Stafford
Clear Sky Chart
History

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

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	14% 6% 2 6% 4 4 4 4 6% 8% 42%	126592
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 44%	123446
Transparency	Day	37% 6% 12% 23% 17% 4	130189
	Night	38% 6% 11% 23% 18% 4	127919
Seeing	Day	18% 11% 26% 25% 15% 5%	16341
	Night	17% 10% 29% 29% 13% 1	25888

By Month

Percentage Cloud Cover SE (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% 5% 2 4 3 3 4 3 5% 6% 50%	3827
	11th to 20th	Day	14% 8% 2 6% 3 4 3 3 3 5% 49%	4060
	21st to end	Day	15% 7% 3 6% 4 4 4 3 4 5% 45%	4466
February	1st to 10th	Day	14% 8% 2 7% 5% 4 5% 3 5% 6% 39%	1800
	11th to 20th	Day	19% 7% 2 5% 4 4 4 2 5% 6% 42%	1804
	21st to end	Day	18% 8% 2 6% 4 5% 4 3 5% 6% 39%	1650
March	1st to 10th	Day	18% 6% 2 5% 4 4 4 4 5% 7% 41%	4907
	11th to 20th	Day	11% 6% 2 5% 3 4 4 3 5% 6% 51%	4980
	21st to end	Day	17% 6% 2 7% 4 4 4 4 5% 7% 41%	5524
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	12% 6% 2 5% 5% 3 4 3 5% 6% 48%	2374
	21st to end	Day	16% 6% 2 5% 5% 4 5% 3 6% 7% 41%	2400
May	1st to 10th	Day	18% 7% 3 6% 4 5% 5% 4 7% 9% 32%	5763
	11th to 20th	Day	21% 7% 2 6% 4 3 4 4 6% 10% 32%	6038
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 7% 8% 40%	6934
June	1st to 10th	Day	11% 4 2 5% 5% 5% 5% 5% 6% 10% 44%	2800
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 5% 6% 9% 43%	2800
	21st to end	Day	8% 3 2 5% 5% 6% 5% 4 7% 10% 45%	2800
July	1st to 10th	Day	7% 5% 2 6% 5% 4 5% 5% 7% 11% 43%	6360
	11th to 20th	Day	9% 6% 2 7% 6% 5% 7% 6% 10% 13% 29%	6002
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 42%	6424
August	1st to 10th	Day	15% 5% 2 7% 5% 5% 5% 4 7% 12% 33%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 9% 40%	3992
	21st to end	Day	9% 4 2 5% 3 4 4 4 7% 8% 50%	4312
September	1st to 10th	Day	14% 4 2 5% 4 4 4 4 6% 8% 47%	2400
	11th to 20th	Day	12% 6% 2 4 3 3 3 4 6% 8% 49%	2400
	21st to end	Day	21% 7% 2 6% 4 4 4 4 5% 6% 36%	2093
October	1st to 10th	Day	16% 5% 2 5% 4 4 4 3 4 6% 49%	2000
	11th to 20th	Day	17% 4 2 4 3 3 4 3 4 6% 52%	2000
	21st to end	Day	14% 5% 2 5% 3 3 3 3 4 7% 51%	2200
November	1st to 10th	Day	12% 5% 1 4 4 4 4 3 5% 6% 53%	1913
	11th to 20th	Day	15% 6% 1 3 3 3 3 3 5% 6% 51%	1785
	21st to end	Day	16% 5% 1 5% 4 3 3 3 5% 5% 50%	1600
December	1st to 10th	Day	12% 5% 2 5% 3 4 4 4 5% 7% 50%	2600
	11th to 20th	Day	18% 8% 2 5% 3 2 3 2 4 6% 46%	2990
	21st to end	Day	17% 6% 2 6% 5% 4 4 3 6% 7% 40%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 3 3 4 5% 52%	6101
	11th to 20th	Night	13% 7% 2 5% 3 2 2 2 4 5% 54%	5900
	21st to end	Night	15% 7% 3 6% 4 3 3 2 4 5% 48%	6490
February	1st to 10th	Night	16% 7% 2 5% 4 4 4 3 6% 6% 42%	2600
	11th to 20th	Night	20% 7% 2 6% 4 3 3 2 4 5% 44%	2480
	21st to end	Night	15% 7% 3 5% 4 3 4 3 5% 6% 44%	1980
March	1st to 10th	Night	16% 6% 2 5% 4 3 4 3 4 5% 48%	4928
	11th to 20th	Night	12% 6% 3 4 3 3 4 4 5% 7% 50%	4980
	21st to end	Night	12% 7% 2 6% 4 4 4 4 4 6% 46%	5406
April	1st to 10th	Night	15% 7% 3 5% 4 4 4 3 5% 7% 43%	2000
	11th to 20th	Night	12% 5% 2 6% 6% 3 4 3 5% 6% 49%	1975
	21st to end	Night	17% 5% 1 5% 4 3 5% 4 6% 7% 43%	2000
May	1st to 10th	Night	18% 10% 2 8% 5% 4 5% 3 5% 9% 31%	4173
	11th to 20th	Night	23% 8% 2 6% 4 4 4 4 5% 8% 32%	3854
	21st to end	Night	17% 6% 2 7% 4 5% 5% 3 6% 9% 36%	3920
June	1st to 10th	Night	9% 5% 2 6% 4 5% 5% 5% 7% 10% 42%	1600
	11th to 20th	Night	11% 5% 2 6% 5% 5% 5% 5% 6% 8% 41%	1600
	21st to end	Night	9% 5% 1 5% 5% 5% 5% 5% 8% 10% 44%	1600
July	1st to 10th	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 40%	3598
	11th to 20th	Night	11% 7% 3 8% 6% 6% 6% 6% 7% 10% 30%	3951
	21st to end	Night	7% 6% 2 7% 6% 5% 6% 4 9% 9% 39%	4427
August	1st to 10th	Night	16% 6% 3 8% 5% 5% 4 5% 6% 9% 32%	2915
	11th to 20th	Night	11% 6% 3 9% 6% 5% 6% 5% 6% 8% 36%	3022
	21st to end	Night	11% 5% 2 5% 4 3 4 3 5% 8% 50%	3586
September	1st to 10th	Night	15% 5% 2 5% 4 4 5% 4 5% 7% 46%	2000
	11th to 20th	Night	11% 5% 3 6% 5% 4 4 3 4 8% 48%	2000
	21st to end	Night	21% 7% 2 6% 4 3 4 3 5% 7% 37%	2302
October	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 4 5% 47%	2400
	11th to 20th	Night	16% 5% 2 6% 3 3 3 3 5% 6% 50%	2400
	21st to end	Night	14% 5% 2 5% 3 4 4 3 5% 6% 50%	2640
November	1st to 10th	Night	12% 6% 2 4 4 3 3 2 4 5% 54%	2426
	11th to 20th	Night	15% 6% 2 4 4 3 3 3 4 6% 50%	2611
	21st to end	Night	12% 4 1 4 4 4 4 4 5% 7% 50%	2800
December	1st to 10th	Night	10% 5% 2 5% 3 4 3 3 5% 6% 55%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 4 5% 41%	5205
	21st to end	Night	17% 6% 2 5% 4 4 4 4 5% 7% 41%	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	35% 3 6% 25% 22% 9%	3828
	11th to 20th	Day	39% 4 6% 21% 20% 11%	4060
	21st to end	Day	42% 4 8% 26% 17% 3	4466
February	1st to 10th	Day	44% 5% 11% 25% 13% 2	1800
	11th to 20th	Day	44% 4 7% 23% 18% 4	1813
	21st to end	Day	45% 4 12% 17% 17% 4	1650
March	1st to 10th	Day	41% 5% 9% 21% 16% 9%	4907
	11th to 20th	Day	34% 4 9% 25% 22% 7%	4980
	21st to end	Day	42% 5% 11% 18% 17% 7%	5524
April	1st to 10th	Day	37% 6% 11% 21% 19% 7%	2400
	11th to 20th	Day	36% 5% 11% 20% 21% 6%	2374
	21st to end	Day	41% 7% 10% 24% 15% 5%	2400
May	1st to 10th	Day	45% 9% 14% 19% 12% 1	5797
	11th to 20th	Day	45% 8% 11% 25% 10% 1	6042
	21st to end	Day	38% 8% 14% 22% 16% 2	6949
June	1st to 10th	Day	34% 7% 15% 27% 16% 1	2800
	11th to 20th	Day	35% 8% 14% 25% 17% 2	2800
	21st to end	Day	31% 8% 15% 25% 18% 2	2800
July	1st to 10th	Day	32% 10% 20% 20% 13% 5%	6290
	11th to 20th	Day	39% 12% 20% 22% 6% 1	5930
	21st to end	Day	35% 8% 13% 22% 22% 1	6240
August	1st to 10th	Day	43% 8% 16% 26% 7%	4221
	11th to 20th	Day	37% 8% 14% 26% 14% 1	4031
	21st to end	Day	29% 7% 16% 28% 18% 2	4312
September	1st to 10th	Day	34% 7% 14% 20% 20% 5%	2400
	11th to 20th	Day	33% 5% 9% 28% 21% 4	2400
	21st to end	Day	47% 5% 10% 19% 17% 2	2093
October	1st to 10th	Day	35% 5% 7% 19% 30% 5%	3260
	11th to 20th	Day	26% 3 7% 26% 29% 10%	3240
	21st to end	Day	22% 3 9% 28% 26% 11%	3538
November	1st to 10th	Day	32% 4 10% 27% 22% 5%	1913
	11th to 20th	Day	34% 5% 10% 23% 22% 7%	1785
	21st to end	Day	36% 4 10% 26% 21% 3	1600
December	1st to 10th	Day	33% 6% 11% 27% 19% 5%	2600
	11th to 20th	Day	41% 3 14% 25% 13% 3	2986
	21st to end	Day	42% 5% 13% 23% 16% 1	3960
January	1st to 10th	Night	35% 3 7% 23% 27% 6%	6101
	11th to 20th	Night	34% 3 7% 24% 24% 8%	5900
	21st to end	Night	40% 3 7% 28% 19% 3	6490
February	1st to 10th	Night	41% 5% 11% 24% 17% 2	2600
	11th to 20th	Night	44% 3 7% 21% 19% 6%	2482
	21st to end	Night	41% 4 10% 21% 17% 6%	1980
March	1st to 10th	Night	38% 4 7% 24% 19% 8%	4928
	11th to 20th	Night	32% 5% 10% 23% 23% 7%	4980
	21st to end	Night	38% 5% 8% 22% 20% 6%	5406
April	1st to 10th	Night	40% 6% 8% 19% 23% 4	2000
	11th to 20th	Night	36% 6% 11% 21% 21% 5%	1975
	21st to end	Night	38% 6% 11% 25% 14% 5%	2000
May	1st to 10th	Night	50% 8% 13% 18% 8% 4	4202
	11th to 20th	Night	49% 8% 11% 20% 11% 1	3855
	21st to end	Night	44% 7% 13% 24% 10% 2	3927
June	1st to 10th	Night	35% 8% 13% 26% 16% 2	1600
	11th to 20th	Night	38% 9% 15% 22% 14% 2	1600
	21st to end	Night	34% 8% 16% 20% 19% 3	1600
July	1st to 10th	Night	39% 10% 18% 18% 13% 2	3560
	11th to 20th	Night	47% 11% 18% 17% 7% 1	3903
	21st to end	Night	37% 9% 15% 21% 17%	4297
August	1st to 10th	Night	46% 8% 16% 21% 8% 1	2904
	11th to 20th	Night	43% 9% 12% 22% 14% 1	3049
	21st to end	Night	33% 5% 11% 29% 20% 1	3586
September	1st to 10th	Night	36% 7% 11% 22% 17% 6%	2000
	11th to 20th	Night	35% 5% 10% 26% 22% 3	2000
	21st to end	Night	46% 6% 12% 17% 15% 3	2302
October	1st to 10th	Night	40% 5% 9% 20% 20% 6%	3920
	11th to 20th	Night	25% 4 9% 22% 32% 9%	3899
	21st to end	Night	22% 4 8% 31% 27% 9%	4252
November	1st to 10th	Night	34% 4 9% 25% 23% 6%	2426
	11th to 20th	Night	36% 4 8% 22% 22% 7%	2611
	21st to end	Night	33% 4 10% 26% 22% 4	2800
December	1st to 10th	Night	31% 4 10% 29% 22% 4	4580
	11th to 20th	Night	45% 4 11% 19% 16% 5%	5208
	21st to end	Night	41% 6% 15% 23% 15% 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	25% 22% 30% 18% 4	250
	11th to 20th	Day	26% 25% 28% 18% 2	300
	21st to end	Day	21% 20% 38% 16% 4	330
February	1st to 10th	Day	23% 20% 37% 13% 6%	300
	11th to 20th	Day	29% 26% 31% 11% 2	351
	21st to end	Day	26% 19% 30% 21% 3	414
March	1st to 10th	Day	22% 19% 34% 20% 4	500
	11th to 20th	Day	23% 18% 35% 18% 5% 1	500
	21st to end	Day	23% 15% 37% 19% 5% 1	550
April	1st to 10th	Day	23% 19% 32% 19% 6% 1	500
	11th to 20th	Day	17% 16% 30% 24% 11% 3	492
	21st to end	Day	21% 13% 31% 25% 9% 2	500
May	1st to 10th	Day	22% 13% 33% 19% 10% 3	503
	11th to 20th	Day	19% 11% 30% 23% 14% 3	527
	21st to end	Day	18% 5% 25% 23% 22% 6%	573
June	1st to 10th	Day	12% 5% 28% 28% 21% 7%	530
	11th to 20th	Day	12% 7% 22% 34% 18% 7%	530
	21st to end	Day	9% 4 21% 35% 22% 8%	530
July	1st to 10th	Day	11% 4 16% 31% 27% 11%	525
	11th to 20th	Day	5% 2 13% 35% 32% 13%	524
	21st to end	Day	11% 3 14% 35% 26% 11%	576
August	1st to 10th	Day	14% 3 17% 30% 27% 10%	526
	11th to 20th	Day	13% 4 16% 31% 25% 11%	519
	21st to end	Day	10% 3 18% 31% 22% 15%	583
September	1st to 10th	Day	17% 3 20% 26% 19% 16%	530
	11th to 20th	Day	14% 3 17% 31% 24% 11%	530
	21st to end	Day	27% 7% 24% 27% 9% 5%	530
October	1st to 10th	Day	21% 6% 19% 28% 18% 8%	530
	11th to 20th	Day	18% 13% 23% 26% 16% 3	530
	21st to end	Day	16% 13% 33% 26% 9% 3	583
November	1st to 10th	Day	16% 9% 32% 26% 14% 2	470
	11th to 20th	Day	19% 11% 37% 20% 11% 1	385
	21st to end	Day	21% 17% 30% 21% 12% 1	200
December	1st to 10th	Day	20% 16% 38% 21% 6% 2	200
	11th to 20th	Day	21% 13% 35% 24% 8%	200
	21st to end	Day	27% 13% 31% 21% 8%	220
January	1st to 10th	Night	21% 19% 37% 20% 3	800
	11th to 20th	Night	21% 15% 35% 25% 4	800
	21st to end	Night	19% 16% 40% 23% 2	880
February	1st to 10th	Night	21% 13% 39% 24% 3	800
	11th to 20th	Night	25% 19% 38% 17% 1	783
	21st to end	Night	21% 18% 36% 22% 2	660
March	1st to 10th	Night	20% 16% 38% 24% 3	800
	11th to 20th	Night	22% 16% 36% 20% 5% 1	800
	21st to end	Night	18% 16% 38% 23% 5%	880
April	1st to 10th	Night	18% 19% 38% 21% 4	800
	11th to 20th	Night	15% 12% 35% 29% 8%	791
	21st to end	Night	19% 11% 32% 30% 9%	800
May	1st to 10th	Night	21% 9% 31% 28% 11% 1	664
	11th to 20th	Night	19% 5% 30% 31% 15%	517
	21st to end	Night	17% 5% 20% 36% 21% 1	433
June	1st to 10th	Night	10% 3 23% 42% 21% 1	400
	11th to 20th	Night	14% 5% 21% 38% 22% 2	400
	21st to end	Night	10% 3 19% 47% 21% 1	400
July	1st to 10th	Night	10% 3 17% 38% 27% 4	390
	11th to 20th	Night	8% 2 13% 37% 34% 6%	531
	21st to end	Night	9% 1 16% 42% 28% 4	657
August	1st to 10th	Night	12% 2 12% 39% 30% 4	597
	11th to 20th	Night	13% 3 13% 41% 27% 4	676
	21st to end	Night	12% 3 15% 36% 29% 6%	880
September	1st to 10th	Night	17% 2 17% 33% 27% 5%	800
	11th to 20th	Night	12% 3 17% 35% 29% 4	800
	21st to end	Night	24% 5% 18% 33% 19% 1	800
October	1st to 10th	Night	20% 5% 20% 29% 23% 4	800
	11th to 20th	Night	17% 10% 28% 29% 16% 1	800
	21st to end	Night	16% 10% 32% 28% 13% 1	880
November	1st to 10th	Night	15% 10% 31% 30% 12% 2	789
	11th to 20th	Night	18% 13% 33% 28% 8%	800
	21st to end	Night	16% 12% 35% 26% 10% 1	800
December	1st to 10th	Night	20% 12% 35% 24% 9%	800
	11th to 20th	Night	20% 10% 39% 26% 5%	800
	21st to end	Night	19% 12% 33% 32% 4	880

Page generated at 2025-10-31 02:02:38. Version ac07

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