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 2025-07-21.

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% 34%	124431
	Night	27% 12% 2 6% 3 3 3 2 3 5% 33%	123503
Transparency	Day	52% 6% 10% 16% 12% 4	124175
	Night	55% 5% 10% 16% 11% 3	123346
Seeing	Day	30% 11% 23% 23% 10% 2	15442
	Night	34% 7% 17% 24% 15% 2	24392

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	44% 20% 3 6% 3 3 2 2 3 3 12%	3639
	11th to 20th	Day	29% 16% 2 5% 3 3 3 2 3 4 29%	3640
	21st to end	Day	34% 10% 2 5% 3 3 3 2 3 4 31%	4305
February	1st to 10th	Day	36% 14% 3 7% 5% 3 4 3 4 5% 16%	1800
	11th to 20th	Day	34% 18% 3 8% 4 4 3 2 2 3 20%	1755
	21st to end	Day	34% 20% 3 6% 4 2 2 3 3 4 19%	1582
March	1st to 10th	Day	22% 17% 5% 10% 5% 5% 4 3 5% 6% 19%	4680
	11th to 20th	Day	31% 16% 4 7% 4 3 2 2 2 2 26%	5000
	21st to end	Day	33% 16% 4 9% 5% 5% 4 3 4 5% 12%	5651
April	1st to 10th	Day	34% 16% 3 8% 6% 4 3 3 4 4 16%	2400
	11th to 20th	Day	28% 15% 3 7% 4 4 3 2 3 4 25%	2369
	21st to end	Day	28% 13% 4 7% 4 4 4 3 4 5% 23%	2570
May	1st to 10th	Day	18% 11% 2 7% 4 4 4 3 4 5% 38%	6293
	11th to 20th	Day	22% 12% 4 9% 6% 4 4 3 4 5% 25%	6277
	21st to end	Day	24% 9% 2 6% 4 4 5% 3 5% 7% 30%	6902
June	1st to 10th	Day	27% 10% 2 5% 5% 4 4 3 4 6% 30%	2800
	11th to 20th	Day	25% 12% 3 8% 5% 4 4 4 6% 6% 25%	2800
	21st to end	Day	12% 8% 3 6% 4 3 4 3 5% 7% 46%	2800
July	1st to 10th	Day	7% 4 1 3 2 2 3 2 4 5% 66%	4560
	11th to 20th	Day	6% 3 1 2 2 2 2 2 3 4 73%	4560
	21st to end	Day	8% 4 1 3 3 2 3 3 4 6% 65%	4791
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% 11% 3 8% 4 3 3 3 4 5% 16%	1674
	11th to 20th	Day	41% 11% 3 6% 3 3 3 2 4 4 18%	1600
	21st to end	Day	34% 14% 3 6% 4 3 3 3 3 4 23%	1600
December	1st to 10th	Day	26% 10% 2 4 3 3 3 2 4 4 37%	2620
	11th to 20th	Day	41% 14% 4 8% 4 3 3 3 4 4 12%	3028
	21st to end	Day	40% 16% 4 9% 4 3 3 3 3 3 12%	4004
January	1st to 10th	Night	43% 17% 2 6% 3 3 3 2 3 3 13%	6306
	11th to 20th	Night	29% 18% 3 5% 2 2 2 2 3 4 30%	6320
	21st to end	Night	33% 10% 2 5% 3 2 2 2 3 4 33%	6661
February	1st to 10th	Night	34% 17% 3 7% 4 3 3 3 4 5% 18%	2600
	11th to 20th	Night	35% 16% 3 6% 3 3 3 2 3 3 23%	2532
	21st to end	Night	34% 19% 3 5% 4 2 3 2 3 5% 20%	2046
March	1st to 10th	Night	24% 20% 3 8% 4 4 5% 3 5% 5% 20%	5210
	11th to 20th	Night	32% 17% 3 6% 3 2 2 2 2 3 28%	5000
	21st to end	Night	38% 17% 3 7% 5% 4 3 3 4 4 12%	5324
April	1st to 10th	Night	32% 17% 3 8% 4 4 4 2 3 3 20%	2000
	11th to 20th	Night	25% 14% 2 6% 5% 2 3 3 4 5% 30%	1979
	21st to end	Night	29% 13% 3 6% 3 4 3 3 3 6% 26%	1827
May	1st to 10th	Night	23% 11% 2 4 3 2 3 2 3 5% 44%	3668
	11th to 20th	Night	20% 10% 4 8% 5% 5% 4 3 4 4 34%	3616
	21st to end	Night	21% 8% 2 6% 4 4 4 4 5% 7% 33%	3972
June	1st to 10th	Night	24% 8% 2 5% 5% 3 4 2 5% 6% 37%	1600
	11th to 20th	Night	21% 11% 3 6% 4 4 3 3 4 5% 35%	1600
	21st to end	Night	12% 6% 2 4 3 3 3 3 3 5% 54%	1600
July	1st to 10th	Night	5% 3 1 2 2 2 2 1 3 3 76%	2620
	11th to 20th	Night	5% 2 1 3 2 2 2 1 3 3 77%	2620
	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	38% 12% 3 6% 4 3 3 3 3 4 20%	2620
	11th to 20th	Night	44% 11% 3 4 4 3 3 2 3 4 19%	2800
	21st to end	Night	34% 12% 3 4 4 4 2 2 3 5% 26%	2800
December	1st to 10th	Night	25% 10% 2 5% 3 3 2 2 4 6% 38%	4560
	11th to 20th	Night	42% 14% 4 8% 4 3 3 2 3 4 12%	5176
	21st to end	Night	41% 17% 3 8% 5% 4 3 2 2 3 12%	6952

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	80% 3 5% 7% 5%	3639
	11th to 20th	Day	59% 4 8% 18% 9% 2	3640
	21st to end	Day	59% 3 5% 13% 16% 4	4305
February	1st to 10th	Day	71% 5% 9% 9% 6%	1800
	11th to 20th	Day	73% 4 6% 11% 4 2	1764
	21st to end	Day	71% 4 8% 7% 8% 2	1582
March	1st to 10th	Day	64% 6% 13% 12% 6%	4680
	11th to 20th	Day	66% 4 6% 18% 6%	5000
	21st to end	Day	75% 7% 9% 6% 2	5651
April	1st to 10th	Day	72% 5% 11% 10% 3	2400
	11th to 20th	Day	65% 5% 8% 12% 9% 2	2372
	21st to end	Day	64% 7% 9% 12% 7% 1	2570
May	1st to 10th	Day	47% 5% 10% 21% 16%	6293
	11th to 20th	Day	57% 8% 15% 14% 5% 1	6280
	21st to end	Day	53% 10% 18% 16% 3	6904
June	1st to 10th	Day	55% 8% 13% 17% 7%	2800
	11th to 20th	Day	59% 8% 15% 13% 6%	2800
	21st to end	Day	38% 8% 15% 25% 10% 5%	2800
July	1st to 10th	Day	21% 5% 9% 17% 20% 27%	4491
	11th to 20th	Day	17% 4 6% 19% 37% 18%	4485
	21st to end	Day	22% 5% 10% 26% 32% 5%	4640
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% 12% 4 1	1674
	11th to 20th	Day	69% 6% 9% 9% 6% 2	1600
	21st to end	Day	67% 5% 6% 8% 13% 2	1600
December	1st to 10th	Day	52% 4 7% 13% 20% 3	2620
	11th to 20th	Day	77% 4 9% 7% 2	3028
	21st to end	Day	79% 4 7% 8% 2	4004
January	1st to 10th	Night	78% 3 6% 8% 5% 1	6306
	11th to 20th	Night	60% 4 6% 19% 9% 2	6320
	21st to end	Night	57% 3 5% 14% 16% 6%	6661
February	1st to 10th	Night	70% 7% 8% 9% 6% 1	2600
	11th to 20th	Night	66% 6% 8% 11% 6% 3	2534
	21st to end	Night	68% 4 9% 10% 7% 1	2046
March	1st to 10th	Night	64% 7% 11% 13% 5% 1	5210
	11th to 20th	Night	63% 4 8% 15% 9% 1	5000
	21st to end	Night	76% 5% 10% 8% 1	5324
April	1st to 10th	Night	69% 6% 10% 10% 4	2000
	11th to 20th	Night	57% 8% 10% 13% 9% 3	1979
	21st to end	Night	60% 6% 12% 13% 5% 3	1827
May	1st to 10th	Night	44% 5% 8% 26% 16% 1	3665
	11th to 20th	Night	48% 9% 19% 18% 5% 1	3616
	21st to end	Night	47% 12% 21% 15% 4	3972
June	1st to 10th	Night	49% 7% 14% 21% 8%	1600
	11th to 20th	Night	50% 7% 17% 21% 5%	1600
	21st to end	Night	33% 6% 18% 29% 10% 5%	1600
July	1st to 10th	Night	15% 4 8% 24% 24% 25%	2580
	11th to 20th	Night	15% 3 8% 22% 35% 18%	2572
	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	2620
	11th to 20th	Night	70% 5% 7% 8% 8% 2	2800
	21st to end	Night	62% 5% 7% 10% 13% 2	2800
December	1st to 10th	Night	49% 4 8% 14% 23% 1	4560
	11th to 20th	Night	77% 4 7% 7% 4	5176
	21st to end	Night	79% 4 8% 6% 2	6952

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	55% 14% 19% 10% 4	200
	11th to 20th	Day	48% 12% 25% 10% 6%	200
	21st to end	Day	49% 10% 17% 18% 6%	352
February	1st to 10th	Day	47% 13% 19% 17% 4 1	400
	11th to 20th	Day	49% 15% 19% 13% 4 1	395
	21st to end	Day	49% 13% 22% 14% 3	381
March	1st to 10th	Day	43% 13% 25% 14% 5%	500
	11th to 20th	Day	39% 15% 25% 14% 6% 2	500
	21st to end	Day	45% 17% 17% 16% 4 1	550
April	1st to 10th	Day	44% 13% 21% 14% 7% 2	500
	11th to 20th	Day	33% 20% 23% 15% 5% 2	492
	21st to end	Day	32% 15% 25% 21% 6% 1	500
May	1st to 10th	Day	23% 17% 26% 22% 8% 2	503
	11th to 20th	Day	34% 15% 20% 20% 7% 4	528
	21st to end	Day	30% 10% 22% 23% 12% 4	573
June	1st to 10th	Day	28% 9% 22% 23% 15% 4	530
	11th to 20th	Day	25% 14% 30% 20% 8% 2	530
	21st to end	Day	12% 9% 31% 29% 15% 4	530
July	1st to 10th	Day	13% 6% 26% 37% 13% 4	526
	11th to 20th	Day	8% 7% 27% 36% 17% 5%	524
	21st to end	Day	7% 5% 22% 41% 19% 5%	532
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% 16% 4 1	255
	11th to 20th	Day	49% 13% 21% 13% 5% 1	200
	21st to end	Day	43% 13% 22% 19% 4	200
December	1st to 10th	Day	41% 16% 22% 15% 6%	200
	11th to 20th	Day	61% 12% 18% 9% 2	200
	21st to end	Day	57% 9% 20% 10% 4	220
January	1st to 10th	Night	50% 13% 21% 12% 3 1	800
	11th to 20th	Night	47% 9% 21% 17% 5%	800
	21st to end	Night	43% 9% 24% 18% 6%	880
February	1st to 10th	Night	44% 9% 21% 19% 6% 1	800
	11th to 20th	Night	47% 10% 17% 20% 6%	784
	21st to end	Night	48% 9% 18% 19% 6%	660
March	1st to 10th	Night	40% 10% 17% 23% 10% 1	800
	11th to 20th	Night	40% 10% 20% 21% 9%	800
	21st to end	Night	48% 7% 15% 21% 8% 2	880
April	1st to 10th	Night	41% 9% 14% 24% 11% 1	800
	11th to 20th	Night	32% 10% 21% 23% 13% 2	791
	21st to end	Night	35% 9% 15% 26% 14%	627
May	1st to 10th	Night	26% 7% 17% 29% 20% 2	415
	11th to 20th	Night	25% 7% 14% 27% 24% 3	394
	21st to end	Night	25% 5% 16% 26% 25% 3	433
June	1st to 10th	Night	28% 5% 15% 27% 24% 2	400
	11th to 20th	Night	24% 7% 20% 29% 19% 1	400
	21st to end	Night	13% 6% 15% 33% 32% 1	400
July	1st to 10th	Night	9% 3 10% 35% 41% 2	390
	11th to 20th	Night	7% 2 12% 43% 34% 3	388
	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	45% 7% 20% 20% 7%	780
	11th to 20th	Night	51% 9% 20% 16% 4	800
	21st to end	Night	42% 9% 23% 22% 4	800
December	1st to 10th	Night	40% 10% 26% 18% 6% 1	800
	11th to 20th	Night	52% 10% 19% 17% 3	800
	21st to end	Night	54% 12% 16% 15% 3	880

Page generated at 2025-07-17 22:09:58. Version ac07

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