Silent Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Silent Lake clear sky chart from 2005-11-06 to 2026-04-17.

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	16% 12% 4 9% 5% 5% 4 3 5% 6% 31%	131910
	Night	15% 12% 4 10% 6% 5% 4 3 5% 6% 30%	130233
Transparency	Day	53% 4 10% 21% 11% 1	135463
	Night	55% 4 10% 20% 11% 1	134766
Seeing	Day	25% 15% 31% 18% 8% 3	16138
	Night	23% 13% 31% 24% 9% 1	25524

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	14% 15% 4 13% 6% 5% 5% 3 6% 6% 23%	4661
	11th to 20th	Day	9% 24% 10% 19% 7% 6% 4 3 3 3 12%	4668
	21st to end	Day	11% 15% 8% 18% 8% 6% 5% 4 4 4 17%	5780
February	1st to 10th	Day	12% 16% 7% 16% 7% 4 4 4 4 4 21%	1890
	11th to 20th	Day	12% 16% 6% 13% 7% 4 5% 3 4 5% 25%	1843
	21st to end	Day	16% 17% 5% 11% 7% 4 5% 3 4 5% 22%	1630
March	1st to 10th	Day	18% 15% 5% 10% 5% 4 3 3 4 6% 27%	4902
	11th to 20th	Day	12% 15% 5% 10% 6% 6% 5% 4 5% 7% 26%	5090
	21st to end	Day	18% 15% 4 8% 4 4 3 2 3 4 36%	5669
April	1st to 10th	Day	20% 14% 4 7% 5% 4 5% 3 4 5% 29%	2520
	11th to 20th	Day	23% 12% 3 6% 5% 3 4 3 4 4 35%	2462
	21st to end	Day	22% 11% 3 8% 4 4 4 3 4 5% 32%	2600
May	1st to 10th	Day	22% 9% 3 7% 4 4 4 3 5% 7% 33%	6159
	11th to 20th	Day	19% 9% 3 7% 5% 4 4 3 5% 7% 35%	6318
	21st to end	Day	20% 8% 2 7% 5% 5% 4 4 5% 6% 34%	6949
June	1st to 10th	Day	21% 9% 3 7% 6% 5% 4 4 5% 7% 31%	2800
	11th to 20th	Day	16% 6% 2 7% 5% 6% 5% 4 6% 7% 36%	2800
	21st to end	Day	17% 8% 3 5% 5% 5% 4 4 5% 9% 34%	2800
July	1st to 10th	Day	13% 6% 3 7% 5% 5% 5% 4 7% 9% 36%	6360
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 5% 7% 46%	6360
	21st to end	Day	7% 5% 2 6% 4 5% 5% 5% 6% 9% 46%	6882
August	1st to 10th	Day	16% 8% 3 8% 4 5% 6% 4 6% 7% 34%	4234
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 5% 7% 8% 33%	4174
	21st to end	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 40%	4420
September	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 6% 7% 40%	2400
	11th to 20th	Day	13% 5% 2 6% 5% 4 4 3 6% 7% 46%	2385
	21st to end	Day	19% 7% 2 7% 5% 4 5% 4 5% 7% 35%	2095
October	1st to 10th	Day	20% 9% 3 8% 4 4 4 3 5% 7% 34%	2000
	11th to 20th	Day	23% 12% 3 9% 5% 4 4 3 4 6% 26%	2000
	21st to end	Day	27% 12% 4 9% 5% 4 4 3 4 5% 23%	2165
November	1st to 10th	Day	17% 13% 4 8% 5% 5% 4 4 4 5% 31%	1740
	11th to 20th	Day	16% 15% 6% 8% 6% 5% 4 3 6% 6% 25%	1680
	21st to end	Day	20% 21% 7% 11% 6% 3 5% 3 4 5% 16%	1680
December	1st to 10th	Day	16% 25% 7% 12% 6% 4 4 3 3 5% 14%	2680
	11th to 20th	Day	17% 22% 7% 14% 4 4 3 2 3 4 21%	3066
	21st to end	Day	21% 22% 7% 12% 5% 3 3 1 2 4 20%	4048
January	1st to 10th	Night	13% 16% 8% 16% 6% 5% 4 3 5% 5% 19%	8234
	11th to 20th	Night	9% 18% 8% 20% 10% 6% 5% 3 4 4 13%	8201
	21st to end	Night	11% 14% 7% 16% 8% 6% 5% 3 5% 5% 21%	8401
February	1st to 10th	Night	11% 16% 6% 12% 8% 5% 5% 4 5% 6% 22%	2730
	11th to 20th	Night	12% 14% 6% 13% 7% 5% 5% 3 4 5% 26%	2663
	21st to end	Night	13% 15% 5% 10% 7% 5% 4 4 6% 6% 25%	2176
March	1st to 10th	Night	16% 15% 4 12% 5% 4 4 3 4 4 29%	5155
	11th to 20th	Night	10% 13% 5% 9% 6% 6% 5% 4 6% 7% 28%	5089
	21st to end	Night	19% 9% 4 9% 6% 4 4 4 5% 6% 31%	5504
April	1st to 10th	Night	20% 13% 4 8% 5% 4 4 3 5% 6% 30%	2100
	11th to 20th	Night	18% 10% 3 7% 5% 3 4 3 5% 5% 36%	2036
	21st to end	Night	19% 10% 3 8% 5% 5% 5% 3 6% 5% 32%	1800
May	1st to 10th	Night	24% 8% 3 6% 4 4 4 4 5% 6% 32%	3779
	11th to 20th	Night	16% 7% 2 6% 4 4 4 3 5% 8% 42%	3575
	21st to end	Night	18% 8% 3 7% 5% 5% 4 4 4 6% 37%	3926
June	1st to 10th	Night	15% 9% 2 7% 5% 5% 4 3 6% 7% 37%	1600
	11th to 20th	Night	12% 5% 3 5% 5% 4 5% 3 6% 8% 43%	1600
	21st to end	Night	16% 8% 2 8% 4 5% 4 4 5% 6% 39%	1600
July	1st to 10th	Night	12% 6% 2 7% 5% 6% 5% 5% 6% 8% 38%	3600
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 5% 7% 49%	3600
	21st to end	Night	8% 6% 2 5% 4 3 4 3 5% 7% 52%	3966
August	1st to 10th	Night	11% 5% 2 5% 4 4 4 5% 6% 7% 47%	2915
	11th to 20th	Night	14% 7% 2 7% 4 4 4 3 5% 6% 44%	2875
	21st to end	Night	12% 5% 2 5% 4 3 4 3 6% 8% 47%	3444
September	1st to 10th	Night	11% 5% 2 6% 4 3 5% 4 6% 9% 46%	2000
	11th to 20th	Night	12% 5% 1 5% 3 3 4 4 5% 7% 50%	2012
	21st to end	Night	20% 7% 3 5% 3 4 5% 3 5% 5% 41%	2304
October	1st to 10th	Night	17% 7% 3 7% 4 5% 4 3 5% 6% 40%	2400
	11th to 20th	Night	21% 11% 3 8% 5% 5% 4 4 5% 5% 30%	2400
	21st to end	Night	21% 13% 3 8% 5% 5% 4 3 5% 5% 27%	2673
November	1st to 10th	Night	17% 11% 3 8% 5% 5% 3 3 6% 6% 33%	2777
	11th to 20th	Night	17% 15% 4 8% 5% 4 4 4 5% 7% 28%	2940
	21st to end	Night	19% 17% 5% 10% 6% 5% 5% 4 4 5% 21%	2940
December	1st to 10th	Night	18% 21% 7% 12% 5% 5% 4 4 4 5% 15%	4720
	11th to 20th	Night	16% 22% 6% 14% 5% 4 4 3 4 4 18%	5348
	21st to end	Night	23% 20% 7% 13% 5% 3 4 3 3 4 16%	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	61% 3 9% 18% 7% 1	4662
	11th to 20th	Day	78% 2 5% 7% 7% 1	4698
	21st to end	Day	71% 3 6% 11% 8% 1	5780
February	1st to 10th	Day	67% 3 7% 15% 7% 1	1890
	11th to 20th	Day	63% 3 9% 14% 10% 2	1852
	21st to end	Day	64% 3 8% 14% 11% 1	1630
March	1st to 10th	Day	59% 3 11% 11% 15% 2	4905
	11th to 20th	Day	57% 3 13% 17% 8% 2	5090
	21st to end	Day	54% 2 5% 20% 16% 3	5669
April	1st to 10th	Day	58% 4 6% 18% 12% 2	2520
	11th to 20th	Day	54% 3 6% 21% 14% 1	2462
	21st to end	Day	55% 3 7% 20% 13% 3	2600
May	1st to 10th	Day	52% 4 7% 23% 10% 5%	6168
	11th to 20th	Day	51% 4 8% 19% 17% 1	6322
	21st to end	Day	51% 4 11% 20% 12% 2	6949
June	1st to 10th	Day	53% 4 9% 20% 13% 1	2800
	11th to 20th	Day	45% 4 11% 26% 13% 2	2800
	21st to end	Day	47% 5% 12% 23% 12% 1	2800
July	1st to 10th	Day	42% 5% 13% 27% 12% 1	6290
	11th to 20th	Day	37% 5% 12% 30% 16%	6285
	21st to end	Day	33% 5% 13% 36% 12%	6684
August	1st to 10th	Day	48% 4 13% 24% 10% 1	4221
	11th to 20th	Day	46% 5% 13% 28% 8%	4213
	21st to end	Day	38% 5% 12% 34% 10% 1	4420
September	1st to 10th	Day	42% 5% 11% 30% 11% 1	2400
	11th to 20th	Day	38% 3 12% 32% 14% 1	2385
	21st to end	Day	48% 4 12% 23% 12%	2095
October	1st to 10th	Day	45% 4 10% 29% 11% 1	3260
	11th to 20th	Day	51% 4 11% 21% 12% 1	3240
	21st to end	Day	59% 4 11% 19% 6%	3479
November	1st to 10th	Day	54% 4 13% 21% 7% 1	1740
	11th to 20th	Day	58% 4 11% 15% 10% 1	1680
	21st to end	Day	70% 4 9% 11% 6% 1	1680
December	1st to 10th	Day	74% 3 11% 8% 3	2680
	11th to 20th	Day	69% 3 6% 14% 7%	3066
	21st to end	Day	71% 3 5% 14% 7%	4048
January	1st to 10th	Night	68% 2 7% 14% 7%	8234
	11th to 20th	Night	75% 3 6% 8% 7% 1	8246
	21st to end	Night	66% 3 7% 12% 13% 1	8401
February	1st to 10th	Night	62% 3 9% 18% 8% 1	2730
	11th to 20th	Night	61% 2 9% 18% 9% 1	2665
	21st to end	Night	58% 2 9% 20% 9% 1	2176
March	1st to 10th	Night	58% 3 6% 15% 12% 5%	5165
	11th to 20th	Night	53% 5% 15% 18% 8% 1	5090
	21st to end	Night	52% 5% 7% 19% 15% 2	5504
April	1st to 10th	Night	56% 3 7% 21% 12% 2	2100
	11th to 20th	Night	50% 3 8% 23% 15% 1	2036
	21st to end	Night	52% 3 8% 18% 16% 2	1800
May	1st to 10th	Night	53% 3 8% 19% 12% 5%	3786
	11th to 20th	Night	43% 4 13% 22% 17% 2	3576
	21st to end	Night	48% 4 10% 22% 14% 1	3927
June	1st to 10th	Night	46% 4 12% 23% 15% 1	1600
	11th to 20th	Night	39% 5% 11% 27% 17% 1	1600
	21st to end	Night	45% 4 13% 24% 13% 1	1600
July	1st to 10th	Night	42% 5% 14% 23% 15% 1	3560
	11th to 20th	Night	35% 4 12% 33% 15%	3552
	21st to end	Night	32% 5% 11% 37% 15%	3849
August	1st to 10th	Night	35% 5% 15% 27% 16% 1	2904
	11th to 20th	Night	40% 5% 17% 26% 11% 1	2902
	21st to end	Night	35% 4 13% 34% 11% 2	3444
September	1st to 10th	Night	34% 5% 17% 29% 14% 1	2000
	11th to 20th	Night	33% 4 14% 36% 13% 1	2012
	21st to end	Night	45% 4 10% 28% 13%	2304
October	1st to 10th	Night	38% 4 10% 29% 19%	3920
	11th to 20th	Night	48% 4 13% 25% 10%	3899
	21st to end	Night	51% 5% 15% 21% 7%	4309
November	1st to 10th	Night	52% 4 12% 23% 9% 1	2777
	11th to 20th	Night	56% 4 12% 19% 8% 1	2940
	21st to end	Night	65% 4 11% 15% 5%	2940
December	1st to 10th	Night	70% 4 11% 12% 3	4720
	11th to 20th	Night	70% 3 7% 12% 8%	5348
	21st to end	Night	73% 3 5% 11% 8%	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	22% 21% 38% 14% 4	220
	11th to 20th	Day	27% 19% 35% 17% 2	223
	21st to end	Day	28% 20% 34% 14% 4	363
February	1st to 10th	Day	29% 22% 35% 11% 3	330
	11th to 20th	Day	32% 26% 35% 6% 1	330
	21st to end	Day	35% 19% 36% 9% 1	346
March	1st to 10th	Day	27% 19% 36% 14% 3 1	540
	11th to 20th	Day	31% 18% 32% 14% 4 1	540
	21st to end	Day	32% 18% 30% 13% 5% 1	594
April	1st to 10th	Day	31% 18% 32% 12% 6%	540
	11th to 20th	Day	38% 18% 24% 10% 8% 2	516
	21st to end	Day	31% 18% 31% 14% 5% 1	500
May	1st to 10th	Day	28% 14% 32% 17% 6% 4	503
	11th to 20th	Day	21% 16% 35% 20% 7% 2	527
	21st to end	Day	21% 16% 31% 19% 10% 3	573
June	1st to 10th	Day	26% 11% 32% 23% 7% 1	530
	11th to 20th	Day	17% 13% 35% 23% 9% 2	530
	21st to end	Day	23% 13% 33% 20% 9% 2	530
July	1st to 10th	Day	15% 9% 33% 27% 13% 3	525
	11th to 20th	Day	16% 11% 31% 26% 12% 4	524
	21st to end	Day	14% 10% 35% 27% 11% 3	576
August	1st to 10th	Day	15% 6% 30% 30% 14% 5%	526
	11th to 20th	Day	17% 5% 25% 33% 14% 6%	519
	21st to end	Day	17% 9% 31% 24% 13% 6%	583
September	1st to 10th	Day	17% 9% 32% 22% 14% 6%	530
	11th to 20th	Day	14% 8% 24% 22% 19% 13%	530
	21st to end	Day	21% 12% 28% 23% 12% 4	530
October	1st to 10th	Day	24% 15% 26% 19% 13% 3	530
	11th to 20th	Day	30% 17% 32% 14% 5% 1	530
	21st to end	Day	35% 15% 30% 16% 4	564
November	1st to 10th	Day	32% 14% 32% 16% 5% 1	314
	11th to 20th	Day	32% 20% 31% 11% 5% 1	220
	21st to end	Day	32% 21% 36% 8% 3	220
December	1st to 10th	Day	32% 19% 30% 16% 2	220
	11th to 20th	Day	38% 21% 29% 10% 1 1	220
	21st to end	Day	35% 20% 28% 12% 5% 1	242
January	1st to 10th	Night	28% 19% 33% 16% 4	840
	11th to 20th	Night	28% 16% 31% 19% 5%	840
	21st to end	Night	20% 16% 34% 22% 6% 1	924
February	1st to 10th	Night	23% 20% 33% 22% 2	840
	11th to 20th	Night	22% 18% 38% 18% 4	823
	21st to end	Night	24% 19% 34% 18% 4	692
March	1st to 10th	Night	25% 18% 35% 18% 3	840
	11th to 20th	Night	24% 16% 35% 19% 4 1	840
	21st to end	Night	23% 14% 34% 20% 7% 1	924
April	1st to 10th	Night	28% 13% 32% 22% 5%	840
	11th to 20th	Night	26% 17% 29% 19% 8%	816
	21st to end	Night	25% 10% 31% 23% 11%	600
May	1st to 10th	Night	27% 7% 27% 26% 12% 1	482
	11th to 20th	Night	17% 10% 33% 32% 8%	394
	21st to end	Night	15% 7% 34% 30% 12% 1	433
June	1st to 10th	Night	19% 7% 33% 33% 8% 1	400
	11th to 20th	Night	15% 10% 29% 35% 11% 1	400
	21st to end	Night	19% 7% 27% 35% 12%	400
July	1st to 10th	Night	7% 7% 32% 38% 15% 1	390
	11th to 20th	Night	13% 5% 28% 36% 18% 1	388
	21st to end	Night	11% 4 30% 38% 15% 1	453
August	1st to 10th	Night	14% 3 21% 36% 24% 2	597
	11th to 20th	Night	12% 4 20% 36% 24% 3	594
	21st to end	Night	13% 5% 30% 31% 19% 2	790
September	1st to 10th	Night	14% 6% 30% 30% 17% 3	800
	11th to 20th	Night	14% 5% 25% 30% 23% 5%	800
	21st to end	Night	23% 9% 21% 28% 16% 4	800
October	1st to 10th	Night	21% 10% 28% 24% 15% 2	800
	11th to 20th	Night	29% 13% 30% 20% 8% 1	800
	21st to end	Night	29% 14% 30% 22% 5%	880
November	1st to 10th	Night	25% 15% 29% 24% 8%	820
	11th to 20th	Night	26% 15% 33% 17% 8% 1	840
	21st to end	Night	31% 16% 28% 20% 5% 1	840
December	1st to 10th	Night	32% 16% 32% 16% 5%	840
	11th to 20th	Night	27% 19% 35% 16% 3 1	840
	21st to end	Night	34% 14% 32% 16% 3	924

Page generated at 2026-04-14 02:44:26. Version ac07

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