Booker
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Booker 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	6% 4 2 4 4 2 4 4 5% 7% 58%	75159
	Night	6% 4 2 4 4 2 4 4 5% 7% 59%	74648
Transparency	Day	25% 6% 11% 24% 25% 10%	74886
	Night	24% 5% 10% 24% 26% 11%	74445
Seeing	Day	8% 8% 31% 33% 17% 3	14279
	Night	8% 9% 39% 35% 9% 1	24351

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	4 5% 2 3 3 2 3 3 5% 5% 64%	1707
	11th to 20th	Day	6% 5% 2 5% 4 3 3 3 3 6% 61%	1665
	21st to end	Day	4 5% 2 3 4 2 4 5% 5% 7% 60%	1854
February	1st to 10th	Day	6% 7% 3 4 4 2 3 3 4 7% 58%	1800
	11th to 20th	Day	5% 7% 2 5% 4 3 3 3 4 6% 59%	1749
	21st to end	Day	6% 5% 3 4 3 2 3 3 4 5% 63%	1480
March	1st to 10th	Day	6% 6% 2 4 4 2 5% 5% 5% 7% 54%	1800
	11th to 20th	Day	10% 4 2 3 3 2 3 4 4 6% 58%	1952
	21st to end	Day	9% 5% 3 5% 4 2 4 4 4 7% 54%	2178
April	1st to 10th	Day	6% 4 1 4 3 2 3 3 4 6% 64%	2160
	11th to 20th	Day	10% 5% 2 4 3 2 3 4 5% 7% 56%	2131
	21st to end	Day	10% 4 1 4 4 3 4 5% 6% 6% 54%	2278
May	1st to 10th	Day	10% 5% 2 4 3 3 4 4 5% 8% 53%	2340
	11th to 20th	Day	8% 4 1 4 4 2 4 5% 5% 8% 54%	2302
	21st to end	Day	7% 5% 1 4 4 3 5% 6% 6% 8% 53%	2531
June	1st to 10th	Day	5% 3 1 4 4 3 3 5% 6% 10% 56%	2340
	11th to 20th	Day	4 3 1 3 3 2 3 4 6% 9% 62%	2340
	21st to end	Day	3 3 1 3 3 3 5% 4 6% 10% 59%	2496
July	1st to 10th	Day	6% 4 2 4 5% 2 4 5% 8% 9% 51%	2448
	11th to 20th	Day	4 2 1 2 3 2 5% 6% 6% 9% 59%	2340
	21st to end	Day	5% 2 1 3 4 3 4 5% 6% 10% 56%	2546
August	1st to 10th	Day	4 3 1 3 4 3 5% 6% 7% 10% 55%	2328
	11th to 20th	Day	6% 5% 3 4 4 3 5% 4 6% 9% 52%	2136
	21st to end	Day	5% 3 1 3 4 3 5% 5% 6% 8% 58%	2376
September	1st to 10th	Day	8% 5% 2 3 5% 2 4 3 3 6% 60%	2093
	11th to 20th	Day	6% 5% 1 3 4 3 3 4 5% 7% 59%	1980
	21st to end	Day	7% 3 1 3 2 2 3 4 5% 5% 66%	1980
October	1st to 10th	Day	11% 5% 2 4 3 2 2 3 3 6% 59%	1980
	11th to 20th	Day	7% 2 1 3 2 2 1 2 3 5% 71%	1918
	21st to end	Day	8% 5% 2 4 3 2 2 2 3 5% 65%	1832
November	1st to 10th	Day	4 4 1 3 3 2 2 3 3 7% 68%	1673
	11th to 20th	Day	3 4 1 3 2 2 3 3 4 5% 70%	1710
	21st to end	Day	8% 5% 1 3 4 2 4 3 4 5% 63%	1710
December	1st to 10th	Day	4 4 2 4 4 2 4 4 5% 8% 59%	1710
	11th to 20th	Day	7% 5% 2 3 3 2 3 4 4 7% 60%	2161
	21st to end	Day	5% 9% 4 6% 5% 3 4 4 4 5% 50%	3135
January	1st to 10th	Night	4 5% 1 4 4 2 3 4 5% 7% 61%	2458
	11th to 20th	Night	6% 4 2 4 4 3 3 4 4 6% 62%	2418
	21st to end	Night	5% 4 2 3 3 3 3 3 4 6% 64%	2516
February	1st to 10th	Night	6% 5% 4 6% 5% 2 4 4 3 5% 56%	2160
	11th to 20th	Night	6% 5% 2 6% 5% 3 4 4 5% 7% 53%	2098
	21st to end	Night	6% 5% 2 4 3 2 3 5% 4 5% 61%	1776
March	1st to 10th	Night	5% 4 3 4 4 2 4 4 6% 8% 57%	2160
	11th to 20th	Night	10% 5% 2 3 4 2 3 3 4 6% 59%	2008
	21st to end	Night	8% 4 2 4 3 3 4 5% 6% 7% 53%	2174
April	1st to 10th	Night	6% 3 2 4 4 2 4 4 5% 6% 61%	1800
	11th to 20th	Night	7% 5% 2 5% 4 2 3 4 6% 8% 53%	1777
	21st to end	Night	10% 5% 2 3 4 1 3 4 5% 8% 55%	1679
May	1st to 10th	Night	8% 5% 2 5% 4 3 3 5% 5% 8% 52%	1620
	11th to 20th	Night	9% 4 1 3 4 2 4 5% 5% 7% 55%	1593
	21st to end	Night	8% 4 2 3 3 3 6% 4 7% 9% 50%	1745
June	1st to 10th	Night	4 2 1 3 3 2 3 5% 6% 11% 59%	1620
	11th to 20th	Night	4 2 1 3 3 2 5% 5% 6% 10% 58%	1620
	21st to end	Night	4 3 2 3 4 2 4 4 8% 10% 56%	1451
July	1st to 10th	Night	4 3 1 3 4 3 5% 5% 7% 11% 55%	1498
	11th to 20th	Night	3 2 1 3 3 3 4 5% 6% 10% 59%	1620
	21st to end	Night	5% 2 2 3 4 4 5% 4 6% 11% 53%	1772
August	1st to 10th	Night	4 3 1 4 4 3 4 7% 8% 11% 50%	1611
	11th to 20th	Night	6% 4 2 5% 5% 3 5% 5% 6% 9% 51%	1761
	21st to end	Night	3 3 2 4 3 3 4 4 7% 8% 58%	1980
September	1st to 10th	Night	7% 4 1 3 3 2 3 4 4 8% 62%	1866
	11th to 20th	Night	7% 4 1 4 4 2 3 4 5% 7% 58%	1980
	21st to end	Night	4 3 1 3 4 2 4 4 5% 6% 65%	1980
October	1st to 10th	Night	10% 4 2 3 2 2 3 3 4 5% 61%	1980
	11th to 20th	Night	7% 3 1 3 3 1 2 2 3 4 71%	2023
	21st to end	Night	7% 4 2 3 3 1 2 2 4 4 68%	2521
November	1st to 10th	Night	4 4 1 2 2 1 2 3 4 6% 70%	2405
	11th to 20th	Night	3 2 1 2 2 2 3 3 4 6% 71%	2470
	21st to end	Night	7% 4 2 4 3 2 3 3 4 6% 63%	2470
December	1st to 10th	Night	5% 3 2 5% 5% 2 3 4 5% 7% 60%	2470
	11th to 20th	Night	6% 5% 2 3 3 2 3 3 5% 7% 60%	3036
	21st to end	Night	6% 6% 2 4 5% 3 5% 5% 5% 6% 52%	4532

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 10% 27% 30% 8%	1708
	11th to 20th	Day	27% 3 9% 23% 29% 9%	1665
	21st to end	Day	24% 4 8% 29% 28% 7%	1854
February	1st to 10th	Day	29% 4 12% 25% 22% 9%	1800
	11th to 20th	Day	28% 4 9% 26% 21% 12%	1759
	21st to end	Day	25% 3 5% 24% 29% 14%	1480
March	1st to 10th	Day	28% 5% 8% 22% 22% 15%	1800
	11th to 20th	Day	28% 4 10% 20% 26% 12%	1952
	21st to end	Day	32% 4 9% 25% 21% 9%	2178
April	1st to 10th	Day	23% 3 9% 23% 26% 16%	2160
	11th to 20th	Day	29% 6% 9% 22% 23% 12%	2134
	21st to end	Day	28% 6% 11% 22% 25% 8%	2278
May	1st to 10th	Day	30% 6% 11% 20% 21% 12%	2340
	11th to 20th	Day	27% 7% 14% 27% 20% 4	2306
	21st to end	Day	27% 8% 14% 24% 21% 7%	2531
June	1st to 10th	Day	22% 8% 17% 26% 18% 8%	2340
	11th to 20th	Day	18% 6% 13% 24% 26% 13%	2340
	21st to end	Day	20% 7% 12% 25% 25% 11%	2496
July	1st to 10th	Day	26% 9% 13% 25% 20% 6%	2383
	11th to 20th	Day	18% 8% 12% 26% 26% 9%	2271
	21st to end	Day	23% 9% 15% 29% 22% 2	2405
August	1st to 10th	Day	21% 8% 17% 31% 17% 6%	2314
	11th to 20th	Day	28% 8% 10% 26% 19% 8%	2136
	21st to end	Day	24% 7% 13% 24% 23% 9%	2376
September	1st to 10th	Day	28% 5% 9% 21% 23% 14%	2093
	11th to 20th	Day	25% 6% 12% 18% 29% 9%	1980
	21st to end	Day	19% 5% 11% 24% 29% 12%	1980
October	1st to 10th	Day	29% 5% 10% 19% 26% 12%	1980
	11th to 20th	Day	18% 3 6% 21% 31% 22%	1918
	21st to end	Day	25% 3 8% 20% 29% 15%	1832
November	1st to 10th	Day	18% 3 8% 23% 31% 18%	1673
	11th to 20th	Day	18% 3 8% 27% 33% 11%	1710
	21st to end	Day	25% 3 5% 20% 30% 15%	1710
December	1st to 10th	Day	23% 4 11% 28% 26% 7%	1710
	11th to 20th	Day	24% 4 10% 23% 29% 10%	2159
	21st to end	Day	34% 7% 9% 17% 25% 9%	3135
January	1st to 10th	Night	23% 5% 9% 26% 29% 7%	2458
	11th to 20th	Night	25% 4 8% 24% 30% 10%	2418
	21st to end	Night	23% 2 7% 28% 31% 9%	2516
February	1st to 10th	Night	31% 4 9% 21% 24% 11%	2160
	11th to 20th	Night	31% 5% 8% 21% 24% 11%	2098
	21st to end	Night	25% 4 7% 20% 27% 17%	1776
March	1st to 10th	Night	25% 5% 8% 24% 26% 12%	2160
	11th to 20th	Night	28% 4 8% 22% 25% 13%	2008
	21st to end	Night	28% 6% 12% 25% 19% 10%	2174
April	1st to 10th	Night	23% 5% 9% 22% 24% 17%	1800
	11th to 20th	Night	28% 7% 11% 23% 22% 10%	1777
	21st to end	Night	28% 5% 12% 26% 21% 7%	1679
May	1st to 10th	Night	29% 6% 10% 20% 22% 13%	1620
	11th to 20th	Night	27% 8% 13% 23% 21% 8%	1594
	21st to end	Night	28% 9% 14% 24% 17% 7%	1746
June	1st to 10th	Night	19% 9% 18% 31% 15% 8%	1620
	11th to 20th	Night	20% 8% 14% 24% 23% 11%	1620
	21st to end	Night	21% 7% 15% 24% 24% 10%	1451
July	1st to 10th	Night	21% 8% 17% 29% 19% 6%	1455
	11th to 20th	Night	17% 7% 16% 25% 26% 9%	1566
	21st to end	Night	23% 8% 13% 28% 23% 4	1673
August	1st to 10th	Night	22% 10% 15% 27% 18% 7%	1601
	11th to 20th	Night	30% 6% 13% 25% 20% 7%	1761
	21st to end	Night	21% 8% 12% 27% 23% 9%	1980
September	1st to 10th	Night	22% 6% 13% 22% 26% 11%	1866
	11th to 20th	Night	25% 6% 12% 17% 32% 8%	1980
	21st to end	Night	18% 6% 10% 22% 30% 14%	1980
October	1st to 10th	Night	25% 6% 9% 17% 29% 13%	1980
	11th to 20th	Night	20% 3 7% 17% 33% 20%	2023
	21st to end	Night	21% 3 6% 23% 29% 18%	2521
November	1st to 10th	Night	17% 3 9% 23% 28% 19%	2405
	11th to 20th	Night	16% 3 7% 27% 34% 14%	2470
	21st to end	Night	25% 4 4 22% 28% 18%	2470
December	1st to 10th	Night	24% 4 9% 26% 28% 9%	2470
	11th to 20th	Night	24% 4 8% 24% 31% 9%	3037
	21st to end	Night	29% 7% 11% 20% 24% 9%	4532

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	8% 8% 44% 30% 9% 1	270
	11th to 20th	Day	13% 17% 33% 27% 9% 1	255
	21st to end	Day	6% 11% 43% 32% 9%	264
February	1st to 10th	Day	6% 14% 47% 30% 4	240
	11th to 20th	Day	10% 14% 49% 21% 5% 1	240
	21st to end	Day	9% 15% 54% 19% 4	198
March	1st to 10th	Day	6% 14% 48% 25% 7%	240
	11th to 20th	Day	18% 15% 35% 21% 8% 2	240
	21st to end	Day	19% 14% 40% 20% 8%	264
April	1st to 10th	Day	10% 18% 46% 19% 6%	420
	11th to 20th	Day	13% 18% 40% 26% 2	412
	21st to end	Day	14% 18% 40% 20% 7% 2	538
May	1st to 10th	Day	12% 10% 41% 28% 8% 1	603
	11th to 20th	Day	11% 9% 31% 33% 15% 1	625
	21st to end	Day	10% 10% 36% 31% 11% 1	679
June	1st to 10th	Day	5% 7% 30% 37% 14% 6%	630
	11th to 20th	Day	5% 6% 36% 35% 17% 1	630
	21st to end	Day	3 5% 29% 42% 19% 3	630
July	1st to 10th	Day	6% 2 15% 37% 32% 9%	621
	11th to 20th	Day	3 1 13% 43% 32% 8%	618
	21st to end	Day	5% 1 9% 41% 36% 8%	684
August	1st to 10th	Day	5% 1 14% 41% 33% 5%	626
	11th to 20th	Day	5% 2 16% 42% 30% 4	450
	21st to end	Day	5% 2 15% 41% 28% 8%	495
September	1st to 10th	Day	8% 3 16% 39% 28% 6%	383
	11th to 20th	Day	4 2 21% 44% 25% 4	270
	21st to end	Day	9% 5% 27% 33% 20% 6%	270
October	1st to 10th	Day	14% 6% 28% 33% 16% 3	270
	11th to 20th	Day	7% 6% 35% 31% 18% 2	270
	21st to end	Day	16% 8% 34% 27% 12% 3	297
November	1st to 10th	Day	6% 9% 36% 33% 14% 1	270
	11th to 20th	Day	7% 11% 39% 28% 11% 3	270
	21st to end	Day	14% 14% 33% 25% 12% 3	270
December	1st to 10th	Day	4 9% 44% 30% 12% 1	270
	11th to 20th	Day	10% 11% 37% 31% 11% 1	270
	21st to end	Day	15% 18% 38% 22% 6%	297
January	1st to 10th	Night	7% 10% 42% 35% 7%	950
	11th to 20th	Night	8% 11% 37% 33% 10% 1	930
	21st to end	Night	7% 10% 42% 35% 6% 1	932
February	1st to 10th	Night	9% 10% 43% 32% 5% 1	720
	11th to 20th	Night	8% 13% 48% 26% 5%	704
	21st to end	Night	10% 14% 46% 25% 5%	592
March	1st to 10th	Night	8% 11% 47% 29% 5%	720
	11th to 20th	Night	13% 13% 41% 26% 6%	568
	21st to end	Night	10% 18% 49% 20% 3	594
April	1st to 10th	Night	8% 15% 46% 30% 1	540
	11th to 20th	Night	11% 17% 50% 20% 2	533
	21st to end	Night	11% 15% 46% 22% 5% 1	540
May	1st to 10th	Night	10% 13% 49% 24% 4	540
	11th to 20th	Night	10% 11% 40% 29% 9% 1	531
	21st to end	Night	10% 9% 45% 26% 10%	584
June	1st to 10th	Night	4 7% 43% 34% 11% 1	540
	11th to 20th	Night	4 6% 48% 34% 7%	540
	21st to end	Night	3 4 40% 42% 10% 1	540
July	1st to 10th	Night	5% 2 24% 48% 20% 2	525
	11th to 20th	Night	4 1 19% 58% 16% 1	522
	21st to end	Night	5% 1 18% 53% 21% 2	591
August	1st to 10th	Night	4 1 21% 50% 23% 1	537
	11th to 20th	Night	7% 2 24% 51% 16%	534
	21st to end	Night	4 1 22% 47% 26% 1	594
September	1st to 10th	Night	8% 1 28% 48% 14% 1	540
	11th to 20th	Night	9% 2 36% 42% 9% 1	540
	21st to end	Night	4 4 36% 41% 14% 1	540
October	1st to 10th	Night	11% 6% 34% 37% 11% 1	540
	11th to 20th	Night	8% 10% 40% 34% 7% 1	583
	21st to end	Night	9% 11% 36% 32% 12% 1	937
November	1st to 10th	Night	6% 7% 38% 38% 10% 1	925
	11th to 20th	Night	5% 11% 43% 33% 8%	950
	21st to end	Night	9% 11% 34% 36% 9% 1	950
December	1st to 10th	Night	7% 9% 40% 37% 8% 1	950
	11th to 20th	Night	11% 10% 44% 30% 5%	950
	21st to end	Night	11% 14% 38% 31% 6%	1045

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