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 2024-11-05.

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%	100954
	Night	10% 6% 2 6% 4 4 4 3 5% 6% 50%	98182
Transparency	Day	35% 6% 12% 25% 18% 3	104631
	Night	32% 6% 12% 26% 20% 4	102632
Seeing	Day	13% 10% 28% 29% 16% 4	15624
	Night	13% 10% 33% 32% 11% 1	25451

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	14% 9% 3 9% 6% 5% 3 4 5% 6% 38%	2847
	11th to 20th	Day	10% 9% 3 8% 5% 7% 5% 2 5% 6% 39%	2850
	21st to end	Day	11% 13% 6% 9% 4 3 3 2 4 5% 39%	3135
February	1st to 10th	Day	13% 10% 4 8% 5% 4 3 2 4 6% 41%	1710
	11th to 20th	Day	10% 8% 4 9% 5% 4 3 2 4 7% 44%	1730
	21st to end	Day	10% 8% 4 6% 4 3 4 4 6% 6% 44%	1594
March	1st to 10th	Day	11% 10% 2 6% 3 3 3 3 4 6% 51%	3436
	11th to 20th	Day	11% 6% 2 3 2 2 3 3 5% 7% 56%	3480
	21st to end	Day	13% 12% 4 7% 4 3 4 3 5% 6% 38%	3814
April	1st to 10th	Day	13% 7% 2 5% 4 3 4 3 5% 7% 47%	2137
	11th to 20th	Day	14% 8% 3 5% 4 3 5% 3 5% 6% 43%	2262
	21st to end	Day	18% 8% 3 6% 5% 3 4 3 4 6% 39%	2470
May	1st to 10th	Day	16% 10% 3 8% 5% 4 4 3 5% 6% 38%	4082
	11th to 20th	Day	14% 7% 2 6% 4 4 4 3 6% 7% 43%	4082
	21st to end	Day	13% 6% 2 6% 5% 5% 4 4 5% 7% 43%	4477
June	1st to 10th	Day	6% 4 2 4 4 4 5% 4 7% 11% 49%	2470
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 6% 8% 50%	2470
	21st to end	Day	6% 3 1 4 4 4 5% 4 6% 9% 55%	2470
July	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 4 7% 8% 41%	4120
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 10% 53%	4120
	21st to end	Day	6% 4 2 6% 5% 5% 6% 6% 8% 10% 42%	4465
August	1st to 10th	Day	7% 3 1 6% 4 4 4 4 6% 7% 54%	4104
	11th to 20th	Day	10% 6% 2 5% 4 4 5% 4 6% 8% 47%	4041
	21st to end	Day	5% 3 1 4 3 3 4 4 6% 8% 57%	4345
September	1st to 10th	Day	12% 5% 1 4 3 4 3 3 4 8% 54%	2199
	11th to 20th	Day	9% 5% 2 5% 4 3 4 4 5% 7% 51%	2090
	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 4 4 2 3 3 4 6% 57%	1851
	11th to 20th	Day	10% 7% 2 4 3 3 3 2 5% 6% 56%	1710
	21st to end	Day	14% 7% 2 5% 3 3 2 2 4 6% 53%	1654
December	1st to 10th	Day	10% 8% 3 6% 4 3 3 3 6% 7% 48%	1520
	11th to 20th	Day	11% 9% 4 7% 5% 4 4 2 4 5% 44%	2065
	21st to end	Day	14% 12% 8% 13% 4 4 4 4 5% 6% 27%	3135
January	1st to 10th	Night	9% 10% 3 13% 5% 4 3 2 3 5% 43%	4108
	11th to 20th	Night	9% 7% 3 8% 6% 7% 6% 3 4 5% 43%	4120
	21st to end	Night	15% 11% 4 7% 3 3 3 2 4 5% 42%	4532
February	1st to 10th	Night	10% 11% 3 7% 5% 5% 3 3 5% 6% 41%	2470
	11th to 20th	Night	12% 9% 2 6% 4 4 3 3 5% 6% 46%	2353
	21st to end	Night	11% 6% 3 7% 5% 3 3 3 4 5% 49%	1860
March	1st to 10th	Night	12% 7% 3 5% 3 2 3 3 4 6% 54%	3419
	11th to 20th	Night	9% 5% 2 4 3 3 3 3 5% 7% 57%	3480
	21st to end	Night	14% 8% 3 7% 4 4 4 3 4 6% 43%	3817
April	1st to 10th	Night	11% 6% 3 5% 4 3 3 3 5% 6% 52%	2043
	11th to 20th	Night	13% 7% 2 6% 4 3 4 3 4 7% 48%	1865
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	1710
May	1st to 10th	Night	15% 10% 2 6% 4 3 6% 3 5% 6% 41%	2823
	11th to 20th	Night	16% 6% 2 6% 3 3 3 3 6% 8% 45%	2823
	21st to end	Night	11% 4 1 5% 3 3 3 3 5% 7% 55%	3090
June	1st to 10th	Night	6% 2 1 4 4 4 5% 4 6% 11% 53%	1710
	11th to 20th	Night	4 2 1 5% 4 4 4 5% 7% 11% 51%	1710
	21st to end	Night	6% 3 1 3 3 4 4 3 5% 9% 60%	1710
July	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 6% 7% 50%	2850
	11th to 20th	Night	2 2 1 4 4 3 4 4 7% 10% 58%	2850
	21st to end	Night	6% 4 1 6% 5% 5% 5% 5% 8% 11% 44%	3098
August	1st to 10th	Night	3 2 1 4 4 4 4 4 6% 9% 58%	2836
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 3 5% 8% 51%	2798
	21st to end	Night	4 3 1 4 3 3 3 3 4 7% 63%	3316
September	1st to 10th	Night	10% 5% 1 4 3 3 3 2 4 5% 60%	1976
	11th to 20th	Night	10% 4 2 5% 3 4 4 2 6% 7% 53%	2090
	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%	2325
	11th to 20th	Night	8% 6% 2 5% 3 3 3 3 5% 6% 55%	2470
	21st to end	Night	13% 8% 2 4 3 2 2 2 4 5% 55%	2517
December	1st to 10th	Night	10% 7% 2 6% 4 3 4 3 5% 7% 48%	2660
	11th to 20th	Night	11% 8% 3 5% 3 3 4 3 5% 6% 48%	3123
	21st to end	Night	16% 13% 6% 9% 5% 4 3 3 3 5% 33%	4532

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	46% 7% 11% 23% 12% 1	2848
	11th to 20th	Day	47% 5% 15% 20% 12% 1	2850
	21st to end	Day	48% 4 12% 19% 15% 1	3135
February	1st to 10th	Day	45% 5% 12% 25% 11% 3	1710
	11th to 20th	Day	42% 4 12% 24% 15% 3	1739
	21st to end	Day	38% 6% 11% 23% 16% 6%	1594
March	1st to 10th	Day	35% 5% 8% 23% 24% 4	3436
	11th to 20th	Day	27% 6% 14% 28% 23% 2	3480
	21st to end	Day	44% 8% 14% 26% 7% 1	3814
April	1st to 10th	Day	37% 6% 11% 24% 19% 3	2137
	11th to 20th	Day	41% 6% 12% 23% 17% 2	2262
	21st to end	Day	45% 6% 11% 25% 11% 1	2470
May	1st to 10th	Day	47% 8% 10% 19% 15% 2	4120
	11th to 20th	Day	37% 9% 14% 25% 15%	4086
	21st to end	Day	38% 9% 13% 24% 14% 2	4489
June	1st to 10th	Day	26% 10% 18% 28% 17% 1	2470
	11th to 20th	Day	31% 7% 13% 28% 18% 2	2470
	21st to end	Day	24% 8% 16% 29% 20% 4	2470
July	1st to 10th	Day	38% 7% 12% 22% 18% 4	4056
	11th to 20th	Day	26% 7% 12% 33% 19% 3	4051
	21st to end	Day	30% 10% 17% 27% 14% 1	4325
August	1st to 10th	Day	28% 6% 12% 26% 22% 6%	4090
	11th to 20th	Day	35% 6% 16% 26% 16% 2	4080
	21st to end	Day	23% 6% 16% 34% 17% 5%	4344
September	1st to 10th	Day	30% 6% 13% 25% 19% 8%	2199
	11th to 20th	Day	30% 7% 10% 28% 23% 3	2090
	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% 21% 5%	1851
	11th to 20th	Day	30% 4 9% 27% 25% 4	1710
	21st to end	Day	35% 3 9% 27% 23% 2	1654
December	1st to 10th	Day	35% 6% 13% 30% 15% 1	1520
	11th to 20th	Day	43% 5% 10% 25% 14% 2	2063
	21st to end	Day	56% 7% 11% 17% 8% 2	3135
January	1st to 10th	Night	43% 6% 12% 24% 13% 2	4108
	11th to 20th	Night	42% 6% 13% 22% 15% 1	4120
	21st to end	Night	45% 5% 13% 21% 14% 2	4532
February	1st to 10th	Night	43% 5% 12% 22% 15% 3	2470
	11th to 20th	Night	39% 5% 11% 25% 17% 3	2354
	21st to end	Night	37% 5% 8% 24% 19% 8%	1860
March	1st to 10th	Night	32% 5% 9% 26% 23% 5%	3419
	11th to 20th	Night	26% 6% 12% 25% 28% 3	3480
	21st to end	Night	39% 9% 16% 24% 11% 1	3817
April	1st to 10th	Night	33% 6% 10% 23% 23% 6%	2043
	11th to 20th	Night	36% 6% 10% 25% 20% 4	1865
	21st to end	Night	38% 7% 13% 24% 15% 3	1710
May	1st to 10th	Night	41% 10% 13% 19% 13% 3	2850
	11th to 20th	Night	37% 8% 15% 27% 14% 1	2824
	21st to end	Night	28% 8% 15% 29% 19% 1	3099
June	1st to 10th	Night	23% 9% 21% 29% 15% 3	1710
	11th to 20th	Night	24% 9% 16% 31% 18% 2	1710
	21st to end	Night	22% 6% 16% 32% 21% 4	1710
July	1st to 10th	Night	32% 7% 14% 23% 20% 5%	2805
	11th to 20th	Night	19% 7% 15% 35% 23% 2	2796
	21st to end	Night	30% 9% 18% 29% 13% 1	2999
August	1st to 10th	Night	22% 6% 17% 28% 19% 8%	2826
	11th to 20th	Night	31% 6% 16% 25% 20% 2	2825
	21st to end	Night	21% 5% 13% 39% 17% 4	3316
September	1st to 10th	Night	27% 5% 10% 27% 22% 9%	1976
	11th to 20th	Night	30% 6% 11% 25% 24% 4	2090
	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 10% 28% 25% 6%	2325
	11th to 20th	Night	29% 5% 9% 26% 26% 4	2470
	21st to end	Night	33% 4 9% 28% 21% 4	2517
December	1st to 10th	Night	35% 6% 13% 26% 19% 1	2660
	11th to 20th	Night	36% 6% 9% 29% 17% 3	3124
	21st to end	Night	52% 7% 9% 21% 9% 2	4532

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% 15% 36% 20% 9%	270
	11th to 20th	Day	21% 17% 42% 18% 2	270
	21st to end	Day	12% 12% 48% 23% 4	297
February	1st to 10th	Day	20% 20% 37% 20% 2 1	270
	11th to 20th	Day	19% 17% 43% 16% 6%	270
	21st to end	Day	15% 26% 40% 15% 4	225
March	1st to 10th	Day	15% 20% 39% 24% 2	270
	11th to 20th	Day	24% 15% 36% 20% 5%	270
	21st to end	Day	27% 20% 36% 15% 2 1	297
April	1st to 10th	Day	19% 20% 35% 19% 7% 1	317
	11th to 20th	Day	20% 16% 38% 21% 4 1	460
	21st to end	Day	21% 16% 34% 20% 8% 1	650
May	1st to 10th	Day	17% 15% 37% 22% 9% 1	653
	11th to 20th	Day	18% 11% 28% 28% 13% 2	675
	21st to end	Day	17% 11% 27% 24% 18% 4	734
June	1st to 10th	Day	5% 4 21% 38% 25% 6%	680
	11th to 20th	Day	8% 4 29% 38% 18% 4	680
	21st to end	Day	5% 5% 34% 40% 15% 2	680
July	1st to 10th	Day	10% 3 15% 34% 28% 10%	671
	11th to 20th	Day	4 3 17% 41% 28% 8%	668
	21st to end	Day	8% 3 13% 37% 31% 8%	739
August	1st to 10th	Day	9% 2 16% 38% 29% 6%	676
	11th to 20th	Day	7% 3 16% 37% 31% 6%	665
	21st to end	Day	8% 2 14% 32% 35% 10%	639
September	1st to 10th	Day	12% 1 12% 41% 27% 6%	409
	11th to 20th	Day	6% 2 19% 39% 29% 5%	300
	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% 11% 34% 27% 10% 2	282
	11th to 20th	Day	16% 18% 34% 23% 9% 1	270
	21st to end	Day	21% 11% 40% 20% 7%	270
December	1st to 10th	Day	10% 12% 44% 25% 6% 4	270
	11th to 20th	Day	15% 19% 34% 23% 7% 1	270
	21st to end	Day	16% 23% 42% 17% 1	297
January	1st to 10th	Night	11% 13% 39% 30% 6% 1	950
	11th to 20th	Night	15% 13% 39% 27% 5% 1	950
	21st to end	Night	13% 14% 39% 29% 5%	1045
February	1st to 10th	Night	17% 13% 41% 27% 2	950
	11th to 20th	Night	16% 15% 39% 28% 2	879
	21st to end	Night	16% 14% 41% 24% 4	604
March	1st to 10th	Night	14% 12% 43% 27% 5%	570
	11th to 20th	Night	18% 12% 39% 26% 6%	570
	21st to end	Night	22% 13% 38% 23% 4	627
April	1st to 10th	Night	15% 14% 42% 22% 7%	570
	11th to 20th	Night	16% 16% 39% 25% 4	563
	21st to end	Night	17% 15% 37% 24% 7% 1	570
May	1st to 10th	Night	18% 9% 35% 32% 6% 1	570
	11th to 20th	Night	16% 6% 31% 34% 12%	561
	21st to end	Night	10% 7% 32% 34% 15% 2	617
June	1st to 10th	Night	6% 3 23% 44% 24% 1	570
	11th to 20th	Night	5% 4 29% 45% 17% 1	570
	21st to end	Night	5% 3 35% 44% 13%	570
July	1st to 10th	Night	8% 3 16% 41% 31% 2	555
	11th to 20th	Night	3 2 17% 49% 26% 3	552
	21st to end	Night	7% 1 15% 44% 32% 2	624
August	1st to 10th	Night	5% 3 17% 46% 29% 1	567
	11th to 20th	Night	6% 4 16% 46% 28% 1	564
	21st to end	Night	6% 1 14% 42% 33% 3	627
September	1st to 10th	Night	12% 1 21% 42% 21% 3	570
	11th to 20th	Night	9% 3 24% 38% 22% 4	570
	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%	805
	11th to 20th	Night	12% 14% 37% 30% 7%	950
	21st to end	Night	18% 12% 35% 29% 6%	950
December	1st to 10th	Night	14% 12% 39% 28% 6% 1	950
	11th to 20th	Night	17% 12% 39% 28% 4	950
	21st to end	Night	17% 16% 37% 27% 4	1045

Page generated at 2024-11-03 02:11:28. Version ac07

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