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 2024-11-30.

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 7% 5% 5% 5% 4 6% 7% 35%	101566
	Night	16% 8% 3 7% 5% 4 5% 4 6% 7% 37%	98016
Transparency	Day	46% 7% 12% 21% 12% 1	105091
	Night	45% 8% 12% 22% 12% 2	102580
Seeing	Day	23% 18% 28% 19% 9% 3	14937
	Night	21% 14% 28% 27% 9% 1	23914

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	12% 8% 3 8% 7% 6% 5% 5% 6% 6% 36%	2517
	11th to 20th	Day	13% 10% 5% 12% 7% 5% 5% 4 6% 5% 29%	2520
	21st to end	Day	15% 12% 4 12% 6% 5% 4 3 5% 6% 27%	2772
February	1st to 10th	Day	13% 11% 3 10% 7% 5% 6% 4 6% 7% 29%	1714
	11th to 20th	Day	9% 8% 4 10% 6% 6% 6% 5% 6% 7% 33%	1848
	21st to end	Day	17% 11% 3 9% 6% 4 4 3 5% 5% 32%	1570
March	1st to 10th	Day	19% 15% 5% 9% 6% 5% 4 3 4 5% 27%	3116
	11th to 20th	Day	12% 9% 3 7% 6% 6% 6% 5% 6% 7% 33%	3242
	21st to end	Day	18% 9% 3 7% 5% 5% 4 4 4 7% 35%	4166
April	1st to 10th	Day	17% 10% 3 7% 6% 4 4 3 4 6% 35%	2280
	11th to 20th	Day	17% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	2254
	21st to end	Day	19% 8% 3 8% 5% 5% 5% 4 5% 5% 32%	2437
May	1st to 10th	Day	22% 10% 3 8% 5% 5% 4 3 5% 6% 28%	4106
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 3 6% 8% 30%	4366
	21st to end	Day	12% 5% 2 5% 4 5% 5% 4 6% 9% 42%	4837
June	1st to 10th	Day	17% 6% 3 7% 6% 6% 5% 4 7% 8% 31%	2660
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 6% 8% 36%	2660
	21st to end	Day	17% 7% 2 6% 6% 6% 5% 6% 6% 8% 31%	2660
July	1st to 10th	Day	11% 5% 3 6% 5% 6% 5% 5% 7% 10% 36%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 10% 37%	4440
	21st to end	Day	16% 6% 2 7% 6% 5% 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	14% 5% 2 7% 5% 5% 6% 4 7% 9% 35%	4330
	11th to 20th	Day	13% 6% 2 7% 6% 5% 6% 5% 6% 9% 34%	3885
	21st to end	Day	8% 5% 2 6% 4 5% 5% 5% 8% 11% 42%	4180
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 8% 45%	2280
	11th to 20th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 48%	2144
	21st to end	Day	16% 6% 2 6% 5% 4 4 3 6% 8% 42%	2090
October	1st to 10th	Day	21% 7% 2 6% 5% 4 5% 3 5% 6% 36%	2057
	11th to 20th	Day	15% 7% 2 6% 4 5% 5% 3 6% 7% 40%	1843
	21st to end	Day	22% 9% 3 7% 5% 5% 4 3 5% 6% 33%	1881
November	1st to 10th	Day	11% 7% 2 6% 5% 5% 4 4 5% 7% 44%	1761
	11th to 20th	Day	15% 7% 2 6% 5% 4 5% 3 5% 6% 40%	1800
	21st to end	Day	18% 9% 2 7% 4 3 5% 4 6% 7% 36%	1698
December	1st to 10th	Day	16% 11% 3 10% 6% 5% 5% 3 6% 5% 30%	1520
	11th to 20th	Day	20% 10% 2 8% 5% 5% 3 3 4 4 36%	1906
	21st to end	Day	20% 12% 4 9% 6% 5% 5% 3 4 5% 27%	2772
January	1st to 10th	Night	15% 7% 3 8% 5% 4 4 4 6% 6% 38%	4429
	11th to 20th	Night	11% 10% 5% 12% 7% 6% 6% 4 5% 6% 29%	4440
	21st to end	Night	18% 13% 4 9% 5% 5% 5% 4 5% 5% 26%	4884
February	1st to 10th	Night	12% 9% 4 9% 6% 6% 5% 4 6% 6% 33%	2463
	11th to 20th	Night	9% 8% 4 6% 6% 6% 5% 6% 7% 7% 35%	2218
	21st to end	Night	14% 9% 2 7% 5% 5% 5% 3 7% 7% 37%	1884
March	1st to 10th	Night	22% 12% 3 6% 5% 4 4 3 5% 5% 31%	3724
	11th to 20th	Night	11% 9% 3 8% 5% 6% 5% 4 5% 6% 37%	3697
	21st to end	Night	17% 11% 3 7% 5% 4 4 3 5% 6% 35%	3466
April	1st to 10th	Night	17% 7% 3 6% 4 5% 4 4 6% 7% 37%	1900
	11th to 20th	Night	17% 9% 2 5% 5% 4 4 3 7% 7% 37%	1875
	21st to end	Night	20% 8% 2 6% 4 3 4 3 5% 7% 37%	1743
May	1st to 10th	Night	24% 9% 3 6% 4 3 5% 4 6% 7% 28%	2834
	11th to 20th	Night	15% 8% 4 10% 6% 6% 6% 4 6% 7% 28%	2542
	21st to end	Night	14% 5% 2 5% 4 4 5% 3 6% 9% 42%	2738
June	1st to 10th	Night	18% 6% 2 5% 4 5% 4 3 5% 8% 40%	1520
	11th to 20th	Night	18% 5% 2 5% 4 4 4 4 6% 7% 40%	1520
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 5% 7% 7% 39%	1520
July	1st to 10th	Night	10% 5% 2 7% 6% 5% 4 5% 7% 9% 41%	2520
	11th to 20th	Night	14% 6% 2 6% 4 3 4 4 6% 8% 42%	2520
	21st to end	Night	15% 8% 2 6% 3 4 5% 3 5% 6% 42%	2739
August	1st to 10th	Night	20% 6% 3 6% 4 4 4 3 6% 7% 36%	2567
	11th to 20th	Night	15% 7% 2 6% 5% 5% 4 3 6% 7% 40%	2938
	21st to end	Night	9% 4 1 3 3 3 4 3 6% 9% 55%	3476
September	1st to 10th	Night	11% 5% 2 4 3 3 4 4 5% 8% 51%	1900
	11th to 20th	Night	12% 5% 1 4 3 3 4 3 4 6% 53%	2035
	21st to end	Night	15% 5% 2 5% 3 4 4 3 5% 7% 47%	2090
October	1st to 10th	Night	20% 7% 2 7% 4 3 4 3 6% 6% 39%	2119
	11th to 20th	Night	15% 5% 2 7% 4 4 5% 4 5% 7% 42%	2336
	21st to end	Night	21% 8% 3 6% 4 4 5% 4 5% 6% 35%	2717
November	1st to 10th	Night	13% 6% 3 6% 4 4 5% 4 5% 7% 44%	2537
	11th to 20th	Night	15% 7% 2 5% 4 4 5% 4 6% 6% 41%	2600
	21st to end	Night	13% 8% 3 6% 5% 4 5% 4 6% 6% 40%	2693
December	1st to 10th	Night	16% 10% 3 8% 6% 6% 4 4 5% 7% 31%	2660
	11th to 20th	Night	19% 9% 2 6% 5% 5% 5% 3 6% 6% 34%	3288
	21st to end	Night	21% 12% 4 9% 5% 4 4 4 5% 6% 27%	4884

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	45% 7% 12% 28% 8%	2518
	11th to 20th	Day	55% 8% 10% 19% 8% 1	2520
	21st to end	Day	57% 7% 9% 18% 7% 1	2772
February	1st to 10th	Day	52% 7% 13% 17% 9% 1	1714
	11th to 20th	Day	49% 7% 11% 19% 12% 2	1857
	21st to end	Day	54% 5% 10% 17% 12% 2	1570
March	1st to 10th	Day	61% 5% 9% 12% 10% 2	3119
	11th to 20th	Day	46% 10% 13% 16% 12% 2	3242
	21st to end	Day	51% 5% 6% 15% 19% 4	4166
April	1st to 10th	Day	51% 5% 9% 21% 12% 3	2280
	11th to 20th	Day	46% 7% 11% 18% 16% 2	2254
	21st to end	Day	52% 6% 9% 17% 14% 3	2437
May	1st to 10th	Day	55% 8% 10% 17% 9% 1	4120
	11th to 20th	Day	50% 9% 14% 16% 10% 2	4370
	21st to end	Day	36% 9% 13% 26% 15% 1	4837
June	1st to 10th	Day	49% 7% 13% 20% 11% 1	2660
	11th to 20th	Day	44% 8% 13% 19% 15% 2	2660
	21st to end	Day	48% 9% 13% 19% 10% 1	2660
July	1st to 10th	Day	39% 9% 15% 21% 16% 1	4370
	11th to 20th	Day	37% 9% 17% 25% 11%	4365
	21st to end	Day	44% 7% 13% 21% 14% 1	4661
August	1st to 10th	Day	44% 9% 16% 21% 11%	4316
	11th to 20th	Day	43% 9% 16% 25% 7% 1	3924
	21st to end	Day	32% 9% 18% 27% 13% 1	4180
September	1st to 10th	Day	36% 8% 12% 27% 16% 1	2280
	11th to 20th	Day	35% 7% 11% 26% 20% 2	2144
	21st to end	Day	41% 6% 12% 23% 15% 3	2090
October	1st to 10th	Day	45% 8% 8% 22% 16% 2	3421
	11th to 20th	Day	38% 6% 11% 27% 17% 1	3046
	21st to end	Day	39% 5% 10% 30% 13% 3	3081
November	1st to 10th	Day	39% 7% 14% 22% 16% 1	1761
	11th to 20th	Day	44% 7% 10% 18% 18% 2	1800
	21st to end	Day	48% 6% 16% 22% 8% 1	1698
December	1st to 10th	Day	55% 7% 12% 15% 9% 1	1520
	11th to 20th	Day	53% 5% 11% 18% 13% 1	1906
	21st to end	Day	59% 7% 10% 15% 9%	2772
January	1st to 10th	Night	45% 9% 14% 21% 10% 1	4429
	11th to 20th	Night	55% 8% 10% 16% 9% 2	4440
	21st to end	Night	58% 7% 9% 16% 9% 1	4884
February	1st to 10th	Night	49% 8% 13% 18% 10% 1	2463
	11th to 20th	Night	45% 8% 11% 21% 13% 2	2220
	21st to end	Night	46% 7% 11% 23% 9% 4	1884
March	1st to 10th	Night	55% 6% 9% 19% 10% 2	3734
	11th to 20th	Night	45% 10% 11% 21% 12% 2	3698
	21st to end	Night	49% 6% 7% 12% 20% 5%	3466
April	1st to 10th	Night	45% 6% 11% 21% 15% 2	1900
	11th to 20th	Night	45% 7% 9% 22% 16% 1	1875
	21st to end	Night	46% 7% 9% 18% 15% 4	1743
May	1st to 10th	Night	53% 9% 9% 17% 9% 1	2840
	11th to 20th	Night	53% 11% 13% 14% 8% 2	2543
	21st to end	Night	39% 10% 14% 21% 14% 2	2739
June	1st to 10th	Night	43% 7% 14% 23% 12% 1	1520
	11th to 20th	Night	42% 9% 15% 20% 13% 1	1520
	21st to end	Night	43% 9% 13% 22% 11% 1	1520
July	1st to 10th	Night	38% 9% 17% 22% 14% 1	2480
	11th to 20th	Night	38% 9% 15% 25% 14%	2472
	21st to end	Night	42% 6% 11% 28% 13% 1	2651
August	1st to 10th	Night	47% 9% 14% 21% 9%	2557
	11th to 20th	Night	42% 9% 17% 22% 9%	2965
	21st to end	Night	26% 8% 17% 34% 14%	3476
September	1st to 10th	Night	33% 8% 13% 28% 18% 1	1900
	11th to 20th	Night	33% 7% 10% 29% 20% 1	2035
	21st to end	Night	37% 8% 10% 28% 16% 2	2090
October	1st to 10th	Night	42% 8% 14% 23% 12% 1	3535
	11th to 20th	Night	31% 8% 12% 26% 21% 3	3872
	21st to end	Night	40% 6% 10% 33% 11% 1	4467
November	1st to 10th	Night	40% 7% 13% 22% 16% 2	2537
	11th to 20th	Night	42% 7% 10% 22% 16% 3	2600
	21st to end	Night	44% 8% 16% 22% 10% 1	2693
December	1st to 10th	Night	52% 7% 13% 19% 8% 1	2660
	11th to 20th	Night	51% 7% 12% 15% 14% 1	3288
	21st to end	Night	58% 6% 11% 19% 6%	4884

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	24% 26% 27% 18% 6%	180
	11th to 20th	Day	30% 25% 28% 14% 2 1	180
	21st to end	Day	21% 30% 31% 13% 5%	198
February	1st to 10th	Day	28% 34% 24% 11% 2	287
	11th to 20th	Day	29% 24% 32% 14% 2	456
	21st to end	Day	29% 24% 28% 17% 2	382
March	1st to 10th	Day	28% 20% 33% 16% 4	460
	11th to 20th	Day	30% 25% 28% 13% 3 1	460
	21st to end	Day	30% 25% 26% 11% 6% 3	506
April	1st to 10th	Day	35% 21% 24% 11% 7% 1	460
	11th to 20th	Day	34% 21% 23% 13% 7% 2	452
	21st to end	Day	28% 20% 31% 13% 5% 2	460
May	1st to 10th	Day	29% 21% 27% 15% 7% 2	463
	11th to 20th	Day	23% 17% 32% 20% 5% 2	487
	21st to end	Day	18% 16% 31% 25% 9% 2	529
June	1st to 10th	Day	18% 18% 34% 19% 10% 1	490
	11th to 20th	Day	18% 16% 33% 21% 10% 2	490
	21st to end	Day	20% 16% 32% 20% 8% 4	490
July	1st to 10th	Day	15% 11% 30% 27% 13% 4	485
	11th to 20th	Day	14% 7% 35% 29% 12% 3	484
	21st to end	Day	14% 10% 36% 26% 12% 2	532
August	1st to 10th	Day	12% 13% 28% 28% 15% 4	486
	11th to 20th	Day	15% 10% 28% 25% 14% 8%	479
	21st to end	Day	13% 12% 28% 25% 15% 7%	539
September	1st to 10th	Day	16% 11% 25% 23% 16% 8%	490
	11th to 20th	Day	15% 13% 24% 20% 17% 11%	490
	21st to end	Day	16% 13% 28% 24% 12% 7%	490
October	1st to 10th	Day	26% 15% 23% 21% 11% 5%	490
	11th to 20th	Day	22% 17% 26% 24% 9% 3	460
	21st to end	Day	32% 14% 29% 17% 8% 1	429
November	1st to 10th	Day	23% 18% 25% 23% 10% 1	395
	11th to 20th	Day	30% 19% 26% 19% 5% 1	400
	21st to end	Day	33% 19% 25% 17% 5% 1	300
December	1st to 10th	Day	25% 32% 22% 16% 5%	180
	11th to 20th	Day	37% 23% 26% 11% 2	180
	21st to end	Day	32% 20% 29% 15% 4	198
January	1st to 10th	Night	22% 19% 33% 23% 3	760
	11th to 20th	Night	25% 19% 31% 22% 2	760
	21st to end	Night	19% 18% 35% 24% 4	836
February	1st to 10th	Night	22% 18% 37% 21% 2	760
	11th to 20th	Night	20% 22% 37% 19% 2	743
	21st to end	Night	25% 22% 31% 20% 2	628
March	1st to 10th	Night	26% 17% 31% 23% 2	760
	11th to 20th	Night	24% 22% 29% 21% 4	760
	21st to end	Night	26% 18% 29% 21% 6%	836
April	1st to 10th	Night	26% 18% 30% 22% 5%	760
	11th to 20th	Night	23% 20% 28% 22% 7%	751
	21st to end	Night	26% 16% 30% 24% 4	603
May	1st to 10th	Night	27% 14% 27% 26% 6%	570
	11th to 20th	Night	21% 11% 31% 29% 7%	408
	21st to end	Night	18% 8% 28% 33% 13% 1	411
June	1st to 10th	Night	21% 7% 27% 34% 12%	380
	11th to 20th	Night	18% 9% 28% 32% 12%	380
	21st to end	Night	14% 7% 27% 41% 11% 1	380
July	1st to 10th	Night	14% 6% 22% 42% 14% 1	370
	11th to 20th	Night	13% 4 21% 42% 18% 2	368
	21st to end	Night	14% 3 24% 39% 18% 1	416
August	1st to 10th	Night	12% 4 19% 46% 19% 1	413
	11th to 20th	Night	15% 7% 24% 33% 19% 3	653
	21st to end	Night	16% 6% 20% 37% 20% 2	836
September	1st to 10th	Night	13% 6% 28% 33% 19% 1	760
	11th to 20th	Night	16% 8% 19% 32% 20% 5%	760
	21st to end	Night	18% 9% 23% 32% 17% 2	760
October	1st to 10th	Night	24% 11% 18% 30% 14% 2	760
	11th to 20th	Night	19% 12% 28% 28% 12% 1	760
	21st to end	Night	26% 16% 27% 25% 6%	836
November	1st to 10th	Night	19% 15% 28% 28% 9%	780
	11th to 20th	Night	20% 18% 29% 25% 7% 1	800
	21st to end	Night	22% 17% 30% 24% 6% 1	800
December	1st to 10th	Night	23% 21% 33% 20% 4	760
	11th to 20th	Night	25% 18% 31% 23% 3	760
	21st to end	Night	28% 17% 34% 19% 2	836

Page generated at 2024-11-27 17:30:50. Version ac07

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