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 2026-04-17.

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	12% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	131565
	Night	12% 8% 3 7% 5% 4 5% 4 6% 7% 40%	130623
Transparency	Day	42% 7% 12% 22% 15% 3	135303
	Night	41% 7% 11% 22% 16% 3	134954
Seeing	Day	16% 11% 23% 23% 18% 9%	16691
	Night	15% 9% 22% 31% 19% 3	26809

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 5% 4 6% 5% 31%	5258
	11th to 20th	Day	11% 7% 5% 10% 5% 5% 5% 3 5% 5% 40%	5280
	21st to end	Day	16% 10% 4 8% 4 4 4 3 5% 7% 34%	5780
February	1st to 10th	Day	17% 9% 4 8% 5% 5% 4 3 5% 5% 35%	2074
	11th to 20th	Day	19% 9% 4 9% 4 4 4 3 4 5% 33%	2048
	21st to end	Day	17% 9% 3 8% 4 5% 4 3 5% 5% 36%	1730
March	1st to 10th	Day	17% 11% 3 8% 5% 4 4 4 4 6% 34%	4716
	11th to 20th	Day	10% 7% 2 6% 4 4 5% 4 5% 7% 45%	5090
	21st to end	Day	13% 8% 3 6% 4 4 4 3 5% 6% 44%	5586
April	1st to 10th	Day	10% 8% 3 6% 4 5% 5% 4 6% 8% 41%	2389
	11th to 20th	Day	12% 8% 2 6% 3 4 4 4 6% 6% 44%	2471
	21st to end	Day	13% 6% 3 7% 5% 4 4 4 6% 7% 41%	2600
May	1st to 10th	Day	14% 9% 4 8% 6% 5% 6% 5% 7% 8% 27%	5862
	11th to 20th	Day	13% 8% 4 9% 6% 6% 6% 5% 6% 8% 30%	5861
	21st to end	Day	13% 8% 3 8% 5% 5% 5% 4 6% 8% 37%	6433
June	1st to 10th	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 9% 38%	2600
	11th to 20th	Day	5% 5% 2 7% 6% 6% 6% 6% 9% 10% 39%	2600
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 9% 11% 39%	2600
July	1st to 10th	Day	5% 5% 2 7% 5% 6% 6% 5% 9% 10% 40%	5900
	11th to 20th	Day	7% 6% 3 8% 6% 7% 7% 6% 9% 11% 30%	5900
	21st to end	Day	11% 9% 4 10% 6% 7% 6% 6% 7% 9% 25%	6424
August	1st to 10th	Day	6% 5% 2 7% 5% 6% 7% 6% 9% 10% 37%	4234
	11th to 20th	Day	6% 5% 2 6% 4 5% 6% 6% 9% 11% 40%	4108
	21st to end	Day	5% 4 2 5% 5% 4 4 4 6% 9% 53%	4312
September	1st to 10th	Day	11% 6% 3 6% 5% 4 5% 3 5% 7% 44%	2220
	11th to 20th	Day	13% 5% 2 6% 3 4 4 4 6% 9% 44%	2200
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 46%	2200
October	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 4 7% 53%	2200
	11th to 20th	Day	12% 5% 2 5% 3 3 3 3 4 5% 54%	2044
	21st to end	Day	15% 6% 1 6% 3 3 4 3 6% 6% 47%	2165
November	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 6% 7% 52%	1852
	11th to 20th	Day	11% 6% 2 5% 5% 3 4 4 5% 7% 48%	1890
	21st to end	Day	13% 8% 4 6% 5% 5% 4 3 4 5% 43%	1890
December	1st to 10th	Day	12% 9% 2 6% 5% 4 5% 5% 5% 6% 41%	3020
	11th to 20th	Day	14% 15% 5% 10% 6% 5% 5% 3 5% 6% 28%	3463
	21st to end	Day	15% 11% 4 8% 5% 5% 5% 4 5% 8% 29%	4565
January	1st to 10th	Night	14% 9% 4 9% 5% 4 4 4 5% 5% 36%	7635
	11th to 20th	Night	12% 9% 3 8% 6% 5% 4 4 5% 5% 40%	7680
	21st to end	Night	14% 11% 4 8% 5% 4 5% 4 5% 5% 35%	8401
February	1st to 10th	Night	13% 10% 4 6% 6% 5% 5% 4 6% 6% 34%	2543
	11th to 20th	Night	21% 8% 4 8% 5% 5% 4 3 5% 5% 32%	2458
	21st to end	Night	19% 9% 3 8% 5% 4 5% 4 6% 5% 33%	2076
March	1st to 10th	Night	16% 10% 3 8% 5% 5% 5% 3 5% 7% 34%	5356
	11th to 20th	Night	9% 5% 2 5% 4 3 4 4 6% 7% 51%	5090
	21st to end	Night	12% 6% 2 7% 4 5% 5% 4 6% 7% 41%	5588
April	1st to 10th	Night	12% 5% 2 5% 3 4 5% 4 7% 7% 47%	2247
	11th to 20th	Night	11% 7% 2 5% 4 4 4 3 5% 8% 48%	2009
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 6% 7% 44%	1800
May	1st to 10th	Night	11% 8% 4 9% 5% 5% 5% 5% 8% 10% 29%	4033
	11th to 20th	Night	14% 10% 3 9% 5% 5% 6% 5% 6% 7% 30%	4032
	21st to end	Night	12% 8% 3 7% 4 5% 5% 4 6% 8% 39%	4421
June	1st to 10th	Night	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 40%	1800
	11th to 20th	Night	4 4 2 5% 4 4 5% 6% 7% 11% 48%	1800
	21st to end	Night	6% 6% 2 6% 5% 5% 6% 4 6% 12% 42%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 6% 5% 8% 12% 39%	4058
	11th to 20th	Night	7% 5% 2 7% 5% 5% 6% 5% 8% 12% 39%	4057
	21st to end	Night	10% 9% 3 9% 6% 6% 6% 5% 8% 12% 25%	4427
August	1st to 10th	Night	5% 4 3 6% 5% 5% 6% 5% 7% 11% 42%	2915
	11th to 20th	Night	6% 5% 2 5% 5% 4 4 4 7% 9% 48%	2936
	21st to end	Night	5% 3 1 3 3 3 5% 3 5% 9% 58%	3586
September	1st to 10th	Night	10% 6% 2 7% 4 4 6% 4 5% 6% 48%	2158
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 4 5% 8% 45%	2200
	21st to end	Night	12% 5% 3 6% 4 4 4 3 5% 6% 49%	2200
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 2 4 6% 55%	2200
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 5% 6% 53%	2355
	21st to end	Night	13% 6% 2 6% 3 3 3 3 4 5% 52%	2671
November	1st to 10th	Night	10% 6% 3 6% 4 4 3 3 5% 5% 52%	2666
	11th to 20th	Night	11% 6% 3 6% 4 3 5% 3 5% 7% 47%	2730
	21st to end	Night	12% 8% 2 7% 4 4 5% 4 5% 5% 43%	2730
December	1st to 10th	Night	13% 9% 3 9% 6% 5% 6% 5% 7% 5% 32%	4380
	11th to 20th	Night	15% 12% 4 10% 5% 4 4 3 5% 5% 33%	4952
	21st to end	Night	15% 10% 3 8% 5% 5% 5% 4 6% 8% 29%	6633

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	51% 6% 9% 23% 11% 1	5259
	11th to 20th	Day	44% 5% 5% 21% 22% 3	5280
	21st to end	Day	48% 5% 8% 25% 12% 1	5780
February	1st to 10th	Day	52% 5% 8% 18% 15% 2	2074
	11th to 20th	Day	53% 4 8% 19% 13% 2	2058
	21st to end	Day	49% 5% 8% 17% 15% 5%	1730
March	1st to 10th	Day	50% 6% 6% 15% 15% 8%	4716
	11th to 20th	Day	38% 5% 7% 21% 23% 6%	5090
	21st to end	Day	42% 4 9% 30% 14% 1	5586
April	1st to 10th	Day	39% 7% 13% 23% 15% 3	2389
	11th to 20th	Day	39% 6% 11% 18% 23% 4	2471
	21st to end	Day	41% 7% 12% 26% 13% 2	2600
May	1st to 10th	Day	51% 8% 12% 19% 9% 1	5897
	11th to 20th	Day	51% 8% 14% 17% 9% 1	5865
	21st to end	Day	44% 7% 12% 20% 14% 3	6447
June	1st to 10th	Day	40% 10% 14% 20% 14% 2	2600
	11th to 20th	Day	34% 12% 17% 21% 14% 2	2600
	21st to end	Day	34% 11% 17% 20% 14% 4	2600
July	1st to 10th	Day	34% 11% 18% 21% 14% 1	5835
	11th to 20th	Day	42% 11% 21% 20% 5% 2	5831
	21st to end	Day	51% 10% 14% 19% 6%	6240
August	1st to 10th	Day	37% 10% 17% 26% 10%	4221
	11th to 20th	Day	33% 9% 19% 25% 14% 1	4147
	21st to end	Day	27% 7% 15% 35% 14% 2	4311
September	1st to 10th	Day	39% 8% 12% 19% 16% 6%	2220
	11th to 20th	Day	36% 8% 11% 24% 17% 3	2200
	21st to end	Day	37% 6% 12% 21% 19% 5%	2200
October	1st to 10th	Day	26% 5% 7% 20% 33% 9%	3590
	11th to 20th	Day	24% 4 9% 23% 30% 10%	3310
	21st to end	Day	27% 4 11% 28% 20% 9%	3479
November	1st to 10th	Day	33% 4 9% 24% 27% 4	1852
	11th to 20th	Day	36% 4 7% 25% 24% 5%	1890
	21st to end	Day	43% 5% 8% 23% 20% 2	1890
December	1st to 10th	Day	44% 5% 7% 21% 21% 1	3020
	11th to 20th	Day	57% 5% 9% 17% 10% 2	3460
	21st to end	Day	52% 5% 14% 20% 8% 1	4565
January	1st to 10th	Night	46% 6% 13% 23% 11% 1	7636
	11th to 20th	Night	44% 6% 5% 21% 20% 4	7680
	21st to end	Night	49% 5% 7% 18% 16% 4	8401
February	1st to 10th	Night	49% 5% 9% 20% 15% 1	2543
	11th to 20th	Night	53% 5% 8% 18% 12% 4	2459
	21st to end	Night	51% 6% 8% 17% 14% 4	2076
March	1st to 10th	Night	48% 5% 8% 18% 16% 5%	5356
	11th to 20th	Night	32% 5% 7% 25% 23% 9%	5090
	21st to end	Night	41% 5% 11% 27% 14% 1	5588
April	1st to 10th	Night	34% 6% 14% 24% 19% 3	2247
	11th to 20th	Night	36% 5% 10% 22% 21% 6%	2009
	21st to end	Night	37% 9% 12% 26% 14% 2	1800
May	1st to 10th	Night	48% 8% 15% 20% 8% 1	4060
	11th to 20th	Night	52% 7% 14% 18% 8%	4033
	21st to end	Night	41% 8% 13% 23% 13% 1	4429
June	1st to 10th	Night	35% 10% 17% 21% 15% 2	1800
	11th to 20th	Night	26% 12% 19% 25% 16% 2	1800
	21st to end	Night	35% 9% 16% 22% 15% 3	1800
July	1st to 10th	Night	36% 10% 20% 21% 11% 1	4015
	11th to 20th	Night	35% 11% 24% 22% 6% 2	4006
	21st to end	Night	49% 11% 18% 17% 5%	4297
August	1st to 10th	Night	34% 9% 18% 28% 10%	2904
	11th to 20th	Night	31% 9% 18% 29% 13% 1	2963
	21st to end	Night	24% 6% 15% 39% 15% 2	3586
September	1st to 10th	Night	36% 8% 15% 18% 18% 6%	2158
	11th to 20th	Night	37% 8% 13% 21% 20% 2	2200
	21st to end	Night	37% 6% 11% 21% 21% 4	2200
October	1st to 10th	Night	29% 4 7% 21% 29% 10%	3590
	11th to 20th	Night	21% 4 7% 25% 31% 12%	3828
	21st to end	Night	24% 4 6% 28% 28% 10%	4307
November	1st to 10th	Night	35% 4 8% 23% 23% 7%	2666
	11th to 20th	Night	37% 4 8% 21% 24% 6%	2730
	21st to end	Night	42% 5% 7% 25% 19% 3	2730
December	1st to 10th	Night	51% 7% 7% 17% 17% 1	4380
	11th to 20th	Night	53% 5% 8% 20% 13% 2	4954
	21st to end	Night	50% 6% 15% 18% 10%	6633

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% 23% 33% 16% 3 1	330
	11th to 20th	Day	25% 28% 28% 15% 3 1	330
	21st to end	Day	20% 25% 37% 15% 3	363
February	1st to 10th	Day	22% 24% 34% 14% 5% 1	330
	11th to 20th	Day	33% 26% 28% 10% 3	330
	21st to end	Day	34% 16% 37% 10% 2	273
March	1st to 10th	Day	23% 19% 34% 18% 5% 1	388
	11th to 20th	Day	22% 22% 28% 21% 6% 1	540
	21st to end	Day	21% 19% 31% 22% 7% 1	594
April	1st to 10th	Day	19% 18% 26% 24% 12% 1	540
	11th to 20th	Day	17% 14% 31% 24% 12% 2	516
	21st to end	Day	18% 12% 29% 26% 13% 2	500
May	1st to 10th	Day	16% 14% 33% 23% 11% 4	503
	11th to 20th	Day	13% 9% 27% 26% 18% 7%	527
	21st to end	Day	15% 7% 17% 27% 20% 13%	573
June	1st to 10th	Day	14% 5% 17% 27% 24% 13%	530
	11th to 20th	Day	6% 4 17% 26% 32% 16%	530
	21st to end	Day	7% 2 21% 29% 25% 17%	530
July	1st to 10th	Day	10% 3 11% 25% 29% 23%	525
	11th to 20th	Day	10% 4 11% 24% 29% 21%	524
	21st to end	Day	13% 3 11% 24% 32% 18%	576
August	1st to 10th	Day	11% 3 13% 22% 32% 18%	526
	11th to 20th	Day	9% 4 13% 32% 28% 14%	519
	21st to end	Day	10% 3 8% 25% 31% 23%	583
September	1st to 10th	Day	11% 4 11% 28% 28% 18%	530
	11th to 20th	Day	13% 4 10% 22% 32% 20%	530
	21st to end	Day	17% 5% 18% 25% 25% 10%	530
October	1st to 10th	Day	11% 6% 18% 26% 28% 12%	530
	11th to 20th	Day	12% 10% 24% 28% 23% 3	530
	21st to end	Day	20% 10% 21% 30% 16% 3	548
November	1st to 10th	Day	15% 11% 32% 22% 16% 3	330
	11th to 20th	Day	14% 16% 33% 27% 8% 2	330
	21st to end	Day	17% 13% 34% 24% 10% 2	330
December	1st to 10th	Day	25% 18% 34% 16% 6% 1	330
	11th to 20th	Day	21% 17% 30% 24% 7% 1	330
	21st to end	Day	22% 18% 31% 20% 9% 1	363
January	1st to 10th	Night	21% 17% 32% 26% 4	840
	11th to 20th	Night	22% 18% 31% 24% 4	840
	21st to end	Night	18% 15% 38% 27% 2	924
February	1st to 10th	Night	21% 14% 33% 29% 4	840
	11th to 20th	Night	26% 17% 34% 21% 1	823
	21st to end	Night	23% 18% 32% 24% 2	692
March	1st to 10th	Night	19% 15% 30% 31% 5%	840
	11th to 20th	Night	18% 13% 31% 30% 7% 1	840
	21st to end	Night	17% 12% 32% 31% 7%	924
April	1st to 10th	Night	15% 13% 28% 33% 10%	777
	11th to 20th	Night	14% 10% 30% 32% 13%	611
	21st to end	Night	14% 8% 28% 38% 11% 1	600
May	1st to 10th	Night	13% 7% 26% 35% 18% 2	600
	11th to 20th	Night	14% 4 21% 34% 25% 3	591
	21st to end	Night	10% 3 15% 33% 33% 6%	650
June	1st to 10th	Night	9% 3 13% 38% 32% 5%	600
	11th to 20th	Night	6% 2 12% 34% 37% 9%	600
	21st to end	Night	7% 1 11% 35% 39% 8%	600
July	1st to 10th	Night	11% 3 7% 26% 41% 11%	585
	11th to 20th	Night	9% 1 7% 27% 43% 13%	582
	21st to end	Night	8% 3 8% 27% 43% 11%	657
August	1st to 10th	Night	9% 4 6% 30% 43% 9%	597
	11th to 20th	Night	11% 2 8% 32% 41% 6%	594
	21st to end	Night	10% 2 7% 28% 43% 10%	660
September	1st to 10th	Night	10% 3 10% 29% 42% 6%	758
	11th to 20th	Night	11% 3 11% 29% 39% 7%	800
	21st to end	Night	14% 3 19% 32% 30% 3	800
October	1st to 10th	Night	12% 6% 15% 28% 32% 7%	800
	11th to 20th	Night	15% 7% 24% 33% 18% 3	800
	21st to end	Night	17% 7% 21% 36% 17% 2	880
November	1st to 10th	Night	14% 9% 19% 37% 18% 3	820
	11th to 20th	Night	14% 13% 26% 34% 11% 1	840
	21st to end	Night	16% 13% 26% 33% 11%	840
December	1st to 10th	Night	22% 15% 27% 29% 6%	840
	11th to 20th	Night	20% 12% 29% 31% 7%	840
	21st to end	Night	22% 14% 26% 32% 6%	924

Page generated at 2026-04-14 02:44:24. Version ac07

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