Mt. Norway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mt. Norway Observatory clear sky chart from 2005-11-06 to 2024-11-04.

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	24% 11% 3 6% 4 3 3 3 4 5% 35%	105437
	Night	27% 12% 2 5% 3 3 3 2 3 5% 34%	102263
Transparency	Day	52% 6% 10% 16% 12% 5%	105180
	Night	54% 6% 10% 16% 11% 4	102109
Seeing	Day	30% 11% 23% 23% 10% 3	14597
	Night	34% 7% 17% 24% 16% 2	23528

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	41% 24% 3 6% 3 3 3 2 3 3 9%	2539
	11th to 20th	Day	41% 17% 2 5% 3 3 3 2 2 3 18%	2540
	21st to end	Day	46% 14% 3 7% 3 3 3 2 3 4 13%	2997
February	1st to 10th	Day	38% 13% 3 7% 5% 3 4 3 4 5% 16%	1710
	11th to 20th	Day	34% 18% 3 8% 4 4 3 2 2 3 19%	1665
	21st to end	Day	34% 20% 3 6% 4 2 2 3 3 5% 19%	1505
March	1st to 10th	Day	25% 20% 5% 9% 5% 5% 4 2 4 5% 16%	3248
	11th to 20th	Day	25% 14% 3 6% 4 3 2 2 2 3 37%	3490
	21st to end	Day	33% 19% 4 8% 5% 4 3 3 3 4 14%	3938
April	1st to 10th	Day	34% 17% 3 8% 6% 3 3 3 3 4 16%	2280
	11th to 20th	Day	30% 16% 4 7% 5% 4 3 2 3 4 23%	2249
	21st to end	Day	29% 13% 3 7% 4 4 4 3 4 5% 22%	2441
May	1st to 10th	Day	24% 14% 3 7% 4 4 4 2 3 4 30%	4406
	11th to 20th	Day	15% 11% 3 7% 5% 4 4 4 5% 6% 36%	4377
	21st to end	Day	30% 11% 3 7% 5% 5% 4 3 4 5% 21%	4813
June	1st to 10th	Day	28% 10% 2 6% 5% 4 4 3 4 6% 28%	2660
	11th to 20th	Day	26% 12% 3 7% 5% 4 4 4 5% 5% 24%	2660
	21st to end	Day	12% 7% 2 6% 4 3 4 3 5% 7% 46%	2660
July	1st to 10th	Day	7% 4 1 3 2 2 3 2 4 5% 66%	4420
	11th to 20th	Day	6% 3 1 2 2 2 2 2 3 4 73%	4420
	21st to end	Day	8% 4 1 3 3 2 3 3 4 6% 65%	4790
August	1st to 10th	Day	6% 4 1 4 2 3 4 3 3 5% 63%	4190
	11th to 20th	Day	7% 5% 2 5% 4 4 4 4 6% 7% 52%	4026
	21st to end	Day	12% 6% 2 5% 3 3 3 3 5% 6% 51%	4277
September	1st to 10th	Day	12% 3 1 4 3 2 3 3 4 6% 60%	2280
	11th to 20th	Day	17% 8% 3 6% 3 4 4 3 4 6% 42%	2280
	21st to end	Day	19% 7% 2 6% 4 3 4 2 4 6% 43%	2009
October	1st to 10th	Day	21% 6% 2 4 3 2 3 3 4 5% 48%	3180
	11th to 20th	Day	24% 7% 2 7% 4 4 4 4 6% 7% 32%	3161
	21st to end	Day	37% 9% 3 6% 3 3 3 3 4 5% 23%	3328
November	1st to 10th	Day	39% 12% 3 8% 4 3 4 3 4 5% 16%	1616
	11th to 20th	Day	41% 11% 3 6% 3 3 3 2 5% 4 19%	1520
	21st to end	Day	35% 13% 3 6% 4 3 3 3 3 3 23%	1520
December	1st to 10th	Day	32% 14% 3 6% 5% 3 3 2 4 4 25%	1520
	11th to 20th	Day	39% 13% 3 6% 3 3 3 3 4 5% 18%	1928
	21st to end	Day	35% 16% 3 9% 4 4 4 3 4 3 16%	2794
January	1st to 10th	Night	42% 20% 2 6% 3 3 3 2 3 3 11%	4406
	11th to 20th	Night	41% 18% 3 5% 3 2 2 2 3 4 17%	4420
	21st to end	Night	45% 13% 3 6% 3 2 3 2 4 4 15%	4662
February	1st to 10th	Night	35% 16% 3 6% 4 3 3 3 4 4 19%	2470
	11th to 20th	Night	35% 15% 3 6% 3 3 3 2 3 3 23%	2402
	21st to end	Night	33% 19% 3 6% 4 2 3 2 3 5% 19%	1947
March	1st to 10th	Night	24% 23% 3 7% 4 4 4 3 4 4 18%	3622
	11th to 20th	Night	26% 13% 2 4 3 2 2 2 3 4 39%	3490
	21st to end	Night	36% 21% 4 7% 4 3 3 3 3 3 13%	3715
April	1st to 10th	Night	32% 17% 3 7% 4 4 4 2 3 3 20%	1900
	11th to 20th	Night	26% 15% 2 6% 5% 3 3 2 4 5% 28%	1879
	21st to end	Night	30% 14% 3 6% 3 4 3 3 3 6% 25%	1736
May	1st to 10th	Night	29% 14% 2 5% 3 2 3 2 3 3 35%	2554
	11th to 20th	Night	15% 7% 2 5% 4 5% 5% 3 4 5% 46%	2516
	21st to end	Night	25% 10% 2 7% 5% 4 4 4 4 6% 30%	2762
June	1st to 10th	Night	25% 8% 2 6% 5% 3 4 2 5% 6% 35%	1520
	11th to 20th	Night	22% 11% 3 7% 4 4 3 3 4 5% 34%	1520
	21st to end	Night	12% 6% 2 4 3 3 3 3 3 6% 54%	1520
July	1st to 10th	Night	5% 3 1 2 2 2 2 1 3 3 77%	2540
	11th to 20th	Night	5% 2 1 3 2 2 2 2 3 4 76%	2540
	21st to end	Night	6% 2 1 2 1 1 2 2 4 5% 75%	2761
August	1st to 10th	Night	4 3 1 3 2 2 2 2 3 6% 73%	2742
	11th to 20th	Night	9% 5% 2 4 4 3 3 3 5% 6% 55%	2812
	21st to end	Night	14% 5% 1 4 3 3 2 2 3 5% 59%	3397
September	1st to 10th	Night	10% 3 1 3 3 3 3 2 4 5% 64%	1900
	11th to 20th	Night	18% 7% 2 6% 4 3 4 2 3 5% 47%	1900
	21st to end	Night	17% 7% 2 5% 3 3 4 3 4 6% 47%	2169
October	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 3 7% 49%	3800
	11th to 20th	Night	23% 7% 2 6% 4 4 4 3 5% 7% 34%	3777
	21st to end	Night	37% 11% 3 6% 3 3 3 2 3 5% 24%	4236
November	1st to 10th	Night	39% 12% 3 6% 4 2 3 3 3 4 20%	2530
	11th to 20th	Night	42% 11% 3 4 4 3 3 2 3 4 20%	2660
	21st to end	Night	34% 12% 3 4 4 4 2 2 3 5% 26%	2660
December	1st to 10th	Night	34% 12% 3 6% 3 3 3 2 4 5% 25%	2660
	11th to 20th	Night	36% 14% 3 7% 3 3 3 2 4 5% 18%	3276
	21st to end	Night	35% 17% 3 8% 5% 4 4 2 3 3 16%	4862

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	82% 4 5% 6% 3 1	2539
	11th to 20th	Day	72% 4 6% 10% 5% 3	2540
	21st to end	Day	77% 3 5% 8% 5% 1	2997
February	1st to 10th	Day	71% 5% 9% 9% 6%	1710
	11th to 20th	Day	73% 4 6% 11% 4 2	1674
	21st to end	Day	71% 5% 8% 7% 7% 2	1505
March	1st to 10th	Day	68% 5% 10% 12% 4	3248
	11th to 20th	Day	55% 4 6% 25% 9%	3490
	21st to end	Day	74% 7% 9% 7% 3	3938
April	1st to 10th	Day	72% 5% 11% 9% 3	2280
	11th to 20th	Day	67% 5% 8% 10% 8% 2	2252
	21st to end	Day	65% 7% 9% 11% 6% 2	2441
May	1st to 10th	Day	56% 6% 11% 15% 12%	4406
	11th to 20th	Day	45% 8% 19% 19% 8% 1	4380
	21st to end	Day	63% 11% 14% 11% 1	4814
June	1st to 10th	Day	57% 8% 13% 16% 6%	2660
	11th to 20th	Day	60% 8% 15% 12% 5%	2660
	21st to end	Day	37% 8% 15% 26% 10% 4	2660
July	1st to 10th	Day	22% 5% 9% 17% 21% 28%	4351
	11th to 20th	Day	17% 4 6% 18% 37% 18%	4345
	21st to end	Day	22% 5% 10% 26% 32% 5%	4639
August	1st to 10th	Day	23% 5% 9% 25% 26% 12%	4174
	11th to 20th	Day	32% 7% 16% 29% 11% 5%	4065
	21st to end	Day	34% 6% 10% 23% 15% 12%	4276
September	1st to 10th	Day	28% 5% 11% 24% 19% 13%	2280
	11th to 20th	Day	44% 6% 11% 16% 14% 9%	2280
	21st to end	Day	42% 6% 13% 18% 14% 6%	2009
October	1st to 10th	Day	40% 4 6% 22% 25% 3	3180
	11th to 20th	Day	51% 6% 13% 15% 12% 2	3161
	21st to end	Day	62% 4 8% 16% 9% 1	3328
November	1st to 10th	Day	71% 5% 7% 11% 4 1	1616
	11th to 20th	Day	68% 6% 9% 9% 6% 2	1520
	21st to end	Day	67% 4 6% 8% 13% 2	1520
December	1st to 10th	Day	66% 5% 7% 11% 10% 2	1520
	11th to 20th	Day	69% 4 13% 10% 4	1928
	21st to end	Day	74% 5% 9% 9% 3	2794
January	1st to 10th	Night	80% 3 6% 7% 3 1	4406
	11th to 20th	Night	72% 5% 6% 11% 4 2	4420
	21st to end	Night	75% 4 6% 9% 6% 1	4662
February	1st to 10th	Night	70% 6% 8% 9% 6% 1	2470
	11th to 20th	Night	66% 6% 8% 11% 6% 2	2404
	21st to end	Night	69% 4 8% 10% 7% 1	1947
March	1st to 10th	Night	67% 6% 10% 13% 3	3622
	11th to 20th	Night	50% 5% 10% 21% 12% 1	3490
	21st to end	Night	76% 6% 8% 8% 2	3715
April	1st to 10th	Night	69% 6% 11% 10% 5%	1900
	11th to 20th	Night	59% 8% 10% 12% 8% 3	1879
	21st to end	Night	62% 6% 12% 13% 5% 3	1736
May	1st to 10th	Night	55% 6% 9% 20% 9% 1	2554
	11th to 20th	Night	37% 9% 21% 25% 7% 2	2516
	21st to end	Night	54% 13% 20% 10% 3	2762
June	1st to 10th	Night	51% 8% 14% 20% 7%	1520
	11th to 20th	Night	52% 7% 17% 19% 5%	1520
	21st to end	Night	32% 6% 18% 30% 10% 5%	1520
July	1st to 10th	Night	14% 4 8% 23% 24% 26%	2500
	11th to 20th	Night	16% 3 8% 21% 35% 18%	2492
	21st to end	Night	12% 4 13% 29% 37% 6%	2674
August	1st to 10th	Night	15% 4 9% 33% 30% 9%	2734
	11th to 20th	Night	27% 10% 21% 27% 10% 6%	2839
	21st to end	Night	28% 6% 14% 26% 14% 12%	3397
September	1st to 10th	Night	23% 6% 13% 28% 19% 12%	1900
	11th to 20th	Night	40% 6% 12% 20% 11% 11%	1900
	21st to end	Night	38% 7% 13% 18% 15% 8%	2169
October	1st to 10th	Night	34% 5% 9% 25% 24% 3	3800
	11th to 20th	Night	49% 6% 13% 16% 14% 2	3777
	21st to end	Night	64% 4 7% 14% 9% 2	4236
November	1st to 10th	Night	68% 5% 6% 13% 7% 1	2530
	11th to 20th	Night	69% 5% 7% 9% 8% 2	2660
	21st to end	Night	62% 4 6% 11% 14% 3	2660
December	1st to 10th	Night	63% 5% 10% 10% 11% 1	2660
	11th to 20th	Night	69% 5% 9% 10% 7%	3276
	21st to end	Night	74% 5% 10% 8% 3	4862

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	54% 12% 19% 10% 4	180
	11th to 20th	Day	52% 11% 22% 10% 4	180
	21st to end	Day	51% 10% 17% 16% 6%	323
February	1st to 10th	Day	48% 12% 19% 17% 4 1	370
	11th to 20th	Day	48% 14% 19% 14% 4 1	365
	21st to end	Day	49% 13% 22% 14% 3	352
March	1st to 10th	Day	45% 13% 24% 13% 5%	460
	11th to 20th	Day	36% 15% 26% 15% 6% 2	460
	21st to end	Day	45% 16% 17% 17% 5% 1	506
April	1st to 10th	Day	44% 13% 21% 13% 7% 2	460
	11th to 20th	Day	36% 21% 21% 14% 5% 2	452
	21st to end	Day	34% 16% 25% 18% 6% 1	460
May	1st to 10th	Day	25% 18% 25% 22% 8% 2	463
	11th to 20th	Day	33% 14% 21% 21% 7% 4	488
	21st to end	Day	32% 10% 21% 22% 11% 4	529
June	1st to 10th	Day	29% 9% 20% 23% 14% 3	490
	11th to 20th	Day	27% 14% 29% 19% 9% 2	490
	21st to end	Day	11% 10% 31% 29% 15% 4	490
July	1st to 10th	Day	14% 7% 26% 36% 13% 4	486
	11th to 20th	Day	8% 7% 27% 36% 17% 5%	484
	21st to end	Day	7% 5% 22% 41% 19% 5%	531
August	1st to 10th	Day	9% 5% 21% 39% 18% 8%	487
	11th to 20th	Day	6% 5% 23% 39% 21% 6%	477
	21st to end	Day	11% 11% 27% 34% 14% 4	539
September	1st to 10th	Day	14% 4 30% 34% 14% 3	490
	11th to 20th	Day	20% 8% 24% 28% 18% 1	490
	21st to end	Day	21% 11% 26% 30% 10% 2	490
October	1st to 10th	Day	26% 8% 26% 27% 12% 2	490
	11th to 20th	Day	31% 10% 22% 25% 10% 3	490
	21st to end	Day	40% 13% 21% 16% 7% 2	468
November	1st to 10th	Day	46% 16% 17% 15% 5% 1	239
	11th to 20th	Day	46% 14% 20% 14% 6% 1	180
	21st to end	Day	45% 12% 23% 18% 2	180
December	1st to 10th	Day	44% 16% 19% 15% 7%	180
	11th to 20th	Day	59% 12% 18% 9% 2	180
	21st to end	Day	54% 10% 21% 12% 5%	198
January	1st to 10th	Night	50% 12% 21% 13% 3 1	760
	11th to 20th	Night	49% 9% 21% 16% 5%	760
	21st to end	Night	44% 8% 23% 17% 7%	836
February	1st to 10th	Night	44% 9% 22% 19% 6% 1	760
	11th to 20th	Night	46% 9% 17% 21% 7%	744
	21st to end	Night	48% 10% 17% 19% 6%	628
March	1st to 10th	Night	40% 10% 16% 22% 10% 1	760
	11th to 20th	Night	39% 9% 20% 22% 10%	760
	21st to end	Night	47% 7% 14% 21% 8% 2	836
April	1st to 10th	Night	42% 9% 14% 23% 11% 1	760
	11th to 20th	Night	33% 10% 20% 21% 13% 3	751
	21st to end	Night	37% 9% 16% 26% 13% 1	596
May	1st to 10th	Night	26% 7% 18% 27% 20% 2	394
	11th to 20th	Night	24% 7% 13% 28% 25% 3	374
	21st to end	Night	25% 5% 16% 26% 25% 3	411
June	1st to 10th	Night	28% 5% 14% 28% 22% 2	380
	11th to 20th	Night	25% 7% 19% 28% 20% 1	380
	21st to end	Night	12% 6% 16% 33% 32% 1	380
July	1st to 10th	Night	9% 3 8% 36% 42% 2	370
	11th to 20th	Night	8% 2 11% 43% 34% 3	368
	21st to end	Night	3 3 8% 40% 41% 5%	416
August	1st to 10th	Night	7% 1 9% 29% 48% 6%	500
	11th to 20th	Night	7% 2 7% 32% 43% 9%	564
	21st to end	Night	10% 4 13% 39% 29% 4	774
September	1st to 10th	Night	12% 3 16% 34% 31% 4	760
	11th to 20th	Night	19% 3 13% 30% 29% 5%	760
	21st to end	Night	20% 4 16% 33% 24% 3	760
October	1st to 10th	Night	25% 4 17% 30% 20% 3	760
	11th to 20th	Night	31% 6% 14% 31% 16% 3	760
	21st to end	Night	39% 7% 20% 23% 9% 2	836
November	1st to 10th	Night	46% 7% 19% 20% 8%	754
	11th to 20th	Night	49% 9% 21% 16% 4	760
	21st to end	Night	41% 9% 24% 22% 4	760
December	1st to 10th	Night	42% 10% 24% 17% 6% 1	760
	11th to 20th	Night	51% 9% 19% 18% 3	760
	21st to end	Night	53% 12% 17% 15% 3	836

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