Sutton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sutton clear sky chart from 2005-11-06 to 2025-01-22.

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	19% 13% 4 9% 5% 5% 4 3 5% 6% 27%	105238
	Night	18% 15% 5% 11% 5% 4 4 3 4 5% 26%	104151
Transparency	Day	57% 6% 12% 18% 7% 1	108724
	Night	60% 6% 10% 16% 8% 1	108754
Seeing	Day	28% 17% 28% 18% 7% 2	14849
	Night	26% 16% 26% 23% 8% 1	23882

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	15% 23% 8% 17% 7% 5% 4 2 3 3 13%	2597
	11th to 20th	Day	10% 18% 8% 20% 10% 6% 4 4 4 4 11%	2600
	21st to end	Day	13% 23% 8% 13% 5% 8% 5% 3 4 4 13%	2780
February	1st to 10th	Day	15% 17% 8% 15% 9% 7% 4 3 4 3 17%	1624
	11th to 20th	Day	12% 20% 8% 15% 8% 5% 5% 3 3 3 18%	1824
	21st to end	Day	13% 19% 6% 12% 8% 6% 4 4 4 5% 20%	1570
March	1st to 10th	Day	21% 16% 5% 10% 6% 5% 4 3 4 5% 21%	3116
	11th to 20th	Day	13% 24% 6% 12% 6% 4 3 2 3 4 21%	3259
	21st to end	Day	18% 13% 6% 11% 5% 3 3 2 3 4 32%	4166
April	1st to 10th	Day	22% 18% 4 9% 4 3 4 2 4 4 25%	2280
	11th to 20th	Day	20% 15% 4 7% 4 4 4 3 4 5% 31%	2259
	21st to end	Day	25% 13% 3 7% 4 5% 4 3 4 6% 27%	2470
May	1st to 10th	Day	25% 12% 3 7% 5% 5% 4 4 5% 6% 26%	4224
	11th to 20th	Day	20% 9% 3 9% 6% 6% 5% 4 6% 6% 27%	4398
	21st to end	Day	16% 8% 3 7% 4 5% 5% 5% 7% 9% 33%	4837
June	1st to 10th	Day	28% 8% 2 5% 4 4 5% 4 5% 7% 27%	2660
	11th to 20th	Day	20% 7% 2 6% 4 5% 4 4 6% 7% 34%	2660
	21st to end	Day	25% 8% 3 7% 4 4 5% 4 6% 6% 27%	2660
July	1st to 10th	Day	15% 6% 3 7% 5% 5% 5% 5% 7% 9% 32%	4440
	11th to 20th	Day	16% 7% 3 6% 5% 6% 6% 5% 8% 8% 30%	4440
	21st to end	Day	16% 7% 2 6% 5% 5% 5% 3 6% 7% 37%	4814
August	1st to 10th	Day	20% 9% 4 8% 4 5% 6% 4 7% 7% 28%	4423
	11th to 20th	Day	20% 8% 3 8% 5% 5% 5% 4 6% 7% 29%	3991
	21st to end	Day	20% 7% 2 5% 3 4 5% 3 5% 8% 37%	4180
September	1st to 10th	Day	18% 7% 3 6% 4 3 4 4 5% 7% 40%	2280
	11th to 20th	Day	19% 6% 2 5% 3 3 4 2 5% 7% 45%	2155
	21st to end	Day	19% 8% 2 6% 4 4 4 3 6% 9% 35%	2090
October	1st to 10th	Day	25% 10% 2 7% 6% 4 5% 3 5% 5% 28%	2033
	11th to 20th	Day	22% 13% 3 7% 5% 4 5% 4 5% 5% 28%	1815
	21st to end	Day	27% 18% 4 7% 4 4 3 2 3 4 24%	1881
November	1st to 10th	Day	19% 17% 4 8% 4 4 4 3 4 5% 27%	1761
	11th to 20th	Day	18% 19% 6% 10% 5% 4 3 3 4 4 23%	1800
	21st to end	Day	18% 21% 7% 11% 4 5% 3 3 5% 4 18%	1605
December	1st to 10th	Day	16% 24% 7% 21% 8% 4 3 2 3 3 10%	2600
	11th to 20th	Day	18% 20% 6% 16% 4 5% 3 2 3 4 19%	2986
	21st to end	Day	18% 19% 6% 13% 6% 5% 5% 3 4 4 18%	3960
January	1st to 10th	Night	14% 22% 8% 18% 7% 6% 4 3 4 3 12%	4569
	11th to 20th	Night	11% 18% 8% 20% 9% 6% 5% 3 3 3 13%	4580
	21st to end	Night	15% 22% 7% 13% 8% 5% 4 2 3 3 18%	4899
February	1st to 10th	Night	14% 20% 6% 15% 8% 6% 4 3 3 4 16%	2554
	11th to 20th	Night	13% 18% 7% 14% 8% 6% 5% 3 4 4 19%	2242
	21st to end	Night	13% 18% 6% 12% 6% 6% 4 2 4 4 24%	1884
March	1st to 10th	Night	21% 15% 4 9% 5% 4 3 3 4 5% 28%	3724
	11th to 20th	Night	15% 23% 6% 11% 6% 4 3 2 3 3 24%	3699
	21st to end	Night	15% 21% 5% 11% 5% 3 3 2 3 3 29%	3466
April	1st to 10th	Night	22% 14% 5% 8% 5% 4 3 4 4 4 26%	1900
	11th to 20th	Night	23% 13% 3 7% 5% 4 4 2 3 6% 30%	1870
	21st to end	Night	23% 13% 3 6% 5% 4 4 3 4 5% 32%	1710
May	1st to 10th	Night	24% 12% 4 8% 4 4 4 3 5% 6% 26%	2745
	11th to 20th	Night	21% 9% 3 8% 5% 4 4 3 4 6% 33%	2495
	21st to end	Night	18% 6% 2 7% 5% 5% 4 4 5% 8% 36%	2738
June	1st to 10th	Night	27% 7% 2 7% 4 4 4 3 5% 7% 30%	1520
	11th to 20th	Night	20% 7% 3 5% 3 5% 5% 4 6% 7% 36%	1520
	21st to end	Night	22% 8% 3 6% 5% 4 6% 4 7% 8% 29%	1520
July	1st to 10th	Night	16% 8% 3 8% 6% 5% 5% 4 6% 7% 32%	2520
	11th to 20th	Night	20% 7% 2 7% 5% 4 4 4 6% 8% 34%	2520
	21st to end	Night	19% 7% 2 7% 5% 4 4 4 5% 6% 37%	2739
August	1st to 10th	Night	25% 8% 3 7% 4 4 4 3 5% 7% 32%	2506
	11th to 20th	Night	18% 9% 3 8% 6% 5% 5% 3 6% 6% 32%	2803
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 4 6% 41%	3476
September	1st to 10th	Night	19% 5% 2 6% 5% 4 4 4 4 6% 41%	1900
	11th to 20th	Night	17% 6% 1 5% 4 4 3 3 6% 7% 46%	2024
	21st to end	Night	21% 6% 2 5% 4 4 4 3 4 7% 40%	2090
October	1st to 10th	Night	21% 10% 2 8% 4 4 3 4 5% 6% 33%	2145
	11th to 20th	Night	23% 12% 3 8% 5% 4 4 3 4 5% 29%	2363
	21st to end	Night	27% 15% 3 7% 4 4 4 3 4 6% 24%	2717
November	1st to 10th	Night	21% 15% 4 8% 4 4 4 3 4 5% 26%	2537
	11th to 20th	Night	19% 17% 5% 10% 5% 4 3 3 4 5% 26%	2600
	21st to end	Night	19% 21% 6% 12% 5% 4 4 3 4 3 20%	2792
December	1st to 10th	Night	15% 26% 12% 19% 6% 3 3 2 3 3 9%	4580
	11th to 20th	Night	20% 23% 7% 16% 5% 5% 3 2 3 2 15%	5208
	21st to end	Night	18% 20% 6% 12% 6% 5% 5% 2 3 3 20%	6996

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	75% 4 13% 7% 1	2598
	11th to 20th	Day	74% 7% 9% 7% 3	2600
	21st to end	Day	74% 5% 9% 8% 3 1	2780
February	1st to 10th	Day	73% 5% 6% 12% 4 1	1624
	11th to 20th	Day	71% 4 10% 10% 4 1	1833
	21st to end	Day	66% 5% 9% 12% 6% 1	1570
March	1st to 10th	Day	65% 5% 10% 12% 6% 1	3119
	11th to 20th	Day	66% 6% 10% 9% 9% 1	3259
	21st to end	Day	57% 4 7% 16% 15% 1	4166
April	1st to 10th	Day	63% 4 8% 16% 8% 1	2280
	11th to 20th	Day	56% 5% 7% 19% 12% 1	2259
	21st to end	Day	60% 5% 9% 16% 9% 2	2470
May	1st to 10th	Day	57% 6% 11% 17% 7% 1	4238
	11th to 20th	Day	53% 10% 15% 14% 7% 1	4402
	21st to end	Day	44% 9% 14% 22% 10% 1	4837
June	1st to 10th	Day	54% 7% 13% 18% 8% 1	2660
	11th to 20th	Day	47% 6% 13% 22% 10% 2	2660
	21st to end	Day	56% 7% 11% 18% 7% 1	2660
July	1st to 10th	Day	46% 6% 15% 25% 8%	4370
	11th to 20th	Day	47% 8% 14% 27% 4	4365
	21st to end	Day	44% 8% 13% 29% 6%	4661
August	1st to 10th	Day	54% 7% 15% 19% 6%	4409
	11th to 20th	Day	51% 7% 18% 19% 5%	4030
	21st to end	Day	44% 6% 16% 19% 13% 1	4180
September	1st to 10th	Day	42% 6% 14% 27% 10% 1	2280
	11th to 20th	Day	40% 4 13% 30% 12%	2155
	21st to end	Day	44% 7% 15% 24% 9% 1	2090
October	1st to 10th	Day	48% 6% 14% 25% 7%	3384
	11th to 20th	Day	54% 5% 13% 20% 8%	2992
	21st to end	Day	54% 4 15% 20% 6% 1	3081
November	1st to 10th	Day	58% 7% 11% 17% 7%	1761
	11th to 20th	Day	65% 5% 10% 12% 8% 2	1800
	21st to end	Day	69% 6% 13% 9% 4	1605
December	1st to 10th	Day	81% 4 7% 6% 1	2600
	11th to 20th	Day	70% 4 7% 14% 5%	2986
	21st to end	Day	71% 4 6% 10% 8% 1	3960
January	1st to 10th	Night	74% 6% 12% 6% 1	4569
	11th to 20th	Night	74% 6% 10% 7% 2	4580
	21st to end	Night	71% 5% 11% 8% 4 1	4899
February	1st to 10th	Night	72% 5% 7% 11% 5%	2554
	11th to 20th	Night	68% 5% 9% 12% 5%	2244
	21st to end	Night	64% 4 9% 14% 8% 1	1884
March	1st to 10th	Night	59% 6% 11% 15% 8% 2	3734
	11th to 20th	Night	64% 7% 7% 13% 9% 1	3700
	21st to end	Night	60% 4 6% 18% 10% 2	3466
April	1st to 10th	Night	61% 5% 7% 16% 10% 1	1900
	11th to 20th	Night	55% 6% 8% 18% 13%	1870
	21st to end	Night	55% 6% 8% 17% 11% 2	1710
May	1st to 10th	Night	57% 7% 10% 16% 8% 2	2751
	11th to 20th	Night	50% 8% 14% 17% 9% 1	2496
	21st to end	Night	44% 8% 12% 20% 15% 1	2739
June	1st to 10th	Night	53% 6% 10% 21% 9% 1	1520
	11th to 20th	Night	46% 8% 12% 21% 11% 2	1520
	21st to end	Night	50% 9% 14% 17% 9% 1	1520
July	1st to 10th	Night	49% 6% 12% 21% 11%	2480
	11th to 20th	Night	46% 8% 13% 27% 7%	2472
	21st to end	Night	46% 7% 14% 23% 10%	2651
August	1st to 10th	Night	51% 7% 14% 22% 6%	2496
	11th to 20th	Night	51% 7% 17% 18% 6%	2830
	21st to end	Night	43% 6% 13% 24% 13% 1	3476
September	1st to 10th	Night	43% 7% 12% 26% 11% 1	1900
	11th to 20th	Night	38% 5% 16% 29% 11%	2024
	21st to end	Night	44% 6% 12% 24% 13% 1	2090
October	1st to 10th	Night	53% 5% 10% 23% 9%	3574
	11th to 20th	Night	53% 4 10% 22% 10%	3925
	21st to end	Night	55% 5% 13% 17% 9% 1	4467
November	1st to 10th	Night	59% 6% 11% 16% 7% 1	2537
	11th to 20th	Night	61% 5% 9% 14% 9% 2	2600
	21st to end	Night	69% 5% 10% 12% 3	2792
December	1st to 10th	Night	82% 4 6% 6% 1	4580
	11th to 20th	Night	76% 4 5% 10% 6%	5208
	21st to end	Night	70% 5% 6% 11% 7% 1	6996

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	30% 25% 29% 12% 6%	200
	11th to 20th	Day	28% 31% 28% 10% 3 1	200
	21st to end	Day	26% 24% 32% 15% 4 1	200
February	1st to 10th	Day	38% 27% 26% 7% 2	229
	11th to 20th	Day	33% 27% 30% 8% 2	432
	21st to end	Day	33% 20% 31% 15% 2	382
March	1st to 10th	Day	33% 19% 29% 16% 2	460
	11th to 20th	Day	32% 22% 28% 13% 3 1	460
	21st to end	Day	35% 22% 23% 14% 6% 1	506
April	1st to 10th	Day	37% 18% 25% 15% 5%	460
	11th to 20th	Day	36% 21% 23% 13% 6% 2	452
	21st to end	Day	35% 17% 29% 12% 5% 2	460
May	1st to 10th	Day	34% 19% 25% 14% 6% 2	463
	11th to 20th	Day	23% 18% 31% 17% 9% 2	487
	21st to end	Day	20% 19% 32% 22% 6% 1	529
June	1st to 10th	Day	31% 14% 32% 16% 7% 1	490
	11th to 20th	Day	21% 13% 34% 22% 9% 1	490
	21st to end	Day	29% 12% 33% 17% 7% 2	490
July	1st to 10th	Day	19% 11% 32% 27% 11% 1	485
	11th to 20th	Day	18% 12% 36% 24% 8% 2	484
	21st to end	Day	19% 10% 35% 27% 8% 1	532
August	1st to 10th	Day	19% 12% 29% 26% 14% 1	486
	11th to 20th	Day	21% 8% 29% 24% 13% 4	479
	21st to end	Day	22% 14% 25% 23% 11% 6%	539
September	1st to 10th	Day	20% 13% 29% 21% 12% 5%	490
	11th to 20th	Day	18% 11% 22% 23% 18% 7%	490
	21st to end	Day	20% 13% 27% 24% 11% 5%	490
October	1st to 10th	Day	27% 17% 23% 18% 12% 3	490
	11th to 20th	Day	33% 18% 21% 18% 9% 1	445
	21st to end	Day	40% 16% 24% 14% 5%	429
November	1st to 10th	Day	35% 19% 27% 14% 4 1	395
	11th to 20th	Day	33% 23% 23% 15% 5% 2	400
	21st to end	Day	34% 23% 27% 14% 1	205
December	1st to 10th	Day	33% 21% 30% 12% 5%	200
	11th to 20th	Day	33% 26% 31% 9% 1 1	200
	21st to end	Day	36% 22% 25% 12% 4	220
January	1st to 10th	Night	28% 24% 29% 16% 3	800
	11th to 20th	Night	28% 24% 28% 17% 4	800
	21st to end	Night	21% 20% 32% 22% 5%	841
February	1st to 10th	Night	26% 21% 32% 18% 4	760
	11th to 20th	Night	24% 23% 32% 18% 3	743
	21st to end	Night	27% 19% 30% 21% 2	628
March	1st to 10th	Night	29% 18% 28% 21% 3	760
	11th to 20th	Night	30% 18% 25% 20% 6%	760
	21st to end	Night	28% 18% 25% 19% 8% 1	836
April	1st to 10th	Night	31% 16% 27% 21% 4	760
	11th to 20th	Night	31% 17% 24% 20% 8%	746
	21st to end	Night	32% 15% 28% 19% 6%	570
May	1st to 10th	Night	26% 12% 30% 23% 8% 1	520
	11th to 20th	Night	24% 13% 28% 28% 7% 1	374
	21st to end	Night	20% 11% 29% 32% 7% 1	411
June	1st to 10th	Night	31% 7% 24% 29% 8% 1	380
	11th to 20th	Night	22% 8% 26% 31% 13% 1	380
	21st to end	Night	20% 11% 24% 36% 9% 1	380
July	1st to 10th	Night	16% 4 30% 36% 13%	370
	11th to 20th	Night	13% 6% 23% 40% 17% 1	368
	21st to end	Night	17% 5% 23% 41% 14%	416
August	1st to 10th	Night	18% 6% 18% 37% 20% 1	378
	11th to 20th	Night	16% 7% 21% 36% 18% 2	569
	21st to end	Night	18% 7% 24% 32% 18% 2	836
September	1st to 10th	Night	20% 9% 24% 32% 14% 1	760
	11th to 20th	Night	20% 7% 18% 30% 23% 2	760
	21st to end	Night	22% 10% 25% 25% 16% 1	760
October	1st to 10th	Night	27% 13% 23% 26% 11% 1	760
	11th to 20th	Night	29% 14% 24% 22% 9% 1	760
	21st to end	Night	35% 15% 27% 19% 4	836
November	1st to 10th	Night	31% 16% 27% 19% 6% 1	780
	11th to 20th	Night	27% 21% 26% 16% 8% 2	800
	21st to end	Night	32% 23% 24% 15% 5% 1	800
December	1st to 10th	Night	35% 21% 24% 15% 4	800
	11th to 20th	Night	32% 20% 27% 16% 4 1	800
	21st to end	Night	32% 21% 25% 18% 3	880

Page generated at 2025-01-19 03:06:49. Version ac07

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