th	Day	23% 3 6% 14% 25% 29%	1800
	11th to 20th	Day	22% 4 6% 16% 27% 26%	1760
	21st to end	Day	24% 5% 8% 18% 24% 21%	1480
March	1st to 10th	Day	30% 4 8% 19% 18% 22%	1800
	11th to 20th	Day	24% 4 10% 19% 22% 19%	1858
	21st to end	Day	22% 6% 10% 22% 23% 16%	2376
April	1st to 10th	Day	23% 5% 12% 24% 22% 14%	2160
	11th to 20th	Day	17% 5% 7% 22% 30% 19%	2132
	21st to end	Day	18% 4 8% 25% 31% 14%	2160
May	1st to 10th	Day	20% 5% 11% 22% 28% 14%	2195
	11th to 20th	Day	20% 5% 10% 23% 29% 13%	2304
	21st to end	Day	20% 5% 10% 28% 29% 8%	2530
June	1st to 10th	Day	20% 4 11% 20% 31% 14%	2398
	11th to 20th	Day	15% 3 7% 14% 33% 28%	2520
	21st to end	Day	14% 2 6% 11% 33% 35%	2520
July	1st to 10th	Day	14% 2 4 14% 23% 43%	2451
	11th to 20th	Day	13% 2 7% 19% 32% 27%	2444
	21st to end	Day	13% 3 9% 22% 33% 19%	2233
August	1st to 10th	Day	12% 2 4 13% 32% 37%	2135
	11th to 20th	Day	9% 1 5% 14% 36% 36%	2122
	21st to end	Day	9% 2 4 13% 35% 37%	2376
September	1st to 10th	Day	11% 2 5% 10% 33% 39%	2160
	11th to 20th	Day	10% 2 5% 13% 33% 37%	2044
	21st to end	Day	12% 2 3 17% 35% 31%	1980
October	1st to 10th	Day	15% 3 6% 20% 23% 33%	1980
	11th to 20th	Day	14% 3 5% 13% 28% 37%	1836
	21st to end	Day	14% 4 4 13% 22% 44%	1962
November	1st to 10th	Day	16% 2 6% 16% 24% 35%	1674
	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	27% 3 8% 15% 25% 22%	2646
	11th to 20th	Night	21% 3 8% 16% 22% 30%	2424
	21st to end	Night	25% 2 8% 17% 18% 31%	2399
February	1st to 10th	Night	24% 2 7% 13% 24% 30%	2160
	11th to 20th	Night	24% 3 6% 17% 23% 27%	2099
	21st to end	Night	29% 3 7% 15% 23% 23%	1776
March	1st to 10th	Night	28% 3 10% 18% 20% 20%	2160
	11th to 20th	Night	27% 5% 12% 16% 22% 19%	2098
	21st to end	Night	24% 5% 11% 22% 23% 15%	1980
April	1st to 10th	Night	25% 5% 11% 25% 19% 16%	1800
	11th to 20th	Night	19% 4 10% 23% 25% 19%	1779
	21st to end	Night	19% 4 13% 24% 27% 13%	1800
May	1st to 10th	Night	23% 7% 12% 22% 23% 14%	1764
	11th to 20th	Night	20% 8% 11% 23% 25% 12%	1595
	21st to end	Night	17% 5% 11% 32% 25% 10%	1746
June	1st to 10th	Night	19% 8% 14% 23% 24% 13%	1556
	11th to 20th	Night	13% 3 12% 17% 34% 21%	1440
	21st to end	Night	16% 2 11% 12% 29% 31%	1440
July	1st to 10th	Night	12% 2 5% 19% 24% 39%	1400
	11th to 20th	Night	13% 3 12% 18% 29% 26%	1392
	21st to end	Night	16% 2 14% 26% 26% 16%	1840
August	1st to 10th	Night	14% 2 7% 14% 32% 31%	1780
	11th to 20th	Night	12% 2 8% 13% 34% 32%	1780
	21st to end	Night	11% 1 6% 16% 33% 32%	1980
September	1st to 10th	Night	15% 2 6% 11% 27% 39%	1800
	11th to 20th	Night	11% 2 7% 13% 30% 37%	1916
	21st to end	Night	11% 2 6% 15% 33% 33%	1980
October	1st to 10th	Night	17% 3 7% 17% 22% 34%	1980
	11th to 20th	Night	17% 3 7% 15% 23% 35%	2119
	21st to end	Night	15% 2 6% 11% 21% 44%	2394
November	1st to 10th	Night	16% 3 9% 16% 21% 36%	2405
	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	334
	21st to end	Day	11% 12% 30% 32% 13% 2	450
February	1st to 10th	Day	10% 10% 29% 30% 16% 4	420
	11th to 20th	Day	14% 13% 33% 28% 10% 2	415
	21st to end	Day	11% 14% 36% 29% 10%	346
March	1st to 10th	Day	14% 15% 34% 25% 10% 1	420
	11th to 20th	Day	13% 14% 29% 31% 12% 1	420
	21st to end	Day	11% 15% 35% 26% 12% 2	462
April	1st to 10th	Day	11% 9% 32% 26% 16% 5%	420
	11th to 20th	Day	8% 13% 37% 29% 9% 4	412
	21st to end	Day	5% 12% 32% 29% 14% 7%	420
May	1st to 10th	Day	7% 13% 27% 34% 15% 4	423
	11th to 20th	Day	7% 7% 28% 35% 19% 4	448
	21st to end	Day	6% 7% 30% 36% 16% 5%	485
June	1st to 10th	Day	5% 4 20% 38% 26% 7%	450
	11th to 20th	Day	5% 5% 15% 40% 25% 9%	450
	21st to end	Day	5% 2 15% 38% 29% 12%	450
July	1st to 10th	Day	3 1 5% 40% 39% 11%	446
	11th to 20th	Day	1 1 2 34% 45% 18%	444
	21st to end	Day	2 3 37% 41% 16%	487
August	1st to 10th	Day	3 1 9% 39% 33% 15%	447
	11th to 20th	Day	3 1 5% 36% 42% 13%	437
	21st to end	Day	2 1 7% 41% 36% 14%	495
September	1st to 10th	Day	5% 1 14% 42% 28% 11%	450
	11th to 20th	Day	4 2 18% 41% 28% 8%	450
	21st to end	Day	2 3 22% 37% 26% 9%	450
October	1st to 10th	Day	3 6% 22% 38% 24% 6%	450
	11th to 20th	Day	4 4 25% 40% 20% 6%	450
	21st to end	Day	5% 7% 23% 32% 25% 7%	477
November	1st to 10th	Day	4 6% 18% 37% 26% 9%	270
	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	8% 3 15% 42% 26% 6%	740
	21st to end	Night	12% 4 17% 39% 24% 4	792
February	1st to 10th	Night	8% 3 15% 38% 28% 7%	720
	11th to 20th	Night	10% 4 15% 39% 25% 7%	704
	21st to end	Night	12% 6% 16% 39% 23% 4	592
March	1st to 10th	Night	10% 5% 15% 45% 22% 3	720
	11th to 20th	Night	11% 3 13% 38% 27% 7%	720
	21st to end	Night	9% 5% 16% 42% 23% 5%	792
April	1st to 10th	Night	8% 4 13% 43% 27% 5%	720
	11th to 20th	Night	6% 4 15% 41% 29% 4	711
	21st to end	Night	5% 7% 15% 38% 28% 7%	720
May	1st to 10th	Night	7% 2 14% 39% 32% 5%	684
	11th to 20th	Night	5% 3 15% 39% 35% 4	531
	21st to end	Night	5% 2 17% 40% 33% 4	584
June	1st to 10th	Night	3 2 11% 38% 40% 5%	476
	11th to 20th	Night	3 1 4 34% 51% 7%	360
	21st to end	Night	3 1 2 31% 56% 6%	360
July	1st to 10th	Night	2 2 15% 71% 11%	350
	11th to 20th	Night	1 10% 73% 15%	348
	21st to end	Night	4 1 19% 58% 19%	766
August	1st to 10th	Night	3 1 2 23% 55% 16%	716
	11th to 20th	Night	2 1 2 17% 60% 18%	712
	21st to end	Night	2 1 19% 59% 18%	792
September	1st to 10th	Night	6% 1 3 22% 52% 17%	720
	11th to 20th	Night	3 3 29% 52% 13%	720
	21st to end	Night	2 1 5% 32% 49% 11%	720
October	1st to 10th	Night	4 1 7% 39% 41% 7%	720
	11th to 20th	Night	5% 3 8% 35% 38% 11%	720
	21st to end	Night	5% 3 10% 34% 34% 14%	792
November	1st to 10th	Night	4 3 11% 36% 34% 12%	740
	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 2024-01-12 22:38:13. Version ac07

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