Twin Sugars Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Twin Sugars Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	125747
	Night	14% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	123281
Transparency	Day	45% 8% 12% 20% 13% 3	129366
	Night	44% 8% 12% 20% 14% 3	127731
Seeing	Day	23% 16% 25% 20% 11% 5%	16188
	Night	21% 15% 22% 27% 14% 2	26434

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% 12% 4 8% 5% 5% 4 3 5% 7% 35%	4056
	11th to 20th	Day	11% 12% 6% 14% 5% 5% 4 3 4 5% 33%	4060
	21st to end	Day	16% 10% 4 7% 4 3 4 3 4 6% 40%	4466
February	1st to 10th	Day	13% 10% 3 9% 5% 5% 5% 3 6% 7% 34%	1800
	11th to 20th	Day	16% 9% 3 10% 6% 5% 5% 3 5% 7% 31%	1810
	21st to end	Day	18% 11% 3 9% 4 4 4 3 5% 6% 33%	1693
March	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 6% 7% 35%	4935
	11th to 20th	Day	12% 6% 3 6% 4 3 4 3 5% 8% 46%	4980
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	5465
April	1st to 10th	Day	12% 9% 4 8% 5% 5% 4 4 4 7% 38%	2259
	11th to 20th	Day	14% 6% 3 7% 5% 6% 5% 4 5% 6% 40%	2374
	21st to end	Day	17% 7% 3 7% 5% 5% 4 4 6% 5% 36%	2595
May	1st to 10th	Day	15% 7% 2 9% 6% 7% 7% 6% 8% 8% 27%	5862
	11th to 20th	Day	22% 7% 2 7% 5% 5% 6% 4 6% 9% 27%	5861
	21st to end	Day	15% 5% 2 6% 6% 6% 6% 5% 8% 9% 32%	6433
June	1st to 10th	Day	13% 6% 2 6% 5% 6% 6% 4 6% 8% 37%	2600
	11th to 20th	Day	10% 5% 2 8% 5% 7% 7% 4 7% 10% 34%	2600
	21st to end	Day	9% 6% 2 7% 6% 6% 6% 5% 7% 10% 36%	2600
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 9% 41%	5900
	11th to 20th	Day	8% 5% 2 7% 6% 6% 8% 6% 9% 11% 33%	5900
	21st to end	Day	12% 7% 3 8% 7% 6% 7% 5% 8% 10% 27%	6424
August	1st to 10th	Day	14% 6% 3 7% 5% 6% 7% 5% 7% 10% 29%	4234
	11th to 20th	Day	10% 5% 2 7% 7% 6% 7% 5% 8% 10% 34%	4174
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 6% 9% 47%	4416
September	1st to 10th	Day	14% 6% 2 6% 4 4 5% 5% 5% 7% 43%	2335
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 4 5% 7% 39%	2178
	21st to end	Day	24% 7% 2 6% 4 4 5% 4 5% 6% 33%	2090
October	1st to 10th	Day	16% 6% 1 5% 4 4 4 3 5% 7% 43%	1929
	11th to 20th	Day	15% 9% 2 6% 3 3 4 3 4 6% 44%	1900
	21st to end	Day	16% 8% 2 6% 4 3 4 3 5% 6% 42%	2090
November	1st to 10th	Day	10% 8% 2 5% 4 4 3 4 5% 8% 46%	1937
	11th to 20th	Day	14% 7% 2 7% 6% 5% 4 4 4 7% 40%	1800
	21st to end	Day	13% 8% 2 6% 6% 5% 4 4 6% 6% 39%	1731
December	1st to 10th	Day	13% 11% 5% 6% 5% 4 3 3 5% 6% 39%	2600
	11th to 20th	Day	20% 12% 2 6% 4 4 5% 3 5% 6% 34%	3194
	21st to end	Day	20% 11% 3 8% 5% 5% 5% 4 6% 7% 26%	4466
January	1st to 10th	Night	11% 9% 5% 12% 6% 5% 5% 4 6% 7% 29%	5889
	11th to 20th	Night	12% 9% 4 12% 7% 4 4 3 5% 5% 36%	5900
	21st to end	Night	17% 8% 3 7% 4 5% 4 3 4 5% 40%	6490
February	1st to 10th	Night	13% 7% 4 8% 6% 5% 5% 5% 6% 8% 33%	2600
	11th to 20th	Night	18% 8% 3 9% 6% 6% 6% 5% 6% 5% 29%	2473
	21st to end	Night	16% 8% 4 8% 5% 4 5% 4 6% 7% 32%	1935
March	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 7% 38%	4918
	11th to 20th	Night	11% 4 2 6% 4 4 4 4 5% 7% 47%	4980
	21st to end	Night	14% 6% 2 5% 4 5% 5% 4 7% 8% 39%	5467
April	1st to 10th	Night	13% 8% 4 8% 5% 5% 5% 4 6% 7% 36%	2140
	11th to 20th	Night	11% 7% 3 5% 5% 5% 5% 4 7% 8% 39%	1975
	21st to end	Night	15% 5% 2 5% 4 5% 5% 4 6% 8% 41%	1820
May	1st to 10th	Night	18% 9% 3 7% 5% 4 5% 4 6% 8% 32%	4033
	11th to 20th	Night	23% 8% 3 6% 4 4 4 4 6% 8% 30%	4032
	21st to end	Night	17% 6% 2 6% 5% 5% 5% 5% 7% 10% 31%	4421
June	1st to 10th	Night	9% 6% 2 6% 5% 4 5% 5% 7% 10% 43%	1800
	11th to 20th	Night	13% 5% 2 5% 4 3 4 5% 7% 9% 42%	1800
	21st to end	Night	9% 5% 2 5% 5% 4 5% 5% 7% 11% 43%	1800
July	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 4 7% 9% 43%	4058
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 39%	4057
	21st to end	Night	12% 7% 3 8% 6% 6% 6% 5% 7% 10% 32%	4427
August	1st to 10th	Night	16% 6% 2 6% 5% 5% 5% 4 6% 8% 36%	2915
	11th to 20th	Night	12% 5% 2 6% 4 5% 6% 4 7% 9% 39%	2875
	21st to end	Night	8% 4 2 5% 4 4 3 4 5% 8% 53%	3449
September	1st to 10th	Night	12% 5% 2 6% 4 4 5% 4 6% 7% 46%	2060
	11th to 20th	Night	14% 7% 2 4 4 3 3 4 5% 7% 47%	2179
	21st to end	Night	20% 6% 3 6% 4 4 4 3 5% 6% 39%	2090
October	1st to 10th	Night	14% 7% 2 6% 3 4 4 4 5% 6% 46%	2250
	11th to 20th	Night	12% 7% 2 7% 4 4 4 3 5% 6% 45%	2280
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 45%	2508
November	1st to 10th	Night	12% 7% 2 6% 4 4 3 4 6% 7% 45%	2356
	11th to 20th	Night	14% 6% 2 7% 4 5% 5% 4 6% 7% 39%	2600
	21st to end	Night	13% 7% 3 6% 5% 5% 5% 4 5% 7% 38%	2661
December	1st to 10th	Night	11% 7% 3 8% 5% 5% 5% 4 6% 9% 37%	4580
	11th to 20th	Night	18% 10% 3 10% 5% 5% 4 4 4 6% 32%	4973
	21st to end	Night	18% 6% 5% 8% 4 5% 6% 4 6% 8% 28%	6490

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	50% 6% 6% 18% 19% 2	4057
	11th to 20th	Day	54% 7% 5% 12% 15% 7%	4060
	21st to end	Day	47% 6% 8% 22% 14% 3	4466
February	1st to 10th	Day	50% 7% 10% 20% 11% 2	1800
	11th to 20th	Day	54% 7% 9% 16% 11% 2	1819
	21st to end	Day	53% 8% 7% 16% 13% 3	1693
March	1st to 10th	Day	46% 9% 9% 19% 13% 5%	4935
	11th to 20th	Day	37% 7% 10% 20% 22% 5%	4980
	21st to end	Day	47% 8% 9% 18% 16% 3	5465
April	1st to 10th	Day	45% 9% 10% 18% 14% 4	2259
	11th to 20th	Day	44% 7% 10% 21% 15% 3	2374
	21st to end	Day	47% 9% 12% 20% 11% 2	2595
May	1st to 10th	Day	52% 9% 11% 19% 8% 2	5897
	11th to 20th	Day	53% 9% 12% 18% 7% 1	5865
	21st to end	Day	44% 10% 15% 18% 10% 3	6447
June	1st to 10th	Day	42% 10% 14% 20% 13%	2600
	11th to 20th	Day	43% 11% 14% 17% 14%	2600
	21st to end	Day	41% 12% 14% 21% 11% 1	2600
July	1st to 10th	Day	38% 9% 17% 21% 11% 3	5835
	11th to 20th	Day	42% 9% 18% 24% 7%	5831
	21st to end	Day	51% 6% 14% 20% 8%	6240
August	1st to 10th	Day	48% 8% 18% 20% 7%	4221
	11th to 20th	Day	43% 8% 12% 26% 11%	4213
	21st to end	Day	32% 9% 17% 28% 13% 1	4416
September	1st to 10th	Day	39% 9% 12% 18% 18% 4	2335
	11th to 20th	Day	44% 7% 10% 21% 17% 2	2178
	21st to end	Day	50% 7% 10% 16% 14% 2	2090
October	1st to 10th	Day	30% 5% 10% 21% 27% 7%	3202
	11th to 20th	Day	32% 4 10% 25% 15% 14%	3140
	21st to end	Day	30% 4 10% 30% 15% 12%	3428
November	1st to 10th	Day	37% 7% 10% 21% 21% 4	1937
	11th to 20th	Day	44% 8% 9% 17% 19% 4	1800
	21st to end	Day	44% 9% 13% 18% 15% 2	1731
December	1st to 10th	Day	47% 5% 7% 19% 20% 2	2600
	11th to 20th	Day	53% 4 11% 16% 15% 1	3191
	21st to end	Day	55% 8% 11% 16% 9%	4466
January	1st to 10th	Night	52% 9% 6% 14% 16% 2	5889
	11th to 20th	Night	51% 6% 6% 17% 14% 6%	5900
	21st to end	Night	47% 6% 10% 20% 15% 2	6490
February	1st to 10th	Night	49% 10% 12% 16% 13% 1	2600
	11th to 20th	Night	55% 9% 9% 14% 11% 2	2475
	21st to end	Night	50% 8% 10% 17% 9% 4	1935
March	1st to 10th	Night	46% 6% 9% 19% 16% 5%	4918
	11th to 20th	Night	37% 7% 9% 24% 20% 4	4980
	21st to end	Night	42% 7% 12% 19% 17% 3	5467
April	1st to 10th	Night	47% 10% 11% 17% 14% 2	2140
	11th to 20th	Night	41% 8% 13% 20% 15% 4	1975
	21st to end	Night	40% 9% 15% 24% 11% 1	1820
May	1st to 10th	Night	50% 9% 14% 17% 9% 1	4060
	11th to 20th	Night	51% 9% 16% 17% 6%	4033
	21st to end	Night	46% 11% 19% 18% 4 3	4429
June	1st to 10th	Night	36% 12% 17% 22% 13%	1800
	11th to 20th	Night	36% 12% 19% 20% 12% 1	1800
	21st to end	Night	33% 13% 17% 24% 12% 1	1800
July	1st to 10th	Night	38% 9% 17% 24% 11% 2	4015
	11th to 20th	Night	42% 8% 18% 25% 6% 1	4006
	21st to end	Night	47% 8% 16% 20% 9%	4297
August	1st to 10th	Night	45% 9% 18% 24% 5%	2904
	11th to 20th	Night	42% 7% 14% 24% 12%	2902
	21st to end	Night	29% 8% 15% 32% 15% 1	3449
September	1st to 10th	Night	36% 11% 14% 19% 17% 4	2060
	11th to 20th	Night	36% 8% 12% 22% 19% 2	2179
	21st to end	Night	45% 8% 12% 22% 13% 1	2090
October	1st to 10th	Night	33% 5% 8% 24% 26% 4	3757
	11th to 20th	Night	33% 4 9% 21% 21% 12%	3779
	21st to end	Night	26% 4 12% 28% 19% 11%	4120
November	1st to 10th	Night	38% 7% 12% 19% 19% 5%	2356
	11th to 20th	Night	43% 8% 11% 16% 18% 4	2600
	21st to end	Night	45% 8% 11% 18% 15% 3	2661
December	1st to 10th	Night	44% 6% 9% 25% 15% 1	4580
	11th to 20th	Night	56% 5% 11% 15% 12% 1	4975
	21st to end	Night	53% 9% 12% 17% 9%	6490

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	39% 28% 21% 8% 3 1	300
	11th to 20th	Day	44% 23% 26% 7%	300
	21st to end	Day	39% 25% 25% 8% 3	330
February	1st to 10th	Day	37% 25% 26% 10% 2	300
	11th to 20th	Day	51% 23% 21% 5% 1	300
	21st to end	Day	43% 22% 26% 8% 1	292
March	1st to 10th	Day	32% 26% 22% 15% 4	500
	11th to 20th	Day	34% 26% 23% 12% 4 1	500
	21st to end	Day	29% 27% 25% 15% 4	550
April	1st to 10th	Day	29% 27% 24% 13% 6% 1	500
	11th to 20th	Day	25% 20% 30% 15% 8% 3	492
	21st to end	Day	28% 20% 26% 14% 9% 2	500
May	1st to 10th	Day	25% 22% 28% 14% 8% 3	503
	11th to 20th	Day	23% 17% 30% 16% 11% 4	527
	21st to end	Day	21% 12% 25% 22% 13% 7%	573
June	1st to 10th	Day	14% 13% 29% 24% 13% 7%	530
	11th to 20th	Day	12% 11% 30% 26% 14% 8%	530
	21st to end	Day	10% 11% 30% 27% 14% 8%	530
July	1st to 10th	Day	14% 6% 21% 30% 17% 12%	525
	11th to 20th	Day	6% 7% 26% 33% 18% 11%	524
	21st to end	Day	12% 7% 24% 28% 19% 10%	576
August	1st to 10th	Day	19% 7% 21% 26% 17% 10%	526
	11th to 20th	Day	13% 8% 21% 26% 22% 11%	519
	21st to end	Day	10% 6% 23% 27% 22% 12%	583
September	1st to 10th	Day	14% 7% 21% 26% 19% 12%	530
	11th to 20th	Day	16% 5% 17% 31% 25% 7%	522
	21st to end	Day	27% 12% 21% 24% 12% 3	490
October	1st to 10th	Day	20% 10% 24% 26% 18% 4	490
	11th to 20th	Day	22% 15% 25% 22% 14% 2	490
	21st to end	Day	24% 17% 24% 20% 12% 2	539
November	1st to 10th	Day	19% 18% 27% 24% 9% 3	475
	11th to 20th	Day	33% 16% 28% 18% 4	300
	21st to end	Day	28% 23% 29% 15% 4	265
December	1st to 10th	Day	38% 26% 22% 11% 4 1	200
	11th to 20th	Day	36% 22% 22% 17% 2	247
	21st to end	Day	35% 22% 23% 15% 5%	330
January	1st to 10th	Night	33% 26% 23% 15% 3	800
	11th to 20th	Night	32% 22% 27% 18% 2	800
	21st to end	Night	31% 23% 28% 18% 1	880
February	1st to 10th	Night	26% 22% 30% 20% 3	800
	11th to 20th	Night	34% 26% 25% 14% 1	783
	21st to end	Night	27% 26% 29% 17% 1	660
March	1st to 10th	Night	26% 21% 30% 21% 3	800
	11th to 20th	Night	28% 21% 26% 19% 6%	800
	21st to end	Night	23% 22% 29% 20% 6%	880
April	1st to 10th	Night	22% 23% 28% 23% 4	800
	11th to 20th	Night	21% 15% 27% 25% 11%	791
	21st to end	Night	18% 16% 26% 31% 8% 1	620
May	1st to 10th	Night	20% 15% 24% 29% 12% 1	600
	11th to 20th	Night	20% 12% 24% 29% 15%	591
	21st to end	Night	16% 7% 19% 30% 24% 4	650
June	1st to 10th	Night	10% 6% 21% 35% 25% 3	600
	11th to 20th	Night	13% 6% 16% 36% 28% 2	600
	21st to end	Night	11% 4 16% 39% 26% 4	600
July	1st to 10th	Night	11% 3 12% 36% 33% 5%	585
	11th to 20th	Night	8% 3 10% 34% 38% 7%	582
	21st to end	Night	9% 4 11% 34% 37% 4	657
August	1st to 10th	Night	18% 3 10% 34% 33% 3	597
	11th to 20th	Night	15% 2 11% 33% 35% 3	594
	21st to end	Night	13% 4 11% 35% 31% 6%	795
September	1st to 10th	Night	14% 3 13% 36% 29% 5%	800
	11th to 20th	Night	16% 5% 13% 32% 30% 4	793
	21st to end	Night	24% 8% 16% 27% 23% 2	760
October	1st to 10th	Night	15% 11% 19% 32% 21% 2	760
	11th to 20th	Night	19% 17% 22% 26% 16% 1	760
	21st to end	Night	20% 16% 22% 28% 13% 1	836
November	1st to 10th	Night	18% 12% 26% 32% 11% 2	780
	11th to 20th	Night	24% 16% 22% 29% 8% 1	800
	21st to end	Night	22% 20% 24% 27% 6% 1	800
December	1st to 10th	Night	28% 23% 24% 20% 5%	800
	11th to 20th	Night	24% 24% 25% 24% 4	800
	21st to end	Night	28% 17% 27% 24% 4	880

Page generated at 2025-09-16 01:49:10. Version ac07

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