Spider Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Spider Lake clear sky chart from 2005-11-06 to 2025-12-03.

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% 11% 4 10% 5% 5% 5% 4 5% 7% 33%	127495
	Night	12% 12% 4 11% 6% 5% 5% 4 5% 6% 32%	123501
Transparency	Day	50% 5% 12% 22% 10% 1	130968
	Night	52% 5% 11% 21% 10% 1	128101
Seeing	Day	21% 16% 34% 19% 7% 2	17569
	Night	20% 14% 33% 24% 8% 1	27454

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	7% 13% 6% 21% 10% 8% 6% 4 4 5% 17%	3617
	11th to 20th	Day	6% 15% 7% 16% 8% 10% 10% 5% 9% 5% 10%	3620
	21st to end	Day	6% 17% 10% 18% 5% 4 6% 3 4 5% 20%	4093
February	1st to 10th	Day	8% 15% 5% 14% 7% 5% 5% 5% 6% 4 27%	1800
	11th to 20th	Day	8% 13% 5% 12% 8% 6% 5% 5% 5% 5% 28%	1754
	21st to end	Day	11% 15% 6% 12% 8% 5% 4 4 5% 6% 24%	1637
March	1st to 10th	Day	12% 9% 3 10% 6% 4 4 3 5% 7% 37%	4480
	11th to 20th	Day	9% 13% 4 13% 5% 4 4 4 5% 8% 31%	4975
	21st to end	Day	20% 14% 5% 12% 4 3 4 3 4 5% 27%	5773
April	1st to 10th	Day	15% 18% 5% 9% 5% 4 4 3 5% 6% 27%	2400
	11th to 20th	Day	19% 15% 6% 8% 3 4 3 3 4 6% 29%	2374
	21st to end	Day	19% 13% 4 8% 5% 5% 4 3 4 5% 31%	2574
May	1st to 10th	Day	14% 9% 2 6% 4 4 3 2 4 5% 47%	6319
	11th to 20th	Day	14% 9% 3 8% 4 4 5% 4 5% 6% 38%	6319
	21st to end	Day	16% 9% 3 7% 4 4 4 4 5% 6% 37%	6935
June	1st to 10th	Day	14% 6% 2 7% 5% 4 6% 4 7% 8% 38%	2800
	11th to 20th	Day	15% 7% 4 8% 6% 4 5% 4 6% 7% 35%	2800
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 7% 8% 35%	2800
July	1st to 10th	Day	10% 5% 2 5% 4 4 5% 4 6% 10% 44%	6360
	11th to 20th	Day	11% 6% 2 5% 4 4 5% 4 6% 9% 45%	6360
	21st to end	Day	9% 6% 2 6% 4 4 5% 4 6% 9% 46%	6924
August	1st to 10th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 7% 37%	4540
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 46%	4171
	21st to end	Day	13% 8% 3 7% 5% 4 4 4 6% 7% 39%	4323
September	1st to 10th	Day	14% 8% 2 7% 4 4 4 4 5% 9% 40%	2400
	11th to 20th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 9% 39%	2400
	21st to end	Day	19% 8% 3 6% 3 4 4 3 5% 6% 38%	2129
October	1st to 10th	Day	17% 11% 3 7% 4 3 4 3 5% 7% 36%	2000
	11th to 20th	Day	17% 17% 5% 9% 4 4 4 3 4 4 27%	2000
	21st to end	Day	20% 15% 4 7% 4 4 5% 3 4 5% 27%	2022
November	1st to 10th	Day	12% 19% 5% 9% 5% 3 4 3 5% 5% 29%	1820
	11th to 20th	Day	13% 18% 9% 13% 7% 4 3 3 4 5% 21%	1680
	21st to end	Day	13% 15% 5% 18% 7% 5% 4 2 4 6% 19%	1680
December	1st to 10th	Day	8% 16% 9% 30% 9% 5% 4 2 3 4 10%	2626
	11th to 20th	Day	9% 17% 8% 13% 12% 6% 5% 3 4 4 19%	3008
	21st to end	Day	15% 21% 8% 18% 6% 5% 3 2 3 3 14%	3982
January	1st to 10th	Night	7% 11% 6% 20% 10% 7% 6% 3 4 4 20%	6348
	11th to 20th	Night	6% 13% 7% 16% 9% 8% 8% 7% 7% 4 14%	6360
	21st to end	Night	6% 18% 8% 16% 7% 6% 5% 3 4 5% 21%	6881
February	1st to 10th	Night	7% 15% 5% 13% 8% 7% 6% 4 5% 5% 26%	2600
	11th to 20th	Night	7% 11% 4 11% 8% 8% 7% 6% 5% 6% 28%	2533
	21st to end	Night	11% 16% 4 10% 6% 6% 5% 4 5% 6% 28%	1992
March	1st to 10th	Night	10% 7% 3 8% 5% 4 5% 4 6% 7% 42%	5373
	11th to 20th	Night	10% 11% 5% 15% 6% 4 5% 5% 5% 6% 27%	5027
	21st to end	Night	19% 15% 6% 10% 5% 4 4 3 4 4 26%	5155
April	1st to 10th	Night	18% 14% 4 9% 4 5% 4 4 5% 5% 29%	2000
	11th to 20th	Night	21% 12% 6% 6% 3 3 5% 3 5% 5% 32%	1975
	21st to end	Night	19% 13% 3 7% 4 3 4 3 5% 6% 34%	1824
May	1st to 10th	Night	13% 8% 3 6% 4 3 4 2 4 6% 47%	3596
	11th to 20th	Night	11% 8% 3 6% 4 4 4 3 5% 6% 46%	3596
	21st to end	Night	12% 8% 3 8% 5% 4 5% 4 6% 6% 40%	3942
June	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 7% 8% 42%	1600
	11th to 20th	Night	11% 7% 3 7% 6% 5% 5% 5% 6% 7% 39%	1600
	21st to end	Night	11% 7% 3 6% 5% 5% 5% 4 6% 8% 40%	1600
July	1st to 10th	Night	8% 5% 2 6% 4 4 5% 4 6% 10% 47%	3618
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 8% 53%	3617
	21st to end	Night	9% 6% 3 7% 4 4 4 4 5% 7% 48%	3947
August	1st to 10th	Night	12% 7% 2 6% 5% 3 4 3 6% 8% 44%	2617
	11th to 20th	Night	13% 6% 2 4 3 3 3 3 5% 7% 51%	2888
	21st to end	Night	15% 9% 3 6% 3 3 3 3 4 7% 44%	3574
September	1st to 10th	Night	11% 6% 2 6% 3 4 4 3 5% 8% 49%	2000
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 3 6% 7% 47%	2000
	21st to end	Night	16% 8% 3 5% 4 4 4 3 5% 7% 41%	2245
October	1st to 10th	Night	16% 9% 2 5% 5% 3 4 3 5% 6% 42%	2400
	11th to 20th	Night	17% 12% 5% 8% 5% 4 4 3 5% 6% 32%	2400
	21st to end	Night	19% 15% 4 8% 4 4 4 3 5% 5% 28%	2816
November	1st to 10th	Night	14% 14% 5% 9% 5% 4 4 3 5% 6% 31%	2693
	11th to 20th	Night	12% 17% 6% 12% 6% 5% 4 3 5% 5% 26%	2940
	21st to end	Night	12% 15% 5% 14% 7% 6% 4 4 5% 5% 23%	2940
December	1st to 10th	Night	9% 14% 8% 21% 10% 7% 5% 3 4 4 14%	4609
	11th to 20th	Night	9% 17% 7% 16% 7% 6% 6% 5% 4 4 18%	5199
	21st to end	Night	20% 19% 6% 14% 6% 5% 4 3 3 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	70% 3 6% 14% 7%	3618
	11th to 20th	Day	72% 4 6% 12% 6%	3620
	21st to end	Day	67% 3 8% 17% 5%	4093
February	1st to 10th	Day	58% 4 10% 17% 11%	1800
	11th to 20th	Day	56% 5% 9% 21% 9%	1763
	21st to end	Day	59% 5% 9% 16% 9% 1	1637
March	1st to 10th	Day	46% 5% 14% 20% 13% 3	4480
	11th to 20th	Day	51% 4 14% 19% 11% 1	4975
	21st to end	Day	61% 3 9% 15% 11%	5775
April	1st to 10th	Day	58% 4 10% 17% 10% 1	2400
	11th to 20th	Day	58% 3 8% 21% 8% 1	2374
	21st to end	Day	57% 4 8% 19% 11% 1	2574
May	1st to 10th	Day	40% 5% 10% 26% 18% 1	6357
	11th to 20th	Day	44% 7% 12% 22% 11% 4	6323
	21st to end	Day	47% 5% 8% 21% 17%	6949
June	1st to 10th	Day	41% 8% 12% 26% 12% 1	2800
	11th to 20th	Day	47% 6% 13% 20% 13% 1	2800
	21st to end	Day	43% 8% 15% 25% 9% 1	2800
July	1st to 10th	Day	35% 7% 17% 32% 8% 1	6291
	11th to 20th	Day	33% 6% 18% 31% 11% 1	6285
	21st to end	Day	35% 6% 14% 30% 15%	6728
August	1st to 10th	Day	44% 6% 15% 23% 11% 1	4525
	11th to 20th	Day	37% 6% 16% 31% 10%	4210
	21st to end	Day	44% 6% 15% 24% 10% 1	4322
September	1st to 10th	Day	41% 6% 15% 28% 11%	2400
	11th to 20th	Day	41% 7% 18% 26% 7%	2400
	21st to end	Day	46% 6% 12% 27% 10%	2129
October	1st to 10th	Day	37% 4 14% 30% 15% 1	3260
	11th to 20th	Day	54% 4 8% 22% 12%	3240
	21st to end	Day	53% 5% 16% 20% 7%	3247
November	1st to 10th	Day	57% 5% 11% 18% 8% 1	1820
	11th to 20th	Day	67% 4 9% 14% 5%	1680
	21st to end	Day	68% 4 11% 12% 4 1	1680
December	1st to 10th	Day	80% 3 6% 8% 2	2626
	11th to 20th	Day	70% 3 7% 11% 8%	3005
	21st to end	Day	77% 4 4 11% 5%	3982
January	1st to 10th	Night	68% 4 7% 14% 7%	6348
	11th to 20th	Night	68% 5% 9% 12% 6%	6360
	21st to end	Night	65% 4 9% 16% 6%	6881
February	1st to 10th	Night	58% 4 8% 18% 11% 1	2600
	11th to 20th	Night	54% 6% 11% 19% 9% 1	2534
	21st to end	Night	56% 4 10% 19% 8% 2	1992
March	1st to 10th	Night	40% 5% 16% 20% 17% 3	5373
	11th to 20th	Night	56% 4 12% 20% 7%	5027
	21st to end	Night	61% 4 8% 14% 12% 1	5155
April	1st to 10th	Night	56% 5% 7% 18% 13% 1	2000
	11th to 20th	Night	53% 5% 8% 21% 11% 2	1975
	21st to end	Night	51% 3 9% 23% 13% 1	1824
May	1st to 10th	Night	39% 5% 13% 27% 14% 3	3620
	11th to 20th	Night	38% 6% 11% 24% 17% 5%	3597
	21st to end	Night	43% 7% 11% 24% 14% 1	3950
June	1st to 10th	Night	38% 7% 15% 27% 12% 1	1600
	11th to 20th	Night	43% 7% 11% 21% 16% 2	1600
	21st to end	Night	40% 7% 13% 25% 13% 1	1600
July	1st to 10th	Night	32% 6% 20% 31% 9% 2	3580
	11th to 20th	Night	29% 6% 18% 35% 12% 1	3572
	21st to end	Night	35% 6% 13% 28% 18% 1	3832
August	1st to 10th	Night	38% 5% 15% 26% 14% 2	2608
	11th to 20th	Night	33% 6% 15% 35% 10% 1	2915
	21st to end	Night	41% 5% 16% 27% 10% 1	3574
September	1st to 10th	Night	33% 6% 15% 33% 13% 1	2000
	11th to 20th	Night	34% 7% 17% 34% 8%	2000
	21st to end	Night	42% 6% 13% 28% 10% 1	2245
October	1st to 10th	Night	32% 4 9% 33% 21% 2	3920
	11th to 20th	Night	47% 5% 11% 22% 15%	3899
	21st to end	Night	56% 4 11% 21% 8%	4541
November	1st to 10th	Night	53% 5% 12% 21% 9%	2693
	11th to 20th	Night	60% 5% 13% 14% 7% 1	2940
	21st to end	Night	62% 5% 12% 15% 5% 1	2940
December	1st to 10th	Night	73% 6% 8% 11% 2	4609
	11th to 20th	Night	69% 4 8% 13% 6%	5201
	21st to end	Night	73% 3 5% 12% 7%	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	20% 21% 35% 19% 4 2	300
	11th to 20th	Day	23% 18% 35% 22% 3	300
	21st to end	Day	22% 17% 39% 20% 3	330
February	1st to 10th	Day	22% 25% 38% 12% 3	300
	11th to 20th	Day	19% 29% 37% 11% 4	300
	21st to end	Day	26% 25% 42% 6%	249
March	1st to 10th	Day	20% 21% 43% 12% 4	300
	11th to 20th	Day	24% 21% 38% 12% 4	300
	21st to end	Day	27% 18% 33% 15% 4 2	460
April	1st to 10th	Day	29% 16% 37% 13% 4 1	500
	11th to 20th	Day	31% 17% 29% 16% 5%	492
	21st to end	Day	31% 15% 31% 17% 5% 1	599
May	1st to 10th	Day	24% 18% 35% 17% 6% 2	703
	11th to 20th	Day	23% 17% 31% 22% 6% 2	725
	21st to end	Day	23% 16% 34% 20% 7% 1	789
June	1st to 10th	Day	16% 18% 35% 22% 6% 2	730
	11th to 20th	Day	20% 14% 40% 18% 5% 1	730
	21st to end	Day	21% 12% 37% 19% 8% 2	730
July	1st to 10th	Day	9% 10% 34% 26% 15% 5%	721
	11th to 20th	Day	13% 12% 34% 26% 12% 5%	718
	21st to end	Day	9% 10% 41% 22% 14% 4	794
August	1st to 10th	Day	16% 9% 31% 28% 14% 3	726
	11th to 20th	Day	17% 7% 29% 29% 13% 4	715
	21st to end	Day	18% 11% 28% 28% 12% 3	594
September	1st to 10th	Day	19% 13% 28% 26% 12% 3	530
	11th to 20th	Day	15% 13% 32% 30% 9% 2	530
	21st to end	Day	23% 15% 27% 22% 12% 2	455
October	1st to 10th	Day	25% 18% 31% 17% 8% 2	330
	11th to 20th	Day	31% 22% 34% 11% 2	330
	21st to end	Day	29% 23% 37% 8% 2 1	363
November	1st to 10th	Day	26% 21% 36% 13% 2	330
	11th to 20th	Day	27% 26% 35% 8% 4	330
	21st to end	Day	25% 24% 37% 12% 2	330
December	1st to 10th	Day	17% 22% 41% 17% 3	306
	11th to 20th	Day	27% 23% 35% 12% 2	300
	21st to end	Day	27% 22% 37% 11% 3 1	330
January	1st to 10th	Night	16% 19% 38% 21% 6%	1000
	11th to 20th	Night	20% 15% 40% 19% 6% 1	1000
	21st to end	Night	19% 16% 40% 21% 4	1100
February	1st to 10th	Night	17% 20% 37% 21% 6%	1000
	11th to 20th	Night	14% 24% 37% 18% 6% 1	979
	21st to end	Night	19% 17% 39% 21% 4	672
March	1st to 10th	Night	18% 18% 37% 21% 6% 1	800
	11th to 20th	Night	19% 20% 35% 19% 7%	621
	21st to end	Night	25% 12% 31% 22% 9% 2	660
April	1st to 10th	Night	28% 13% 34% 21% 5% 1	600
	11th to 20th	Night	29% 11% 29% 22% 8%	593
	21st to end	Night	28% 9% 29% 26% 7% 1	600
May	1st to 10th	Night	23% 12% 32% 27% 6% 1	600
	11th to 20th	Night	19% 9% 34% 29% 7% 2	591
	21st to end	Night	16% 10% 31% 33% 9% 1	650
June	1st to 10th	Night	16% 9% 31% 34% 10% 1	600
	11th to 20th	Night	16% 7% 33% 34% 10%	600
	21st to end	Night	15% 8% 29% 35% 12% 2	600
July	1st to 10th	Night	8% 5% 30% 36% 20% 2	585
	11th to 20th	Night	10% 7% 33% 32% 17% 1	582
	21st to end	Night	8% 5% 31% 38% 16% 2	657
August	1st to 10th	Night	12% 4 24% 42% 17% 1	597
	11th to 20th	Night	16% 4 23% 35% 19% 3	594
	21st to end	Night	15% 6% 29% 29% 19% 2	660
September	1st to 10th	Night	13% 9% 26% 34% 15% 3	600
	11th to 20th	Night	12% 9% 34% 29% 15% 2	600
	21st to end	Night	19% 12% 29% 27% 11% 1	775
October	1st to 10th	Night	23% 12% 30% 25% 10% 1	800
	11th to 20th	Night	27% 15% 35% 17% 6% 1	800
	21st to end	Night	28% 13% 31% 22% 6%	880
November	1st to 10th	Night	25% 15% 35% 19% 5%	847
	11th to 20th	Night	23% 22% 33% 17% 5%	1050
	21st to end	Night	24% 21% 34% 17% 4	1050
December	1st to 10th	Night	19% 19% 37% 19% 5%	1011
	11th to 20th	Night	21% 19% 34% 19% 5% 1	1000
	21st to end	Night	30% 17% 33% 14% 5%	1100

Page generated at 2025-11-30 00:00:52. Version ac07

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