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 2026-07-31.

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% 54%	146699
	Night	8% 6% 2 5% 4 3 4 3 5% 7% 54%	143725
Transparency	Day	30% 4 11% 28% 22% 5%	141525
	Night	30% 4 12% 26% 22% 6%	139911
Seeing	Day	11% 9% 31% 32% 15% 2	18100
	Night	10% 9% 35% 37% 9%	27952

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% 2 6% 4 3 3 3 5% 7% 51%	4720
	11th to 20th	Day	4 8% 3 7% 5% 4 4 3 5% 6% 52%	4770
	21st to end	Day	5% 8% 4 6% 4 3 3 2 4 5% 55%	5812
February	1st to 10th	Day	7% 9% 3 7% 5% 4 4 4 6% 7% 44%	2011
	11th to 20th	Day	5% 7% 3 6% 5% 5% 4 4 6% 8% 47%	2049
	21st to end	Day	6% 6% 3 5% 3 3 2 3 4 7% 58%	1730
March	1st to 10th	Day	8% 8% 2 5% 3 2 3 2 4 5% 59%	4600
	11th to 20th	Day	7% 5% 2 5% 3 3 3 2 4 7% 60%	5077
	21st to end	Day	8% 6% 2 5% 3 3 4 4 6% 7% 53%	5585
April	1st to 10th	Day	10% 6% 2 4 3 3 3 3 5% 7% 54%	2503
	11th to 20th	Day	10% 6% 2 5% 4 3 3 3 6% 7% 50%	2536
	21st to end	Day	15% 7% 2 4 4 4 3 3 6% 6% 46%	2730
May	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 50%	7655
	11th to 20th	Day	9% 4 1 4 3 3 3 3 5% 6% 59%	7627
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	8404
June	1st to 10th	Day	6% 3 1 3 3 3 4 3 5% 10% 59%	2856
	11th to 20th	Day	7% 4 1 4 4 3 4 3 5% 7% 57%	2940
	21st to end	Day	6% 4 1 4 4 3 4 3 6% 7% 57%	2940
July	1st to 10th	Day	6% 4 2 5% 4 3 4 4 5% 7% 56%	6479
	11th to 20th	Day	6% 2 1 3 3 3 4 4 6% 9% 57%	6480
	21st to end	Day	6% 4 2 5% 3 4 5% 4 7% 8% 54%	6566
August	1st to 10th	Day	11% 6% 2 5% 3 4 4 4 5% 8% 48%	5801
	11th to 20th	Day	8% 3 2 4 3 4 4 4 6% 9% 53%	5821
	21st to end	Day	15% 5% 2 6% 3 3 3 3 4 6% 49%	5984
September	1st to 10th	Day	9% 4 1 4 3 2 4 2 5% 6% 59%	2325
	11th to 20th	Day	9% 4 2 3 3 3 3 3 4 6% 60%	2200
	21st to end	Day	8% 3 1 4 3 2 3 3 4 6% 63%	2200
October	1st to 10th	Day	10% 2 1 1 2 2 2 1 3 4 73%	3590
	11th to 20th	Day	5% 3 1 4 3 3 4 3 4 5% 65%	3313
	21st to end	Day	8% 5% 2 3 2 2 3 2 4 6% 63%	3221
November	1st to 10th	Day	7% 4 2 4 3 4 4 2 5% 6% 60%	1853
	11th to 20th	Day	7% 5% 1 4 3 2 4 3 4 5% 63%	1890
	21st to end	Day	8% 7% 2 5% 3 3 3 3 4 7% 56%	1890
December	1st to 10th	Day	4 4 2 4 3 3 3 3 4 6% 64%	3030
	11th to 20th	Day	5% 6% 2 5% 3 3 3 2 4 6% 61%	3419
	21st to end	Day	9% 12% 4 9% 4 4 3 3 4 5% 42%	4092
January	1st to 10th	Night	6% 9% 3 9% 4 3 3 3 5% 6% 48%	8201
	11th to 20th	Night	3 5% 3 6% 4 4 4 3 5% 7% 58%	8216
	21st to end	Night	6% 9% 2 7% 5% 5% 4 3 4 5% 50%	8402
February	1st to 10th	Night	9% 7% 3 6% 4 4 4 4 5% 6% 49%	2625
	11th to 20th	Night	6% 8% 2 6% 5% 5% 4 4 6% 7% 47%	2458
	21st to end	Night	6% 6% 2 5% 4 4 4 3 5% 5% 58%	2076
March	1st to 10th	Night	8% 6% 1 4 2 2 3 3 4 5% 62%	5494
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 7% 58%	5102
	21st to end	Night	9% 6% 2 6% 4 5% 5% 4 7% 9% 43%	5588
April	1st to 10th	Night	9% 6% 2 5% 3 3 4 4 5% 7% 51%	2114
	11th to 20th	Night	12% 5% 3 5% 3 4 4 3 5% 7% 47%	2031
	21st to end	Night	13% 6% 2 4 4 4 3 4 5% 8% 48%	1890
May	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 47%	5292
	11th to 20th	Night	7% 4 2 4 3 3 4 3 6% 8% 56%	5265
	21st to end	Night	17% 8% 2 6% 3 3 4 3 5% 7% 41%	5803
June	1st to 10th	Night	6% 4 2 4 4 3 4 4 6% 9% 55%	1759
	11th to 20th	Night	7% 4 1 5% 3 3 4 4 6% 8% 54%	1680
	21st to end	Night	6% 4 2 5% 4 3 3 5% 8% 10% 51%	1680
July	1st to 10th	Night	7% 4 2 5% 3 4 4 5% 7% 9% 50%	3720
	11th to 20th	Night	8% 5% 2 6% 4 5% 5% 4 7% 10% 44%	3720
	21st to end	Night	5% 3 2 5% 3 4 4 4 5% 9% 57%	4526
August	1st to 10th	Night	8% 4 2 5% 4 4 5% 4 7% 10% 45%	4011
	11th to 20th	Night	7% 4 2 5% 4 5% 4 4 7% 10% 49%	4028
	21st to end	Night	11% 6% 2 6% 4 5% 5% 4 6% 8% 44%	4988
September	1st to 10th	Night	7% 4 1 3 2 3 3 3 5% 6% 61%	2075
	11th to 20th	Night	8% 4 1 4 3 3 3 3 5% 6% 60%	2200
	21st to end	Night	8% 4 2 4 2 2 3 3 4 6% 62%	2200
October	1st to 10th	Night	7% 2 1 2 2 3 2 2 3 4 72%	3590
	11th to 20th	Night	6% 4 1 3 2 2 2 2 4 5% 68%	3792
	21st to end	Night	7% 3 1 3 3 2 3 2 4 6% 65%	4575
November	1st to 10th	Night	5% 3 1 3 3 2 3 3 4 7% 65%	2665
	11th to 20th	Night	5% 4 1 4 3 3 3 3 5% 6% 62%	2730
	21st to end	Night	8% 6% 2 5% 3 2 3 3 4 6% 59%	2730
December	1st to 10th	Night	3 4 2 4 3 3 3 2 4 6% 67%	4380
	11th to 20th	Night	4 4 1 4 3 3 3 3 6% 7% 61%	4991
	21st to end	Night	9% 13% 3 7% 4 3 3 3 4 6% 45%	7128

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% 2 10% 30% 21% 2	4723
	11th to 20th	Day	34% 5% 8% 28% 20% 5%	4770
	21st to end	Day	34% 2 12% 26% 22% 4	5812
February	1st to 10th	Day	40% 5% 14% 23% 14% 4	2011
	11th to 20th	Day	35% 4 15% 25% 16% 4	2059
	21st to end	Day	28% 2 12% 26% 24% 8%	1730
March	1st to 10th	Day	30% 2 7% 20% 30% 11%	4600
	11th to 20th	Day	27% 2 8% 23% 28% 12%	5077
	21st to end	Day	30% 4 19% 34% 12% 1	5585
April	1st to 10th	Day	30% 4 13% 28% 22% 4	2503
	11th to 20th	Day	34% 4 12% 25% 19% 6%	2539
	21st to end	Day	38% 4 11% 27% 17% 2	2730
May	1st to 10th	Day	33% 4 10% 29% 19% 5%	7655
	11th to 20th	Day	27% 3 10% 27% 25% 8%	7631
	21st to end	Day	42% 5% 13% 24% 15% 2	8405
June	1st to 10th	Day	22% 5% 20% 34% 17% 3	2856
	11th to 20th	Day	27% 4 14% 27% 21% 7%	2940
	21st to end	Day	24% 5% 10% 28% 25% 7%	2940
July	1st to 10th	Day	28% 4 9% 28% 26% 5%	6411
	11th to 20th	Day	22% 6% 17% 28% 23% 4	6405
	21st to end	Day	26% 6% 13% 26% 22% 7%	6381
August	1st to 10th	Day	34% 5% 13% 26% 18% 3	4220
	11th to 20th	Day	31% 7% 16% 24% 20% 2	4210
	21st to end	Day	24% 4 12% 26% 24% 10%	4311
September	1st to 10th	Day	27% 4 10% 25% 25% 9%	2325
	11th to 20th	Day	26% 4 10% 28% 25% 6%	2200
	21st to end	Day	23% 3 9% 29% 31% 5%	2200
October	1st to 10th	Day	19% 2 5% 23% 38% 13%	3590
	11th to 20th	Day	22% 3 10% 35% 21% 9%	3313
	21st to end	Day	25% 2 8% 27% 33% 5%	3221
November	1st to 10th	Day	25% 3 11% 30% 26% 6%	1853
	11th to 20th	Day	24% 2 10% 30% 29% 5%	1890
	21st to end	Day	30% 3 10% 27% 25% 5%	1890
December	1st to 10th	Day	23% 2 10% 35% 24% 6%	3030
	11th to 20th	Day	26% 2 14% 40% 17% 1	3417
	21st to end	Day	46% 2 8% 24% 18% 2	4092
January	1st to 10th	Night	39% 2 12% 23% 20% 3	8202
	11th to 20th	Night	27% 3 13% 29% 24% 5%	8216
	21st to end	Night	37% 3 10% 23% 20% 7%	8402
February	1st to 10th	Night	36% 4 12% 25% 16% 6%	2625
	11th to 20th	Night	36% 4 10% 29% 18% 3	2458
	21st to end	Night	28% 4 10% 22% 26% 10%	2076
March	1st to 10th	Night	26% 2 6% 23% 31% 12%	5494
	11th to 20th	Night	27% 2 7% 20% 27% 17%	5102
	21st to end	Night	37% 4 18% 29% 11% 2	5588
April	1st to 10th	Night	32% 5% 12% 24% 22% 4	2114
	11th to 20th	Night	36% 4 12% 23% 20% 5%	2031
	21st to end	Night	36% 5% 13% 27% 18% 2	1890
May	1st to 10th	Night	37% 5% 10% 28% 18% 3	5293
	11th to 20th	Night	27% 3 12% 29% 23% 6%	5266
	21st to end	Night	44% 5% 13% 24% 13% 1	5805
June	1st to 10th	Night	25% 7% 20% 29% 15% 4	1759
	11th to 20th	Night	28% 5% 15% 27% 19% 6%	1680
	21st to end	Night	27% 5% 15% 26% 21% 6%	1680
July	1st to 10th	Night	30% 5% 15% 27% 20% 3	3680
	11th to 20th	Night	34% 5% 15% 22% 18% 5%	3672
	21st to end	Night	24% 5% 16% 24% 23% 8%	4398
August	1st to 10th	Night	31% 5% 16% 24% 20% 4	2916
	11th to 20th	Night	35% 6% 15% 25% 17% 3	2915
	21st to end	Night	27% 5% 12% 28% 18% 9%	3592
September	1st to 10th	Night	22% 4 11% 26% 25% 11%	2075
	11th to 20th	Night	25% 5% 9% 27% 27% 7%	2200
	21st to end	Night	24% 3 8% 26% 31% 7%	2200
October	1st to 10th	Night	18% 3 5% 18% 44% 12%	3590
	11th to 20th	Night	20% 2 11% 27% 27% 13%	3792
	21st to end	Night	22% 2 11% 26% 32% 8%	4575
November	1st to 10th	Night	20% 3 10% 29% 29% 9%	2665
	11th to 20th	Night	23% 3 9% 29% 28% 8%	2730
	21st to end	Night	28% 3 10% 27% 25% 7%	2730
December	1st to 10th	Night	21% 2 13% 35% 23% 6%	4380
	11th to 20th	Night	23% 2 11% 37% 22% 4	4992
	21st to end	Night	42% 2 9% 29% 15% 2	7128

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	13% 9% 37% 32% 8% 1	330
	11th to 20th	Day	13% 17% 36% 26% 7% 1	330
	21st to end	Day	11% 9% 47% 25% 8% 1	363
February	1st to 10th	Day	12% 12% 44% 26% 5%	330
	11th to 20th	Day	11% 12% 44% 26% 7% 1	330
	21st to end	Day	9% 14% 48% 25% 3	273
March	1st to 10th	Day	12% 14% 44% 22% 7% 1	330
	11th to 20th	Day	16% 12% 33% 28% 9% 2	330
	21st to end	Day	18% 13% 43% 20% 6% 1	363
April	1st to 10th	Day	13% 21% 38% 21% 6%	523
	11th to 20th	Day	15% 16% 39% 24% 6%	577
	21st to end	Day	19% 14% 36% 21% 9% 1	750
May	1st to 10th	Day	14% 10% 34% 28% 13% 1	753
	11th to 20th	Day	17% 11% 30% 26% 14% 2	775
	21st to end	Day	15% 9% 32% 30% 12% 2	844
June	1st to 10th	Day	5% 7% 30% 31% 21% 6%	780
	11th to 20th	Day	7% 8% 33% 37% 13% 2	780
	21st to end	Day	8% 5% 32% 35% 18% 1	780
July	1st to 10th	Day	6% 3 26% 38% 22% 5%	771
	11th to 20th	Day	5% 3 17% 43% 27% 5%	768
	21st to end	Day	7% 3 18% 40% 26% 5%	845
August	1st to 10th	Day	9% 1 18% 41% 26% 4	726
	11th to 20th	Day	6% 3 21% 42% 24% 3	715
	21st to end	Day	9% 3 18% 47% 20% 5%	583
September	1st to 10th	Day	8% 4 25% 44% 16% 4	455
	11th to 20th	Day	6% 6% 24% 37% 22% 4	330
	21st to end	Day	10% 8% 28% 31% 18% 5%	330
October	1st to 10th	Day	19% 6% 25% 32% 14% 4	330
	11th to 20th	Day	7% 13% 40% 27% 10% 3	330
	21st to end	Day	17% 14% 32% 26% 10% 1	363
November	1st to 10th	Day	12% 12% 36% 25% 12% 3	330
	11th to 20th	Day	12% 12% 38% 25% 10% 2	330
	21st to end	Day	17% 10% 35% 24% 12% 2	330
December	1st to 10th	Day	8% 8% 41% 31% 11% 1	330
	11th to 20th	Day	12% 12% 42% 28% 6% 1	330
	21st to end	Day	17% 12% 35% 26% 9%	363
January	1st to 10th	Night	9% 8% 39% 37% 6% 1	1050
	11th to 20th	Night	10% 13% 36% 34% 7% 1	1050
	21st to end	Night	9% 8% 45% 32% 5%	1155
February	1st to 10th	Night	12% 9% 42% 31% 5%	945
	11th to 20th	Night	9% 13% 40% 33% 4	824
	21st to end	Night	8% 12% 41% 33% 5%	692
March	1st to 10th	Night	12% 12% 38% 34% 4	840
	11th to 20th	Night	13% 13% 36% 31% 6% 1	642
	21st to end	Night	15% 14% 40% 26% 5%	693
April	1st to 10th	Night	13% 16% 45% 24% 3	630
	11th to 20th	Night	16% 14% 41% 25% 4	623
	21st to end	Night	16% 15% 33% 29% 7%	630
May	1st to 10th	Night	15% 8% 36% 33% 8%	630
	11th to 20th	Night	15% 9% 36% 31% 8% 1	621
	21st to end	Night	15% 10% 35% 31% 8%	683
June	1st to 10th	Night	7% 8% 32% 40% 12% 1	630
	11th to 20th	Night	9% 8% 37% 37% 9%	630
	21st to end	Night	7% 7% 37% 39% 11%	630
July	1st to 10th	Night	7% 3 26% 48% 16%	615
	11th to 20th	Night	4 3 23% 50% 19% 1	612
	21st to end	Night	6% 2 19% 50% 22% 1	687
August	1st to 10th	Night	7% 2 19% 51% 21% 1	597
	11th to 20th	Night	7% 3 24% 47% 19%	594
	21st to end	Night	7% 1 24% 49% 17% 2	660
September	1st to 10th	Night	8% 2 29% 50% 11% 1	600
	11th to 20th	Night	11% 4 29% 41% 15% 1	600
	21st to end	Night	11% 7% 32% 37% 14% 1	600
October	1st to 10th	Night	14% 7% 31% 37% 11% 1	600
	11th to 20th	Night	10% 9% 38% 35% 7%	732
	21st to end	Night	14% 10% 32% 33% 11%	1077
November	1st to 10th	Night	8% 9% 33% 42% 8%	1025
	11th to 20th	Night	8% 10% 36% 39% 7%	1050
	21st to end	Night	12% 8% 32% 37% 10% 1	1050
December	1st to 10th	Night	8% 9% 39% 38% 5%	1050
	11th to 20th	Night	11% 10% 37% 33% 8%	1050
	21st to end	Night	13% 11% 35% 36% 5%	1155

Page generated at 2026-07-28 02:25:21. Version ac07

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