Spiral Gateway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spiral Gateway 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	11% 7% 3 6% 4 4 4 3 5% 7% 47%	125906
	Night	10% 6% 2 6% 4 3 4 3 5% 7% 50%	123404
Transparency	Day	35% 6% 11% 26% 19% 4	129544
	Night	33% 5% 11% 26% 20% 4	127829
Seeing	Day	13% 10% 29% 29% 16% 4	16858
	Night	13% 10% 33% 32% 11% 1	26618

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	16% 8% 3 8% 5% 4 3 3 5% 6% 39%	4077
	11th to 20th	Day	8% 8% 3 9% 6% 6% 5% 3 5% 6% 42%	4080
	21st to end	Day	10% 11% 5% 7% 4 4 3 3 5% 5% 43%	4488
February	1st to 10th	Day	13% 10% 4 8% 5% 4 3 2 4 6% 40%	1800
	11th to 20th	Day	11% 9% 4 9% 5% 4 3 2 4 6% 43%	1824
	21st to end	Day	10% 8% 3 6% 4 3 4 4 6% 6% 45%	1675
March	1st to 10th	Day	9% 8% 2 6% 3 4 3 3 4 6% 53%	4936
	11th to 20th	Day	9% 6% 2 4 2 2 3 3 5% 6% 58%	4980
	21st to end	Day	11% 10% 4 8% 5% 4 4 4 6% 7% 37%	5462
April	1st to 10th	Day	13% 7% 2 5% 4 3 3 3 5% 6% 47%	2250
	11th to 20th	Day	15% 7% 3 5% 4 4 5% 3 5% 6% 43%	2383
	21st to end	Day	18% 8% 3 6% 5% 3 4 3 4 6% 39%	2600
May	1st to 10th	Day	16% 8% 2 6% 4 4 3 3 4 6% 43%	5862
	11th to 20th	Day	12% 5% 2 5% 4 3 4 3 6% 7% 49%	5862
	21st to end	Day	19% 6% 2 5% 4 4 4 4 5% 7% 40%	6434
June	1st to 10th	Day	6% 4 2 4 4 4 5% 4 7% 10% 49%	2600
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 4 6% 8% 50%	2600
	21st to end	Day	6% 3 1 4 4 4 5% 4 6% 9% 55%	2600
July	1st to 10th	Day	9% 5% 2 6% 4 5% 5% 4 7% 9% 45%	5900
	11th to 20th	Day	8% 5% 2 5% 4 4 4 4 6% 10% 48%	5900
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6422
August	1st to 10th	Day	7% 3 1 6% 4 4 4 4 6% 7% 54%	4234
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 5% 8% 48%	4171
	21st to end	Day	6% 4 1 4 3 3 4 4 6% 8% 56%	4482
September	1st to 10th	Day	12% 5% 1 4 3 4 3 3 4 7% 54%	2315
	11th to 20th	Day	9% 5% 2 5% 4 4 5% 4 6% 8% 50%	2178
	21st to end	Day	9% 5% 2 5% 3 3 4 3 5% 6% 54%	2090
October	1st to 10th	Day	10% 5% 2 5% 3 3 4 4 4 7% 53%	1939
	11th to 20th	Day	11% 5% 2 4 2 2 2 2 4 5% 61%	1900
	21st to end	Day	15% 6% 2 5% 3 2 3 3 4 5% 51%	2090
November	1st to 10th	Day	9% 6% 2 5% 4 2 3 3 5% 6% 57%	1908
	11th to 20th	Day	11% 7% 2 4 3 3 3 2 5% 6% 56%	1800
	21st to end	Day	14% 7% 2 5% 3 3 2 2 4 6% 53%	1741
December	1st to 10th	Day	6% 5% 2 5% 3 3 3 3 5% 6% 58%	2610
	11th to 20th	Day	8% 7% 3 6% 4 4 4 2 3 5% 53%	3225
	21st to end	Day	17% 14% 7% 11% 4 3 4 3 4 5% 27%	4488
January	1st to 10th	Night	12% 9% 3 12% 5% 4 4 3 4 6% 39%	5888
	11th to 20th	Night	7% 6% 2 8% 6% 6% 5% 3 4 6% 47%	5900
	21st to end	Night	13% 9% 3 7% 4 3 3 2 3 5% 47%	6490
February	1st to 10th	Night	11% 11% 3 7% 5% 5% 3 3 5% 6% 41%	2600
	11th to 20th	Night	12% 9% 2 7% 4 4 3 3 5% 6% 45%	2480
	21st to end	Night	10% 6% 2 7% 4 3 3 3 5% 5% 50%	1955
March	1st to 10th	Night	12% 8% 3 5% 2 2 3 2 4 5% 55%	4919
	11th to 20th	Night	7% 4 1 4 2 3 3 3 6% 7% 59%	4980
	21st to end	Night	12% 7% 3 6% 4 4 5% 4 5% 7% 44%	5467
April	1st to 10th	Night	11% 6% 3 5% 4 3 3 3 5% 6% 51%	2150
	11th to 20th	Night	13% 7% 2 6% 4 3 4 2 4 7% 49%	1964
	21st to end	Night	15% 6% 2 7% 4 4 4 3 5% 7% 43%	1800
May	1st to 10th	Night	16% 8% 2 5% 3 3 5% 3 5% 6% 45%	4053
	11th to 20th	Night	14% 6% 2 6% 3 2 3 2 5% 6% 51%	4053
	21st to end	Night	18% 5% 1 5% 3 3 3 3 4 5% 51%	4443
June	1st to 10th	Night	6% 2 1 4 4 4 5% 4 6% 11% 53%	1800
	11th to 20th	Night	5% 3 1 5% 4 4 4 5% 7% 11% 51%	1800
	21st to end	Night	6% 3 1 3 3 4 4 4 5% 9% 59%	1800
July	1st to 10th	Night	7% 5% 2 5% 3 3 4 4 6% 8% 53%	4078
	11th to 20th	Night	4 3 1 5% 4 4 4 4 7% 10% 54%	4077
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 4 8% 10% 44%	4449
August	1st to 10th	Night	3 2 1 4 4 4 4 4 6% 9% 59%	2926
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 3 5% 8% 52%	2888
	21st to end	Night	5% 3 1 4 3 3 3 3 5% 7% 63%	3421
September	1st to 10th	Night	11% 5% 1 4 3 3 3 2 4 5% 59%	2080
	11th to 20th	Night	9% 4 2 5% 3 4 4 3 6% 7% 52%	2178
	21st to end	Night	7% 4 2 5% 4 3 4 3 5% 6% 59%	2090
October	1st to 10th	Night	10% 4 2 4 3 3 3 3 5% 6% 57%	2220
	11th to 20th	Night	11% 5% 1 4 3 3 2 2 4 6% 60%	2280
	21st to end	Night	13% 6% 2 5% 2 2 3 3 4 5% 54%	2508
November	1st to 10th	Night	10% 5% 2 4 3 2 3 2 4 5% 62%	2388
	11th to 20th	Night	9% 6% 2 5% 3 3 3 3 5% 6% 55%	2600
	21st to end	Night	12% 8% 2 4 3 2 2 3 4 5% 55%	2650
December	1st to 10th	Night	6% 5% 2 5% 3 3 3 3 5% 6% 60%	4580
	11th to 20th	Night	11% 7% 3 5% 3 3 4 4 5% 7% 49%	4959
	21st to end	Night	17% 14% 5% 8% 5% 4 3 3 3 5% 34%	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	45% 5% 11% 25% 12% 2	4078
	11th to 20th	Day	44% 4 11% 21% 17% 3	4080
	21st to end	Day	43% 4 10% 21% 16% 5%	4488
February	1st to 10th	Day	46% 5% 11% 25% 10% 3	1800
	11th to 20th	Day	43% 4 11% 24% 15% 3	1833
	21st to end	Day	37% 5% 10% 24% 18% 6%	1675
March	1st to 10th	Day	34% 4 8% 21% 23% 10%	4936
	11th to 20th	Day	27% 5% 10% 21% 26% 12%	4980
	21st to end	Day	43% 7% 13% 28% 8% 1	5464
April	1st to 10th	Day	37% 5% 11% 24% 19% 3	2250
	11th to 20th	Day	41% 6% 12% 22% 18% 2	2383
	21st to end	Day	46% 6% 11% 25% 11% 1	2600
May	1st to 10th	Day	42% 7% 10% 21% 19% 1	5900
	11th to 20th	Day	33% 7% 11% 23% 22% 4	5866
	21st to end	Day	42% 8% 12% 23% 14% 2	6447
June	1st to 10th	Day	26% 10% 17% 27% 17% 2	2600
	11th to 20th	Day	31% 7% 13% 28% 18% 2	2600
	21st to end	Day	24% 8% 17% 28% 19% 4	2600
July	1st to 10th	Day	34% 7% 12% 25% 20% 3	5836
	11th to 20th	Day	32% 7% 12% 31% 18% 2	5831
	21st to end	Day	32% 9% 15% 31% 12% 1	6240
August	1st to 10th	Day	28% 6% 12% 27% 21% 5%	4220
	11th to 20th	Day	34% 6% 15% 27% 16% 2	4210
	21st to end	Day	24% 6% 15% 33% 17% 5%	4481
September	1st to 10th	Day	31% 6% 13% 25% 19% 7%	2315
	11th to 20th	Day	30% 7% 10% 28% 22% 3	2178
	21st to end	Day	30% 6% 8% 24% 27% 4	2090
October	1st to 10th	Day	20% 5% 7% 19% 40% 9%	3225
	11th to 20th	Day	16% 3 5% 31% 33% 12%	3140
	21st to end	Day	28% 4 10% 25% 27% 5%	3428
November	1st to 10th	Day	29% 5% 11% 29% 20% 6%	1908
	11th to 20th	Day	31% 4 9% 27% 25% 5%	1800
	21st to end	Day	35% 3 9% 28% 22% 2	1741
December	1st to 10th	Day	27% 4 11% 39% 18% 1	2610
	11th to 20th	Day	35% 4 9% 33% 18% 2	3223
	21st to end	Day	58% 5% 9% 19% 7% 1	4488
January	1st to 10th	Night	45% 5% 11% 23% 13% 2	5888
	11th to 20th	Night	38% 5% 10% 23% 20% 4	5900
	21st to end	Night	41% 4 10% 21% 18% 7%	6490
February	1st to 10th	Night	43% 5% 12% 22% 14% 3	2600
	11th to 20th	Night	40% 5% 11% 24% 17% 3	2481
	21st to end	Night	35% 5% 7% 24% 20% 8%	1955
March	1st to 10th	Night	33% 4 7% 24% 26% 6%	4919
	11th to 20th	Night	23% 5% 9% 19% 34% 10%	4980
	21st to end	Night	37% 7% 16% 25% 12% 2	5467
April	1st to 10th	Night	34% 6% 10% 22% 23% 5%	2150
	11th to 20th	Night	36% 6% 10% 24% 20% 4	1964
	21st to end	Night	39% 7% 13% 23% 14% 3	1800
May	1st to 10th	Night	38% 8% 12% 23% 16% 2	4080
	11th to 20th	Night	33% 6% 11% 25% 18% 7%	4054
	21st to end	Night	35% 6% 12% 27% 18% 2	4452
June	1st to 10th	Night	23% 9% 20% 29% 16% 3	1800
	11th to 20th	Night	25% 8% 16% 31% 18% 2	1800
	21st to end	Night	22% 6% 16% 31% 21% 4	1800
July	1st to 10th	Night	28% 6% 15% 26% 22% 4	4035
	11th to 20th	Night	25% 6% 13% 35% 19% 2	4026
	21st to end	Night	31% 8% 15% 34% 11% 1	4320
August	1st to 10th	Night	22% 6% 17% 28% 19% 8%	2916
	11th to 20th	Night	30% 6% 15% 27% 20% 2	2915
	21st to end	Night	21% 5% 13% 39% 17% 4	3421
September	1st to 10th	Night	27% 5% 11% 27% 21% 8%	2080
	11th to 20th	Night	30% 6% 11% 25% 23% 4	2178
	21st to end	Night	26% 5% 10% 24% 30% 5%	2090
October	1st to 10th	Night	18% 4 7% 16% 46% 9%	3701
	11th to 20th	Night	19% 3 5% 26% 33% 14%	3779
	21st to end	Night	28% 4 8% 27% 29% 4	4120
November	1st to 10th	Night	27% 4 9% 28% 25% 6%	2388
	11th to 20th	Night	29% 5% 9% 26% 26% 5%	2600
	21st to end	Night	33% 4 10% 29% 21% 4	2650
December	1st to 10th	Night	25% 4 14% 35% 21% 1	4580
	11th to 20th	Night	34% 5% 11% 28% 19% 3	4960
	21st to end	Night	52% 6% 9% 23% 9% 2	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	21% 14% 35% 21% 8%	300
	11th to 20th	Day	19% 17% 44% 17% 2	300
	21st to end	Day	11% 11% 50% 23% 4	330
February	1st to 10th	Day	20% 21% 36% 20% 2 1	300
	11th to 20th	Day	20% 18% 42% 15% 5%	300
	21st to end	Day	14% 24% 42% 15% 4	249
March	1st to 10th	Day	16% 18% 39% 24% 3	300
	11th to 20th	Day	24% 15% 35% 22% 5%	300
	21st to end	Day	25% 21% 37% 15% 2 1	330
April	1st to 10th	Day	20% 21% 34% 18% 7% 1	350
	11th to 20th	Day	20% 18% 38% 20% 4 1	501
	21st to end	Day	21% 16% 34% 21% 7% 1	700
May	1st to 10th	Day	17% 15% 36% 23% 9% 1	703
	11th to 20th	Day	18% 13% 27% 27% 13% 2	725
	21st to end	Day	18% 10% 28% 23% 17% 4	789
June	1st to 10th	Day	5% 5% 22% 36% 25% 6%	730
	11th to 20th	Day	8% 4 29% 38% 18% 4	730
	21st to end	Day	5% 5% 34% 40% 15% 2	730
July	1st to 10th	Day	9% 2 15% 34% 29% 10%	721
	11th to 20th	Day	4 3 18% 41% 27% 7%	718
	21st to end	Day	8% 3 13% 37% 31% 8%	794
August	1st to 10th	Day	9% 2 18% 39% 27% 5%	726
	11th to 20th	Day	7% 3 15% 37% 31% 6%	715
	21st to end	Day	9% 2 14% 33% 33% 9%	688
September	1st to 10th	Day	13% 2 13% 40% 27% 5%	445
	11th to 20th	Day	6% 2 19% 40% 27% 6%	324
	21st to end	Day	14% 5% 22% 38% 15% 5%	300
October	1st to 10th	Day	12% 8% 29% 29% 15% 8%	300
	11th to 20th	Day	9% 15% 44% 23% 10%	300
	21st to end	Day	20% 13% 38% 23% 4 2	330
November	1st to 10th	Day	15% 13% 33% 27% 10% 2	300
	11th to 20th	Day	17% 19% 33% 21% 10% 1	300
	21st to end	Day	20% 12% 40% 19% 8%	300
December	1st to 10th	Day	9% 13% 46% 23% 5% 4	300
	11th to 20th	Day	15% 17% 38% 23% 7% 1	300
	21st to end	Day	18% 22% 41% 17% 2	330
January	1st to 10th	Night	12% 13% 40% 29% 6% 1	1000
	11th to 20th	Night	14% 13% 39% 28% 5% 1	1000
	21st to end	Night	12% 14% 40% 29% 5%	1100
February	1st to 10th	Night	17% 13% 41% 26% 2	1000
	11th to 20th	Night	16% 16% 39% 26% 2	926
	21st to end	Night	15% 14% 41% 25% 4	635
March	1st to 10th	Night	14% 12% 43% 27% 5%	600
	11th to 20th	Night	18% 12% 39% 25% 6%	600
	21st to end	Night	22% 13% 39% 22% 3	660
April	1st to 10th	Night	15% 13% 42% 22% 7%	600
	11th to 20th	Night	16% 17% 38% 24% 4	593
	21st to end	Night	18% 14% 38% 24% 6% 1	600
May	1st to 10th	Night	18% 8% 34% 32% 7% 1	600
	11th to 20th	Night	15% 7% 31% 34% 13%	591
	21st to end	Night	11% 7% 32% 33% 15% 2	650
June	1st to 10th	Night	7% 3 24% 43% 23% 1	600
	11th to 20th	Night	6% 4 28% 46% 16% 1	600
	21st to end	Night	5% 3 36% 44% 12%	600
July	1st to 10th	Night	8% 2 16% 43% 30% 2	585
	11th to 20th	Night	3 2 18% 49% 25% 3	582
	21st to end	Night	7% 1 15% 45% 31% 2	657
August	1st to 10th	Night	5% 3 18% 47% 27% 1	597
	11th to 20th	Night	5% 4 15% 46% 29% 1	594
	21st to end	Night	6% 2 14% 43% 32% 3	660
September	1st to 10th	Night	13% 1 21% 42% 21% 3	600
	11th to 20th	Night	9% 3 23% 38% 23% 4	594
	21st to end	Night	8% 5% 25% 43% 17% 2	570
October	1st to 10th	Night	12% 5% 29% 33% 20% 1	700
	11th to 20th	Night	13% 9% 41% 29% 7%	760
	21st to end	Night	17% 13% 33% 27% 9%	836
November	1st to 10th	Night	11% 10% 36% 30% 12%	828
	11th to 20th	Night	12% 15% 36% 30% 7%	1000
	21st to end	Night	17% 12% 35% 29% 6%	1000
December	1st to 10th	Night	14% 11% 40% 29% 6% 1	1000
	11th to 20th	Night	17% 13% 39% 27% 4	1000
	21st to end	Night	18% 15% 36% 27% 4	1100

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

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