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 2024-11-25.

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 5% 4 5% 7% 45%	105203
	Night	12% 8% 3 7% 5% 5% 5% 4 6% 6% 40%	102882
Transparency	Day	39% 3 12% 23% 16% 8%	104946
	Night	43% 3 13% 22% 13% 6%	102728
Seeing	Day	14% 6% 18% 30% 24% 10%	14597
	Night	17% 3 12% 40% 24% 4	23937

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	15% 15% 5% 11% 7% 7% 6% 5% 6% 7% 16%	2539
	11th to 20th	Day	16% 16% 6% 11% 6% 5% 6% 4 6% 8% 15%	2540
	21st to end	Day	23% 15% 5% 11% 8% 6% 6% 4 4 5% 14%	2794
February	1st to 10th	Day	18% 14% 5% 8% 7% 4 5% 4 5% 7% 24%	1539
	11th to 20th	Day	15% 17% 4 9% 5% 5% 6% 4 5% 6% 25%	1825
	21st to end	Day	14% 15% 3 8% 5% 5% 4 5% 7% 7% 26%	1570
March	1st to 10th	Day	9% 12% 4 11% 6% 6% 6% 5% 6% 8% 27%	3138
	11th to 20th	Day	8% 7% 3 6% 5% 4 5% 4 6% 7% 46%	3294
	21st to end	Day	13% 15% 4 11% 7% 8% 7% 5% 6% 6% 19%	4094
April	1st to 10th	Day	12% 10% 3 7% 5% 5% 8% 5% 8% 9% 28%	2280
	11th to 20th	Day	10% 10% 3 7% 6% 4 5% 4 7% 8% 37%	2305
	21st to end	Day	12% 8% 3 8% 4 6% 6% 4 6% 8% 35%	2470
May	1st to 10th	Day	10% 8% 2 6% 4 4 5% 4 6% 7% 45%	4224
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 5% 8% 54%	4377
	21st to end	Day	10% 7% 3 7% 5% 5% 6% 5% 8% 9% 34%	4813
June	1st to 10th	Day	14% 7% 2 7% 6% 5% 5% 4 6% 7% 37%	2698
	11th to 20th	Day	11% 7% 2 6% 4 4 5% 5% 6% 8% 43%	2850
	21st to end	Day	5% 4 1 4 3 4 3 3 5% 6% 62%	2692
July	1st to 10th	Day	1 1 2 1 2 2 2 4 4 80%	4420
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 81%	4420
	21st to end	Day	1 1 2 2 2 2 3 5% 5% 78%	4790
August	1st to 10th	Day	3 2 1 3 2 3 3 3 4 7% 69%	4400
	11th to 20th	Day	3 2 1 3 2 2 3 2 5% 7% 69%	4066
	21st to end	Day	3 3 1 3 3 3 3 2 4 6% 69%	4242
September	1st to 10th	Day	4 2 1 3 2 2 2 2 5% 6% 71%	2280
	11th to 20th	Day	7% 4 2 5% 3 4 5% 3 6% 7% 54%	2154
	21st to end	Day	7% 4 1 5% 3 5% 5% 3 6% 8% 54%	2090
October	1st to 10th	Day	8% 4 2 5% 3 3 3 3 5% 8% 57%	3411
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	2974
	21st to end	Day	13% 9% 3 8% 6% 5% 4 4 6% 8% 33%	3101
November	1st to 10th	Day	21% 8% 3 7% 6% 6% 5% 4 5% 8% 27%	1764
	11th to 20th	Day	18% 10% 5% 10% 6% 5% 6% 5% 6% 7% 23%	1776
	21st to end	Day	15% 11% 4 11% 7% 6% 6% 5% 7% 8% 22%	1514
December	1st to 10th	Day	17% 11% 4 10% 6% 7% 5% 5% 5% 7% 23%	1330
	11th to 20th	Day	18% 15% 5% 11% 7% 8% 6% 4 6% 6% 15%	1685
	21st to end	Day	15% 13% 4 10% 7% 7% 6% 4 6% 9% 19%	2744
January	1st to 10th	Night	20% 16% 5% 13% 7% 6% 6% 4 5% 6% 12%	4406
	11th to 20th	Night	18% 15% 5% 10% 6% 5% 7% 5% 7% 7% 15%	4420
	21st to end	Night	20% 13% 5% 14% 7% 6% 6% 4 4 5% 16%	4862
February	1st to 10th	Night	19% 16% 4 8% 6% 4 5% 4 5% 5% 23%	2640
	11th to 20th	Night	16% 13% 4 8% 5% 5% 5% 3 6% 6% 30%	2242
	21st to end	Night	14% 14% 4 9% 6% 5% 5% 5% 5% 6% 27%	1884
March	1st to 10th	Night	9% 11% 3 8% 6% 6% 7% 6% 6% 8% 29%	3736
	11th to 20th	Night	9% 9% 3 6% 4 4 3 4 5% 5% 49%	3686
	21st to end	Night	11% 10% 3 9% 7% 8% 8% 6% 8% 8% 22%	3555
April	1st to 10th	Night	13% 11% 2 6% 5% 4 5% 4 7% 9% 33%	1900
	11th to 20th	Night	13% 9% 2 7% 5% 5% 4 3 5% 6% 41%	1822
	21st to end	Night	12% 8% 2 7% 4 5% 5% 5% 5% 7% 40%	1710
May	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 7% 7% 44%	2739
	11th to 20th	Night	8% 4 2 4 4 3 5% 4 6% 8% 54%	2516
	21st to end	Night	16% 8% 3 6% 4 4 5% 4 6% 7% 37%	2762
June	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 3 8% 10% 43%	1473
	11th to 20th	Night	10% 5% 2 6% 4 4 4 4 5% 8% 47%	1330
	21st to end	Night	5% 3 1 4 3 3 4 4 5% 7% 61%	1475
July	1st to 10th	Night	1 1 1 1 1 2 2 3 5% 83%	2540
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 84%	2540
	21st to end	Night	3 2 1 2 2 2 2 2 3 5% 77%	2761
August	1st to 10th	Night	3 2 1 3 2 2 3 3 6% 7% 67%	2529
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 6% 69%	2772
	21st to end	Night	3 2 1 4 3 2 3 3 5% 6% 68%	3430
September	1st to 10th	Night	4 3 1 2 2 3 3 3 5% 6% 69%	1900
	11th to 20th	Night	6% 4 2 5% 4 4 4 3 5% 7% 56%	2025
	21st to end	Night	10% 4 2 4 3 4 3 3 5% 6% 56%	2090
October	1st to 10th	Night	6% 3 2 4 3 3 4 3 5% 5% 61%	3552
	11th to 20th	Night	8% 5% 2 5% 4 4 4 5% 7% 8% 49%	3957
	21st to end	Night	11% 7% 2 7% 6% 6% 6% 5% 7% 6% 36%	4457
November	1st to 10th	Night	20% 10% 3 8% 5% 5% 4 4 5% 7% 29%	2535
	11th to 20th	Night	19% 11% 3 8% 6% 6% 5% 4 5% 6% 27%	2620
	21st to end	Night	16% 11% 3 8% 5% 5% 6% 4 6% 7% 30%	2736
December	1st to 10th	Night	17% 10% 3 9% 6% 6% 6% 5% 7% 7% 25%	2850
	11th to 20th	Night	20% 14% 5% 11% 7% 6% 6% 5% 6% 6% 14%	3506
	21st to end	Night	17% 12% 3 11% 7% 9% 8% 5% 5% 6% 16%	4924

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	66% 4 16% 13% 1	2539
	11th to 20th	Day	67% 3 16% 12% 3	2540
	21st to end	Day	71% 3 16% 9% 1	2794
February	1st to 10th	Day	60% 6% 19% 12% 4	1539
	11th to 20th	Day	61% 3 14% 16% 5%	1835
	21st to end	Day	56% 4 12% 20% 6% 1	1570
March	1st to 10th	Day	52% 2 11% 28% 6%	3138
	11th to 20th	Day	39% 2 18% 32% 9%	3294
	21st to end	Day	65% 2 15% 12% 5%	4094
April	1st to 10th	Day	52% 4 16% 21% 7% 1	2280
	11th to 20th	Day	46% 3 14% 23% 12% 3	2308
	21st to end	Day	47% 3 12% 25% 12% 1	2470
May	1st to 10th	Day	41% 2 9% 30% 16% 2	4224
	11th to 20th	Day	31% 2 11% 31% 21% 5%	4380
	21st to end	Day	47% 2 17% 26% 7% 1	4814
June	1st to 10th	Day	45% 4 12% 27% 11% 1	2698
	11th to 20th	Day	39% 3 12% 28% 15% 3	2850
	21st to end	Day	24% 2 10% 33% 26% 5%	2692
July	1st to 10th	Day	11% 1 4 16% 30% 38%	4351
	11th to 20th	Day	10% 1 5% 11% 26% 47%	4345
	21st to end	Day	10% 2 4 20% 36% 28%	4639
August	1st to 10th	Day	16% 2 5% 19% 40% 18%	4384
	11th to 20th	Day	16% 2 6% 36% 29% 10%	4105
	21st to end	Day	17% 3 11% 33% 29% 7%	4241
September	1st to 10th	Day	16% 2 8% 32% 29% 13%	2280
	11th to 20th	Day	31% 2 12% 27% 21% 6%	2154
	21st to end	Day	29% 3 16% 33% 15% 3	2090
October	1st to 10th	Day	27% 2 16% 35% 18% 3	3411
	11th to 20th	Day	32% 5% 24% 26% 12%	2974
	21st to end	Day	46% 5% 15% 27% 7%	3101
November	1st to 10th	Day	55% 4 16% 21% 3 1	1764
	11th to 20th	Day	59% 4 17% 17% 3 1	1776
	21st to end	Day	55% 6% 19% 16% 3	1514
December	1st to 10th	Day	60% 6% 15% 16% 4	1330
	11th to 20th	Day	68% 4 17% 10% 1	1684
	21st to end	Day	62% 2 18% 16% 2	2744
January	1st to 10th	Night	69% 3 14% 12% 1	4406
	11th to 20th	Night	67% 2 16% 12% 3	4420
	21st to end	Night	68% 4 17% 10% 2	4862
February	1st to 10th	Night	62% 4 16% 14% 4 1	2640
	11th to 20th	Night	54% 3 17% 20% 5%	2243
	21st to end	Night	57% 3 16% 16% 6% 1	1884
March	1st to 10th	Night	47% 2 12% 32% 6%	3736
	11th to 20th	Night	38% 1 15% 37% 8%	3686
	21st to end	Night	56% 3 20% 16% 5%	3555
April	1st to 10th	Night	47% 3 15% 26% 9%	1900
	11th to 20th	Night	46% 2 9% 25% 14% 4	1822
	21st to end	Night	44% 3 13% 23% 16% 2	1710
May	1st to 10th	Night	39% 1 10% 30% 17% 2	2739
	11th to 20th	Night	30% 2 9% 33% 23% 4	2516
	21st to end	Night	47% 2 17% 24% 9%	2762
June	1st to 10th	Night	37% 4 13% 30% 15% 1	1473
	11th to 20th	Night	36% 3 10% 30% 17% 5%	1330
	21st to end	Night	25% 2 9% 29% 27% 8%	1475
July	1st to 10th	Night	8% 1 4 16% 30% 42%	2500
	11th to 20th	Night	6% 2 5% 12% 24% 51%	2492
	21st to end	Night	12% 2 2 16% 33% 35%	2674
August	1st to 10th	Night	16% 2 5% 14% 43% 19%	2521
	11th to 20th	Night	16% 2 8% 27% 34% 12%	2799
	21st to end	Night	17% 5% 8% 27% 34% 10%	3430
September	1st to 10th	Night	17% 2 7% 28% 31% 15%	1900
	11th to 20th	Night	28% 2 10% 27% 26% 8%	2025
	21st to end	Night	30% 2 12% 32% 17% 7%	2090
October	1st to 10th	Night	25% 3 10% 35% 26% 2	3552
	11th to 20th	Night	30% 5% 21% 26% 18% 1	3957
	21st to end	Night	42% 6% 14% 27% 10% 1	4457
November	1st to 10th	Night	53% 3 14% 22% 6%	2535
	11th to 20th	Night	55% 3 18% 19% 3 1	2620
	21st to end	Night	51% 2 19% 23% 5%	2736
December	1st to 10th	Night	56% 7% 17% 17% 4	2850
	11th to 20th	Night	69% 4 17% 10% 1	3507
	21st to end	Night	63% 2 20% 12% 4	4924

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% 4 13% 28% 23% 8%	180
	11th to 20th	Day	29% 4 11% 28% 18% 9%	180
	21st to end	Day	21% 6% 15% 35% 19% 4	198
February	1st to 10th	Day	27% 4 21% 34% 13% 2	199
	11th to 20th	Day	25% 7% 17% 31% 17% 3	443
	21st to end	Day	26% 7% 19% 30% 16% 3	382
March	1st to 10th	Day	18% 6% 25% 29% 18% 4	460
	11th to 20th	Day	17% 6% 18% 25% 24% 10%	460
	21st to end	Day	17% 9% 23% 30% 16% 5%	506
April	1st to 10th	Day	18% 11% 21% 28% 16% 6%	460
	11th to 20th	Day	15% 9% 27% 27% 15% 7%	452
	21st to end	Day	17% 8% 21% 25% 23% 6%	460
May	1st to 10th	Day	12% 11% 18% 30% 23% 6%	463
	11th to 20th	Day	15% 7% 16% 27% 21% 14%	488
	21st to end	Day	11% 5% 19% 25% 27% 12%	529
June	1st to 10th	Day	14% 6% 16% 28% 25% 11%	528
	11th to 20th	Day	13% 6% 20% 34% 20% 6%	680
	21st to end	Day	6% 5% 19% 30% 30% 10%	522
July	1st to 10th	Day	3 4 21% 32% 29% 10%	486
	11th to 20th	Day	2 3 23% 33% 27% 12%	484
	21st to end	Day	1 3 18% 33% 32% 14%	531
August	1st to 10th	Day	3 3 17% 31% 29% 17%	487
	11th to 20th	Day	3 3 14% 27% 32% 20%	477
	21st to end	Day	7% 5% 17% 28% 28% 17%	539
September	1st to 10th	Day	5% 2 18% 25% 30% 20%	490
	11th to 20th	Day	9% 5% 13% 30% 28% 14%	490
	21st to end	Day	10% 4 16% 31% 29% 10%	490
October	1st to 10th	Day	12% 7% 10% 31% 27% 13%	490
	11th to 20th	Day	12% 5% 14% 28% 28% 13%	380
	21st to end	Day	23% 4 14% 34% 21% 5%	330
November	1st to 10th	Day	27% 4 14% 32% 21% 1	300
	11th to 20th	Day	21% 5% 20% 28% 21% 6%	287
	21st to end	Day	18% 3 14% 37% 23% 5%	188
December	1st to 10th	Day	21% 3 12% 35% 19% 11%	180
	11th to 20th	Day	31% 7% 11% 33% 15% 3	180
	21st to end	Day	25% 2 12% 38% 18% 5%	198
January	1st to 10th	Night	27% 5% 15% 34% 16% 4	760
	11th to 20th	Night	24% 4 14% 36% 18% 4	760
	21st to end	Night	22% 4 11% 38% 23% 3	836
February	1st to 10th	Night	26% 6% 14% 38% 14% 2	760
	11th to 20th	Night	22% 4 15% 36% 20% 3	744
	21st to end	Night	20% 5% 17% 39% 17% 2	628
March	1st to 10th	Night	20% 3 15% 40% 19% 3	760
	11th to 20th	Night	14% 5% 17% 37% 24% 3	760
	21st to end	Night	19% 4 12% 47% 15% 3	836
April	1st to 10th	Night	19% 4 13% 39% 23% 2	760
	11th to 20th	Night	17% 4 19% 40% 16% 3	694
	21st to end	Night	14% 4 15% 43% 22% 2	570
May	1st to 10th	Night	10% 5% 15% 42% 27% 2	501
	11th to 20th	Night	13% 5% 6% 34% 37% 5%	374
	21st to end	Night	15% 2 8% 35% 34% 5%	411
June	1st to 10th	Night	12% 3 10% 44% 28% 4	380
	11th to 20th	Night	12% 6% 17% 38% 25% 3	380
	21st to end	Night	6% 1 10% 46% 34% 2	380
July	1st to 10th	Night	2 1 8% 48% 37% 4	370
	11th to 20th	Night	2 1 9% 54% 32% 2	368
	21st to end	Night	2 2 6% 45% 41% 3	416
August	1st to 10th	Night	3 1 5% 46% 40% 5%	378
	11th to 20th	Night	2 1 7% 39% 43% 7%	537
	21st to end	Night	5% 2 10% 39% 39% 6%	794
September	1st to 10th	Night	5% 1 9% 43% 35% 8%	760
	11th to 20th	Night	8% 1 7% 37% 39% 9%	760
	21st to end	Night	11% 2 8% 42% 32% 6%	760
October	1st to 10th	Night	10% 2 11% 46% 28% 4	760
	11th to 20th	Night	14% 3 11% 38% 29% 6%	760
	21st to end	Night	17% 3 14% 41% 20% 4	836
November	1st to 10th	Night	24% 4 11% 42% 18% 1	780
	11th to 20th	Night	23% 7% 15% 35% 17% 3	800
	21st to end	Night	21% 4 12% 37% 21% 4	792
December	1st to 10th	Night	22% 3 12% 40% 20% 4	950
	11th to 20th	Night	29% 4 11% 36% 17% 2	950
	21st to end	Night	31% 4 15% 35% 13% 2	872

Page generated at 2024-11-22 05:02:24. Version ac07

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