Norris Lake
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norris Lake clear sky chart from 2005-11-06 to 2024-12-24.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	101462
	Night	12% 7% 3 7% 5% 4 5% 4 6% 7% 41%	98622
Transparency	Day	41% 7% 12% 22% 15% 3	105244
	Night	39% 7% 12% 22% 16% 4	102978
Seeing	Day	16% 11% 22% 24% 19% 9%	14978
	Night	15% 9% 22% 31% 20% 3	25053

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	15% 12% 3 10% 5% 4 4 4 7% 6% 28%	2836
	11th to 20th	Day	16% 10% 5% 9% 5% 5% 5% 4 5% 4 30%	2840
	21st to end	Day	19% 13% 4 8% 4 4 4 3 5% 6% 31%	3124
February	1st to 10th	Day	17% 10% 4 8% 5% 4 4 3 4 5% 34%	1876
	11th to 20th	Day	19% 9% 4 9% 4 4 4 3 4 6% 34%	1848
	21st to end	Day	19% 10% 4 7% 4 5% 4 3 5% 5% 35%	1570
March	1st to 10th	Day	21% 10% 4 9% 4 4 4 3 3 6% 32%	3211
	11th to 20th	Day	12% 7% 2 6% 4 4 5% 3 4 6% 47%	3480
	21st to end	Day	14% 8% 3 7% 5% 4 4 3 5% 6% 40%	3815
April	1st to 10th	Day	11% 8% 3 6% 4 5% 5% 4 6% 7% 41%	2161
	11th to 20th	Day	13% 8% 2 6% 3 4 4 4 6% 6% 44%	2278
	21st to end	Day	13% 6% 3 6% 5% 4 4 4 6% 7% 42%	2470
May	1st to 10th	Day	15% 9% 4 8% 5% 4 5% 5% 6% 7% 33%	4082
	11th to 20th	Day	14% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	4081
	21st to end	Day	10% 6% 2 7% 5% 5% 6% 4 7% 8% 40%	4476
June	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 8% 38%	2470
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 6% 9% 10% 40%	2470
	21st to end	Day	7% 5% 2 7% 5% 6% 5% 5% 8% 11% 39%	2470
July	1st to 10th	Day	5% 4 2 7% 5% 6% 7% 6% 10% 12% 36%	4120
	11th to 20th	Day	8% 6% 2 7% 6% 6% 6% 5% 8% 11% 36%	4120
	21st to end	Day	15% 11% 4 12% 8% 7% 6% 6% 6% 7% 20%	4466
August	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 9% 10% 38%	4104
	11th to 20th	Day	6% 4 2 6% 4 5% 6% 6% 9% 12% 40%	3980
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 9% 52%	4180
September	1st to 10th	Day	11% 6% 3 6% 5% 4 5% 3 5% 7% 44%	2109
	11th to 20th	Day	13% 5% 2 6% 3 4 4 4 6% 9% 42%	2090
	21st to end	Day	14% 6% 2 6% 4 3 4 3 5% 7% 46%	2090
October	1st to 10th	Day	10% 5% 2 6% 3 4 4 3 4 7% 53%	2090
	11th to 20th	Day	12% 6% 2 5% 3 4 3 3 4 5% 52%	1942
	21st to end	Day	15% 6% 1 6% 3 3 4 2 6% 6% 48%	2057
November	1st to 10th	Day	9% 5% 2 5% 4 3 4 3 6% 6% 53%	1762
	11th to 20th	Day	12% 6% 2 5% 5% 3 4 4 5% 6% 48%	1800
	21st to end	Day	13% 8% 4 6% 5% 4 3 3 4 5% 43%	1800
December	1st to 10th	Day	17% 12% 3 7% 5% 5% 4 4 4 6% 34%	1800
	11th to 20th	Day	12% 12% 3 8% 5% 5% 6% 4 6% 6% 33%	2243
	21st to end	Day	16% 12% 3 8% 5% 5% 5% 4 4 7% 31%	3151
January	1st to 10th	Night	15% 11% 5% 8% 6% 5% 5% 4 6% 5% 31%	4109
	11th to 20th	Night	16% 10% 3 8% 7% 5% 3 3 4 5% 35%	4120
	21st to end	Night	18% 11% 4 8% 5% 4 5% 4 5% 5% 31%	4532
February	1st to 10th	Night	14% 10% 4 6% 6% 5% 5% 4 6% 6% 35%	2301
	11th to 20th	Night	21% 7% 3 7% 5% 4 4 4 5% 5% 33%	2218
	21st to end	Night	20% 9% 3 7% 5% 5% 5% 4 6% 6% 31%	1884
March	1st to 10th	Night	20% 9% 3 7% 5% 5% 5% 3 6% 7% 29%	3641
	11th to 20th	Night	10% 5% 2 6% 4 3 4 4 6% 7% 50%	3480
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 6% 42%	3817
April	1st to 10th	Night	12% 5% 2 5% 3 4 5% 4 7% 7% 48%	2033
	11th to 20th	Night	12% 7% 2 6% 4 4 4 3 5% 7% 47%	1851
	21st to end	Night	11% 6% 2 5% 5% 4 5% 4 6% 7% 45%	1710
May	1st to 10th	Night	11% 8% 4 9% 5% 5% 5% 4 7% 9% 34%	2813
	11th to 20th	Night	14% 7% 3 7% 4 3 5% 4 6% 8% 38%	2812
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 5% 6% 9% 45%	3079
June	1st to 10th	Night	7% 5% 2 7% 5% 4 6% 5% 8% 10% 40%	1710
	11th to 20th	Night	3 4 2 5% 3 4 5% 6% 8% 12% 49%	1710
	21st to end	Night	6% 6% 2 6% 6% 5% 6% 4 6% 12% 42%	1710
July	1st to 10th	Night	6% 5% 2 5% 5% 6% 6% 5% 9% 12% 38%	2840
	11th to 20th	Night	8% 5% 2 5% 4 4 5% 5% 6% 10% 48%	2840
	21st to end	Night	12% 10% 4 10% 7% 6% 6% 5% 7% 9% 24%	3087
August	1st to 10th	Night	5% 4 3 6% 5% 5% 6% 5% 8% 11% 43%	2825
	11th to 20th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 48%	2844
	21st to end	Night	5% 3 1 3 3 3 5% 3 5% 9% 58%	3476
September	1st to 10th	Night	10% 6% 2 7% 4 4 6% 3 5% 6% 48%	2050
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 4 5% 8% 44%	2090
	21st to end	Night	12% 5% 3 6% 4 4 4 3 5% 6% 50%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 2 4 6% 56%	2090
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 52%	2237
	21st to end	Night	13% 6% 2 6% 3 3 3 3 4 5% 52%	2537
November	1st to 10th	Night	10% 5% 3 5% 4 4 3 3 5% 5% 52%	2536
	11th to 20th	Night	12% 7% 3 6% 4 3 5% 3 5% 7% 47%	2600
	21st to end	Night	12% 8% 3 7% 4 4 5% 3 5% 5% 44%	2600
December	1st to 10th	Night	16% 10% 3 8% 6% 5% 5% 4 6% 5% 33%	2600
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 4 5% 5% 37%	3172
	21st to end	Night	17% 10% 3 8% 6% 6% 5% 5% 7% 7% 27%	4578

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	50% 7% 8% 20% 12% 2	2837
	11th to 20th	Day	54% 7% 7% 19% 13% 1	2840
	21st to end	Day	52% 7% 8% 21% 12%	3124
February	1st to 10th	Day	52% 5% 8% 18% 15% 2	1876
	11th to 20th	Day	53% 5% 8% 19% 13% 2	1858
	21st to end	Day	51% 5% 9% 15% 15% 5%	1570
March	1st to 10th	Day	53% 6% 7% 18% 11% 5%	3211
	11th to 20th	Day	37% 5% 9% 26% 18% 4	3480
	21st to end	Day	45% 5% 9% 27% 13% 2	3815
April	1st to 10th	Day	40% 7% 12% 22% 15% 3	2161
	11th to 20th	Day	39% 6% 9% 17% 23% 4	2278
	21st to end	Day	40% 7% 12% 26% 14% 2	2470
May	1st to 10th	Day	48% 8% 11% 21% 11% 1	4120
	11th to 20th	Day	46% 8% 12% 22% 11% 1	4085
	21st to end	Day	38% 8% 14% 21% 16% 2	4489
June	1st to 10th	Day	40% 10% 14% 20% 14% 2	2470
	11th to 20th	Day	33% 12% 17% 22% 15% 2	2470
	21st to end	Day	36% 11% 16% 19% 14% 4	2470
July	1st to 10th	Day	35% 13% 22% 19% 11% 1	4055
	11th to 20th	Day	40% 8% 18% 25% 7% 2	4051
	21st to end	Day	61% 9% 10% 14% 5% 1	4324
August	1st to 10th	Day	37% 10% 17% 26% 10%	4091
	11th to 20th	Day	33% 9% 19% 25% 14% 1	4019
	21st to end	Day	27% 8% 15% 35% 13% 2	4179
September	1st to 10th	Day	39% 8% 13% 19% 15% 6%	2109
	11th to 20th	Day	38% 9% 11% 23% 17% 3	2090
	21st to end	Day	37% 6% 11% 21% 19% 6%	2090
October	1st to 10th	Day	26% 5% 7% 20% 33% 9%	3480
	11th to 20th	Day	24% 5% 9% 23% 30% 10%	3208
	21st to end	Day	26% 5% 11% 28% 20% 10%	3371
November	1st to 10th	Day	32% 4 8% 24% 27% 4	1762
	11th to 20th	Day	37% 4 6% 24% 24% 5%	1800
	21st to end	Day	43% 5% 8% 22% 20% 2	1800
December	1st to 10th	Day	52% 5% 8% 21% 12% 2	1800
	11th to 20th	Day	49% 6% 11% 18% 14% 2	2240
	21st to end	Day	52% 5% 11% 22% 9% 1	3151
January	1st to 10th	Night	49% 8% 10% 19% 13% 2	4109
	11th to 20th	Night	50% 5% 8% 17% 16% 4	4120
	21st to end	Night	53% 6% 8% 17% 15% 1	4532
February	1st to 10th	Night	49% 5% 9% 20% 16% 1	2301
	11th to 20th	Night	52% 5% 8% 18% 13% 5%	2219
	21st to end	Night	52% 6% 9% 15% 13% 4	1884
March	1st to 10th	Night	51% 7% 11% 15% 14% 3	3641
	11th to 20th	Night	33% 6% 9% 27% 16% 9%	3480
	21st to end	Night	40% 6% 11% 26% 15% 2	3817
April	1st to 10th	Night	34% 7% 13% 24% 19% 3	2033
	11th to 20th	Night	37% 5% 10% 20% 21% 7%	1851
	21st to end	Night	37% 8% 11% 27% 14% 2	1710
May	1st to 10th	Night	45% 8% 14% 23% 8% 1	2840
	11th to 20th	Night	43% 8% 16% 23% 11% 1	2813
	21st to end	Night	33% 10% 14% 26% 16% 1	3087
June	1st to 10th	Night	35% 10% 16% 21% 15% 2	1710
	11th to 20th	Night	25% 12% 20% 25% 16% 2	1710
	21st to end	Night	36% 9% 16% 22% 15% 4	1710
July	1st to 10th	Night	36% 11% 22% 21% 9% 1	2795
	11th to 20th	Night	31% 8% 20% 29% 9% 3	2786
	21st to end	Night	53% 10% 15% 15% 6% 1	2988
August	1st to 10th	Night	33% 9% 18% 29% 10%	2814
	11th to 20th	Night	31% 8% 18% 28% 13% 1	2871
	21st to end	Night	23% 6% 15% 40% 14% 2	3476
September	1st to 10th	Night	35% 8% 14% 19% 18% 6%	2050
	11th to 20th	Night	38% 8% 12% 20% 20% 2	2090
	21st to end	Night	36% 6% 11% 22% 21% 5%	2090
October	1st to 10th	Night	28% 4 7% 21% 29% 11%	3480
	11th to 20th	Night	22% 4 7% 25% 31% 12%	3710
	21st to end	Night	24% 4 6% 28% 28% 10%	4173
November	1st to 10th	Night	34% 3 9% 22% 24% 7%	2536
	11th to 20th	Night	38% 4 8% 20% 24% 6%	2600
	21st to end	Night	42% 5% 7% 25% 19% 3	2600
December	1st to 10th	Night	51% 6% 9% 19% 12% 3	2600
	11th to 20th	Night	46% 5% 9% 22% 16% 2	3174
	21st to end	Night	51% 8% 12% 17% 11% 1	4578

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% 24% 32% 17% 3 1	270
	11th to 20th	Day	27% 27% 27% 15% 4 1	270
	21st to end	Day	19% 24% 39% 14% 4	297
February	1st to 10th	Day	24% 24% 32% 14% 5% 1	270
	11th to 20th	Day	32% 26% 28% 10% 3	270
	21st to end	Day	37% 18% 35% 8% 1	225
March	1st to 10th	Day	23% 17% 34% 20% 6% 1	322
	11th to 20th	Day	20% 22% 29% 22% 6% 1	460
	21st to end	Day	22% 18% 29% 23% 8% 1	506
April	1st to 10th	Day	19% 18% 25% 24% 12% 2	460
	11th to 20th	Day	18% 13% 29% 25% 12% 2	452
	21st to end	Day	18% 13% 28% 25% 13% 2	460
May	1st to 10th	Day	14% 13% 32% 25% 12% 5%	463
	11th to 20th	Day	12% 8% 26% 28% 19% 8%	487
	21st to end	Day	13% 6% 17% 28% 21% 15%	529
June	1st to 10th	Day	14% 5% 17% 26% 24% 14%	490
	11th to 20th	Day	4 4 16% 27% 32% 17%	490
	21st to end	Day	8% 2 22% 30% 24% 15%	490
July	1st to 10th	Day	10% 4 11% 24% 28% 23%	485
	11th to 20th	Day	10% 4 12% 25% 29% 20%	484
	21st to end	Day	13% 3 11% 25% 31% 17%	532
August	1st to 10th	Day	10% 3 14% 22% 33% 18%	486
	11th to 20th	Day	10% 4 14% 33% 26% 13%	479
	21st to end	Day	10% 3 7% 24% 32% 24%	539
September	1st to 10th	Day	11% 3 11% 27% 29% 18%	490
	11th to 20th	Day	14% 4 10% 22% 31% 19%	490
	21st to end	Day	17% 5% 17% 25% 25% 10%	490
October	1st to 10th	Day	11% 6% 18% 26% 28% 11%	490
	11th to 20th	Day	12% 10% 24% 27% 22% 3	490
	21st to end	Day	19% 9% 21% 31% 18% 3	506
November	1st to 10th	Day	15% 10% 33% 23% 17% 3	300
	11th to 20th	Day	15% 15% 33% 27% 8% 2	300
	21st to end	Day	15% 13% 34% 25% 11% 2	300
December	1st to 10th	Day	25% 18% 33% 17% 6% 1	300
	11th to 20th	Day	21% 17% 29% 24% 8% 1	300
	21st to end	Day	21% 18% 31% 20% 9%	306
January	1st to 10th	Night	21% 17% 31% 26% 4	760
	11th to 20th	Night	23% 18% 30% 25% 4	760
	21st to end	Night	18% 16% 38% 26% 3	836
February	1st to 10th	Night	20% 13% 34% 29% 4	760
	11th to 20th	Night	25% 16% 35% 22% 2	743
	21st to end	Night	24% 18% 32% 23% 2	628
March	1st to 10th	Night	19% 15% 30% 31% 5%	760
	11th to 20th	Night	18% 13% 32% 28% 7% 1	760
	21st to end	Night	17% 12% 31% 32% 8%	836
April	1st to 10th	Night	15% 13% 28% 34% 10%	703
	11th to 20th	Night	14% 10% 30% 31% 13% 1	563
	21st to end	Night	14% 8% 28% 38% 12% 1	570
May	1st to 10th	Night	12% 7% 26% 35% 18% 2	570
	11th to 20th	Night	13% 4 21% 33% 26% 3	561
	21st to end	Night	8% 2 14% 34% 35% 7%	617
June	1st to 10th	Night	9% 3 14% 38% 32% 5%	570
	11th to 20th	Night	4 2 12% 36% 38% 9%	570
	21st to end	Night	8% 1 11% 36% 38% 6%	570
July	1st to 10th	Night	11% 3 8% 26% 41% 11%	555
	11th to 20th	Night	8% 1 7% 28% 44% 12%	552
	21st to end	Night	8% 3 9% 27% 42% 11%	624
August	1st to 10th	Night	8% 3 6% 29% 45% 8%	567
	11th to 20th	Night	12% 2 8% 33% 40% 5%	564
	21st to end	Night	10% 2 7% 27% 44% 10%	627
September	1st to 10th	Night	10% 3 10% 28% 42% 6%	720
	11th to 20th	Night	12% 3 12% 29% 38% 7%	760
	21st to end	Night	13% 3 18% 32% 30% 3	760
October	1st to 10th	Night	12% 6% 16% 28% 31% 7%	760
	11th to 20th	Night	15% 7% 25% 32% 18% 3	760
	21st to end	Night	16% 7% 21% 37% 18% 2	836
November	1st to 10th	Night	13% 9% 19% 36% 19% 3	780
	11th to 20th	Night	14% 13% 27% 34% 11% 1	800
	21st to end	Night	16% 12% 26% 34% 12%	800
December	1st to 10th	Night	22% 15% 27% 29% 7%	800
	11th to 20th	Night	20% 12% 29% 31% 7% 1	800
	21st to end	Night	22% 15% 25% 32% 7%	851

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