Rustic Lake Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rustic Lake 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	14% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	126184
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 32%	122828
Transparency	Day	50% 6% 11% 20% 11% 1	129816
	Night	50% 6% 11% 20% 12% 2	127266
Seeing	Day	18% 12% 28% 22% 14% 6%	16207
	Night	18% 10% 28% 29% 13% 2	26057

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	13% 18% 9% 21% 7% 5% 4 2 4 5% 12%	4056
	11th to 20th	Day	12% 21% 6% 15% 8% 5% 7% 3 4 4 15%	4060
	21st to end	Day	21% 19% 9% 13% 7% 6% 4 3 3 3 13%	4466
February	1st to 10th	Day	17% 17% 6% 11% 8% 5% 4 3 4 5% 22%	1800
	11th to 20th	Day	14% 12% 6% 14% 7% 5% 5% 4 4 5% 22%	1871
	21st to end	Day	16% 14% 4 11% 6% 4 6% 4 6% 6% 25%	1650
March	1st to 10th	Day	17% 11% 8% 13% 5% 4 4 3 4 5% 26%	4728
	11th to 20th	Day	11% 12% 4 10% 5% 4 4 3 4 6% 36%	4980
	21st to end	Day	17% 13% 5% 11% 5% 5% 4 4 5% 5% 26%	5465
April	1st to 10th	Day	14% 13% 5% 10% 6% 5% 4 4 5% 6% 30%	2225
	11th to 20th	Day	16% 10% 4 8% 6% 5% 4 3 5% 7% 33%	2562
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 5% 7% 33%	2600
May	1st to 10th	Day	20% 10% 3 9% 5% 5% 4 4 5% 6% 29%	5862
	11th to 20th	Day	17% 9% 3 8% 5% 5% 5% 3 6% 8% 32%	5861
	21st to end	Day	16% 6% 3 7% 4 5% 5% 5% 7% 8% 34%	6433
June	1st to 10th	Day	13% 8% 2 7% 5% 5% 5% 4 6% 8% 38%	2600
	11th to 20th	Day	12% 8% 2 7% 5% 5% 5% 4 8% 9% 35%	2654
	21st to end	Day	10% 6% 2 6% 5% 5% 5% 5% 8% 9% 39%	2800
July	1st to 10th	Day	9% 6% 3 8% 5% 5% 5% 4 7% 10% 40%	6250
	11th to 20th	Day	8% 6% 2 7% 6% 6% 7% 5% 7% 10% 36%	5900
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 9% 38%	6424
August	1st to 10th	Day	12% 7% 3 8% 5% 5% 6% 4 6% 9% 36%	4234
	11th to 20th	Day	8% 7% 2 8% 5% 5% 6% 5% 8% 10% 36%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 9% 40%	4655
September	1st to 10th	Day	12% 5% 2 6% 4 5% 4 4 5% 7% 45%	2216
	11th to 20th	Day	9% 8% 2 6% 4 4 4 3 6% 9% 45%	2178
	21st to end	Day	18% 8% 3 7% 4 5% 3 3 5% 6% 39%	2090
October	1st to 10th	Day	13% 7% 3 7% 4 4 4 4 6% 7% 42%	2005
	11th to 20th	Day	15% 10% 2 7% 4 4 5% 4 6% 7% 37%	1900
	21st to end	Day	19% 11% 3 8% 5% 5% 4 3 5% 6% 30%	2042
November	1st to 10th	Day	9% 10% 3 7% 6% 4 4 4 6% 7% 40%	1762
	11th to 20th	Day	17% 14% 4 9% 5% 4 4 3 4 7% 30%	1800
	21st to end	Day	17% 17% 5% 10% 5% 5% 5% 3 5% 5% 24%	1733
December	1st to 10th	Day	16% 14% 6% 14% 6% 4 4 4 4 6% 22%	2600
	11th to 20th	Day	20% 17% 8% 12% 5% 4 3 3 5% 6% 18%	3082
	21st to end	Day	21% 23% 8% 12% 6% 5% 3 2 3 3 13%	4466
January	1st to 10th	Night	13% 18% 7% 21% 8% 5% 3 3 3 4 14%	5889
	11th to 20th	Night	10% 17% 9% 18% 8% 8% 7% 5% 4 4 11%	5900
	21st to end	Night	19% 17% 6% 13% 6% 4 4 2 4 4 20%	6490
February	1st to 10th	Night	18% 14% 5% 12% 7% 5% 4 3 4 5% 22%	2600
	11th to 20th	Night	15% 15% 5% 11% 6% 4 4 4 6% 6% 23%	2410
	21st to end	Night	17% 15% 4 11% 7% 4 4 4 4 5% 25%	1980
March	1st to 10th	Night	18% 13% 5% 9% 4 4 3 3 4 4 33%	5111
	11th to 20th	Night	11% 8% 3 7% 5% 4 5% 4 6% 8% 40%	4980
	21st to end	Night	16% 10% 4 9% 5% 5% 4 3 5% 5% 35%	5467
April	1st to 10th	Night	15% 13% 3 8% 5% 4 5% 4 5% 6% 32%	2191
	11th to 20th	Night	14% 9% 3 8% 5% 5% 4 4 6% 6% 36%	1785
	21st to end	Night	15% 8% 4 7% 5% 4 4 4 6% 7% 36%	1800
May	1st to 10th	Night	21% 11% 4 8% 6% 4 5% 4 5% 6% 26%	4033
	11th to 20th	Night	16% 8% 3 6% 4 4 3 3 5% 9% 37%	4032
	21st to end	Night	13% 7% 2 6% 4 4 5% 4 7% 8% 40%	4421
June	1st to 10th	Night	10% 7% 3 7% 5% 5% 4 4 5% 7% 44%	1800
	11th to 20th	Night	8% 5% 3 7% 5% 4 5% 5% 8% 8% 42%	1736
	21st to end	Night	9% 5% 2 6% 6% 4 5% 5% 7% 9% 44%	1600
July	1st to 10th	Night	8% 4 2 6% 5% 5% 5% 4 6% 11% 44%	3706
	11th to 20th	Night	8% 5% 2 7% 5% 5% 6% 4 7% 9% 44%	4057
	21st to end	Night	7% 6% 3 7% 4 4 5% 4 7% 9% 45%	4427
August	1st to 10th	Night	11% 7% 3 7% 5% 5% 5% 4 6% 9% 39%	2915
	11th to 20th	Night	10% 8% 3 8% 4 4 4 3 5% 8% 44%	2875
	21st to end	Night	5% 5% 2 5% 4 5% 5% 4 6% 9% 49%	3223
September	1st to 10th	Night	11% 5% 2 5% 3 3 4 4 5% 7% 52%	2165
	11th to 20th	Night	8% 5% 1 4 4 3 4 3 4 8% 56%	2179
	21st to end	Night	14% 8% 2 6% 4 4 4 3 6% 6% 42%	2090
October	1st to 10th	Night	12% 6% 2 5% 4 3 5% 3 6% 6% 47%	2174
	11th to 20th	Night	16% 8% 3 7% 5% 5% 3 3 4 6% 40%	2280
	21st to end	Night	19% 9% 3 6% 5% 5% 4 3 4 7% 34%	2555
November	1st to 10th	Night	12% 9% 3 6% 4 4 5% 4 5% 7% 42%	2536
	11th to 20th	Night	14% 13% 4 9% 5% 4 3 3 5% 6% 34%	2600
	21st to end	Night	18% 14% 4 8% 5% 4 4 4 5% 6% 27%	2661
December	1st to 10th	Night	14% 16% 5% 12% 6% 5% 5% 4 5% 6% 23%	4580
	11th to 20th	Night	18% 17% 7% 13% 5% 4 3 3 5% 5% 20%	5090
	21st to end	Night	20% 21% 7% 12% 6% 4 4 3 4 5% 14%	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	75% 4 8% 8% 4	4057
	11th to 20th	Day	72% 5% 10% 7% 7%	4060
	21st to end	Day	76% 4 6% 9% 5%	4466
February	1st to 10th	Day	66% 5% 10% 13% 6% 1	1800
	11th to 20th	Day	63% 6% 8% 15% 8% 1	1880
	21st to end	Day	58% 7% 13% 14% 7% 2	1650
March	1st to 10th	Day	60% 5% 8% 21% 6%	4728
	11th to 20th	Day	48% 5% 11% 26% 9% 2	4980
	21st to end	Day	58% 4 10% 18% 10%	5465
April	1st to 10th	Day	54% 6% 11% 17% 11% 1	2225
	11th to 20th	Day	51% 6% 11% 17% 13% 2	2562
	21st to end	Day	50% 6% 10% 21% 11% 2	2600
May	1st to 10th	Day	55% 7% 9% 17% 11% 2	5897
	11th to 20th	Day	49% 6% 13% 21% 10% 1	5865
	21st to end	Day	43% 9% 10% 22% 11% 4	6447
June	1st to 10th	Day	42% 7% 11% 25% 14% 1	2600
	11th to 20th	Day	44% 9% 12% 21% 13% 1	2654
	21st to end	Day	39% 9% 14% 21% 16% 2	2800
July	1st to 10th	Day	38% 7% 10% 29% 15% 1	6183
	11th to 20th	Day	39% 8% 15% 22% 15% 1	5831
	21st to end	Day	40% 8% 12% 19% 19% 2	6240
August	1st to 10th	Day	42% 8% 14% 22% 14% 1	4221
	11th to 20th	Day	38% 9% 11% 29% 11% 1	4213
	21st to end	Day	37% 8% 14% 27% 12% 2	4655
September	1st to 10th	Day	36% 7% 13% 23% 17% 4	2216
	11th to 20th	Day	36% 7% 11% 26% 18% 2	2178
	21st to end	Day	46% 6% 9% 20% 18% 1	2090
October	1st to 10th	Day	29% 6% 13% 31% 21% 1	3330
	11th to 20th	Day	41% 6% 10% 22% 19% 2	3140
	21st to end	Day	40% 5% 12% 24% 18% 1	3344
November	1st to 10th	Day	43% 5% 12% 23% 15% 2	1762
	11th to 20th	Day	54% 5% 9% 19% 10% 2	1800
	21st to end	Day	62% 5% 10% 15% 7% 1	1733
December	1st to 10th	Day	63% 5% 14% 15% 3	2600
	11th to 20th	Day	68% 3 10% 14% 4	3078
	21st to end	Day	78% 4 6% 8% 4	4466
January	1st to 10th	Night	74% 4 7% 12% 3	5889
	11th to 20th	Night	77% 4 7% 7% 5%	5900
	21st to end	Night	68% 3 10% 11% 6% 1	6490
February	1st to 10th	Night	64% 6% 9% 14% 6% 1	2600
	11th to 20th	Night	61% 6% 9% 15% 8% 1	2412
	21st to end	Night	61% 5% 9% 14% 8% 2	1980
March	1st to 10th	Night	54% 4 9% 22% 10% 1	5111
	11th to 20th	Night	41% 6% 12% 29% 10% 2	4980
	21st to end	Night	50% 4 9% 21% 14% 1	5467
April	1st to 10th	Night	51% 6% 11% 16% 14% 2	2191
	11th to 20th	Night	47% 6% 11% 18% 17% 3	1785
	21st to end	Night	46% 5% 11% 21% 14% 2	1800
May	1st to 10th	Night	58% 7% 9% 14% 10% 3	4060
	11th to 20th	Night	44% 6% 11% 23% 15% 1	4033
	21st to end	Night	39% 8% 13% 26% 12% 2	4429
June	1st to 10th	Night	38% 7% 11% 22% 19% 3	1800
	11th to 20th	Night	36% 8% 14% 23% 17% 2	1736
	21st to end	Night	34% 8% 14% 25% 16% 3	1600
July	1st to 10th	Night	34% 7% 14% 25% 16% 4	3666
	11th to 20th	Night	34% 9% 15% 27% 14% 1	4006
	21st to end	Night	34% 7% 13% 21% 23% 2	4297
August	1st to 10th	Night	40% 7% 13% 23% 15% 1	2904
	11th to 20th	Night	37% 7% 10% 28% 16% 1	2902
	21st to end	Night	31% 7% 14% 30% 15% 3	3223
September	1st to 10th	Night	31% 7% 12% 28% 17% 5%	2165
	11th to 20th	Night	27% 6% 10% 28% 26% 2	2179
	21st to end	Night	41% 7% 10% 21% 20% 2	2090
October	1st to 10th	Night	29% 5% 8% 30% 27% 1	3629
	11th to 20th	Night	37% 5% 11% 23% 21% 3	3779
	21st to end	Night	38% 4 10% 28% 18% 3	4203
November	1st to 10th	Night	40% 7% 11% 22% 17% 3	2536
	11th to 20th	Night	51% 4 10% 20% 13% 3	2600
	21st to end	Night	57% 5% 12% 18% 7% 1	2661
December	1st to 10th	Night	60% 5% 12% 17% 5%	4580
	11th to 20th	Night	67% 4 11% 14% 3	5093
	21st to end	Night	72% 5% 9% 10% 3	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	24% 22% 38% 10% 4 1	300
	11th to 20th	Day	25% 27% 36% 8% 4	300
	21st to end	Day	27% 21% 36% 14% 2	330
February	1st to 10th	Day	30% 22% 32% 13% 2 1	300
	11th to 20th	Day	27% 28% 31% 12% 2	300
	21st to end	Day	26% 29% 35% 9%	249
March	1st to 10th	Day	24% 20% 35% 18% 4	405
	11th to 20th	Day	24% 17% 33% 19% 6% 1	500
	21st to end	Day	29% 14% 32% 20% 5%	550
April	1st to 10th	Day	21% 19% 32% 18% 8% 2	500
	11th to 20th	Day	22% 21% 26% 17% 10% 4	492
	21st to end	Day	22% 15% 32% 20% 9% 2	500
May	1st to 10th	Day	23% 14% 31% 22% 7% 3	503
	11th to 20th	Day	19% 10% 32% 20% 13% 6%	527
	21st to end	Day	13% 8% 26% 26% 20% 8%	573
June	1st to 10th	Day	15% 7% 28% 30% 16% 4	530
	11th to 20th	Day	14% 9% 23% 30% 18% 5%	584
	21st to end	Day	10% 9% 29% 32% 16% 5%	730
July	1st to 10th	Day	13% 3 20% 27% 24% 13%	678
	11th to 20th	Day	11% 4 21% 28% 23% 12%	524
	21st to end	Day	11% 4 20% 27% 26% 12%	576
August	1st to 10th	Day	11% 5% 15% 29% 26% 14%	526
	11th to 20th	Day	12% 5% 20% 27% 24% 13%	519
	21st to end	Day	10% 4 17% 27% 26% 15%	583
September	1st to 10th	Day	12% 5% 20% 24% 24% 14%	530
	11th to 20th	Day	9% 3 14% 25% 33% 16%	522
	21st to end	Day	21% 11% 22% 20% 19% 8%	490
October	1st to 10th	Day	11% 8% 28% 25% 20% 8%	490
	11th to 20th	Day	15% 16% 32% 23% 9% 4	490
	21st to end	Day	25% 16% 33% 20% 5% 1	491
November	1st to 10th	Day	14% 18% 37% 18% 11% 2	300
	11th to 20th	Day	29% 16% 34% 15% 7%	300
	21st to end	Day	27% 19% 38% 14% 2	265
December	1st to 10th	Day	22% 20% 38% 15% 4 2	200
	11th to 20th	Day	24% 20% 35% 18% 3	220
	21st to end	Day	28% 22% 32% 16% 2	330
January	1st to 10th	Night	23% 18% 36% 18% 5%	800
	11th to 20th	Night	26% 19% 35% 16% 4	800
	21st to end	Night	24% 17% 34% 22% 4	880
February	1st to 10th	Night	24% 17% 36% 21% 2	800
	11th to 20th	Night	25% 16% 34% 21% 3	783
	21st to end	Night	25% 16% 37% 19% 3	660
March	1st to 10th	Night	23% 14% 33% 24% 6%	800
	11th to 20th	Night	18% 15% 35% 24% 7% 1	800
	21st to end	Night	23% 11% 34% 26% 6% 1	880
April	1st to 10th	Night	22% 14% 34% 25% 5% 1	791
	11th to 20th	Night	19% 12% 31% 26% 9% 2	593
	21st to end	Night	18% 9% 36% 29% 7% 1	600
May	1st to 10th	Night	19% 10% 29% 32% 10% 1	600
	11th to 20th	Night	20% 7% 26% 33% 13% 2	591
	21st to end	Night	11% 3 21% 42% 22% 3	650
June	1st to 10th	Night	12% 3 26% 40% 19% 1	600
	11th to 20th	Night	10% 4 21% 43% 20% 3	600
	21st to end	Night	10% 3 22% 42% 23% 1	600
July	1st to 10th	Night	8% 2 15% 42% 29% 4	585
	11th to 20th	Night	8% 3 15% 41% 29% 4	582
	21st to end	Night	8% 2 15% 41% 31% 2	657
August	1st to 10th	Night	9% 1 14% 43% 30% 4	597
	11th to 20th	Night	10% 2 13% 39% 32% 5%	594
	21st to end	Night	8% 2 14% 38% 32% 5%	660
September	1st to 10th	Night	12% 4 18% 32% 30% 5%	785
	11th to 20th	Night	10% 2 13% 34% 35% 6%	793
	21st to end	Night	18% 7% 21% 31% 20% 4	760
October	1st to 10th	Night	14% 6% 24% 35% 19% 3	760
	11th to 20th	Night	20% 10% 30% 27% 10% 2	760
	21st to end	Night	23% 12% 29% 28% 8%	836
November	1st to 10th	Night	16% 12% 29% 29% 12% 1	780
	11th to 20th	Night	20% 15% 33% 23% 7% 1	800
	21st to end	Night	25% 14% 32% 24% 6%	800
December	1st to 10th	Night	26% 15% 33% 21% 4	800
	11th to 20th	Night	25% 15% 34% 22% 4	800
	21st to end	Night	26% 17% 32% 22% 3	880

Page generated at 2025-09-16 01:49:11. Version ac07

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