Lewiston Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lewiston Lake clear sky chart from 2005-11-06 to 2025-11-18.

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	9% 4 1 3 2 3 3 3 4 6% 62%	135818
	Night	12% 5% 1 4 3 3 3 3 4 5% 58%	131922
Transparency	Day	25% 5% 8% 20% 24% 18%	130674
	Night	29% 6% 9% 22% 22% 13%	128057
Seeing	Day	13% 12% 25% 29% 16% 4	16431
	Night	13% 5% 19% 35% 24% 3	25538

By Month

Percentage Cloud Cover SW (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	19% 8% 2 6% 4 4 5% 4 5% 7% 37%	3639
	11th to 20th	Day	20% 7% 2 4 3 2 3 2 3 4 51%	4036
	21st to end	Day	24% 6% 2 3 2 2 3 2 4 5% 48%	4510
February	1st to 10th	Day	20% 7% 2 4 3 4 5% 4 6% 5% 41%	1800
	11th to 20th	Day	16% 8% 2 4 3 3 3 3 5% 7% 44%	1755
	21st to end	Day	16% 7% 3 4 4 3 4 4 6% 7% 42%	1639
March	1st to 10th	Day	17% 14% 3 7% 3 3 4 3 4 6% 36%	4868
	11th to 20th	Day	17% 10% 2 5% 3 4 3 3 4 6% 43%	5000
	21st to end	Day	19% 10% 2 6% 5% 5% 5% 3 5% 7% 33%	5529
April	1st to 10th	Day	18% 6% 2 4 4 4 4 4 5% 8% 42%	2400
	11th to 20th	Day	13% 5% 1 5% 4 3 4 3 5% 7% 50%	2369
	21st to end	Day	6% 4 1 5% 4 3 4 3 5% 8% 56%	2450
May	1st to 10th	Day	5% 3 1 3 2 3 3 3 4 6% 66%	5880
	11th to 20th	Day	4 2 1 3 2 3 3 3 4 6% 69%	6221
	21st to end	Day	4 2 1 2 2 2 2 3 4 6% 74%	6902
June	1st to 10th	Day	6% 3 1 3 3 3 4 3 6% 6% 60%	2800
	11th to 20th	Day	3 2 1 3 2 3 2 2 4 6% 73%	2800
	21st to end	Day	4 2 1 2 1 2 2 2 2 3 79%	2800
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 89%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 88%	6263
	21st to end	Day	1 1 1 2 2 4 4 83%	6402
August	1st to 10th	Day	1 1 1 1 2 2 2 3 5% 82%	5777
	11th to 20th	Day	1 1 1 1 2 2 2 3 4 83%	5761
	21st to end	Day	2 1 1 1 2 3 2 4 5% 79%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 3 4 85%	2400
	11th to 20th	Day	4 2 1 2 2 3 3 3 5% 6% 68%	2390
	21st to end	Day	3 2 2 2 2 2 2 3 5% 76%	2106
October	1st to 10th	Day	4 2 1 2 2 2 3 2 3 4 75%	3280
	11th to 20th	Day	7% 3 1 3 2 3 3 4 5% 7% 62%	3261
	21st to end	Day	10% 5% 2 5% 4 4 5% 4 6% 8% 48%	3558
November	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 5% 8% 48%	1948
	11th to 20th	Day	15% 7% 2 5% 3 4 4 4 7% 8% 41%	1719
	21st to end	Day	14% 5% 1 4 4 4 4 4 4 6% 51%	1600
December	1st to 10th	Day	12% 5% 1 4 2 3 5% 4 5% 6% 52%	2620
	11th to 20th	Day	31% 9% 2 6% 4 4 4 3 6% 7% 22%	3028
	21st to end	Day	27% 9% 2 6% 5% 4 5% 3 5% 6% 27%	4004
January	1st to 10th	Night	19% 9% 3 7% 4 4 4 4 6% 7% 34%	6306
	11th to 20th	Night	18% 7% 2 5% 3 2 3 4 4 4 48%	5924
	21st to end	Night	24% 6% 2 5% 3 3 3 3 3 4 45%	6468
February	1st to 10th	Night	20% 7% 2 4 3 3 4 3 5% 6% 43%	2600
	11th to 20th	Night	17% 7% 2 5% 3 4 3 4 6% 7% 43%	2532
	21st to end	Night	14% 6% 2 5% 3 3 4 3 6% 8% 47%	1990
March	1st to 10th	Night	15% 9% 2 7% 4 4 5% 4 5% 6% 38%	5014
	11th to 20th	Night	17% 8% 2 5% 3 4 3 3 4 6% 45%	5000
	21st to end	Night	16% 9% 3 7% 5% 5% 5% 3 5% 6% 36%	5432
April	1st to 10th	Night	14% 8% 3 5% 4 3 3 4 5% 7% 45%	2000
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 5% 7% 56%	1979
	21st to end	Night	7% 5% 1 3 2 2 3 3 4 8% 61%	1946
May	1st to 10th	Night	5% 3 1 3 2 2 3 2 3 5% 72%	4100
	11th to 20th	Night	4 2 1 3 2 2 3 3 4 4 72%	3672
	21st to end	Night	3 1 1 1 1 2 2 4 6% 78%	3972
June	1st to 10th	Night	7% 4 1 4 3 3 3 4 6% 6% 59%	1600
	11th to 20th	Night	4 2 1 2 1 1 2 1 3 4 79%	1600
	21st to end	Night	3 1 1 2 2 1 1 2 3 3 81%	1600
July	1st to 10th	Night	1 1 1 1 1 1 1 2 2 90%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 88%	3682
	21st to end	Night	1 1 1 2 2 3 4 85%	4472
August	1st to 10th	Night	1 1 1 1 2 2 2 3 3 84%	4035
	11th to 20th	Night	1 1 1 1 1 1 2 1 2 3 86%	4088
	21st to end	Night	2 1 2 1 1 2 1 2 3 83%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 4 86%	2000
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 6% 76%	2005
	21st to end	Night	3 2 1 2 2 2 3 2 4 5% 75%	2292
October	1st to 10th	Night	3 2 1 2 2 3 3 2 2 3 78%	3920
	11th to 20th	Night	6% 3 1 3 3 3 4 3 4 6% 64%	3897
	21st to end	Night	8% 2 1 4 3 4 5% 4 6% 8% 55%	4253
November	1st to 10th	Night	14% 4 2 5% 4 4 4 3 5% 7% 47%	2570
	11th to 20th	Night	15% 6% 1 4 3 4 4 5% 7% 9% 42%	2830
	21st to end	Night	14% 4 2 5% 3 3 3 3 5% 7% 50%	2800
December	1st to 10th	Night	14% 4 1 3 3 4 5% 4 5% 6% 52%	4560
	11th to 20th	Night	31% 8% 2 6% 5% 5% 4 4 6% 5% 25%	5176
	21st to end	Night	29% 11% 3 6% 4 3 5% 3 4 4 29%	6952

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% 10% 12% 19% 12% 2	3639
	11th to 20th	Day	39% 4 8% 22% 18% 8%	4036
	21st to end	Day	40% 5% 9% 11% 22% 14%	4510
February	1st to 10th	Day	43% 7% 10% 18% 14% 7%	1800
	11th to 20th	Day	39% 6% 10% 23% 19% 4	1764
	21st to end	Day	39% 7% 11% 21% 16% 5%	1639
March	1st to 10th	Day	47% 7% 10% 17% 16% 3	4868
	11th to 20th	Day	41% 7% 12% 17% 21% 2	5000
	21st to end	Day	47% 9% 12% 24% 6% 1	5529
April	1st to 10th	Day	40% 7% 15% 19% 16% 3	2400
	11th to 20th	Day	33% 7% 12% 25% 18% 6%	2372
	21st to end	Day	25% 6% 14% 29% 22% 4	2450
May	1st to 10th	Day	17% 6% 11% 25% 29% 12%	5880
	11th to 20th	Day	16% 5% 8% 23% 36% 13%	6224
	21st to end	Day	12% 4 10% 41% 31% 3	6904
June	1st to 10th	Day	22% 7% 10% 28% 25% 8%	2800
	11th to 20th	Day	14% 3 9% 23% 35% 16%	2800
	21st to end	Day	13% 2 4 18% 34% 29%	2800
July	1st to 10th	Day	5% 1 3 13% 26% 52%	6251
	11th to 20th	Day	6% 1 2 8% 23% 60%	6188
	21st to end	Day	6% 2 7% 20% 31% 34%	6217
August	1st to 10th	Day	9% 1 4 13% 28% 45%	4206
	11th to 20th	Day	6% 2 4 23% 34% 30%	4172
	21st to end	Day	9% 3 5% 18% 39% 27%	4311
September	1st to 10th	Day	5% 2 4 13% 27% 48%	2400
	11th to 20th	Day	16% 5% 7% 21% 35% 17%	2390
	21st to end	Day	13% 3 6% 19% 33% 26%	2106
October	1st to 10th	Day	13% 4 5% 16% 23% 38%	3280
	11th to 20th	Day	20% 5% 9% 27% 26% 12%	3261
	21st to end	Day	31% 7% 9% 21% 21% 11%	3558
November	1st to 10th	Day	35% 6% 10% 23% 17% 9%	1948
	11th to 20th	Day	39% 8% 14% 18% 14% 8%	1719
	21st to end	Day	34% 5% 12% 17% 21% 11%	1600
December	1st to 10th	Day	31% 5% 7% 21% 25% 10%	2620
	11th to 20th	Day	60% 7% 10% 13% 8% 2	3028
	21st to end	Day	55% 8% 9% 16% 10% 2	4004
January	1st to 10th	Night	46% 10% 12% 21% 9% 2	6306
	11th to 20th	Night	38% 8% 7% 22% 17% 7%	5924
	21st to end	Night	44% 5% 6% 10% 22% 12%	6468
February	1st to 10th	Night	42% 5% 10% 20% 16% 7%	2600
	11th to 20th	Night	40% 7% 10% 20% 18% 5%	2534
	21st to end	Night	35% 6% 14% 21% 20% 4	1990
March	1st to 10th	Night	41% 9% 15% 18% 14% 3	5014
	11th to 20th	Night	37% 8% 13% 23% 17% 4	5000
	21st to end	Night	44% 11% 15% 21% 7% 1	5432
April	1st to 10th	Night	39% 7% 16% 20% 16% 3	2000
	11th to 20th	Night	28% 7% 16% 24% 19% 6%	1979
	21st to end	Night	22% 7% 17% 32% 19% 3	1946
May	1st to 10th	Night	16% 4 10% 29% 32% 9%	4097
	11th to 20th	Night	15% 4 12% 26% 33% 10%	3672
	21st to end	Night	10% 4 13% 50% 22% 1	3972
June	1st to 10th	Night	23% 8% 13% 31% 22% 4	1600
	11th to 20th	Night	11% 4 10% 29% 37% 10%	1600
	21st to end	Night	11% 2 6% 23% 40% 19%	1600
July	1st to 10th	Night	4 1 3 19% 28% 45%	3600
	11th to 20th	Night	5% 1 2 10% 31% 51%	3634
	21st to end	Night	5% 3 8% 27% 31% 27%	4345
August	1st to 10th	Night	10% 1 4 16% 32% 38%	2929
	11th to 20th	Night	5% 2 5% 27% 38% 23%	2952
	21st to end	Night	8% 2 4 27% 39% 20%	3608
September	1st to 10th	Night	6% 2 3 16% 29% 45%	2000
	11th to 20th	Night	12% 3 8% 25% 38% 15%	2005
	21st to end	Night	13% 4 6% 22% 30% 25%	2292
October	1st to 10th	Night	14% 3 5% 13% 31% 34%	3920
	11th to 20th	Night	19% 6% 10% 35% 19% 11%	3897
	21st to end	Night	25% 7% 10% 24% 23% 11%	4253
November	1st to 10th	Night	35% 7% 13% 21% 15% 9%	2570
	11th to 20th	Night	37% 9% 13% 20% 14% 6%	2830
	21st to end	Night	33% 6% 12% 19% 20% 10%	2800
December	1st to 10th	Night	31% 6% 8% 22% 23% 10%	4560
	11th to 20th	Night	58% 7% 8% 15% 10% 2	5176
	21st to end	Night	56% 8% 9% 14% 10% 2	6952

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	36% 11% 24% 21% 8% 2	200
	11th to 20th	Day	27% 12% 23% 21% 14% 4	375
	21st to end	Day	23% 11% 24% 23% 15% 3	440
February	1st to 10th	Day	24% 15% 25% 23% 12% 2	400
	11th to 20th	Day	25% 16% 29% 21% 8% 1	395
	21st to end	Day	20% 17% 27% 27% 9% 1	409
March	1st to 10th	Day	31% 19% 26% 19% 6%	500
	11th to 20th	Day	27% 16% 24% 22% 9% 2	500
	21st to end	Day	26% 20% 28% 17% 8% 1	550
April	1st to 10th	Day	22% 18% 29% 22% 8% 1	500
	11th to 20th	Day	23% 18% 25% 21% 11% 2	492
	21st to end	Day	10% 22% 31% 21% 13% 3	500
May	1st to 10th	Day	13% 19% 29% 27% 9% 3	503
	11th to 20th	Day	14% 17% 29% 24% 13% 3	528
	21st to end	Day	9% 15% 27% 30% 14% 5%	573
June	1st to 10th	Day	9% 17% 23% 32% 17% 3	530
	11th to 20th	Day	5% 21% 30% 29% 13% 2	530
	21st to end	Day	3 11% 28% 34% 20% 5%	530
July	1st to 10th	Day	3 7% 27% 38% 18% 6%	526
	11th to 20th	Day	1 4 23% 42% 22% 8%	524
	21st to end	Day	3 21% 41% 23% 11%	575
August	1st to 10th	Day	1 3 25% 39% 23% 9%	527
	11th to 20th	Day	2 6% 23% 36% 23% 10%	517
	21st to end	Day	2 6% 20% 37% 28% 7%	583
September	1st to 10th	Day	2 6% 26% 37% 24% 5%	530
	11th to 20th	Day	6% 8% 24% 37% 20% 5%	530
	21st to end	Day	5% 9% 27% 35% 20% 4	530
October	1st to 10th	Day	8% 12% 27% 29% 20% 4	530
	11th to 20th	Day	11% 10% 22% 31% 22% 5%	530
	21st to end	Day	15% 10% 24% 28% 18% 4	583
November	1st to 10th	Day	15% 12% 22% 29% 19% 4	424
	11th to 20th	Day	18% 7% 26% 28% 17% 5%	247
	21st to end	Day	19% 11% 22% 28% 18% 4	200
December	1st to 10th	Day	25% 8% 21% 22% 17% 9%	200
	11th to 20th	Day	29% 7% 22% 27% 12% 5%	200
	21st to end	Day	28% 9% 27% 22% 13% 1	220
January	1st to 10th	Night	24% 5% 24% 29% 14% 4	800
	11th to 20th	Night	21% 6% 22% 31% 16% 4	800
	21st to end	Night	21% 5% 20% 31% 19% 3	880
February	1st to 10th	Night	23% 4 20% 35% 17% 1	800
	11th to 20th	Night	21% 7% 21% 35% 14% 2	784
	21st to end	Night	18% 10% 23% 37% 11% 1	660
March	1st to 10th	Night	24% 9% 22% 30% 13% 3	800
	11th to 20th	Night	22% 8% 20% 30% 18% 1	800
	21st to end	Night	20% 11% 25% 31% 12%	880
April	1st to 10th	Night	18% 10% 26% 31% 15% 2	800
	11th to 20th	Night	13% 10% 26% 31% 16% 3	791
	21st to end	Night	9% 8% 29% 33% 20% 1	746
May	1st to 10th	Night	8% 6% 32% 34% 20% 2	600
	11th to 20th	Night	6% 7% 24% 38% 25% 1	424
	21st to end	Night	9% 4 22% 36% 27% 1	433
June	1st to 10th	Night	8% 6% 22% 37% 26% 1	400
	11th to 20th	Night	5% 6% 26% 41% 23% 1	400
	21st to end	Night	2 2 15% 48% 32% 1	400
July	1st to 10th	Night	1 1 11% 45% 40% 2	390
	11th to 20th	Night	1 9% 41% 46% 2	404
	21st to end	Night	1 4 39% 49% 6%	657
August	1st to 10th	Night	1 1 6% 38% 49% 6%	597
	11th to 20th	Night	1 1 9% 40% 44% 5%	604
	21st to end	Night	3 1 8% 39% 45% 4	880
September	1st to 10th	Night	1 3 11% 41% 41% 3	800
	11th to 20th	Night	4 3 11% 38% 39% 5%	800
	21st to end	Night	4 4 15% 38% 36% 4	800
October	1st to 10th	Night	6% 4 18% 38% 32% 3	800
	11th to 20th	Night	8% 4 17% 37% 30% 4	800
	21st to end	Night	13% 5% 15% 38% 24% 5%	880
November	1st to 10th	Night	15% 4 19% 36% 21% 5%	820
	11th to 20th	Night	17% 6% 20% 36% 19% 2	828
	21st to end	Night	16% 5% 23% 35% 18% 4	800
December	1st to 10th	Night	25% 3 18% 33% 17% 5%	800
	11th to 20th	Night	28% 4 19% 33% 12% 4	800
	21st to end	Night	26% 7% 26% 30% 9% 1	880

Page generated at 2025-11-14 21:55:59. Version ac07

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