Big Cigar Astronomy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Big Cigar Astronomy clear sky chart from 2005-11-06 to 2026-08-15.

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	13% 8% 3 8% 5% 4 4 4 5% 7% 38%	144114
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 39%	138098
Transparency	Day	29% 35% 8% 17% 10% 1	147608
	Night	31% 35% 7% 16% 10% 1	142681
Seeing	Day	17% 9% 29% 26% 14% 5%	18345
	Night	18% 8% 30% 31% 11% 1	28122

By Month

Percentage Cloud Cover SE (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	11% 13% 7% 14% 7% 5% 5% 4 5% 5% 24%	4691
	11th to 20th	Day	7% 11% 6% 13% 4 4 5% 7% 6% 6% 31%	4710
	21st to end	Day	10% 12% 7% 10% 5% 5% 4 3 4 6% 34%	5414
February	1st to 10th	Day	11% 15% 5% 9% 5% 5% 6% 4 6% 7% 28%	1890
	11th to 20th	Day	13% 11% 3 8% 6% 6% 4 3 4 6% 37%	2011
	21st to end	Day	13% 13% 5% 10% 6% 4 4 3 5% 6% 32%	1730
March	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 4 5% 44%	4580
	11th to 20th	Day	12% 7% 3 7% 5% 4 4 3 5% 8% 43%	4855
	21st to end	Day	20% 12% 4 11% 5% 5% 4 3 5% 5% 25%	6067
April	1st to 10th	Day	19% 12% 4 8% 5% 4 3 4 4 6% 30%	2520
	11th to 20th	Day	17% 10% 3 7% 5% 4 4 4 5% 8% 33%	2494
	21st to end	Day	19% 8% 4 8% 5% 4 4 4 5% 7% 32%	2595
May	1st to 10th	Day	13% 7% 2 8% 5% 5% 5% 4 5% 7% 38%	7730
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 8% 41%	8222
	21st to end	Day	14% 7% 2 7% 4 5% 5% 4 6% 8% 39%	9047
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 5% 6% 8% 44%	2940
	11th to 20th	Day	11% 6% 2 5% 4 5% 4 4 6% 8% 44%	2940
	21st to end	Day	12% 6% 3 6% 5% 4 5% 5% 8% 9% 39%	2940
July	1st to 10th	Day	10% 7% 2 7% 4 5% 5% 4 7% 9% 39%	8280
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 5% 9% 50%	8280
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	8990
August	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 4 7% 9% 41%	4642
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 9% 44%	3932
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 8% 47%	4312
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 47%	2400
	11th to 20th	Day	12% 4 1 4 4 3 3 3 5% 8% 52%	2317
	21st to end	Day	11% 7% 2 8% 4 4 4 3 5% 6% 45%	2200
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 4 6% 48%	2072
	11th to 20th	Day	20% 10% 2 6% 4 4 4 3 5% 6% 36%	2000
	21st to end	Day	19% 10% 3 7% 4 3 4 3 4 6% 38%	2005
November	1st to 10th	Day	13% 9% 3 5% 4 3 4 4 5% 5% 44%	1852
	11th to 20th	Day	14% 13% 4 9% 5% 3 5% 4 5% 6% 32%	1890
	21st to end	Day	16% 14% 4 10% 5% 3 2 2 3 6% 34%	1718
December	1st to 10th	Day	8% 10% 4 9% 5% 4 6% 3 5% 7% 39%	2690
	11th to 20th	Day	13% 15% 5% 12% 5% 5% 5% 4 5% 6% 24%	3088
	21st to end	Day	20% 22% 8% 13% 5% 4 4 3 3 4 13%	4070
January	1st to 10th	Night	10% 14% 7% 14% 7% 4 3 2 4 4 31%	8232
	11th to 20th	Night	9% 14% 7% 16% 7% 5% 5% 5% 6% 4 22%	8280
	21st to end	Night	11% 14% 6% 10% 6% 5% 4 3 5% 6% 32%	8785
February	1st to 10th	Night	13% 14% 4 10% 5% 5% 4 3 4 5% 33%	2730
	11th to 20th	Night	12% 11% 3 8% 5% 4 4 3 5% 6% 39%	2493
	21st to end	Night	16% 12% 4 8% 3 3 4 3 4 6% 37%	2076
March	1st to 10th	Night	16% 9% 4 7% 4 3 3 3 4 5% 43%	5493
	11th to 20th	Night	9% 7% 2 6% 4 4 4 4 5% 8% 47%	5355
	21st to end	Night	21% 13% 5% 9% 5% 4 4 3 4 5% 29%	5102
April	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 37%	2100
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 4 6% 38%	2075
	21st to end	Night	17% 9% 2 7% 4 4 4 4 5% 7% 37%	2023
May	1st to 10th	Night	14% 6% 2 6% 4 4 4 4 6% 8% 42%	5125
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 6% 8% 44%	4670
	21st to end	Night	14% 8% 2 7% 5% 5% 5% 4 5% 6% 40%	5139
June	1st to 10th	Night	9% 4 1 5% 5% 4 5% 4 7% 8% 47%	1680
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 6% 10% 46%	1680
	21st to end	Night	10% 7% 2 6% 4 3 4 4 6% 8% 45%	1680
July	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 5% 8% 44%	4708
	11th to 20th	Night	6% 5% 2 5% 3 3 4 3 6% 8% 55%	4707
	21st to end	Night	8% 5% 2 5% 4 5% 5% 5% 6% 7% 48%	5117
August	1st to 10th	Night	9% 4 2 5% 4 5% 4 4 5% 8% 51%	2734
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 49%	3204
	21st to end	Night	9% 5% 2 4 4 3 4 3 5% 9% 52%	3586
September	1st to 10th	Night	9% 5% 2 6% 4 3 4 2 4 6% 55%	2000
	11th to 20th	Night	11% 3 2 5% 3 4 4 3 5% 7% 53%	2062
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 6% 50%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 4 5% 51%	2325
	11th to 20th	Night	18% 8% 2 6% 3 4 3 2 4 6% 44%	2400
	21st to end	Night	19% 9% 3 6% 4 4 4 3 5% 6% 38%	2834
November	1st to 10th	Night	13% 9% 2 6% 3 4 3 3 5% 6% 47%	2666
	11th to 20th	Night	14% 11% 4 8% 4 5% 4 3 5% 5% 37%	2730
	21st to end	Night	17% 12% 4 8% 5% 3 3 3 4 5% 35%	2898
December	1st to 10th	Night	10% 10% 3 7% 4 5% 5% 4 5% 6% 41%	4720
	11th to 20th	Night	16% 18% 5% 9% 5% 4 4 3 4 4 27%	5339
	21st to end	Night	21% 19% 7% 11% 5% 4 4 3 4 4 17%	7150

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	51% 22% 10% 12% 4	4692
	11th to 20th	Day	39% 23% 8% 14% 14% 2	4710
	21st to end	Day	40% 23% 10% 19% 8%	5414
February	1st to 10th	Day	23% 59% 6% 6% 6%	1890
	11th to 20th	Day	21% 57% 3 12% 5% 2	2021
	21st to end	Day	21% 59% 4 10% 4 1	1730
March	1st to 10th	Day	28% 30% 7% 23% 8% 4	4580
	11th to 20th	Day	27% 30% 8% 25% 9% 1	4855
	21st to end	Day	44% 32% 8% 9% 6% 1	6069
April	1st to 10th	Day	25% 59% 4 8% 4	2520
	11th to 20th	Day	21% 60% 5% 9% 5%	2494
	21st to end	Day	20% 59% 5% 9% 6% 1	2595
May	1st to 10th	Day	32% 25% 7% 20% 16%	7765
	11th to 20th	Day	31% 26% 8% 21% 14% 1	8226
	21st to end	Day	31% 25% 9% 19% 12% 4	9061
June	1st to 10th	Day	13% 60% 5% 13% 8% 1	2940
	11th to 20th	Day	18% 60% 6% 10% 6% 1	2940
	21st to end	Day	17% 59% 6% 12% 6% 1	2940
July	1st to 10th	Day	33% 24% 12% 21% 10% 1	8211
	11th to 20th	Day	24% 23% 10% 28% 15%	8205
	21st to end	Day	30% 23% 10% 20% 16% 1	8794
August	1st to 10th	Day	25% 37% 10% 19% 8%	4627
	11th to 20th	Day	24% 38% 8% 23% 7%	3968
	21st to end	Day	23% 37% 9% 19% 11%	4311
September	1st to 10th	Day	17% 58% 5% 13% 7%	2400
	11th to 20th	Day	12% 57% 5% 15% 9% 2	2317
	21st to end	Day	18% 57% 4 12% 9% 1	2200
October	1st to 10th	Day	13% 35% 3 28% 20% 1	3371
	11th to 20th	Day	16% 37% 5% 21% 20% 1	3240
	21st to end	Day	27% 36% 5% 20% 9% 2	3217
November	1st to 10th	Day	22% 54% 6% 10% 7% 1	1852
	11th to 20th	Day	23% 54% 4 11% 6% 1	1890
	21st to end	Day	28% 54% 5% 10% 2	1718
December	1st to 10th	Day	27% 35% 15% 18% 4 1	2690
	11th to 20th	Day	40% 32% 8% 13% 5% 1	3085
	21st to end	Day	61% 26% 3 6% 4	4070
January	1st to 10th	Night	47% 22% 8% 16% 7%	8233
	11th to 20th	Night	51% 22% 7% 8% 11% 1	8280
	21st to end	Night	41% 23% 9% 18% 9%	8785
February	1st to 10th	Night	23% 59% 5% 6% 6% 1	2730
	11th to 20th	Night	21% 57% 3 13% 6%	2494
	21st to end	Night	19% 59% 4 11% 5% 3	2076
March	1st to 10th	Night	32% 29% 8% 17% 8% 6%	5493
	11th to 20th	Night	24% 30% 7% 27% 9% 3	5355
	21st to end	Night	44% 29% 6% 12% 8% 1	5102
April	1st to 10th	Night	22% 59% 3 10% 5% 1	2100
	11th to 20th	Night	20% 60% 3 8% 8% 1	2075
	21st to end	Night	18% 59% 5% 10% 8% 1	2023
May	1st to 10th	Night	30% 26% 8% 18% 17% 2	5152
	11th to 20th	Night	27% 27% 11% 19% 13% 3	4671
	21st to end	Night	33% 25% 7% 17% 13% 5%	5149
June	1st to 10th	Night	12% 60% 5% 11% 10% 1	1680
	11th to 20th	Night	17% 60% 4 11% 7% 1	1680
	21st to end	Night	16% 59% 6% 11% 7% 1	1680
July	1st to 10th	Night	32% 23% 12% 20% 10% 2	4670
	11th to 20th	Night	20% 23% 10% 26% 19% 1	4662
	21st to end	Night	26% 24% 10% 23% 16% 1	5002
August	1st to 10th	Night	22% 36% 9% 18% 14%	2725
	11th to 20th	Night	25% 38% 7% 21% 9%	3234
	21st to end	Night	20% 38% 9% 21% 11% 1	3586
September	1st to 10th	Night	13% 58% 6% 14% 9%	2000
	11th to 20th	Night	12% 57% 5% 16% 8% 2	2062
	21st to end	Night	17% 57% 4 12% 9%	2200
October	1st to 10th	Night	15% 35% 3 26% 20% 1	3806
	11th to 20th	Night	19% 37% 5% 19% 17% 2	3899
	21st to end	Night	27% 37% 5% 17% 13% 2	4572
November	1st to 10th	Night	23% 53% 4 11% 8% 1	2666
	11th to 20th	Night	22% 54% 4 12% 7% 1	2730
	21st to end	Night	27% 54% 5% 10% 4 1	2898
December	1st to 10th	Night	26% 36% 11% 19% 7% 1	4720
	11th to 20th	Night	42% 32% 7% 11% 8% 1	5341
	21st to end	Night	53% 28% 6% 8% 4	7150

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% 12% 32% 28% 8% 1	330
	11th to 20th	Day	20% 16% 36% 23% 4 1	330
	21st to end	Day	21% 14% 35% 25% 5% 1	363
February	1st to 10th	Day	16% 17% 41% 21% 5% 1	330
	11th to 20th	Day	18% 18% 38% 20% 5% 1	330
	21st to end	Day	23% 18% 42% 15% 2	273
March	1st to 10th	Day	18% 19% 40% 16% 6%	330
	11th to 20th	Day	26% 17% 33% 17% 6% 1	330
	21st to end	Day	28% 13% 32% 20% 5% 2	575
April	1st to 10th	Day	27% 13% 35% 21% 4	540
	11th to 20th	Day	25% 17% 28% 19% 9% 2	532
	21st to end	Day	24% 11% 30% 23% 10% 2	540
May	1st to 10th	Day	20% 13% 32% 24% 9% 3	580
	11th to 20th	Day	22% 9% 30% 24% 12% 3	775
	21st to end	Day	17% 8% 28% 27% 14% 6%	844
June	1st to 10th	Day	11% 7% 29% 31% 17% 5%	780
	11th to 20th	Day	12% 7% 31% 29% 16% 5%	780
	21st to end	Day	15% 6% 30% 30% 15% 4	780
July	1st to 10th	Day	10% 4 17% 33% 23% 13%	771
	11th to 20th	Day	10% 4 25% 29% 23% 9%	768
	21st to end	Day	9% 3 23% 36% 21% 8%	849
August	1st to 10th	Day	10% 3 17% 34% 24% 12%	776
	11th to 20th	Day	13% 3 17% 27% 25% 15%	558
	21st to end	Day	11% 5% 16% 34% 25% 9%	583
September	1st to 10th	Day	14% 6% 22% 26% 24% 8%	530
	11th to 20th	Day	11% 2 18% 31% 26% 12%	530
	21st to end	Day	15% 9% 25% 25% 18% 8%	530
October	1st to 10th	Day	16% 7% 27% 26% 19% 4	402
	11th to 20th	Day	26% 14% 32% 21% 7% 1	330
	21st to end	Day	28% 12% 31% 21% 6% 1	363
November	1st to 10th	Day	20% 16% 36% 19% 8% 2	330
	11th to 20th	Day	23% 16% 34% 19% 6% 1	330
	21st to end	Day	28% 10% 37% 20% 5%	330
December	1st to 10th	Day	17% 17% 38% 18% 7% 2	330
	11th to 20th	Day	18% 12% 41% 20% 8% 1	330
	21st to end	Day	29% 13% 37% 18% 2	363
January	1st to 10th	Night	18% 12% 35% 28% 7%	1050
	11th to 20th	Night	21% 12% 36% 24% 6% 1	1050
	21st to end	Night	20% 10% 35% 29% 6%	1155
February	1st to 10th	Night	21% 9% 38% 26% 6%	1050
	11th to 20th	Night	19% 13% 34% 29% 5%	858
	21st to end	Night	23% 12% 38% 23% 4	692
March	1st to 10th	Night	21% 10% 36% 26% 7%	840
	11th to 20th	Night	18% 12% 33% 29% 8%	752
	21st to end	Night	24% 9% 32% 29% 6% 1	693
April	1st to 10th	Night	24% 11% 37% 24% 4	630
	11th to 20th	Night	23% 10% 31% 29% 6%	623
	21st to end	Night	23% 7% 32% 29% 8% 1	630
May	1st to 10th	Night	18% 8% 34% 33% 7%	630
	11th to 20th	Night	20% 5% 29% 35% 11%	621
	21st to end	Night	15% 5% 25% 40% 15% 1	683
June	1st to 10th	Night	10% 3 26% 46% 14% 1	630
	11th to 20th	Night	13% 4 31% 41% 11%	630
	21st to end	Night	12% 4 25% 43% 16%	630
July	1st to 10th	Night	9% 2 14% 44% 30% 2	615
	11th to 20th	Night	9% 1 20% 44% 24% 2	612
	21st to end	Night	8% 1 17% 50% 23% 1	690
August	1st to 10th	Night	7% 1 13% 49% 27% 4	627
	11th to 20th	Night	10% 2 14% 39% 30% 5%	606
	21st to end	Night	10% 1 15% 45% 26% 3	660
September	1st to 10th	Night	10% 3 20% 38% 24% 5%	600
	11th to 20th	Night	12% 2 20% 36% 26% 4	662
	21st to end	Night	12% 6% 26% 32% 22% 2	800
October	1st to 10th	Night	14% 5% 30% 33% 16% 2	800
	11th to 20th	Night	22% 10% 28% 31% 9% 1	800
	21st to end	Night	25% 9% 31% 26% 9%	880
November	1st to 10th	Night	17% 10% 37% 25% 10% 1	820
	11th to 20th	Night	21% 13% 35% 24% 6%	840
	21st to end	Night	23% 12% 33% 26% 6%	1008
December	1st to 10th	Night	22% 11% 37% 24% 6%	1050
	11th to 20th	Night	24% 10% 34% 25% 6% 1	1050
	21st to end	Night	31% 9% 34% 23% 3	1155

Page generated at 2026-08-12 02:37:18. Version ac07

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