The Geek Shed
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Geek Shed clear sky chart from 2005-11-06 to 2026-01-17.

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	15% 8% 3 8% 5% 5% 5% 4 6% 7% 34%	127579
	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 37%	124098
Transparency	Day	46% 6% 12% 22% 13% 2	131171
	Night	45% 6% 12% 22% 14% 2	128592
Seeing	Day	22% 19% 27% 19% 10% 3	16252
	Night	21% 16% 27% 25% 10% 1	25736

By Month

Percentage Cloud Cover NE (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% 10% 4 11% 6% 4 5% 4 5% 6% 33%	3677
	11th to 20th	Day	11% 14% 6% 15% 9% 7% 4 3 5% 6% 18%	3956
	21st to end	Day	15% 13% 6% 9% 6% 5% 5% 3 4 6% 29%	4466
February	1st to 10th	Day	16% 14% 4 9% 6% 5% 4 3 6% 7% 27%	1800
	11th to 20th	Day	13% 10% 4 10% 6% 5% 5% 5% 5% 7% 31%	1769
	21st to end	Day	16% 11% 4 9% 7% 5% 4 4 5% 6% 30%	1650
March	1st to 10th	Day	19% 9% 3 7% 4 4 5% 3 5% 5% 37%	4792
	11th to 20th	Day	12% 8% 2 7% 6% 5% 5% 4 6% 7% 38%	4980
	21st to end	Day	12% 9% 3 8% 5% 5% 4 4 5% 6% 39%	5590
April	1st to 10th	Day	16% 9% 3 9% 5% 5% 4 3 6% 6% 33%	2400
	11th to 20th	Day	15% 10% 3 7% 5% 5% 5% 5% 5% 6% 36%	2374
	21st to end	Day	17% 8% 3 7% 5% 4 4 3 6% 7% 36%	2429
May	1st to 10th	Day	26% 6% 2 7% 5% 5% 5% 4 5% 7% 27%	5929
	11th to 20th	Day	24% 8% 2 6% 4 5% 5% 4 6% 8% 29%	6318
	21st to end	Day	18% 6% 2 6% 4 5% 6% 5% 7% 9% 33%	6949
June	1st to 10th	Day	12% 6% 3 7% 5% 5% 5% 6% 6% 8% 37%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 6% 5% 3 6% 9% 36%	2800
	21st to end	Day	11% 5% 2 5% 5% 5% 5% 5% 7% 11% 39%	2800
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 5% 7% 8% 40%	6360
	11th to 20th	Day	8% 6% 2 7% 6% 7% 6% 6% 9% 11% 32%	6360
	21st to end	Day	11% 7% 3 7% 5% 5% 6% 5% 7% 8% 39%	6520
August	1st to 10th	Day	20% 6% 2 7% 5% 5% 5% 4 7% 8% 32%	4234
	11th to 20th	Day	14% 6% 2 7% 4 6% 6% 5% 7% 8% 35%	4076
	21st to end	Day	16% 4 2 4 3 4 4 4 6% 9% 43%	4312
September	1st to 10th	Day	16% 5% 2 6% 5% 4 4 3 5% 6% 45%	2400
	11th to 20th	Day	13% 4 2 5% 4 4 3 3 5% 7% 50%	2400
	21st to end	Day	23% 7% 2 7% 3 4 3 3 4 6% 37%	2101
October	1st to 10th	Day	17% 7% 2 6% 4 4 4 3 5% 7% 42%	2000
	11th to 20th	Day	17% 7% 2 6% 4 5% 4 4 5% 7% 39%	2000
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 5% 6% 37%	2200
November	1st to 10th	Day	11% 7% 2 6% 4 3 4 4 6% 7% 45%	1883
	11th to 20th	Day	14% 10% 3 9% 4 4 6% 4 6% 6% 35%	1780
	21st to end	Day	17% 8% 3 6% 5% 5% 5% 6% 6% 6% 32%	1680
December	1st to 10th	Day	14% 11% 5% 10% 6% 5% 5% 4 5% 8% 27%	2680
	11th to 20th	Day	21% 12% 3 8% 5% 4 4 3 6% 6% 29%	3066
	21st to end	Day	17% 12% 4 12% 7% 6% 5% 4 5% 6% 21%	4048
January	1st to 10th	Night	11% 8% 3 11% 7% 5% 5% 4 6% 8% 32%	6489
	11th to 20th	Night	11% 10% 4 10% 6% 5% 4 4 6% 8% 30%	6060
	21st to end	Night	16% 9% 3 8% 5% 5% 6% 4 6% 7% 31%	6490
February	1st to 10th	Night	15% 10% 3 7% 7% 5% 4 4 6% 6% 32%	2600
	11th to 20th	Night	15% 9% 4 8% 6% 5% 5% 4 6% 7% 32%	2515
	21st to end	Night	15% 9% 3 9% 5% 5% 5% 4 4 6% 32%	1980
March	1st to 10th	Night	17% 10% 3 7% 5% 4 4 3 4 5% 37%	5045
	11th to 20th	Night	10% 6% 3 7% 4 5% 4 3 6% 7% 44%	4979
	21st to end	Night	14% 8% 2 7% 4 5% 4 4 5% 6% 40%	5341
April	1st to 10th	Night	18% 10% 4 8% 6% 4 4 4 5% 6% 33%	2000
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 5% 6% 7% 37%	1975
	21st to end	Night	18% 5% 2 6% 4 4 4 4 6% 7% 40%	1989
May	1st to 10th	Night	31% 8% 2 5% 4 3 4 4 4 7% 28%	4009
	11th to 20th	Night	24% 8% 2 6% 4 4 4 4 5% 6% 34%	3575
	21st to end	Night	20% 6% 2 6% 4 4 4 3 5% 8% 37%	3926
June	1st to 10th	Night	14% 6% 2 7% 5% 4 4 3 5% 9% 42%	1600
	11th to 20th	Night	14% 5% 2 6% 5% 3 5% 4 6% 8% 42%	1600
	21st to end	Night	11% 4 2 6% 5% 4 5% 4 7% 9% 43%	1600
July	1st to 10th	Night	9% 6% 3 6% 4 5% 5% 4 6% 10% 42%	3600
	11th to 20th	Night	11% 6% 2 7% 6% 6% 6% 5% 7% 10% 35%	3600
	21st to end	Night	11% 6% 2 6% 4 4 5% 3 5% 8% 47%	4328
August	1st to 10th	Night	18% 6% 2 6% 4 5% 4 4 6% 8% 38%	2915
	11th to 20th	Night	15% 7% 2 7% 5% 3 3 4 5% 6% 44%	2938
	21st to end	Night	15% 4 1 4 3 3 4 4 5% 9% 47%	3586
September	1st to 10th	Night	16% 5% 2 6% 4 4 4 3 5% 6% 47%	2000
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 52%	2000
	21st to end	Night	20% 6% 3 5% 4 3 4 3 5% 7% 39%	2292
October	1st to 10th	Night	19% 6% 2 5% 3 3 3 3 5% 7% 45%	2400
	11th to 20th	Night	15% 7% 1 6% 4 3 4 4 5% 8% 43%	2400
	21st to end	Night	18% 7% 2 5% 4 4 4 3 5% 6% 41%	2640
November	1st to 10th	Night	9% 7% 3 6% 4 4 4 3 5% 5% 50%	2633
	11th to 20th	Night	16% 8% 3 5% 5% 4 5% 4 5% 7% 37%	2835
	21st to end	Night	15% 8% 3 8% 4 4 4 4 6% 8% 35%	2940
December	1st to 10th	Night	11% 9% 3 9% 5% 6% 5% 4 5% 8% 34%	4720
	11th to 20th	Night	21% 9% 2 7% 5% 4 4 3 5% 6% 32%	5348
	21st to end	Night	17% 11% 4 10% 6% 6% 5% 4 5% 6% 27%	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	50% 4 12% 22% 10% 1	3678
	11th to 20th	Day	64% 4 8% 13% 9% 2	3990
	21st to end	Day	56% 5% 10% 19% 11% 1	4466
February	1st to 10th	Day	56% 5% 11% 18% 8% 1	1800
	11th to 20th	Day	52% 6% 10% 23% 9% 1	1778
	21st to end	Day	54% 5% 11% 19% 8% 2	1650
March	1st to 10th	Day	48% 5% 9% 23% 10% 5%	4795
	11th to 20th	Day	44% 4 15% 23% 9% 4	4980
	21st to end	Day	45% 5% 9% 19% 18% 4	5590
April	1st to 10th	Day	51% 5% 9% 20% 13% 3	2400
	11th to 20th	Day	46% 6% 10% 21% 15% 3	2374
	21st to end	Day	45% 6% 10% 21% 14% 4	2429
May	1st to 10th	Day	54% 6% 10% 17% 11% 2	5938
	11th to 20th	Day	52% 7% 11% 19% 10% 1	6322
	21st to end	Day	45% 7% 11% 23% 12% 2	6949
June	1st to 10th	Day	42% 7% 12% 22% 14% 2	2800
	11th to 20th	Day	43% 8% 13% 21% 13% 2	2800
	21st to end	Day	37% 8% 13% 27% 14% 1	2800
July	1st to 10th	Day	38% 9% 12% 26% 14% 1	6290
	11th to 20th	Day	41% 9% 17% 22% 11% 1	6285
	21st to end	Day	40% 7% 11% 23% 19%	6333
August	1st to 10th	Day	48% 7% 13% 21% 11%	4221
	11th to 20th	Day	43% 7% 11% 28% 10% 1	4115
	21st to end	Day	36% 5% 12% 32% 13% 2	4312
September	1st to 10th	Day	39% 7% 10% 23% 18% 3	2400
	11th to 20th	Day	34% 4 12% 26% 21% 2	2400
	21st to end	Day	49% 4 10% 19% 17% 2	2101
October	1st to 10th	Day	39% 5% 8% 25% 18% 5%	3260
	11th to 20th	Day	37% 5% 9% 26% 21% 1	3240
	21st to end	Day	30% 4 15% 31% 18% 1	3538
November	1st to 10th	Day	36% 5% 15% 24% 17% 3	1883
	11th to 20th	Day	47% 5% 12% 22% 11% 2	1780
	21st to end	Day	48% 7% 13% 23% 8% 1	1680
December	1st to 10th	Day	54% 5% 13% 20% 7% 2	2680
	11th to 20th	Day	53% 5% 14% 21% 6%	3066
	21st to end	Day	60% 6% 15% 12% 6% 2	4048
January	1st to 10th	Night	50% 5% 11% 20% 12% 2	6489
	11th to 20th	Night	50% 5% 14% 16% 12% 4	6101
	21st to end	Night	52% 5% 10% 17% 13% 2	6490
February	1st to 10th	Night	50% 6% 14% 22% 8% 1	2600
	11th to 20th	Night	50% 6% 12% 20% 12% 1	2517
	21st to end	Night	52% 4 10% 20% 13% 2	1980
March	1st to 10th	Night	50% 4 10% 20% 11% 5%	5055
	11th to 20th	Night	40% 5% 15% 27% 9% 4	4980
	21st to end	Night	45% 5% 9% 18% 21% 4	5341
April	1st to 10th	Night	52% 4 9% 19% 14% 2	2000
	11th to 20th	Night	43% 6% 10% 22% 16% 2	1975
	21st to end	Night	42% 6% 11% 22% 15% 3	1989
May	1st to 10th	Night	56% 5% 9% 17% 9% 3	4016
	11th to 20th	Night	50% 9% 11% 16% 12% 1	3576
	21st to end	Night	46% 7% 12% 19% 14% 3	3927
June	1st to 10th	Night	40% 6% 12% 24% 17% 2	1600
	11th to 20th	Night	40% 7% 13% 22% 16% 2	1600
	21st to end	Night	35% 7% 14% 27% 16% 1	1600
July	1st to 10th	Night	38% 8% 13% 23% 16% 3	3560
	11th to 20th	Night	43% 9% 13% 22% 11% 1	3552
	21st to end	Night	36% 7% 10% 22% 24% 1	4200
August	1st to 10th	Night	44% 9% 13% 23% 10% 1	2904
	11th to 20th	Night	41% 5% 12% 26% 16%	2965
	21st to end	Night	35% 6% 11% 32% 15% 1	3586
September	1st to 10th	Night	38% 5% 10% 24% 19% 3	2000
	11th to 20th	Night	32% 4 11% 30% 20% 2	2000
	21st to end	Night	44% 5% 10% 21% 18% 1	2292
October	1st to 10th	Night	38% 4 11% 24% 21% 1	3920
	11th to 20th	Night	32% 5% 10% 28% 23% 3	3899
	21st to end	Night	31% 4 14% 29% 20% 2	4252
November	1st to 10th	Night	35% 4 12% 26% 19% 3	2633
	11th to 20th	Night	45% 5% 11% 22% 14% 3	2835
	21st to end	Night	46% 5% 14% 24% 9% 1	2940
December	1st to 10th	Night	46% 5% 15% 26% 7%	4720
	11th to 20th	Night	52% 4 14% 20% 9% 1	5348
	21st to end	Night	57% 5% 11% 19% 7%	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	30% 34% 25% 9% 1 1	220
	11th to 20th	Day	34% 29% 28% 7% 1	288
	21st to end	Day	25% 36% 28% 8% 3	330
February	1st to 10th	Day	30% 34% 27% 9% 1	300
	11th to 20th	Day	34% 30% 32% 4	316
	21st to end	Day	33% 26% 26% 14% 1	414
March	1st to 10th	Day	31% 25% 29% 12% 3	500
	11th to 20th	Day	29% 28% 27% 12% 3 1	500
	21st to end	Day	28% 26% 28% 13% 3 2	550
April	1st to 10th	Day	28% 28% 26% 13% 5% 1	500
	11th to 20th	Day	25% 29% 22% 15% 6% 3	492
	21st to end	Day	27% 22% 31% 12% 7% 1	500
May	1st to 10th	Day	31% 20% 28% 12% 7% 3	503
	11th to 20th	Day	23% 22% 26% 18% 8% 2	527
	21st to end	Day	20% 13% 29% 19% 15% 4	573
June	1st to 10th	Day	17% 18% 28% 24% 11% 2	530
	11th to 20th	Day	20% 15% 26% 25% 12% 2	530
	21st to end	Day	14% 13% 34% 26% 10% 2	530
July	1st to 10th	Day	12% 7% 28% 30% 17% 6%	525
	11th to 20th	Day	8% 9% 28% 33% 17% 5%	524
	21st to end	Day	14% 9% 31% 28% 14% 5%	576
August	1st to 10th	Day	14% 6% 28% 28% 19% 6%	526
	11th to 20th	Day	16% 8% 24% 27% 19% 7%	519
	21st to end	Day	15% 9% 26% 25% 16% 9%	583
September	1st to 10th	Day	19% 11% 20% 24% 18% 8%	530
	11th to 20th	Day	14% 6% 24% 26% 22% 7%	530
	21st to end	Day	29% 12% 25% 17% 14% 3	530
October	1st to 10th	Day	22% 13% 23% 21% 17% 4	530
	11th to 20th	Day	22% 18% 30% 20% 9% 1	530
	21st to end	Day	22% 20% 31% 19% 7%	583
November	1st to 10th	Day	21% 20% 27% 21% 10% 1	441
	11th to 20th	Day	26% 26% 26% 17% 5%	320
	21st to end	Day	27% 30% 29% 11% 2	220
December	1st to 10th	Day	29% 31% 27% 9% 3	220
	11th to 20th	Day	34% 25% 29% 11% 1	220
	21st to end	Day	28% 27% 31% 12% 2	242
January	1st to 10th	Night	20% 28% 33% 15% 3	840
	11th to 20th	Night	26% 23% 30% 18% 4	825
	21st to end	Night	18% 26% 35% 19% 2	880
February	1st to 10th	Night	23% 22% 33% 21% 1	800
	11th to 20th	Night	25% 27% 32% 15% 1	783
	21st to end	Night	25% 25% 32% 16% 3	660
March	1st to 10th	Night	25% 23% 30% 20% 3	800
	11th to 20th	Night	24% 23% 29% 20% 5%	800
	21st to end	Night	22% 19% 35% 18% 5% 1	880
April	1st to 10th	Night	25% 22% 30% 18% 5%	800
	11th to 20th	Night	21% 20% 31% 21% 7% 1	791
	21st to end	Night	22% 17% 29% 26% 6%	789
May	1st to 10th	Night	26% 12% 26% 25% 11%	582
	11th to 20th	Night	21% 10% 30% 29% 9%	394
	21st to end	Night	18% 10% 22% 33% 16%	433
June	1st to 10th	Night	15% 6% 26% 36% 17%	400
	11th to 20th	Night	15% 9% 23% 39% 15%	400
	21st to end	Night	11% 3 30% 42% 14% 1	400
July	1st to 10th	Night	12% 5% 24% 38% 20% 1	390
	11th to 20th	Night	8% 3 19% 44% 24% 1	388
	21st to end	Night	13% 3 20% 42% 21% 1	609
August	1st to 10th	Night	13% 2 18% 41% 24% 2	597
	11th to 20th	Night	14% 5% 17% 33% 28% 3	631
	21st to end	Night	15% 3 19% 35% 26% 2	880
September	1st to 10th	Night	18% 5% 22% 30% 23% 2	800
	11th to 20th	Night	15% 5% 19% 31% 26% 5%	800
	21st to end	Night	24% 8% 19% 28% 19% 2	800
October	1st to 10th	Night	22% 10% 22% 25% 19% 3	800
	11th to 20th	Night	19% 14% 27% 29% 10% 1	800
	21st to end	Night	24% 14% 29% 26% 7%	880
November	1st to 10th	Night	16% 15% 30% 27% 11%	820
	11th to 20th	Night	25% 21% 23% 24% 6% 1	840
	21st to end	Night	23% 22% 29% 21% 4	840
December	1st to 10th	Night	25% 22% 32% 16% 4	840
	11th to 20th	Night	25% 22% 30% 19% 4	840
	21st to end	Night	25% 20% 29% 24% 2	924

Page generated at 2026-01-14 01:14:24. Version ac07

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