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 2025-04-11.

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% 9% 3 8% 5% 4 4 4 5% 7% 37%	113439
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	115051
Transparency	Day	26% 43% 7% 15% 8% 1	116980
	Night	29% 41% 6% 14% 9% 1	119653
Seeing	Day	18% 9% 28% 26% 14% 5%	16360
	Night	18% 8% 30% 31% 12% 1	26390

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	9% 11% 8% 15% 7% 6% 6% 4 5% 5% 25%	3617
	11th to 20th	Day	9% 11% 6% 12% 4 3 5% 7% 6% 6% 30%	3620
	21st to end	Day	12% 14% 8% 10% 4 4 3 3 4 6% 32%	4184
February	1st to 10th	Day	11% 15% 5% 9% 5% 5% 6% 4 6% 6% 28%	1800
	11th to 20th	Day	14% 11% 4 8% 6% 6% 4 3 4 6% 36%	1913
	21st to end	Day	13% 13% 5% 9% 6% 4 4 3 5% 6% 32%	1650
March	1st to 10th	Day	15% 8% 3 6% 4 4 4 3 5% 5% 44%	4480
	11th to 20th	Day	12% 6% 3 7% 5% 4 4 3 5% 8% 43%	4750
	21st to end	Day	20% 13% 4 11% 5% 5% 4 3 5% 5% 24%	5936
April	1st to 10th	Day	19% 12% 4 8% 5% 4 3 4 4 6% 30%	2400
	11th to 20th	Day	17% 10% 3 7% 5% 3 4 3 5% 7% 34%	2254
	21st to end	Day	20% 9% 4 7% 5% 4 3 4 5% 6% 32%	2347
May	1st to 10th	Day	17% 7% 3 8% 5% 4 4 4 5% 6% 36%	4141
	11th to 20th	Day	15% 7% 2 5% 4 4 4 3 5% 7% 44%	4399
	21st to end	Day	13% 6% 2 6% 4 4 5% 4 7% 8% 39%	4824
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 5% 6% 8% 44%	2660
	11th to 20th	Day	11% 6% 2 5% 4 5% 4 4 6% 8% 44%	2660
	21st to end	Day	12% 6% 3 6% 5% 4 5% 5% 8% 9% 38%	2660
July	1st to 10th	Day	12% 8% 2 8% 4 5% 5% 5% 8% 10% 33%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 6% 9% 51%	4440
	21st to end	Day	9% 6% 2 7% 5% 6% 5% 4 6% 9% 42%	4813
August	1st to 10th	Day	10% 5% 2 6% 4 5% 6% 4 7% 9% 41%	4366
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 6% 9% 45%	3762
	21st to end	Day	10% 6% 2 6% 4 4 4 4 5% 8% 47%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 47%	2280
	11th to 20th	Day	12% 4 1 4 4 3 3 3 6% 8% 51%	2201
	21st to end	Day	12% 7% 2 8% 5% 4 4 3 5% 7% 44%	2090
October	1st to 10th	Day	14% 6% 2 5% 4 3 3 3 4 6% 48%	1969
	11th to 20th	Day	20% 10% 2 6% 4 4 3 3 4 6% 36%	1900
	21st to end	Day	20% 10% 3 7% 4 3 4 3 4 6% 37%	1905
November	1st to 10th	Day	13% 9% 3 5% 4 3 4 4 5% 5% 44%	1762
	11th to 20th	Day	14% 13% 4 9% 5% 4 5% 4 5% 6% 32%	1800
	21st to end	Day	17% 13% 4 10% 5% 3 2 2 4 6% 35%	1636
December	1st to 10th	Day	8% 10% 4 9% 4 4 6% 3 5% 7% 39%	2610
	11th to 20th	Day	13% 15% 5% 12% 5% 5% 6% 4 5% 6% 23%	3008
	21st to end	Day	20% 22% 9% 13% 5% 4 4 3 3 4 13%	3982
January	1st to 10th	Night	9% 13% 7% 14% 7% 4 3 2 4 4 32%	6348
	11th to 20th	Night	11% 14% 7% 14% 6% 5% 5% 5% 6% 4 23%	6360
	21st to end	Night	13% 15% 7% 11% 5% 5% 3 3 4 5% 30%	6792
February	1st to 10th	Night	13% 14% 4 9% 5% 5% 4 3 5% 4 33%	2600
	11th to 20th	Night	12% 11% 3 8% 5% 4 4 3 4 6% 38%	2372
	21st to end	Night	17% 12% 4 8% 3 3 4 3 4 6% 37%	1980
March	1st to 10th	Night	17% 9% 4 7% 4 3 3 3 4 5% 43%	5373
	11th to 20th	Night	9% 7% 2 6% 4 4 4 4 5% 8% 48%	5239
	21st to end	Night	22% 13% 5% 9% 5% 3 4 3 4 5% 29%	4991
April	1st to 10th	Night	19% 9% 3 6% 4 4 4 3 4 6% 39%	2000
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 4 5% 38%	1881
	21st to end	Night	18% 8% 2 7% 4 4 4 4 5% 7% 38%	1831
May	1st to 10th	Night	17% 6% 3 6% 4 4 4 4 6% 8% 37%	2752
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 5% 6% 7% 40%	2506
	21st to end	Night	11% 8% 2 6% 4 4 4 4 5% 7% 43%	2743
June	1st to 10th	Night	9% 4 1 5% 5% 4 5% 4 7% 8% 46%	1520
	11th to 20th	Night	11% 5% 2 5% 3 4 4 3 6% 10% 47%	1520
	21st to end	Night	11% 7% 2 6% 4 3 4 4 6% 7% 46%	1520
July	1st to 10th	Night	15% 7% 2 6% 4 4 5% 4 5% 8% 40%	2530
	11th to 20th	Night	5% 4 2 4 4 4 5% 4 6% 8% 55%	2530
	21st to end	Night	7% 5% 2 5% 5% 5% 5% 5% 6% 8% 46%	2750
August	1st to 10th	Night	8% 4 2 5% 4 4 4 4 5% 8% 52%	2570
	11th to 20th	Night	10% 6% 2 6% 3 4 4 3 6% 7% 50%	3066
	21st to end	Night	9% 5% 2 4 4 3 4 3 5% 9% 51%	3476
September	1st to 10th	Night	9% 5% 3 5% 4 3 4 2 4 6% 55%	1900
	11th to 20th	Night	11% 4 2 5% 3 4 4 3 5% 8% 53%	1959
	21st to end	Night	11% 6% 2 5% 4 4 4 3 5% 7% 50%	2090
October	1st to 10th	Night	12% 6% 2 6% 4 3 4 3 4 6% 51%	2209
	11th to 20th	Night	18% 8% 2 6% 3 4 3 2 4 6% 44%	2280
	21st to end	Night	20% 9% 2 6% 4 4 4 3 5% 6% 38%	2692
November	1st to 10th	Night	13% 8% 2 6% 3 3 3 3 5% 6% 48%	2536
	11th to 20th	Night	14% 11% 4 8% 4 5% 4 3 6% 5% 36%	2600
	21st to end	Night	17% 12% 4 8% 5% 3 3 3 4 5% 36%	2760
December	1st to 10th	Night	10% 9% 3 7% 4 5% 5% 4 5% 6% 41%	4580
	11th to 20th	Night	16% 18% 4 9% 5% 4 4 3 4 4 28%	5199
	21st to end	Night	21% 19% 7% 12% 5% 4 4 3 4 4 17%	6996

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	47% 28% 7% 14% 4	3618
	11th to 20th	Day	36% 28% 9% 8% 16% 2	3620
	21st to end	Day	37% 29% 10% 14% 9%	4184
February	1st to 10th	Day	22% 61% 5% 5% 6%	1800
	11th to 20th	Day	20% 60% 3 11% 5% 1	1923
	21st to end	Day	19% 62% 4 9% 4 1	1650
March	1st to 10th	Day	28% 30% 7% 22% 9% 4	4480
	11th to 20th	Day	27% 30% 8% 25% 9% 1	4750
	21st to end	Day	44% 32% 8% 9% 6% 1	5938
April	1st to 10th	Day	24% 62% 3 8% 3	2400
	11th to 20th	Day	18% 65% 4 7% 5% 1	2254
	21st to end	Day	17% 65% 4 6% 7% 1	2347
May	1st to 10th	Day	26% 43% 6% 13% 11%	4179
	11th to 20th	Day	23% 42% 6% 20% 9%	4403
	21st to end	Day	20% 43% 9% 14% 11% 3	4837
June	1st to 10th	Day	11% 65% 5% 12% 7% 1	2660
	11th to 20th	Day	16% 65% 5% 8% 5% 1	2660
	21st to end	Day	15% 64% 6% 10% 5% 1	2660
July	1st to 10th	Day	33% 39% 8% 15% 5% 1	4371
	11th to 20th	Day	17% 38% 11% 23% 11%	4365
	21st to end	Day	25% 38% 9% 11% 15% 2	4662
August	1st to 10th	Day	25% 39% 10% 18% 8%	4351
	11th to 20th	Day	22% 40% 8% 23% 7%	3798
	21st to end	Day	24% 38% 9% 18% 11%	4179
September	1st to 10th	Day	16% 61% 5% 11% 7%	2280
	11th to 20th	Day	11% 60% 5% 14% 9% 2	2201
	21st to end	Day	18% 59% 4 11% 7% 1	2090
October	1st to 10th	Day	12% 36% 3 28% 19% 1	3268
	11th to 20th	Day	15% 37% 5% 21% 20% 1	3140
	21st to end	Day	27% 37% 5% 20% 9% 2	3117
November	1st to 10th	Day	22% 56% 5% 9% 6% 1	1762
	11th to 20th	Day	23% 57% 4 10% 6% 1	1800
	21st to end	Day	26% 56% 5% 10% 2	1636
December	1st to 10th	Day	25% 36% 15% 18% 4 1	2610
	11th to 20th	Day	40% 33% 8% 13% 5% 1	3005
	21st to end	Day	60% 26% 3 6% 4	3982
January	1st to 10th	Night	42% 28% 6% 15% 9%	6348
	11th to 20th	Night	47% 28% 6% 6% 13% 1	6360
	21st to end	Night	41% 29% 9% 12% 9%	6792
February	1st to 10th	Night	22% 61% 5% 5% 6% 1	2600
	11th to 20th	Night	20% 60% 3 11% 6%	2373
	21st to end	Night	17% 62% 4 9% 5% 3	1980
March	1st to 10th	Night	32% 30% 7% 17% 8% 6%	5373
	11th to 20th	Night	23% 30% 7% 28% 10% 3	5239
	21st to end	Night	44% 30% 6% 12% 7% 1	4991
April	1st to 10th	Night	20% 62% 3 9% 5% 1	2000
	11th to 20th	Night	18% 65% 2 6% 8% 1	1881
	21st to end	Night	15% 64% 4 9% 7% 1	1831
May	1st to 10th	Night	26% 43% 7% 10% 12% 2	2779
	11th to 20th	Night	23% 45% 8% 17% 6% 1	2507
	21st to end	Night	21% 44% 7% 10% 13% 4	2751
June	1st to 10th	Night	10% 66% 5% 10% 8% 1	1520
	11th to 20th	Night	14% 66% 3 10% 6% 1	1520
	21st to end	Night	14% 64% 5% 10% 7% 1	1520
July	1st to 10th	Night	33% 37% 10% 12% 6% 3	2490
	11th to 20th	Night	15% 38% 11% 25% 11%	2482
	21st to end	Night	22% 39% 9% 12% 16% 2	2662
August	1st to 10th	Night	21% 38% 9% 18% 14%	2561
	11th to 20th	Night	24% 39% 7% 21% 9%	3096
	21st to end	Night	20% 39% 9% 20% 10% 1	3476
September	1st to 10th	Night	12% 61% 6% 12% 9%	1900
	11th to 20th	Night	12% 60% 5% 14% 7% 3	1959
	21st to end	Night	16% 60% 4 12% 8%	2090
October	1st to 10th	Night	15% 36% 3 26% 19% 1	3690
	11th to 20th	Night	18% 38% 5% 19% 17% 2	3779
	21st to end	Night	26% 38% 5% 17% 12% 2	4430
November	1st to 10th	Night	22% 56% 4 10% 7% 1	2536
	11th to 20th	Night	21% 57% 3 11% 7% 1	2600
	21st to end	Night	25% 56% 5% 9% 4 1	2760
December	1st to 10th	Night	25% 37% 11% 19% 7% 1	4580
	11th to 20th	Night	41% 33% 7% 11% 8%	5201
	21st to end	Night	53% 28% 6% 8% 4	6996

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	19% 12% 31% 28% 9% 1	300
	11th to 20th	Day	20% 15% 36% 23% 4 1	300
	21st to end	Day	22% 12% 34% 25% 5% 1	330
February	1st to 10th	Day	18% 17% 40% 20% 4 1	300
	11th to 20th	Day	18% 19% 37% 21% 4 1	300
	21st to end	Day	23% 18% 41% 15% 2	249
March	1st to 10th	Day	17% 20% 40% 17% 6%	300
	11th to 20th	Day	25% 18% 32% 18% 7% 1	300
	21st to end	Day	29% 12% 30% 22% 5% 2	532
April	1st to 10th	Day	28% 12% 34% 21% 4	500
	11th to 20th	Day	24% 17% 28% 19% 10% 2	452
	21st to end	Day	25% 11% 28% 23% 11% 2	460
May	1st to 10th	Day	22% 12% 30% 24% 9% 2	496
	11th to 20th	Day	23% 6% 30% 25% 12% 3	675
	21st to end	Day	17% 8% 27% 26% 15% 6%	734
June	1st to 10th	Day	11% 6% 29% 30% 17% 5%	680
	11th to 20th	Day	12% 7% 29% 30% 17% 5%	680
	21st to end	Day	15% 6% 28% 29% 16% 5%	680
July	1st to 10th	Day	10% 3 17% 32% 24% 14%	671
	11th to 20th	Day	9% 4 27% 29% 20% 10%	668
	21st to end	Day	8% 3 23% 35% 22% 9%	739
August	1st to 10th	Day	9% 3 18% 33% 24% 12%	676
	11th to 20th	Day	11% 3 17% 27% 25% 15%	500
	21st to end	Day	12% 4 16% 34% 24% 9%	539
September	1st to 10th	Day	15% 5% 22% 26% 24% 7%	490
	11th to 20th	Day	12% 2 19% 31% 24% 11%	490
	21st to end	Day	15% 10% 25% 26% 17% 7%	490
October	1st to 10th	Day	18% 7% 27% 26% 18% 4	369
	11th to 20th	Day	27% 14% 32% 20% 7% 1	300
	21st to end	Day	30% 12% 30% 21% 5% 1	330
November	1st to 10th	Day	20% 14% 37% 20% 8% 2	300
	11th to 20th	Day	23% 15% 36% 18% 6% 1	300
	21st to end	Day	28% 8% 38% 21% 5%	300
December	1st to 10th	Day	18% 17% 38% 18% 7% 2	300
	11th to 20th	Day	19% 12% 40% 20% 9% 1	300
	21st to end	Day	29% 13% 37% 19% 2	330
January	1st to 10th	Night	18% 12% 35% 28% 7%	1000
	11th to 20th	Night	22% 11% 35% 25% 6% 1	1000
	21st to end	Night	20% 9% 34% 30% 6%	1100
February	1st to 10th	Night	21% 9% 37% 26% 6%	1000
	11th to 20th	Night	19% 13% 35% 28% 5%	817
	21st to end	Night	24% 11% 38% 23% 4	660
March	1st to 10th	Night	21% 10% 35% 26% 7%	800
	11th to 20th	Night	16% 12% 33% 30% 8%	716
	21st to end	Night	25% 8% 30% 29% 6% 1	660
April	1st to 10th	Night	23% 11% 37% 25% 4	600
	11th to 20th	Night	23% 10% 32% 29% 6%	565
	21st to end	Night	23% 7% 32% 28% 9% 1	570
May	1st to 10th	Night	19% 8% 33% 33% 7% 1	570
	11th to 20th	Night	20% 4 28% 36% 12%	561
	21st to end	Night	15% 5% 25% 39% 15% 1	617
June	1st to 10th	Night	10% 3 26% 46% 15% 1	570
	11th to 20th	Night	13% 2 31% 41% 12% 1	570
	21st to end	Night	13% 4 25% 42% 17% 1	570
July	1st to 10th	Night	9% 1 12% 44% 31% 2	555
	11th to 20th	Night	9% 1 20% 42% 25% 3	552
	21st to end	Night	8% 1 16% 50% 24% 2	624
August	1st to 10th	Night	6% 1 13% 50% 27% 4	567
	11th to 20th	Night	9% 2 15% 39% 31% 5%	564
	21st to end	Night	10% 1 15% 44% 26% 3	627
September	1st to 10th	Night	10% 3 20% 38% 24% 5%	570
	11th to 20th	Night	12% 2 20% 36% 26% 3	629
	21st to end	Night	13% 6% 26% 32% 21% 2	760
October	1st to 10th	Night	14% 5% 30% 33% 16% 2	760
	11th to 20th	Night	22% 10% 28% 31% 9% 1	760
	21st to end	Night	25% 8% 31% 26% 9%	836
November	1st to 10th	Night	17% 10% 37% 26% 10% 1	780
	11th to 20th	Night	22% 13% 35% 25% 7%	800
	21st to end	Night	22% 12% 33% 26% 7%	960
December	1st to 10th	Night	22% 11% 37% 24% 6%	1000
	11th to 20th	Night	24% 10% 34% 26% 6% 1	1000
	21st to end	Night	31% 9% 33% 24% 3	1100

Page generated at 2025-04-08 08:17:27. Version ac07

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