Grafton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grafton clear sky chart from 2005-11-06 to 2026-02-16.

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	15% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	130612
	Night	15% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	130218
Transparency	Day	46% 7% 12% 21% 13% 2	134119
	Night	45% 7% 12% 21% 13% 2	134798
Seeing	Day	23% 18% 28% 19% 8% 3	16390
	Night	21% 14% 28% 27% 8% 1	25480

By Month

Percentage Cloud Cover NE (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% 8% 3 15% 8% 8% 7% 5% 6% 5% 27%	4661
	11th to 20th	Day	8% 11% 5% 12% 6% 5% 5% 4 6% 5% 31%	4650
	21st to end	Day	11% 10% 3 8% 5% 5% 4 3 5% 7% 39%	5124
February	1st to 10th	Day	12% 10% 4 10% 6% 5% 6% 4 6% 8% 30%	1896
	11th to 20th	Day	9% 8% 3 10% 6% 6% 5% 5% 6% 7% 34%	1998
	21st to end	Day	17% 11% 3 9% 6% 4 4 3 5% 6% 32%	1650
March	1st to 10th	Day	17% 12% 4 10% 5% 5% 4 3 4 5% 30%	4476
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	4644
	21st to end	Day	21% 10% 3 7% 4 4 4 3 4 6% 34%	5970
April	1st to 10th	Day	17% 10% 3 7% 6% 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	17% 8% 3 6% 5% 5% 5% 4 5% 7% 37%	2374
	21st to end	Day	19% 9% 3 8% 5% 5% 5% 4 5% 5% 33%	2565
May	1st to 10th	Day	31% 10% 3 8% 5% 5% 4 3 5% 6% 23%	5886
	11th to 20th	Day	20% 10% 4 8% 5% 4 5% 4 5% 7% 28%	6272
	21st to end	Day	19% 7% 3 6% 4 5% 5% 4 6% 8% 32%	6949
June	1st to 10th	Day	17% 6% 3 7% 6% 5% 5% 4 7% 8% 31%	2800
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 6% 8% 36%	2800
	21st to end	Day	17% 7% 2 7% 6% 6% 5% 5% 6% 8% 32%	2800
July	1st to 10th	Day	12% 6% 3 7% 5% 6% 6% 5% 7% 10% 34%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 8% 10% 37%	6360
	21st to end	Day	15% 5% 2 6% 5% 4 5% 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 5% 2 7% 5% 5% 6% 4 7% 8% 36%	4467
	11th to 20th	Day	13% 6% 2 7% 6% 5% 6% 5% 6% 9% 35%	4010
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 7% 11% 43%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 3 6% 8% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 3 4 4 3 5% 8% 49%	2257
	21st to end	Day	16% 5% 2 5% 5% 4 4 3 6% 8% 42%	2200
October	1st to 10th	Day	20% 7% 2 6% 4 4 5% 3 5% 6% 38%	2165
	11th to 20th	Day	15% 7% 2 6% 4 5% 5% 3 6% 7% 40%	1940
	21st to end	Day	21% 9% 3 7% 5% 5% 4 3 4 6% 33%	1980
November	1st to 10th	Day	11% 8% 2 6% 5% 5% 4 4 5% 7% 44%	1851
	11th to 20th	Day	15% 7% 2 7% 5% 4 5% 3 5% 6% 40%	1890
	21st to end	Day	17% 9% 2 7% 4 4 5% 4 6% 7% 35%	1785
December	1st to 10th	Day	14% 11% 4 11% 6% 5% 5% 3 5% 6% 31%	2680
	11th to 20th	Day	19% 8% 3 8% 5% 4 5% 4 5% 5% 35%	3066
	21st to end	Day	17% 9% 4 9% 6% 5% 5% 3 4 5% 32%	4048
January	1st to 10th	Night	11% 7% 4 13% 7% 6% 6% 4 6% 6% 30%	8234
	11th to 20th	Night	10% 10% 5% 13% 7% 6% 6% 4 6% 6% 28%	8235
	21st to end	Night	11% 9% 3 7% 5% 6% 6% 4 5% 7% 37%	9057
February	1st to 10th	Night	11% 9% 4 8% 5% 6% 5% 4 6% 6% 35%	2721
	11th to 20th	Night	9% 9% 4 7% 6% 6% 5% 6% 7% 7% 35%	2399
	21st to end	Night	14% 9% 2 7% 5% 5% 5% 3 7% 7% 36%	1980
March	1st to 10th	Night	19% 10% 3 7% 5% 4 5% 4 6% 7% 32%	5364
	11th to 20th	Night	13% 10% 3 8% 5% 5% 4 4 5% 7% 37%	5295
	21st to end	Night	21% 11% 3 6% 4 4 4 3 5% 6% 33%	4962
April	1st to 10th	Night	17% 8% 3 7% 4 5% 4 4 6% 7% 36%	2000
	11th to 20th	Night	17% 9% 2 5% 5% 4 4 3 7% 7% 37%	1975
	21st to end	Night	19% 8% 2 6% 4 3 4 3 5% 7% 38%	1835
May	1st to 10th	Night	33% 8% 3 5% 4 3 4 3 6% 7% 23%	4054
	11th to 20th	Night	14% 10% 4 10% 6% 5% 6% 4 6% 7% 28%	3650
	21st to end	Night	20% 7% 3 7% 5% 5% 5% 3 5% 8% 33%	3926
June	1st to 10th	Night	19% 6% 2 4 4 5% 4 3 5% 7% 40%	1600
	11th to 20th	Night	18% 5% 2 5% 5% 4 4 4 7% 7% 39%	1600
	21st to end	Night	14% 7% 2 6% 5% 4 5% 5% 7% 7% 39%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 5% 4 5% 6% 9% 42%	3600
	11th to 20th	Night	13% 7% 2 7% 5% 4 5% 4 7% 8% 38%	3600
	21st to end	Night	11% 6% 2 5% 3 4 4 3 4 6% 50%	3926
August	1st to 10th	Night	20% 6% 3 6% 4 4 4 3 5% 7% 37%	2649
	11th to 20th	Night	14% 7% 2 6% 5% 5% 4 3 5% 7% 41%	3033
	21st to end	Night	9% 4 1 4 3 3 4 3 6% 9% 55%	3586
September	1st to 10th	Night	11% 5% 2 4 3 3 4 4 5% 8% 51%	2000
	11th to 20th	Night	12% 5% 1 4 3 3 4 3 5% 6% 53%	2142
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 6% 47%	2200
October	1st to 10th	Night	19% 7% 2 6% 4 3 4 3 6% 6% 41%	2231
	11th to 20th	Night	15% 5% 2 7% 4 4 4 4 5% 7% 42%	2459
	21st to end	Night	21% 8% 3 6% 4 4 5% 4 5% 6% 35%	2860
November	1st to 10th	Night	13% 6% 3 6% 4 4 5% 4 5% 7% 44%	2667
	11th to 20th	Night	15% 7% 2 6% 4 4 5% 4 6% 7% 41%	2730
	21st to end	Night	13% 8% 3 6% 5% 4 5% 4 6% 6% 39%	2830
December	1st to 10th	Night	13% 12% 4 10% 6% 5% 5% 4 6% 7% 30%	4720
	11th to 20th	Night	21% 9% 3 7% 4 4 4 3 5% 5% 34%	5348
	21st to end	Night	18% 12% 4 9% 4 4 4 3 5% 5% 31%	7150

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	53% 8% 10% 21% 7%	4662
	11th to 20th	Day	51% 8% 9% 15% 14% 2	4680
	21st to end	Day	42% 8% 9% 25% 15% 2	5124
February	1st to 10th	Day	51% 6% 13% 18% 10% 2	1896
	11th to 20th	Day	48% 7% 11% 19% 12% 2	2007
	21st to end	Day	53% 5% 10% 17% 13% 2	1650
March	1st to 10th	Day	56% 5% 9% 12% 10% 8%	4479
	11th to 20th	Day	45% 8% 15% 19% 10% 3	4644
	21st to end	Day	51% 4 6% 15% 18% 5%	5970
April	1st to 10th	Day	51% 5% 9% 20% 12% 3	2400
	11th to 20th	Day	46% 7% 10% 18% 16% 2	2374
	21st to end	Day	51% 6% 8% 17% 14% 3	2565
May	1st to 10th	Day	62% 8% 9% 14% 7% 1	5897
	11th to 20th	Day	54% 8% 13% 16% 9% 1	6276
	21st to end	Day	46% 8% 13% 22% 11% 1	6949
June	1st to 10th	Day	49% 7% 12% 20% 11% 1	2800
	11th to 20th	Day	45% 8% 13% 18% 15% 2	2800
	21st to end	Day	47% 9% 13% 20% 11% 1	2800
July	1st to 10th	Day	43% 8% 15% 20% 14%	6290
	11th to 20th	Day	37% 9% 17% 23% 13% 1	6285
	21st to end	Day	39% 6% 14% 27% 14% 1	6728
August	1st to 10th	Day	43% 8% 16% 21% 12% 1	4453
	11th to 20th	Day	42% 9% 15% 26% 7% 1	4049
	21st to end	Day	32% 9% 18% 27% 14% 1	4312
September	1st to 10th	Day	35% 8% 12% 27% 17% 1	2400
	11th to 20th	Day	35% 6% 11% 26% 21% 2	2257
	21st to end	Day	41% 6% 12% 24% 15% 2	2200
October	1st to 10th	Day	44% 8% 8% 22% 16% 2	3529
	11th to 20th	Day	38% 6% 11% 27% 17% 1	3143
	21st to end	Day	39% 5% 10% 31% 13% 3	3180
November	1st to 10th	Day	40% 7% 14% 22% 16% 1	1851
	11th to 20th	Day	45% 7% 10% 19% 17% 2	1890
	21st to end	Day	48% 6% 15% 22% 8% 1	1785
December	1st to 10th	Day	52% 7% 12% 21% 7% 1	2680
	11th to 20th	Day	50% 6% 10% 18% 14% 2	3066
	21st to end	Day	53% 5% 8% 17% 14% 3	4048
January	1st to 10th	Night	50% 10% 12% 18% 10% 1	8234
	11th to 20th	Night	54% 8% 10% 13% 11% 3	8280
	21st to end	Night	44% 7% 9% 18% 17% 4	9057
February	1st to 10th	Night	48% 7% 12% 20% 11% 1	2721
	11th to 20th	Night	45% 7% 11% 21% 13% 2	2401
	21st to end	Night	46% 7% 11% 23% 9% 4	1980
March	1st to 10th	Night	50% 6% 10% 16% 14% 4	5374
	11th to 20th	Night	46% 8% 16% 18% 11% 2	5296
	21st to end	Night	51% 6% 6% 13% 20% 3	4962
April	1st to 10th	Night	46% 6% 11% 20% 15% 2	2000
	11th to 20th	Night	46% 7% 9% 21% 16% 1	1975
	21st to end	Night	46% 7% 9% 19% 16% 3	1835
May	1st to 10th	Night	60% 8% 10% 14% 8% 1	4060
	11th to 20th	Night	52% 10% 12% 16% 8% 1	3651
	21st to end	Night	49% 9% 13% 17% 11% 1	3927
June	1st to 10th	Night	44% 7% 13% 23% 12% 1	1600
	11th to 20th	Night	43% 9% 15% 20% 12% 1	1600
	21st to end	Night	42% 9% 13% 23% 11% 1	1600
July	1st to 10th	Night	37% 8% 17% 25% 12% 1	3560
	11th to 20th	Night	40% 10% 14% 20% 14%	3552
	21st to end	Night	34% 6% 13% 34% 13% 1	3809
August	1st to 10th	Night	46% 9% 14% 21% 9%	2639
	11th to 20th	Night	42% 8% 17% 22% 10%	3060
	21st to end	Night	26% 8% 17% 35% 14% 1	3586
September	1st to 10th	Night	32% 8% 13% 28% 18% 1	2000
	11th to 20th	Night	32% 7% 10% 29% 21% 1	2142
	21st to end	Night	37% 8% 10% 28% 16% 2	2200
October	1st to 10th	Night	41% 8% 14% 23% 13% 1	3647
	11th to 20th	Night	31% 8% 12% 26% 21% 2	3995
	21st to end	Night	40% 6% 10% 33% 11% 1	4610
November	1st to 10th	Night	40% 7% 13% 22% 16% 2	2667
	11th to 20th	Night	42% 7% 10% 23% 16% 3	2730
	21st to end	Night	44% 8% 15% 22% 10% 1	2830
December	1st to 10th	Night	51% 8% 12% 22% 7%	4720
	11th to 20th	Night	52% 6% 11% 14% 16% 1	5348
	21st to end	Night	54% 5% 9% 17% 12% 3	7150

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% 28% 25% 18% 5%	220
	11th to 20th	Day	30% 24% 30% 12% 2 1	220
	21st to end	Day	25% 30% 28% 13% 4	242
February	1st to 10th	Day	28% 33% 25% 12% 2	349
	11th to 20th	Day	30% 23% 31% 14% 2	516
	21st to end	Day	29% 24% 29% 16% 2	414
March	1st to 10th	Day	29% 21% 32% 15% 3	500
	11th to 20th	Day	29% 26% 28% 13% 4 1	500
	21st to end	Day	31% 24% 27% 10% 6% 2	550
April	1st to 10th	Day	37% 22% 23% 10% 7% 1	500
	11th to 20th	Day	34% 23% 23% 12% 6% 2	492
	21st to end	Day	29% 21% 30% 13% 5% 2	500
May	1st to 10th	Day	31% 20% 26% 14% 7% 2	503
	11th to 20th	Day	23% 19% 31% 20% 5% 2	527
	21st to end	Day	19% 16% 31% 24% 9% 2	573
June	1st to 10th	Day	19% 17% 33% 19% 10% 1	530
	11th to 20th	Day	19% 16% 33% 20% 10% 2	530
	21st to end	Day	20% 16% 32% 21% 8% 3	530
July	1st to 10th	Day	15% 11% 31% 26% 13% 3	525
	11th to 20th	Day	14% 7% 35% 29% 12% 3	524
	21st to end	Day	14% 11% 35% 26% 11% 2	576
August	1st to 10th	Day	11% 12% 27% 29% 16% 5%	526
	11th to 20th	Day	14% 10% 28% 25% 15% 9%	519
	21st to end	Day	12% 13% 28% 26% 15% 7%	583
September	1st to 10th	Day	16% 11% 26% 23% 16% 8%	530
	11th to 20th	Day	14% 12% 25% 21% 17% 11%	530
	21st to end	Day	17% 13% 28% 25% 11% 6%	530
October	1st to 10th	Day	25% 14% 22% 21% 12% 5%	530
	11th to 20th	Day	23% 18% 26% 23% 9% 2	497
	21st to end	Day	30% 15% 30% 17% 7% 1	462
November	1st to 10th	Day	26% 17% 24% 22% 9% 1	425
	11th to 20th	Day	29% 19% 27% 18% 6% 1	430
	21st to end	Day	33% 19% 26% 17% 5% 1	325
December	1st to 10th	Day	27% 27% 24% 16% 5%	220
	11th to 20th	Day	37% 24% 25% 12% 2	220
	21st to end	Day	32% 17% 31% 15% 4	242
January	1st to 10th	Night	22% 21% 33% 22% 2	840
	11th to 20th	Night	25% 20% 31% 21% 2	840
	21st to end	Night	19% 18% 36% 23% 4	924
February	1st to 10th	Night	21% 19% 36% 22% 2	840
	11th to 20th	Night	22% 22% 36% 19% 2	804
	21st to end	Night	24% 22% 32% 21% 2	660
March	1st to 10th	Night	27% 18% 31% 22% 2	800
	11th to 20th	Night	24% 21% 29% 22% 4	800
	21st to end	Night	26% 18% 29% 21% 6%	880
April	1st to 10th	Night	27% 19% 29% 22% 4	800
	11th to 20th	Night	24% 20% 28% 21% 7%	791
	21st to end	Night	26% 17% 30% 23% 4	635
May	1st to 10th	Night	28% 13% 27% 25% 6%	600
	11th to 20th	Night	20% 11% 32% 30% 7%	430
	21st to end	Night	19% 7% 27% 33% 13% 1	433
June	1st to 10th	Night	21% 7% 27% 34% 11%	400
	11th to 20th	Night	18% 9% 29% 33% 12%	400
	21st to end	Night	13% 7% 27% 41% 12% 1	400
July	1st to 10th	Night	14% 7% 22% 44% 13% 1	390
	11th to 20th	Night	13% 4 22% 41% 18% 2	388
	21st to end	Night	14% 3 25% 40% 18% 1	438
August	1st to 10th	Night	12% 4 19% 45% 20% 1	435
	11th to 20th	Night	15% 7% 24% 33% 19% 3	688
	21st to end	Night	15% 5% 21% 37% 20% 2	880
September	1st to 10th	Night	13% 7% 27% 33% 19% 1	800
	11th to 20th	Night	15% 7% 19% 33% 20% 5%	800
	21st to end	Night	18% 9% 23% 32% 17% 2	800
October	1st to 10th	Night	23% 12% 18% 30% 15% 2	800
	11th to 20th	Night	20% 12% 27% 28% 12% 1	800
	21st to end	Night	26% 15% 27% 25% 6%	880
November	1st to 10th	Night	20% 16% 28% 27% 8%	820
	11th to 20th	Night	20% 18% 29% 25% 7% 1	840
	21st to end	Night	22% 17% 30% 24% 6% 1	840
December	1st to 10th	Night	23% 20% 32% 21% 4	840
	11th to 20th	Night	26% 18% 31% 23% 3	840
	21st to end	Night	28% 18% 33% 20% 2	924

Page generated at 2026-02-13 01:42:14. Version ac07

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