Lewiston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lewiston clear sky chart from 2005-11-06 to 2026-01-17.

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	9% 7% 2 6% 4 4 4 4 5% 7% 46%	135810
	Night	11% 8% 3 7% 5% 5% 5% 4 6% 7% 40%	133125
Transparency	Day	37% 4 13% 24% 16% 7%	130489
	Night	43% 4 14% 22% 14% 5%	129451
Seeing	Day	13% 6% 18% 30% 24% 10%	15974
	Night	16% 4 12% 40% 24% 4	25428

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% 17% 5% 11% 7% 6% 5% 4 5% 6% 15%	3719
	11th to 20th	Day	11% 14% 5% 10% 5% 4 5% 3 6% 8% 29%	3688
	21st to end	Day	17% 12% 4 8% 6% 5% 5% 4 4 5% 31%	4004
February	1st to 10th	Day	17% 14% 5% 8% 7% 4 5% 4 5% 7% 23%	1620
	11th to 20th	Day	16% 17% 4 9% 5% 5% 6% 4 5% 6% 25%	1924
	21st to end	Day	14% 15% 3 8% 5% 5% 4 5% 7% 7% 26%	1650
March	1st to 10th	Day	8% 10% 3 9% 6% 6% 6% 6% 7% 9% 28%	4518
	11th to 20th	Day	9% 9% 4 7% 5% 4 5% 4 6% 8% 39%	4726
	21st to end	Day	12% 15% 5% 11% 7% 7% 6% 5% 6% 6% 20%	5872
April	1st to 10th	Day	12% 10% 2 7% 5% 5% 8% 5% 8% 9% 29%	2400
	11th to 20th	Day	10% 9% 3 7% 6% 4 5% 4 7% 8% 38%	2428
	21st to end	Day	12% 8% 3 8% 4 5% 6% 4 6% 8% 36%	2600
May	1st to 10th	Day	8% 6% 2 5% 3 3 4 4 6% 6% 53%	6046
	11th to 20th	Day	6% 6% 3 6% 5% 5% 6% 5% 6% 9% 43%	6277
	21st to end	Day	8% 6% 2 5% 4 4 5% 4 7% 9% 46%	6902
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 7% 39%	2840
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 5% 6% 8% 43%	3000
	21st to end	Day	5% 4 2 4 3 3 3 3 5% 6% 62%	2833
July	1st to 10th	Day	3 2 1 2 2 2 2 2 4 5% 76%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 82%	6320
	21st to end	Day	2 1 2 1 2 2 2 5% 7% 75%	6880
August	1st to 10th	Day	3 4 2 5% 3 3 3 4 5% 7% 61%	6210
	11th to 20th	Day	3 2 1 3 2 2 2 2 4 6% 72%	5849
	21st to end	Day	4 3 1 4 3 3 3 3 5% 6% 65%	6072
September	1st to 10th	Day	4 2 1 3 2 2 3 3 5% 6% 70%	2400
	11th to 20th	Day	7% 4 2 5% 3 4 5% 3 6% 7% 55%	2267
	21st to end	Day	7% 4 1 6% 3 5% 5% 3 5% 7% 54%	2200
October	1st to 10th	Day	7% 4 2 5% 3 3 3 3 5% 8% 57%	3519
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	3068
	21st to end	Day	13% 9% 3 8% 6% 5% 4 4 6% 8% 33%	3200
November	1st to 10th	Day	20% 9% 3 7% 6% 6% 5% 4 5% 8% 28%	1854
	11th to 20th	Day	17% 11% 5% 10% 7% 5% 6% 5% 6% 6% 23%	1865
	21st to end	Day	14% 11% 4 11% 7% 6% 6% 5% 7% 8% 22%	1638
December	1st to 10th	Day	12% 9% 3 9% 7% 8% 6% 4 5% 6% 30%	2360
	11th to 20th	Day	18% 15% 4 11% 7% 7% 6% 4 6% 5% 16%	2715
	21st to end	Day	18% 14% 4 11% 7% 6% 6% 4 6% 7% 18%	4027
January	1st to 10th	Night	20% 18% 6% 13% 7% 6% 5% 4 5% 5% 11%	6446
	11th to 20th	Night	13% 13% 4 9% 6% 5% 7% 5% 8% 8% 22%	6404
	21st to end	Night	14% 10% 4 11% 6% 5% 5% 3 5% 5% 30%	6952
February	1st to 10th	Night	19% 16% 5% 8% 6% 4 5% 4 5% 5% 22%	2778
	11th to 20th	Night	17% 13% 4 8% 5% 5% 5% 3 6% 6% 29%	2363
	21st to end	Night	14% 13% 4 9% 6% 5% 5% 5% 5% 6% 28%	1980
March	1st to 10th	Night	8% 10% 3 8% 5% 6% 7% 6% 7% 9% 32%	5376
	11th to 20th	Night	11% 12% 4 8% 5% 4 4 4 5% 5% 40%	5274
	21st to end	Night	11% 10% 4 10% 7% 7% 7% 6% 7% 7% 22%	5099
April	1st to 10th	Night	13% 10% 2 6% 5% 5% 5% 4 8% 9% 33%	2000
	11th to 20th	Night	13% 9% 2 7% 5% 5% 4 3 5% 6% 42%	1919
	21st to end	Night	11% 8% 2 7% 4 5% 5% 5% 5% 7% 41%	1800
May	1st to 10th	Night	11% 4 2 5% 3 3 4 3 6% 6% 53%	3923
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 6% 7% 46%	3616
	21st to end	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	3972
June	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 3 7% 9% 45%	1550
	11th to 20th	Night	10% 5% 2 6% 4 4 4 5% 5% 8% 47%	1400
	21st to end	Night	5% 3 2 5% 3 3 4 4 5% 7% 60%	1553
July	1st to 10th	Night	1 1 1 2 1 1 2 2 4 6% 78%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 85%	3640
	21st to end	Night	3 2 1 2 2 2 2 2 4 7% 75%	3970
August	1st to 10th	Night	3 2 1 5% 3 3 4 4 6% 8% 61%	3582
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 6% 70%	3998
	21st to end	Night	4 3 2 4 3 2 3 3 5% 7% 63%	4919
September	1st to 10th	Night	4 3 1 2 2 3 3 3 5% 6% 69%	2000
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 5% 7% 57%	2132
	21st to end	Night	10% 4 2 4 3 4 3 3 5% 6% 56%	2200
October	1st to 10th	Night	6% 3 2 4 3 3 4 4 5% 5% 61%	3664
	11th to 20th	Night	8% 5% 2 5% 4 4 4 5% 7% 8% 48%	4083
	21st to end	Night	11% 7% 2 7% 6% 6% 6% 5% 7% 7% 36%	4600
November	1st to 10th	Night	20% 10% 3 7% 5% 5% 5% 4 5% 7% 29%	2665
	11th to 20th	Night	19% 11% 3 8% 6% 6% 5% 4 5% 5% 26%	2751
	21st to end	Night	16% 11% 4 8% 5% 5% 6% 4 6% 6% 28%	2956
December	1st to 10th	Night	11% 7% 2 7% 8% 6% 6% 5% 8% 9% 32%	5040
	11th to 20th	Night	19% 15% 5% 12% 7% 6% 6% 5% 6% 6% 14%	5696
	21st to end	Night	18% 13% 4 13% 8% 8% 7% 5% 5% 5% 14%	7184

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	68% 5% 14% 11% 1	3719
	11th to 20th	Day	53% 4 21% 17% 5%	3688
	21st to end	Day	54% 4 15% 20% 7%	4004
February	1st to 10th	Day	60% 6% 18% 12% 4	1620
	11th to 20th	Day	61% 4 14% 15% 5%	1934
	21st to end	Day	56% 4 13% 19% 6% 1	1650
March	1st to 10th	Day	46% 5% 13% 26% 10%	4518
	11th to 20th	Day	43% 3 17% 28% 8%	4726
	21st to end	Day	62% 4 15% 14% 4	5872
April	1st to 10th	Day	50% 4 17% 22% 7% 1	2400
	11th to 20th	Day	45% 3 14% 24% 12% 3	2431
	21st to end	Day	46% 3 12% 26% 12% 1	2600
May	1st to 10th	Day	33% 3 9% 30% 24% 2	6046
	11th to 20th	Day	37% 5% 13% 26% 16% 3	6280
	21st to end	Day	35% 3 17% 32% 12% 1	6904
June	1st to 10th	Day	43% 4 12% 27% 13% 1	2840
	11th to 20th	Day	38% 3 12% 28% 15% 3	3000
	21st to end	Day	24% 3 10% 33% 25% 5%	2833
July	1st to 10th	Day	13% 2 5% 21% 31% 29%	6251
	11th to 20th	Day	9% 1 4 15% 32% 37%	6245
	21st to end	Day	10% 3 5% 29% 34% 20%	6681
August	1st to 10th	Day	16% 2 5% 19% 39% 18%	4524
	11th to 20th	Day	16% 2 6% 36% 29% 10%	4236
	21st to end	Day	18% 3 10% 33% 29% 7%	4375
September	1st to 10th	Day	16% 2 9% 31% 29% 13%	2400
	11th to 20th	Day	30% 2 12% 28% 21% 7%	2267
	21st to end	Day	29% 3 16% 33% 16% 3	2200
October	1st to 10th	Day	27% 2 15% 35% 18% 3	3519
	11th to 20th	Day	33% 5% 24% 26% 12%	3068
	21st to end	Day	46% 5% 15% 26% 7%	3200
November	1st to 10th	Day	55% 4 17% 21% 3	1854
	11th to 20th	Day	59% 4 17% 16% 3 1	1865
	21st to end	Day	56% 7% 19% 15% 3	1638
December	1st to 10th	Day	52% 8% 22% 16% 3	2360
	11th to 20th	Day	67% 6% 16% 10% 1	2714
	21st to end	Day	65% 5% 16% 14% 1	4027
January	1st to 10th	Night	72% 4 13% 10% 1	6446
	11th to 20th	Night	55% 5% 21% 14% 5%	6404
	21st to end	Night	53% 3 18% 19% 6%	6952
February	1st to 10th	Night	63% 4 16% 13% 4 1	2778
	11th to 20th	Night	55% 4 17% 20% 5%	2364
	21st to end	Night	57% 3 16% 16% 6% 1	1980
March	1st to 10th	Night	42% 4 14% 29% 11%	5376
	11th to 20th	Night	47% 3 12% 31% 8%	5274
	21st to end	Night	56% 4 19% 17% 4	5099
April	1st to 10th	Night	46% 3 16% 26% 8%	2000
	11th to 20th	Night	44% 2 9% 26% 14% 4	1919
	21st to end	Night	43% 3 12% 24% 16% 2	1800
May	1st to 10th	Night	32% 2 8% 32% 25% 2	3920
	11th to 20th	Night	36% 4 11% 30% 17% 3	3616
	21st to end	Night	37% 3 15% 30% 15%	3972
June	1st to 10th	Night	35% 4 13% 30% 17% 1	1550
	11th to 20th	Night	36% 3 10% 29% 18% 5%	1400
	21st to end	Night	25% 2 9% 30% 26% 8%	1553
July	1st to 10th	Night	10% 1 4 21% 31% 33%	3600
	11th to 20th	Night	6% 1 4 13% 35% 41%	3592
	21st to end	Night	12% 2 3 24% 33% 25%	3857
August	1st to 10th	Night	17% 2 5% 14% 42% 19%	2601
	11th to 20th	Night	16% 2 8% 28% 34% 12%	2888
	21st to end	Night	17% 4 8% 27% 33% 11%	3538
September	1st to 10th	Night	17% 2 8% 28% 31% 15%	2000
	11th to 20th	Night	27% 2 10% 28% 26% 8%	2132
	21st to end	Night	29% 2 12% 32% 18% 7%	2200
October	1st to 10th	Night	25% 3 10% 35% 26% 2	3664
	11th to 20th	Night	30% 5% 20% 26% 18% 1	4083
	21st to end	Night	42% 6% 15% 27% 10% 1	4600
November	1st to 10th	Night	53% 4 15% 22% 6%	2665
	11th to 20th	Night	56% 4 18% 18% 3 1	2751
	21st to end	Night	52% 4 18% 22% 4	2956
December	1st to 10th	Night	44% 9% 25% 19% 3	5040
	11th to 20th	Night	69% 5% 17% 9% 1	5697
	21st to end	Night	66% 4 17% 10% 3	7184

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	25% 5% 15% 28% 20% 6%	220
	11th to 20th	Day	26% 4 12% 29% 21% 8%	212
	21st to end	Day	20% 5% 15% 37% 19% 5%	220
February	1st to 10th	Day	25% 5% 21% 33% 13% 2	220
	11th to 20th	Day	26% 7% 16% 30% 18% 2	482
	21st to end	Day	25% 8% 19% 31% 15% 3	414
March	1st to 10th	Day	17% 6% 25% 30% 17% 4	500
	11th to 20th	Day	17% 8% 18% 25% 22% 10%	500
	21st to end	Day	16% 11% 23% 30% 16% 4	550
April	1st to 10th	Day	17% 10% 21% 28% 17% 7%	500
	11th to 20th	Day	15% 9% 26% 28% 15% 7%	492
	21st to end	Day	16% 8% 20% 26% 23% 7%	500
May	1st to 10th	Day	11% 10% 18% 31% 24% 6%	503
	11th to 20th	Day	15% 8% 17% 27% 20% 13%	528
	21st to end	Day	11% 5% 19% 25% 27% 14%	573
June	1st to 10th	Day	13% 6% 15% 28% 26% 12%	570
	11th to 20th	Day	13% 6% 21% 34% 21% 6%	730
	21st to end	Day	5% 5% 19% 31% 29% 10%	563
July	1st to 10th	Day	3 4 21% 32% 30% 10%	526
	11th to 20th	Day	2 3 23% 32% 28% 12%	524
	21st to end	Day	1 3 17% 32% 32% 15%	575
August	1st to 10th	Day	4 3 17% 30% 29% 17%	527
	11th to 20th	Day	3 3 15% 27% 32% 19%	517
	21st to end	Day	7% 5% 16% 27% 29% 16%	583
September	1st to 10th	Day	5% 2 17% 24% 30% 22%	530
	11th to 20th	Day	9% 5% 13% 29% 29% 15%	530
	21st to end	Day	10% 4 15% 30% 30% 11%	530
October	1st to 10th	Day	11% 7% 12% 32% 26% 12%	530
	11th to 20th	Day	13% 6% 14% 27% 28% 12%	414
	21st to end	Day	21% 4 14% 34% 21% 5%	363
November	1st to 10th	Day	27% 5% 15% 32% 20% 1	330
	11th to 20th	Day	21% 5% 18% 30% 20% 5%	316
	21st to end	Day	19% 4 15% 37% 22% 4	220
December	1st to 10th	Day	20% 6% 13% 31% 19% 11%	220
	11th to 20th	Day	30% 9% 11% 34% 14% 3	220
	21st to end	Day	24% 5% 14% 36% 17% 4	242
January	1st to 10th	Night	27% 5% 14% 35% 15% 3	840
	11th to 20th	Night	22% 4 15% 37% 18% 4	824
	21st to end	Night	21% 5% 11% 39% 23% 3	880
February	1st to 10th	Night	25% 7% 14% 38% 14% 2	800
	11th to 20th	Night	23% 4 14% 37% 20% 3	784
	21st to end	Night	20% 6% 17% 39% 17% 2	660
March	1st to 10th	Night	19% 4 15% 41% 19% 2	800
	11th to 20th	Night	15% 6% 17% 37% 23% 3	800
	21st to end	Night	19% 5% 13% 46% 15% 2	880
April	1st to 10th	Night	18% 4 13% 40% 23% 2	800
	11th to 20th	Night	16% 5% 20% 40% 16% 3	731
	21st to end	Night	13% 5% 15% 43% 23% 2	600
May	1st to 10th	Night	10% 5% 15% 42% 27% 2	527
	11th to 20th	Night	14% 5% 7% 34% 36% 5%	394
	21st to end	Night	15% 2 9% 35% 34% 6%	433
June	1st to 10th	Night	11% 3 10% 43% 29% 4	400
	11th to 20th	Night	12% 5% 16% 39% 25% 3	400
	21st to end	Night	6% 2 10% 47% 33% 3	400
July	1st to 10th	Night	2 1 8% 48% 37% 4	390
	11th to 20th	Night	2 1 9% 53% 33% 2	388
	21st to end	Night	2 2 6% 45% 42% 3	438
August	1st to 10th	Night	3 1 5% 46% 40% 5%	398
	11th to 20th	Night	2 1 8% 40% 42% 7%	566
	21st to end	Night	5% 2 9% 40% 38% 5%	836
September	1st to 10th	Night	4 1 8% 43% 36% 8%	800
	11th to 20th	Night	8% 1 7% 37% 39% 9%	800
	21st to end	Night	11% 2 9% 41% 32% 5%	800
October	1st to 10th	Night	10% 2 11% 46% 27% 4	800
	11th to 20th	Night	14% 4 12% 37% 28% 5%	800
	21st to end	Night	16% 4 15% 42% 19% 4	880
November	1st to 10th	Night	24% 5% 12% 41% 18% 1	820
	11th to 20th	Night	23% 7% 15% 36% 17% 2	840
	21st to end	Night	21% 4 12% 38% 21% 4	856
December	1st to 10th	Night	22% 3 12% 38% 21% 4	1050
	11th to 20th	Night	28% 5% 13% 36% 16% 2	1050
	21st to end	Night	30% 5% 16% 35% 12% 2	963

Page generated at 2026-01-13 22:14:29. Version ac07

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