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 2026-04-02.

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 3 3 3 3 5% 7% 59%	140699
	Night	6% 4 2 4 3 3 4 3 5% 7% 59%	138527
Transparency	Day	24% 5% 9% 23% 27% 11%	135650
	Night	24% 5% 9% 23% 27% 12%	134586
Seeing	Day	8% 9% 31% 32% 16% 3	17240
	Night	8% 9% 39% 34% 9%	27321

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% 7% 3 6% 4 3 3 3 4 5% 54%	5287
	11th to 20th	Day	3 5% 3 4 2 2 3 2 3 5% 68%	5310
	21st to end	Day	8% 5% 2 5% 3 2 3 3 4 6% 59%	6066
February	1st to 10th	Day	5% 8% 2 5% 3 3 3 3 4 6% 57%	2100
	11th to 20th	Day	5% 7% 2 6% 3 4 3 2 4 6% 58%	2049
	21st to end	Day	5% 5% 2 4 3 2 2 3 3 5% 66%	1730
March	1st to 10th	Day	6% 5% 2 4 3 3 3 3 4 5% 61%	4600
	11th to 20th	Day	5% 3 2 4 4 4 4 3 5% 7% 59%	5034
	21st to end	Day	5% 5% 2 5% 3 4 4 3 5% 7% 57%	5598
April	1st to 10th	Day	7% 5% 1 4 3 2 3 2 4 6% 63%	2413
	11th to 20th	Day	10% 5% 2 4 3 2 4 3 5% 7% 56%	2371
	21st to end	Day	10% 4 1 4 3 4 3 4 6% 6% 54%	2532
May	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 4 6% 52%	5900
	11th to 20th	Day	6% 3 1 4 3 3 4 3 5% 6% 62%	5862
	21st to end	Day	11% 6% 2 4 3 3 3 3 4 5% 54%	6446
June	1st to 10th	Day	5% 4 1 5% 3 4 3 4 7% 9% 54%	2600
	11th to 20th	Day	4 3 1 4 2 3 3 4 7% 8% 61%	2600
	21st to end	Day	3 3 1 4 3 4 5% 4 7% 9% 57%	2774
July	1st to 10th	Day	5% 4 2 6% 4 4 5% 4 7% 9% 50%	6163
	11th to 20th	Day	4 3 1 4 3 3 4 3 6% 8% 61%	5900
	21st to end	Day	4 2 1 3 3 3 4 4 6% 10% 61%	6424
August	1st to 10th	Day	8% 4 2 5% 3 4 4 4 6% 8% 53%	5801
	11th to 20th	Day	3 3 1 4 3 3 4 4 7% 10% 58%	5393
	21st to end	Day	9% 4 2 4 3 4 5% 3 6% 8% 52%	5984
September	1st to 10th	Day	8% 5% 2 4 4 3 4 3 4 6% 58%	2325
	11th to 20th	Day	6% 4 1 4 3 4 3 4 5% 7% 59%	2200
	21st to end	Day	7% 3 1 3 2 3 3 3 5% 6% 65%	2200
October	1st to 10th	Day	6% 4 1 3 2 2 2 2 4 5% 71%	3590
	11th to 20th	Day	4 2 1 4 3 3 2 3 4 7% 67%	3448
	21st to end	Day	5% 4 1 3 1 2 2 1 3 5% 72%	3268
November	1st to 10th	Day	4 4 1 4 2 2 3 2 4 6% 67%	1853
	11th to 20th	Day	3 4 1 4 2 2 3 2 4 5% 70%	1890
	21st to end	Day	7% 4 1 4 3 3 3 2 4 5% 63%	1890
December	1st to 10th	Day	2 2 1 3 2 2 2 2 4 6% 72%	3030
	11th to 20th	Day	4 3 1 3 2 2 3 3 5% 8% 66%	3481
	21st to end	Day	7% 10% 4 8% 4 4 3 3 4 5% 49%	4587
January	1st to 10th	Night	6% 6% 3 6% 4 4 4 3 5% 7% 52%	7634
	11th to 20th	Night	2 3 2 4 3 3 3 3 4 5% 69%	7680
	21st to end	Night	9% 7% 3 6% 3 3 3 3 3 5% 55%	8204
February	1st to 10th	Night	5% 6% 3 6% 5% 3 4 3 4 5% 56%	2520
	11th to 20th	Night	6% 6% 2 7% 4 4 4 3 5% 7% 52%	2458
	21st to end	Night	5% 5% 1 4 2 3 3 4 4 6% 63%	2076
March	1st to 10th	Night	3 5% 2 4 3 2 3 3 4 6% 65%	5494
	11th to 20th	Night	5% 4 1 3 3 3 3 3 5% 7% 64%	5148
	21st to end	Night	4 4 2 5% 3 3 3 3 6% 7% 59%	5572
April	1st to 10th	Night	7% 5% 1 4 3 3 4 3 5% 6% 60%	2010
	11th to 20th	Night	7% 5% 2 5% 4 3 3 3 6% 7% 55%	1977
	21st to end	Night	11% 5% 2 4 3 2 3 3 5% 8% 54%	1865
May	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 47%	4080
	11th to 20th	Night	6% 3 1 4 3 3 4 4 5% 7% 61%	4053
	21st to end	Night	11% 6% 2 4 3 3 3 3 5% 6% 53%	4451
June	1st to 10th	Night	5% 3 1 4 3 3 4 4 7% 11% 56%	1800
	11th to 20th	Night	4 2 1 4 3 3 4 5% 7% 10% 58%	1800
	21st to end	Night	4 4 2 4 4 4 4 4 7% 10% 54%	1611
July	1st to 10th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 51%	3789
	11th to 20th	Night	3 3 2 4 4 4 5% 4 7% 11% 52%	4080
	21st to end	Night	3 2 1 4 4 4 5% 4 7% 9% 56%	4449
August	1st to 10th	Night	6% 3 2 5% 4 5% 5% 5% 9% 11% 46%	4011
	11th to 20th	Night	3 3 2 5% 5% 5% 6% 4 9% 12% 48%	4439
	21st to end	Night	13% 5% 2 4 3 4 4 4 7% 10% 46%	4993
September	1st to 10th	Night	6% 4 1 3 3 3 4 3 5% 7% 62%	2074
	11th to 20th	Night	7% 4 1 5% 3 3 4 3 5% 7% 59%	2200
	21st to end	Night	4 3 1 4 3 3 4 3 5% 6% 65%	2200
October	1st to 10th	Night	6% 3 1 2 1 1 2 2 3 5% 74%	3590
	11th to 20th	Night	5% 3 1 3 3 2 3 3 4 7% 65%	3657
	21st to end	Night	5% 3 1 2 2 2 3 2 4 5% 72%	4528
November	1st to 10th	Night	5% 4 1 3 2 2 2 3 4 6% 69%	2665
	11th to 20th	Night	3 2 1 3 2 3 3 3 4 6% 71%	2730
	21st to end	Night	6% 5% 2 4 3 3 2 2 4 6% 63%	2730
December	1st to 10th	Night	3 2 1 4 3 2 2 2 4 5% 70%	4380
	11th to 20th	Night	4 3 1 3 2 3 2 2 4 6% 69%	4946
	21st to end	Night	8% 8% 2 5% 3 3 4 3 5% 6% 53%	6633

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	32% 3 6% 25% 26% 8%	5290
	11th to 20th	Day	20% 3 8% 19% 34% 16%	5310
	21st to end	Day	27% 3 5% 21% 35% 8%	6066
February	1st to 10th	Day	29% 4 11% 24% 22% 11%	2100
	11th to 20th	Day	29% 3 9% 26% 22% 11%	2059
	21st to end	Day	22% 2 4 24% 30% 17%	1730
March	1st to 10th	Day	26% 3 6% 15% 22% 30%	4600
	11th to 20th	Day	23% 5% 7% 17% 29% 19%	5034
	21st to end	Day	25% 6% 10% 30% 24% 5%	5598
April	1st to 10th	Day	24% 3 8% 23% 26% 16%	2413
	11th to 20th	Day	28% 5% 8% 21% 23% 14%	2374
	21st to end	Day	29% 6% 11% 22% 25% 8%	2532
May	1st to 10th	Day	32% 6% 13% 18% 19% 11%	5900
	11th to 20th	Day	21% 7% 9% 20% 23% 21%	5866
	21st to end	Day	30% 6% 10% 19% 20% 15%	6447
June	1st to 10th	Day	24% 8% 17% 26% 18% 7%	2600
	11th to 20th	Day	19% 6% 13% 24% 26% 12%	2600
	21st to end	Day	22% 7% 12% 25% 23% 10%	2774
July	1st to 10th	Day	29% 7% 13% 21% 25% 6%	6099
	11th to 20th	Day	20% 5% 12% 32% 27% 5%	5831
	21st to end	Day	18% 5% 16% 36% 20% 4	6240
August	1st to 10th	Day	26% 6% 14% 26% 24% 4	4220
	11th to 20th	Day	19% 6% 11% 26% 29% 9%	3896
	21st to end	Day	21% 5% 11% 31% 26% 6%	4311
September	1st to 10th	Day	28% 5% 9% 20% 25% 13%	2325
	11th to 20th	Day	24% 6% 12% 20% 30% 9%	2200
	21st to end	Day	20% 5% 10% 24% 28% 13%	2200
October	1st to 10th	Day	18% 4 5% 14% 42% 16%	3590
	11th to 20th	Day	19% 3 7% 25% 29% 16%	3448
	21st to end	Day	18% 2 5% 21% 37% 18%	3268
November	1st to 10th	Day	19% 3 8% 23% 31% 17%	1853
	11th to 20th	Day	18% 3 7% 27% 34% 12%	1890
	21st to end	Day	25% 3 6% 22% 29% 14%	1890
December	1st to 10th	Day	15% 3 7% 34% 34% 8%	3030
	11th to 20th	Day	18% 3 6% 21% 42% 10%	3479
	21st to end	Day	37% 5% 8% 17% 24% 9%	4587
January	1st to 10th	Night	32% 4 8% 28% 23% 5%	7635
	11th to 20th	Night	17% 4 10% 20% 35% 14%	7680
	21st to end	Night	34% 3 5% 16% 33% 9%	8204
February	1st to 10th	Night	31% 4 8% 21% 24% 12%	2520
	11th to 20th	Night	32% 5% 9% 21% 24% 10%	2458
	21st to end	Night	22% 3 6% 20% 28% 20%	2076
March	1st to 10th	Night	21% 3 5% 18% 24% 29%	5494
	11th to 20th	Night	18% 5% 8% 16% 26% 27%	5148
	21st to end	Night	22% 6% 10% 30% 25% 7%	5572
April	1st to 10th	Night	24% 4 9% 22% 24% 16%	2010
	11th to 20th	Night	27% 7% 10% 22% 23% 12%	1977
	21st to end	Night	29% 6% 12% 26% 21% 7%	1865
May	1st to 10th	Night	35% 7% 12% 17% 16% 14%	4080
	11th to 20th	Night	21% 8% 9% 18% 21% 23%	4054
	21st to end	Night	31% 6% 11% 19% 16% 17%	4452
June	1st to 10th	Night	21% 9% 19% 30% 15% 7%	1800
	11th to 20th	Night	20% 7% 15% 26% 22% 10%	1800
	21st to end	Night	23% 6% 14% 24% 22% 10%	1611
July	1st to 10th	Night	28% 6% 16% 30% 17% 3	3746
	11th to 20th	Night	24% 5% 14% 28% 25% 4	4026
	21st to end	Night	23% 5% 13% 33% 21% 4	4320
August	1st to 10th	Night	29% 7% 12% 22% 24% 5%	2916
	11th to 20th	Night	26% 5% 9% 30% 23% 6%	3211
	21st to end	Night	22% 6% 13% 31% 22% 6%	3597
September	1st to 10th	Night	23% 5% 13% 22% 26% 11%	2074
	11th to 20th	Night	24% 6% 11% 20% 31% 7%	2200
	21st to end	Night	19% 6% 10% 22% 29% 15%	2200
October	1st to 10th	Night	15% 4 5% 16% 39% 21%	3590
	11th to 20th	Night	20% 3 10% 20% 31% 16%	3657
	21st to end	Night	16% 2 5% 23% 38% 17%	4528
November	1st to 10th	Night	18% 3 9% 22% 29% 18%	2665
	11th to 20th	Night	16% 3 7% 25% 35% 14%	2730
	21st to end	Night	24% 3 5% 25% 27% 16%	2730
December	1st to 10th	Night	17% 2 6% 35% 34% 5%	4380
	11th to 20th	Night	18% 3 5% 24% 38% 12%	4947
	21st to end	Night	31% 5% 8% 23% 24% 8%	6633

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	9% 9% 44% 29% 8% 1	330
	11th to 20th	Day	12% 16% 36% 25% 9% 1	330
	21st to end	Day	8% 11% 41% 29% 11%	363
February	1st to 10th	Day	7% 13% 46% 29% 4	330
	11th to 20th	Day	12% 15% 45% 22% 6% 1	330
	21st to end	Day	8% 14% 50% 22% 5%	273
March	1st to 10th	Day	7% 15% 47% 25% 7%	330
	11th to 20th	Day	17% 15% 39% 21% 7% 2	330
	21st to end	Day	16% 14% 40% 23% 8%	364
April	1st to 10th	Day	12% 18% 45% 19% 6%	505
	11th to 20th	Day	12% 19% 42% 25% 2	492
	21st to end	Day	14% 18% 40% 21% 6% 2	632
May	1st to 10th	Day	12% 11% 39% 29% 8% 1	703
	11th to 20th	Day	10% 10% 32% 32% 15% 1	725
	21st to end	Day	11% 11% 35% 31% 11% 1	789
June	1st to 10th	Day	5% 7% 31% 37% 15% 5%	730
	11th to 20th	Day	5% 5% 36% 36% 17% 1	730
	21st to end	Day	3 4 31% 41% 17% 3	730
July	1st to 10th	Day	6% 2 17% 37% 30% 9%	721
	11th to 20th	Day	3 1 13% 44% 32% 7%	718
	21st to end	Day	5% 2 10% 41% 35% 7%	794
August	1st to 10th	Day	5% 2 17% 40% 32% 4	726
	11th to 20th	Day	5% 2 16% 42% 31% 5%	531
	21st to end	Day	7% 2 15% 40% 28% 9%	583
September	1st to 10th	Day	8% 3 17% 40% 27% 5%	455
	11th to 20th	Day	3 3 24% 44% 22% 4	330
	21st to end	Day	9% 4 27% 32% 22% 6%	330
October	1st to 10th	Day	12% 5% 27% 36% 17% 3	330
	11th to 20th	Day	7% 7% 36% 31% 16% 3	330
	21st to end	Day	16% 10% 31% 27% 13% 3	363
November	1st to 10th	Day	7% 11% 35% 33% 13% 1	330
	11th to 20th	Day	8% 11% 38% 29% 12% 3	330
	21st to end	Day	12% 16% 32% 25% 12% 3	330
December	1st to 10th	Day	3 9% 46% 30% 11% 1	330
	11th to 20th	Day	9% 10% 38% 31% 12% 1	330
	21st to end	Day	14% 16% 39% 22% 7% 1	363
January	1st to 10th	Night	8% 9% 43% 33% 7%	1050
	11th to 20th	Night	8% 12% 37% 33% 9%	1050
	21st to end	Night	7% 10% 43% 34% 6% 1	1087
February	1st to 10th	Night	9% 11% 44% 31% 5% 1	840
	11th to 20th	Night	9% 14% 47% 26% 4	824
	21st to end	Night	9% 14% 47% 26% 4	692
March	1st to 10th	Night	8% 13% 47% 28% 4	840
	11th to 20th	Night	12% 13% 42% 27% 5%	662
	21st to end	Night	10% 17% 49% 20% 3	693
April	1st to 10th	Night	10% 16% 45% 28% 1	603
	11th to 20th	Night	11% 17% 50% 20% 2	593
	21st to end	Night	12% 15% 45% 23% 5% 1	600
May	1st to 10th	Night	10% 13% 49% 24% 4	600
	11th to 20th	Night	10% 11% 40% 29% 9% 1	591
	21st to end	Night	10% 10% 46% 26% 9%	650
June	1st to 10th	Night	4 7% 43% 34% 11% 1	600
	11th to 20th	Night	4 6% 48% 35% 7%	600
	21st to end	Night	3 4 42% 41% 10% 1	600
July	1st to 10th	Night	4 2 25% 48% 19% 1	585
	11th to 20th	Night	4 1 20% 58% 15% 1	582
	21st to end	Night	5% 1 20% 52% 21% 1	657
August	1st to 10th	Night	4 1 23% 49% 21% 1	597
	11th to 20th	Night	7% 2 24% 52% 15%	594
	21st to end	Night	5% 1 22% 46% 25% 1	660
September	1st to 10th	Night	8% 2 29% 46% 15% 1	600
	11th to 20th	Night	9% 2 37% 42% 9% 1	600
	21st to end	Night	5% 4 34% 42% 15% 1	600
October	1st to 10th	Night	10% 7% 34% 38% 11% 1	600
	11th to 20th	Night	8% 10% 40% 34% 7% 1	649
	21st to end	Night	9% 11% 35% 32% 12% 1	1042
November	1st to 10th	Night	7% 8% 38% 37% 10%	1025
	11th to 20th	Night	5% 11% 43% 34% 7%	1050
	21st to end	Night	9% 11% 35% 36% 9% 1	1050
December	1st to 10th	Night	6% 9% 40% 37% 7% 1	1050
	11th to 20th	Night	10% 10% 45% 30% 6%	1050
	21st to end	Night	11% 13% 38% 31% 6%	1155

Page generated at 2026-03-30 01:18:45. Version ac07

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