Norton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Norton clear sky chart from 2005-11-06 to 2025-03-27.

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	8% 5% 2 5% 3 3 3 3 5% 7% 55%	113105
	Night	7% 6% 2 5% 3 3 4 3 5% 7% 55%	114997
Transparency	Day	29% 4 12% 28% 22% 5%	112860
	Night	30% 4 12% 27% 22% 6%	114832
Seeing	Day	11% 8% 31% 32% 15% 3	16121
	Night	10% 9% 34% 37% 9%	26225

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	8% 9% 3 5% 4 3 3 3 5% 8% 49%	3638
	11th to 20th	Day	5% 9% 3 7% 5% 4 4 3 5% 5% 49%	3657
	21st to end	Day	4 7% 3 6% 4 3 3 2 3 6% 58%	4488
February	1st to 10th	Day	8% 9% 3 8% 5% 4 4 3 6% 7% 43%	1915
	11th to 20th	Day	5% 8% 3 6% 5% 5% 4 4 6% 8% 47%	1949
	21st to end	Day	6% 6% 3 5% 3 3 2 3 4 7% 57%	1650
March	1st to 10th	Day	9% 6% 1 4 2 2 3 3 4 5% 59%	3230
	11th to 20th	Day	9% 6% 2 5% 3 2 2 2 4 6% 58%	3578
	21st to end	Day	10% 8% 2 5% 3 3 4 4 5% 7% 49%	3880
April	1st to 10th	Day	9% 6% 2 4 3 3 3 3 5% 8% 54%	2265
	11th to 20th	Day	11% 6% 2 6% 4 4 3 3 6% 7% 48%	2292
	21st to end	Day	14% 6% 2 4 4 4 3 3 6% 7% 47%	2470
May	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 6% 9% 48%	4120
	11th to 20th	Day	11% 6% 2 5% 3 4 4 3 6% 7% 50%	4082
	21st to end	Day	12% 5% 2 5% 4 5% 4 4 6% 8% 45%	4489
June	1st to 10th	Day	5% 2 1 3 3 3 4 3 5% 10% 60%	2584
	11th to 20th	Day	7% 4 1 4 4 4 4 4 5% 7% 57%	2660
	21st to end	Day	5% 3 1 4 3 3 4 3 7% 8% 58%	2660
July	1st to 10th	Day	7% 5% 2 5% 4 3 4 4 5% 7% 54%	4430
	11th to 20th	Day	4 2 1 3 4 4 4 4 7% 10% 57%	4430
	21st to end	Day	4 3 1 4 3 3 4 4 7% 9% 58%	4476
August	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 6% 8% 47%	4103
	11th to 20th	Day	10% 4 2 5% 3 4 4 4 6% 8% 50%	4041
	21st to end	Day	5% 4 2 5% 3 3 3 3 5% 7% 61%	4180
September	1st to 10th	Day	10% 5% 1 4 3 2 4 2 5% 5% 60%	2209
	11th to 20th	Day	9% 4 2 3 3 3 3 3 4 6% 61%	2090
	21st to end	Day	8% 3 1 4 3 2 3 3 4 6% 64%	2090
October	1st to 10th	Day	10% 2 1 1 1 2 2 1 3 4 73%	3480
	11th to 20th	Day	5% 3 1 4 3 3 4 3 4 5% 65%	3211
	21st to end	Day	7% 5% 2 3 2 2 3 2 4 6% 63%	3121
November	1st to 10th	Day	7% 4 2 4 3 4 4 2 5% 6% 59%	1763
	11th to 20th	Day	7% 5% 1 4 3 2 4 3 4 5% 63%	1800
	21st to end	Day	8% 7% 2 4 3 3 3 3 4 7% 57%	1800
December	1st to 10th	Day	4 4 2 4 3 3 3 3 4 6% 65%	2940
	11th to 20th	Day	5% 6% 2 5% 3 3 3 2 5% 6% 61%	3330
	21st to end	Day	9% 12% 4 9% 4 4 3 3 4 5% 41%	4004
January	1st to 10th	Night	7% 9% 3 10% 4 3 3 3 5% 6% 46%	6327
	11th to 20th	Night	3 6% 3 6% 4 4 4 3 5% 7% 55%	6319
	21st to end	Night	6% 8% 2 7% 5% 4 4 3 4 5% 53%	6490
February	1st to 10th	Night	9% 8% 3 7% 4 4 5% 4 5% 5% 47%	2500
	11th to 20th	Night	6% 8% 2 6% 5% 5% 4 4 6% 7% 47%	2338
	21st to end	Night	6% 6% 2 5% 4 4 4 3 5% 5% 57%	1980
March	1st to 10th	Night	10% 5% 1 4 2 2 3 3 4 6% 60%	3854
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 6% 7% 57%	3601
	21st to end	Night	11% 7% 2 5% 4 4 4 4 7% 9% 43%	3889
April	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 5% 7% 51%	1913
	11th to 20th	Night	13% 6% 3 5% 3 4 4 3 5% 7% 46%	1835
	21st to end	Night	13% 6% 2 4 4 3 3 4 5% 8% 48%	1710
May	1st to 10th	Night	16% 7% 2 5% 4 4 3 3 5% 6% 44%	2850
	11th to 20th	Night	10% 6% 2 6% 4 4 4 3 5% 8% 47%	2823
	21st to end	Night	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	3098
June	1st to 10th	Night	5% 3 2 4 4 3 5% 4 6% 9% 56%	1591
	11th to 20th	Night	8% 4 1 5% 3 4 4 5% 6% 8% 53%	1520
	21st to end	Night	4 4 2 5% 4 3 3 5% 8% 10% 52%	1520
July	1st to 10th	Night	9% 4 2 6% 4 4 4 5% 6% 7% 50%	2540
	11th to 20th	Night	4 4 2 5% 3 5% 5% 5% 8% 11% 48%	2540
	21st to end	Night	3 2 1 3 2 3 3 4 5% 9% 64%	3085
August	1st to 10th	Night	6% 4 2 6% 4 4 5% 4 7% 9% 49%	2836
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 8% 48%	2798
	21st to end	Night	3 3 1 4 4 5% 5% 4 7% 10% 53%	3482
September	1st to 10th	Night	7% 4 1 3 2 3 3 4 5% 6% 61%	1971
	11th to 20th	Night	8% 4 1 4 3 3 3 3 5% 6% 60%	2090
	21st to end	Night	7% 4 2 4 2 2 3 3 4 6% 63%	2090
October	1st to 10th	Night	7% 2 1 2 2 3 2 2 3 4 73%	3480
	11th to 20th	Night	6% 3 1 3 2 2 2 2 4 5% 68%	3675
	21st to end	Night	6% 3 1 4 3 2 3 2 4 6% 66%	4433
November	1st to 10th	Night	6% 3 1 3 3 2 3 3 4 7% 65%	2535
	11th to 20th	Night	5% 4 1 4 3 3 3 3 5% 6% 62%	2600
	21st to end	Night	7% 6% 2 5% 3 2 3 3 4 6% 59%	2600
December	1st to 10th	Night	3 3 2 4 3 3 3 3 4 6% 67%	4250
	11th to 20th	Night	4 3 1 4 3 3 3 3 6% 7% 61%	4860
	21st to end	Night	9% 13% 3 7% 4 3 3 3 4 6% 44%	6974

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	35% 3 12% 30% 19% 1	3639
	11th to 20th	Day	37% 5% 7% 27% 20% 4	3657
	21st to end	Day	30% 2 12% 28% 22% 5%	4488
February	1st to 10th	Day	41% 4 14% 23% 14% 4	1915
	11th to 20th	Day	36% 4 15% 25% 16% 4	1959
	21st to end	Day	29% 3 12% 25% 23% 8%	1650
March	1st to 10th	Day	29% 3 9% 22% 31% 7%	3230
	11th to 20th	Day	29% 3 10% 28% 25% 5%	3578
	21st to end	Day	35% 4 21% 31% 8% 1	3880
April	1st to 10th	Day	30% 4 13% 27% 22% 4	2265
	11th to 20th	Day	35% 4 13% 25% 18% 5%	2295
	21st to end	Day	37% 4 12% 28% 17% 2	2470
May	1st to 10th	Day	34% 5% 13% 27% 13% 8%	4120
	11th to 20th	Day	34% 5% 15% 31% 15% 1	4086
	21st to end	Day	36% 5% 14% 26% 16% 2	4489
June	1st to 10th	Day	20% 5% 21% 35% 16% 2	2584
	11th to 20th	Day	27% 4 14% 28% 20% 7%	2660
	21st to end	Day	22% 5% 11% 30% 25% 6%	2660
July	1st to 10th	Day	30% 5% 10% 26% 24% 6%	4361
	11th to 20th	Day	21% 6% 14% 30% 25% 4	4355
	21st to end	Day	20% 5% 14% 27% 23% 10%	4334
August	1st to 10th	Day	34% 5% 13% 26% 19% 3	4090
	11th to 20th	Day	32% 7% 16% 23% 20% 2	4080
	21st to end	Day	23% 4 12% 26% 24% 10%	4179
September	1st to 10th	Day	27% 4 9% 25% 26% 9%	2209
	11th to 20th	Day	25% 4 10% 29% 26% 6%	2090
	21st to end	Day	23% 3 9% 29% 31% 5%	2090
October	1st to 10th	Day	18% 2 5% 23% 38% 14%	3480
	11th to 20th	Day	22% 3 10% 36% 20% 9%	3211
	21st to end	Day	24% 2 8% 28% 33% 5%	3121
November	1st to 10th	Day	26% 3 11% 30% 25% 6%	1763
	11th to 20th	Day	25% 2 9% 29% 29% 5%	1800
	21st to end	Day	30% 3 9% 27% 26% 5%	1800
December	1st to 10th	Day	22% 2 11% 35% 24% 6%	2940
	11th to 20th	Day	26% 2 14% 40% 17% 1	3328
	21st to end	Day	46% 2 8% 24% 17% 2	4004
January	1st to 10th	Night	41% 3 12% 25% 17% 2	6327
	11th to 20th	Night	29% 3 12% 27% 25% 4	6319
	21st to end	Night	35% 3 11% 23% 20% 8%	6490
February	1st to 10th	Night	37% 5% 13% 24% 15% 6%	2500
	11th to 20th	Night	36% 4 10% 29% 18% 3	2338
	21st to end	Night	29% 4 10% 23% 24% 10%	1980
March	1st to 10th	Night	27% 2 7% 23% 29% 12%	3854
	11th to 20th	Night	27% 3 9% 27% 28% 7%	3601
	21st to end	Night	37% 4 20% 29% 9% 1	3889
April	1st to 10th	Night	31% 5% 12% 23% 23% 5%	1913
	11th to 20th	Night	37% 4 12% 23% 20% 4	1835
	21st to end	Night	35% 5% 13% 27% 18% 2	1710
May	1st to 10th	Night	40% 5% 12% 22% 15% 5%	2850
	11th to 20th	Night	37% 4 15% 27% 16% 1	2824
	21st to end	Night	38% 6% 16% 26% 12% 2	3099
June	1st to 10th	Night	24% 7% 21% 30% 15% 3	1591
	11th to 20th	Night	29% 5% 15% 26% 19% 6%	1520
	21st to end	Night	25% 5% 16% 27% 22% 6%	1520
July	1st to 10th	Night	31% 5% 14% 23% 22% 4	2500
	11th to 20th	Night	28% 6% 16% 24% 21% 6%	2492
	21st to end	Night	17% 5% 17% 25% 24% 11%	2988
August	1st to 10th	Night	30% 5% 16% 24% 20% 4	2826
	11th to 20th	Night	35% 6% 16% 24% 17% 3	2825
	21st to end	Night	26% 5% 12% 29% 18% 9%	3482
September	1st to 10th	Night	22% 4 10% 26% 26% 11%	1971
	11th to 20th	Night	24% 5% 8% 26% 28% 8%	2090
	21st to end	Night	23% 3 8% 26% 31% 7%	2090
October	1st to 10th	Night	18% 3 5% 18% 44% 12%	3480
	11th to 20th	Night	20% 2 11% 27% 27% 13%	3675
	21st to end	Night	21% 2 11% 26% 32% 8%	4433
November	1st to 10th	Night	21% 3 10% 29% 28% 10%	2535
	11th to 20th	Night	23% 3 9% 28% 29% 8%	2600
	21st to end	Night	27% 3 10% 27% 26% 7%	2600
December	1st to 10th	Night	21% 2 13% 35% 23% 6%	4250
	11th to 20th	Night	24% 2 11% 38% 22% 4	4861
	21st to end	Night	43% 2 9% 30% 14% 1	6974

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	11% 8% 39% 32% 8% 1	300
	11th to 20th	Day	12% 17% 35% 28% 7% 1	300
	21st to end	Day	10% 8% 48% 24% 8% 1	330
February	1st to 10th	Day	14% 13% 43% 26% 5%	300
	11th to 20th	Day	10% 11% 43% 26% 7% 1	300
	21st to end	Day	10% 14% 47% 25% 4	249
March	1st to 10th	Day	13% 13% 42% 23% 7% 1	300
	11th to 20th	Day	16% 11% 34% 28% 9% 2	300
	21st to end	Day	19% 14% 42% 20% 5% 1	315
April	1st to 10th	Day	13% 20% 38% 23% 6%	445
	11th to 20th	Day	17% 15% 39% 24% 6%	493
	21st to end	Day	18% 12% 37% 22% 10% 1	650
May	1st to 10th	Day	15% 10% 34% 29% 12% 1	653
	11th to 20th	Day	18% 10% 29% 27% 16% 2	675
	21st to end	Day	14% 8% 33% 30% 12% 2	734
June	1st to 10th	Day	5% 5% 29% 33% 22% 7%	680
	11th to 20th	Day	7% 7% 33% 38% 14% 2	680
	21st to end	Day	6% 6% 30% 37% 20% 1	680
July	1st to 10th	Day	7% 3 25% 37% 23% 5%	671
	11th to 20th	Day	4 3 18% 45% 26% 4	668
	21st to end	Day	7% 3 18% 40% 27% 5%	739
August	1st to 10th	Day	9% 1 16% 41% 28% 4	676
	11th to 20th	Day	6% 3 22% 42% 24% 3	665
	21st to end	Day	7% 2 18% 48% 20% 5%	539
September	1st to 10th	Day	8% 4 25% 44% 15% 4	419
	11th to 20th	Day	5% 7% 25% 36% 23% 4	300
	21st to end	Day	10% 9% 29% 30% 19% 4	300
October	1st to 10th	Day	20% 6% 23% 32% 15% 4	300
	11th to 20th	Day	6% 12% 38% 29% 11% 4	300
	21st to end	Day	16% 12% 33% 27% 12% 1	330
November	1st to 10th	Day	13% 12% 35% 25% 12% 3	300
	11th to 20th	Day	13% 12% 37% 26% 10% 2	300
	21st to end	Day	16% 10% 36% 23% 13% 2	300
December	1st to 10th	Day	9% 8% 38% 33% 12% 1	300
	11th to 20th	Day	12% 11% 41% 29% 6% 1	300
	21st to end	Day	18% 12% 35% 25% 9%	330
January	1st to 10th	Night	9% 8% 38% 37% 7% 1	1000
	11th to 20th	Night	9% 12% 36% 34% 8% 1	1000
	21st to end	Night	9% 8% 45% 32% 6%	1100
February	1st to 10th	Night	13% 9% 42% 31% 5%	900
	11th to 20th	Night	9% 13% 40% 33% 4	784
	21st to end	Night	8% 13% 40% 33% 5%	660
March	1st to 10th	Night	12% 12% 38% 34% 4	800
	11th to 20th	Night	13% 11% 36% 32% 7% 1	611
	21st to end	Night	15% 14% 40% 25% 5%	647
April	1st to 10th	Night	12% 14% 46% 25% 3	570
	11th to 20th	Night	16% 13% 40% 26% 5%	563
	21st to end	Night	14% 15% 32% 30% 8%	570
May	1st to 10th	Night	15% 8% 36% 32% 8%	570
	11th to 20th	Night	16% 7% 36% 31% 9% 1	561
	21st to end	Night	13% 10% 35% 32% 9%	617
June	1st to 10th	Night	6% 7% 32% 41% 12% 1	570
	11th to 20th	Night	9% 8% 36% 38% 10%	570
	21st to end	Night	5% 6% 36% 42% 12%	570
July	1st to 10th	Night	7% 3 25% 48% 17%	555
	11th to 20th	Night	3 2 24% 52% 17% 1	552
	21st to end	Night	5% 2 19% 49% 24% 1	624
August	1st to 10th	Night	7% 1 18% 52% 21% 1	567
	11th to 20th	Night	8% 3 23% 48% 18%	564
	21st to end	Night	5% 1 25% 50% 17% 2	627
September	1st to 10th	Night	8% 2 28% 50% 11% 1	570
	11th to 20th	Night	11% 4 29% 40% 15% 1	570
	21st to end	Night	10% 6% 32% 37% 14% 1	570
October	1st to 10th	Night	14% 7% 31% 37% 11% 1	570
	11th to 20th	Night	9% 8% 37% 37% 8%	695
	21st to end	Night	13% 10% 32% 33% 12%	1023
November	1st to 10th	Night	8% 9% 32% 43% 8%	975
	11th to 20th	Night	8% 11% 36% 38% 7%	1000
	21st to end	Night	11% 8% 32% 38% 10% 1	1000
December	1st to 10th	Night	7% 9% 38% 39% 6%	1000
	11th to 20th	Night	11% 10% 37% 34% 8%	1000
	21st to end	Night	13% 11% 34% 36% 5%	1100

Page generated at 2025-03-24 08:04:29. Version ac07

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