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 2024-02-20.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	78304
	Night	14% 10% 4 7% 5% 3 4 4 5% 6% 37%	80936
Transparency	Day	32% 37% 8% 14% 8% 1	78005
	Night	33% 35% 8% 14% 8% 1	80756
Seeing	Day	18% 8% 28% 26% 14% 5%	14800
	Night	18% 7% 30% 32% 12% 1	24856

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	12% 14% 10% 12% 7% 3 5% 4 4 4 25%	2527
	11th to 20th	Day	12% 11% 6% 15% 7% 2 5% 10% 7% 5% 19%	2530
	21st to end	Day	17% 17% 10% 11% 6% 3 4 3 4 5% 20%	2915
February	1st to 10th	Day	11% 14% 5% 7% 7% 4 6% 5% 6% 6% 28%	1710
	11th to 20th	Day	14% 10% 4 7% 6% 4 5% 4 4 6% 36%	1804
	21st to end	Day	14% 13% 6% 8% 7% 3 4 4 4 6% 31%	1480
March	1st to 10th	Day	13% 10% 6% 8% 6% 4 5% 3 5% 6% 33%	1800
	11th to 20th	Day	13% 8% 4 6% 6% 3 4 4 4 8% 39%	1890
	21st to end	Day	20% 11% 4 7% 6% 4 4 5% 5% 6% 27%	2359
April	1st to 10th	Day	19% 10% 5% 7% 6% 3 4 5% 4 6% 30%	2160
	11th to 20th	Day	17% 10% 4 7% 6% 3 3 4 5% 8% 33%	2134
	21st to end	Day	20% 8% 4 6% 6% 4 4 4 5% 6% 32%	2223
May	1st to 10th	Day	19% 8% 4 7% 6% 3 5% 5% 6% 7% 30%	2371
	11th to 20th	Day	20% 8% 3 5% 5% 3 4 4 5% 7% 36%	2479
	21st to end	Day	14% 6% 3 6% 5% 3 6% 5% 6% 8% 37%	2725
June	1st to 10th	Day	11% 5% 2 5% 5% 3 5% 6% 6% 8% 44%	2520
	11th to 20th	Day	11% 5% 2 4 5% 3 5% 5% 6% 9% 44%	2520
	21st to end	Day	12% 6% 2 5% 6% 3 5% 6% 9% 9% 38%	2520
July	1st to 10th	Day	8% 5% 2 4 4 3 4 6% 7% 10% 47%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 6% 7% 7% 9% 45%	2520
	21st to end	Day	8% 4 2 6% 5% 4 5% 5% 6% 9% 46%	2743
August	1st to 10th	Day	8% 4 2 5% 6% 3 6% 6% 7% 11% 42%	2478
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 5% 6% 8% 44%	2145
	21st to end	Day	11% 5% 2 4 5% 2 5% 5% 5% 8% 48%	2376
September	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 4 5% 8% 46%	2160
	11th to 20th	Day	13% 4 1 4 4 2 4 3 6% 8% 51%	2086
	21st to end	Day	12% 7% 2 6% 6% 3 4 4 5% 7% 44%	1980
October	1st to 10th	Day	15% 6% 2 5% 5% 2 4 4 4 6% 46%	1866
	11th to 20th	Day	22% 10% 3 5% 5% 3 3 4 4 6% 35%	1800
	21st to end	Day	20% 10% 3 6% 5% 2 4 3 4 6% 37%	1805
November	1st to 10th	Day	12% 9% 3 5% 5% 2 4 5% 5% 5% 45%	1672
	11th to 20th	Day	14% 11% 5% 8% 6% 2 5% 5% 5% 6% 33%	1710
	21st to end	Day	17% 12% 4 8% 6% 2 2 3 4 6% 36%	1555
December	1st to 10th	Day	14% 14% 4 8% 6% 3 6% 4 4 6% 29%	1520
	11th to 20th	Day	14% 14% 5% 9% 6% 3 6% 5% 5% 6% 28%	1918
	21st to end	Day	20% 19% 9% 13% 7% 3 4 4 3 3 15%	2783
January	1st to 10th	Night	13% 13% 9% 10% 7% 3 3 3 4 4 31%	4428
	11th to 20th	Night	14% 16% 8% 12% 7% 3 4 7% 6% 4 20%	4440
	21st to end	Night	17% 17% 9% 11% 6% 4 4 4 4 5% 20%	4750
February	1st to 10th	Night	13% 13% 5% 8% 6% 4 4 4 4 5% 33%	2470
	11th to 20th	Night	12% 10% 4 7% 5% 3 4 4 4 6% 39%	2246
	21st to end	Night	18% 12% 5% 6% 5% 2 3 4 4 6% 35%	1776
March	1st to 10th	Night	17% 9% 4 5% 5% 3 4 4 5% 6% 36%	2160
	11th to 20th	Night	11% 9% 3 6% 6% 4 5% 5% 5% 7% 41%	2085
	21st to end	Night	20% 10% 5% 7% 5% 3 4 3 4 6% 33%	1995
April	1st to 10th	Night	19% 8% 3 5% 5% 4 4 5% 4 6% 37%	1800
	11th to 20th	Night	20% 8% 4 6% 5% 3 4 3 4 6% 38%	1775
	21st to end	Night	19% 8% 2 6% 5% 3 5% 4 5% 7% 37%	1735
May	1st to 10th	Night	16% 7% 3 6% 5% 3 5% 5% 6% 8% 35%	1585
	11th to 20th	Night	16% 7% 3 5% 4 4 5% 5% 5% 7% 38%	1416
	21st to end	Night	12% 6% 3 5% 5% 3 5% 5% 6% 8% 41%	1551
June	1st to 10th	Night	9% 4 2 4 6% 3 6% 5% 7% 9% 47%	1440
	11th to 20th	Night	11% 5% 2 4 4 3 4 4 6% 10% 47%	1440
	21st to end	Night	11% 6% 2 5% 5% 2 4 5% 6% 8% 47%	1440
July	1st to 10th	Night	6% 4 2 3 4 3 4 5% 6% 10% 52%	1440
	11th to 20th	Night	7% 5% 3 3 3 2 5% 4 5% 8% 55%	1440
	21st to end	Night	7% 4 2 4 4 3 4 6% 7% 9% 51%	1575
August	1st to 10th	Night	5% 3 2 4 6% 3 6% 5% 6% 9% 51%	1457
	11th to 20th	Night	9% 4 2 4 4 2 4 3 6% 8% 55%	1754
	21st to end	Night	8% 5% 2 4 4 2 4 4 5% 8% 54%	1980
September	1st to 10th	Night	9% 5% 3 5% 5% 2 4 3 4 6% 54%	1800
	11th to 20th	Night	12% 4 2 4 3 3 4 4 5% 7% 53%	1855
	21st to end	Night	12% 5% 2 4 5% 3 5% 4 5% 6% 50%	1980
October	1st to 10th	Night	13% 6% 2 5% 4 3 4 4 4 6% 50%	2092
	11th to 20th	Night	19% 7% 3 5% 4 2 4 3 4 6% 43%	2160
	21st to end	Night	20% 8% 3 5% 5% 3 4 4 5% 6% 38%	2550
November	1st to 10th	Night	13% 8% 2 5% 4 2 3 3 5% 6% 49%	2406
	11th to 20th	Night	15% 10% 4 6% 6% 3 5% 4 5% 6% 36%	2470
	21st to end	Night	18% 10% 4 6% 5% 2 3 4 4 6% 37%	2622
December	1st to 10th	Night	16% 12% 5% 8% 6% 3 5% 4 4 6% 31%	2660
	11th to 20th	Night	15% 14% 5% 8% 6% 2 5% 4 4 5% 30%	3279
	21st to end	Night	22% 17% 7% 9% 6% 3 4 5% 5% 5% 18%	4884

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% 27% 8% 11% 3	2528
	11th to 20th	Day	47% 28% 10% 10% 5%	2530
	21st to end	Day	55% 23% 7% 11% 3	2915
February	1st to 10th	Day	37% 36% 9% 12% 5% 1	1710
	11th to 20th	Day	32% 36% 9% 15% 6% 1	1814
	21st to end	Day	38% 37% 5% 13% 6%	1480
March	1st to 10th	Day	34% 38% 9% 13% 6%	1800
	11th to 20th	Day	30% 37% 10% 16% 7% 1	1890
	21st to end	Day	40% 38% 8% 11% 4	2359
April	1st to 10th	Day	34% 38% 7% 14% 7%	2160
	11th to 20th	Day	34% 37% 6% 14% 7% 2	2134
	21st to end	Day	35% 39% 8% 12% 6% 1	2223
May	1st to 10th	Day	35% 39% 8% 14% 5%	2371
	11th to 20th	Day	29% 38% 9% 15% 9%	2483
	21st to end	Day	30% 40% 8% 11% 9% 1	2725
June	1st to 10th	Day	27% 40% 9% 14% 10% 1	2520
	11th to 20th	Day	20% 39% 8% 17% 15% 1	2520
	21st to end	Day	27% 39% 10% 13% 9% 1	2520
July	1st to 10th	Day	21% 39% 10% 20% 8% 1	2451
	11th to 20th	Day	21% 40% 10% 15% 12% 1	2445
	21st to end	Day	23% 40% 11% 15% 9% 2	2591
August	1st to 10th	Day	24% 40% 10% 18% 7% 1	2463
	11th to 20th	Day	22% 37% 10% 19% 11% 1	2145
	21st to end	Day	19% 38% 7% 18% 15% 2	2376
September	1st to 10th	Day	23% 37% 8% 18% 13% 1	2160
	11th to 20th	Day	23% 38% 8% 20% 11%	2086
	21st to end	Day	26% 38% 7% 19% 10%	1980
October	1st to 10th	Day	22% 37% 7% 22% 12% 1	1866
	11th to 20th	Day	36% 37% 9% 12% 5%	1800
	21st to end	Day	32% 37% 6% 16% 8% 1	1805
November	1st to 10th	Day	23% 41% 9% 17% 9% 1	1672
	11th to 20th	Day	33% 41% 9% 11% 6%	1710
	21st to end	Day	30% 40% 9% 16% 5%	1555
December	1st to 10th	Day	37% 40% 8% 11% 3	1520
	11th to 20th	Day	40% 33% 12% 11% 4 1	1915
	21st to end	Day	61% 26% 6% 5% 2	2783
January	1st to 10th	Night	47% 27% 7% 15% 4	4428
	11th to 20th	Night	52% 26% 7% 9% 6%	4440
	21st to end	Night	56% 23% 7% 10% 3	4750
February	1st to 10th	Night	35% 35% 8% 14% 7% 1	2470
	11th to 20th	Night	31% 36% 9% 14% 9% 1	2247
	21st to end	Night	35% 37% 9% 11% 6% 1	1776
March	1st to 10th	Night	32% 37% 11% 13% 5% 1	2160
	11th to 20th	Night	28% 38% 10% 14% 8% 2	2085
	21st to end	Night	37% 38% 7% 12% 6%	1995
April	1st to 10th	Night	30% 38% 9% 13% 11% 1	1800
	11th to 20th	Night	32% 35% 5% 14% 12% 3	1775
	21st to end	Night	31% 39% 8% 15% 6% 1	1735
May	1st to 10th	Night	30% 39% 9% 14% 8%	1585
	11th to 20th	Night	27% 39% 8% 15% 10% 1	1417
	21st to end	Night	26% 40% 8% 13% 12% 1	1552
June	1st to 10th	Night	22% 40% 10% 16% 11% 1	1440
	11th to 20th	Night	20% 38% 7% 14% 19% 2	1440
	21st to end	Night	24% 37% 11% 13% 13% 2	1440
July	1st to 10th	Night	19% 40% 10% 17% 12% 2	1400
	11th to 20th	Night	20% 39% 8% 18% 13% 2	1392
	21st to end	Night	19% 40% 12% 16% 10% 3	1487
August	1st to 10th	Night	19% 38% 11% 21% 9% 2	1448
	11th to 20th	Night	16% 37% 10% 21% 15% 1	1754
	21st to end	Night	17% 38% 7% 18% 16% 4	1980
September	1st to 10th	Night	22% 37% 6% 15% 18% 3	1800
	11th to 20th	Night	22% 38% 6% 20% 14%	1855
	21st to end	Night	23% 37% 7% 20% 11% 1	1980
October	1st to 10th	Night	20% 37% 7% 21% 15% 1	2092
	11th to 20th	Night	30% 38% 8% 14% 10% 1	2160
	21st to end	Night	30% 38% 7% 16% 7% 1	2550
November	1st to 10th	Night	19% 42% 7% 19% 11% 2	2406
	11th to 20th	Night	31% 41% 9% 12% 6% 1	2470
	21st to end	Night	30% 41% 9% 14% 7%	2622
December	1st to 10th	Night	34% 41% 7% 13% 4	2660
	11th to 20th	Night	39% 35% 8% 13% 4	3281
	21st to end	Night	54% 27% 8% 9% 2	4884

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% 11% 30% 28% 10% 1	270
	11th to 20th	Day	21% 13% 36% 25% 4 1	270
	21st to end	Day	23% 9% 34% 27% 5% 1	297
February	1st to 10th	Day	17% 16% 40% 21% 5% 1	270
	11th to 20th	Day	19% 18% 37% 21% 4	267
	21st to end	Day	24% 18% 38% 16% 3 1	198
March	1st to 10th	Day	15% 21% 38% 18% 8%	240
	11th to 20th	Day	24% 15% 33% 19% 8% 1	240
	21st to end	Day	29% 12% 28% 22% 6% 3	445
April	1st to 10th	Day	27% 10% 35% 23% 5%	420
	11th to 20th	Day	23% 16% 29% 19% 10% 3	412
	21st to end	Day	25% 10% 27% 23% 11% 3	420
May	1st to 10th	Day	23% 11% 29% 24% 10% 2	454
	11th to 20th	Day	24% 6% 30% 25% 12% 3	625
	21st to end	Day	17% 7% 27% 26% 15% 7%	679
June	1st to 10th	Day	10% 6% 29% 32% 18% 6%	630
	11th to 20th	Day	13% 6% 27% 31% 18% 6%	630
	21st to end	Day	16% 6% 27% 30% 17% 5%	630
July	1st to 10th	Day	10% 3 15% 33% 24% 15%	621
	11th to 20th	Day	10% 4 27% 30% 20% 9%	618
	21st to end	Day	8% 2 24% 36% 21% 8%	684
August	1st to 10th	Day	9% 3 17% 33% 25% 13%	626
	11th to 20th	Day	12% 3 16% 27% 27% 17%	459
	21st to end	Day	13% 5% 16% 34% 22% 10%	495
September	1st to 10th	Day	16% 5% 21% 27% 24% 7%	450
	11th to 20th	Day	13% 3 19% 31% 23% 11%	450
	21st to end	Day	15% 10% 24% 25% 18% 7%	450
October	1st to 10th	Day	19% 6% 26% 26% 18% 4	336
	11th to 20th	Day	28% 14% 31% 19% 6% 1	270
	21st to end	Day	31% 12% 30% 20% 6% 1	297
November	1st to 10th	Day	18% 14% 37% 20% 9% 2	270
	11th to 20th	Day	22% 14% 37% 20% 6% 1	270
	21st to end	Day	28% 7% 37% 21% 6%	270
December	1st to 10th	Day	20% 16% 37% 18% 7% 2	270
	11th to 20th	Day	20% 9% 40% 21% 10% 1	270
	21st to end	Day	28% 13% 38% 19% 2	297
January	1st to 10th	Night	18% 11% 36% 27% 7%	950
	11th to 20th	Night	22% 11% 35% 25% 6% 1	950
	21st to end	Night	21% 9% 34% 30% 7%	1045
February	1st to 10th	Night	21% 9% 36% 27% 6%	950
	11th to 20th	Night	19% 12% 35% 29% 5%	775
	21st to end	Night	24% 12% 36% 23% 5% 1	592
March	1st to 10th	Night	21% 10% 34% 26% 8%	720
	11th to 20th	Night	17% 10% 32% 31% 9%	645
	21st to end	Night	24% 8% 29% 30% 7% 2	594
April	1st to 10th	Night	23% 10% 37% 25% 5%	540
	11th to 20th	Night	24% 9% 30% 30% 7%	533
	21st to end	Night	23% 7% 32% 28% 9% 1	540
May	1st to 10th	Night	18% 7% 34% 33% 7% 1	540
	11th to 20th	Night	21% 4 28% 35% 12%	531
	21st to end	Night	15% 4 25% 39% 16% 1	584
June	1st to 10th	Night	10% 2 26% 46% 15% 1	540
	11th to 20th	Night	13% 2 29% 42% 13% 1	540
	21st to end	Night	12% 3 24% 43% 18% 1	540
July	1st to 10th	Night	8% 1 12% 44% 32% 2	525
	11th to 20th	Night	9% 1 20% 43% 25% 2	522
	21st to end	Night	8% 1 16% 50% 24% 1	591
August	1st to 10th	Night	6% 1 12% 50% 27% 4	537
	11th to 20th	Night	8% 1 14% 39% 32% 6%	534
	21st to end	Night	10% 1 15% 43% 27% 3	594
September	1st to 10th	Night	11% 3 20% 38% 23% 5%	540
	11th to 20th	Night	13% 3 21% 35% 26% 4	595
	21st to end	Night	14% 6% 26% 31% 21% 2	720
October	1st to 10th	Night	15% 5% 29% 33% 17% 2	720
	11th to 20th	Night	23% 10% 28% 30% 9% 1	720
	21st to end	Night	26% 8% 31% 26% 8% 1	792
November	1st to 10th	Night	16% 9% 37% 25% 11% 1	740
	11th to 20th	Night	21% 13% 35% 25% 7%	760
	21st to end	Night	23% 10% 33% 27% 7%	912
December	1st to 10th	Night	23% 11% 36% 24% 6%	950
	11th to 20th	Night	24% 9% 34% 26% 7% 1	950
	21st to end	Night	30% 9% 34% 23% 3	1045

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