Preston
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Preston 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	18% 7% 2 6% 4 4 4 3 5% 7% 39%	126435
	Night	18% 8% 2 6% 4 4 4 3 5% 6% 40%	123623
Transparency	Day	43% 6% 12% 21% 15% 3	129968
	Night	44% 5% 11% 21% 16% 3	128177
Seeing	Day	22% 17% 32% 20% 7% 2	16375
	Night	22% 13% 32% 24% 8% 1	25305

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% 9% 3 5% 4 4 4 3 5% 5% 47%	3597
	11th to 20th	Day	15% 8% 2 5% 3 4 4 3 4 6% 46%	3570
	21st to end	Day	18% 10% 2 5% 3 3 4 3 4 4 42%	3960
February	1st to 10th	Day	18% 8% 2 5% 4 4 5% 5% 6% 6% 37%	1920
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 4 6% 41%	1948
	21st to end	Day	20% 9% 2 6% 4 4 3 3 4 5% 39%	1650
March	1st to 10th	Day	23% 10% 2 5% 3 3 3 2 4 5% 40%	4476
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 6% 7% 41%	4700
	21st to end	Day	23% 9% 2 6% 4 3 4 4 6% 6% 34%	5873
April	1st to 10th	Day	21% 8% 3 6% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	19% 6% 2 6% 3 4 4 4 4 7% 41%	2374
	21st to end	Day	22% 5% 2 6% 3 4 4 3 4 7% 39%	2560
May	1st to 10th	Day	31% 8% 2 6% 4 3 3 4 5% 8% 26%	5886
	11th to 20th	Day	22% 9% 2 7% 4 5% 4 3 4 6% 33%	5986
	21st to end	Day	21% 6% 2 5% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	2800
	11th to 20th	Day	17% 7% 2 6% 5% 4 4 4 7% 9% 36%	2800
	21st to end	Day	14% 7% 2 6% 5% 4 5% 5% 6% 9% 37%	2800
July	1st to 10th	Day	18% 7% 3 7% 5% 4 5% 4 6% 9% 33%	6360
	11th to 20th	Day	14% 6% 2 7% 5% 5% 5% 5% 7% 10% 34%	6360
	21st to end	Day	18% 6% 3 6% 4 3 4 4 5% 7% 41%	6926
August	1st to 10th	Day	20% 7% 3 7% 5% 4 6% 4 6% 8% 31%	4303
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 3 6% 8% 40%	3986
	21st to end	Day	7% 5% 3 7% 4 5% 5% 4 7% 9% 45%	4312
September	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 5% 5% 46%	2400
	11th to 20th	Day	14% 5% 2 5% 3 4 4 3 5% 7% 51%	2220
	21st to end	Day	20% 7% 2 5% 4 4 5% 3 4 6% 41%	2200
October	1st to 10th	Day	20% 8% 2 5% 3 3 3 3 4 5% 44%	2200
	11th to 20th	Day	16% 6% 2 5% 4 3 4 4 4 6% 48%	1966
	21st to end	Day	18% 6% 3 7% 4 4 2 3 5% 6% 44%	1980
November	1st to 10th	Day	14% 7% 1 5% 4 4 4 3 4 5% 49%	1779
	11th to 20th	Day	18% 5% 2 4 3 3 4 3 4 6% 48%	1800
	21st to end	Day	17% 6% 2 6% 5% 4 4 3 5% 5% 45%	1800
December	1st to 10th	Day	15% 8% 2 5% 4 3 3 3 5% 6% 46%	2648
	11th to 20th	Day	20% 6% 2 5% 2 3 2 2 4 5% 48%	2986
	21st to end	Day	21% 10% 3 6% 3 3 3 2 3 5% 41%	3960
January	1st to 10th	Night	14% 5% 2 5% 3 3 3 3 5% 7% 48%	6349
	11th to 20th	Night	15% 9% 3 7% 4 3 3 3 3 5% 45%	6315
	21st to end	Night	17% 9% 3 6% 5% 5% 5% 3 5% 5% 37%	6996
February	1st to 10th	Night	17% 9% 2 7% 4 4 3 3 5% 6% 41%	2475
	11th to 20th	Night	18% 8% 2 5% 4 3 4 3 5% 5% 41%	2338
	21st to end	Night	19% 9% 2 6% 4 4 3 3 4 5% 40%	1980
March	1st to 10th	Night	23% 8% 2 4 4 3 3 3 5% 5% 39%	5364
	11th to 20th	Night	17% 9% 3 6% 4 4 4 3 4 5% 41%	5257
	21st to end	Night	22% 9% 2 5% 4 4 5% 4 5% 5% 35%	5055
April	1st to 10th	Night	20% 8% 3 6% 4 4 4 3 5% 6% 38%	2000
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 39%	1975
	21st to end	Night	22% 7% 2 5% 5% 4 3 3 4 5% 41%	1838
May	1st to 10th	Night	28% 10% 3 7% 4 4 4 3 4 6% 28%	4054
	11th to 20th	Night	22% 10% 3 8% 4 4 4 3 5% 7% 29%	3950
	21st to end	Night	23% 8% 3 6% 5% 4 4 4 5% 6% 33%	3926
June	1st to 10th	Night	17% 8% 2 7% 4 4 5% 3 6% 7% 37%	1600
	11th to 20th	Night	19% 7% 2 5% 4 3 4 4 5% 8% 38%	1600
	21st to end	Night	14% 8% 2 5% 5% 3 5% 3 6% 8% 40%	1600
July	1st to 10th	Night	20% 7% 3 7% 5% 4 5% 4 6% 8% 33%	3600
	11th to 20th	Night	20% 9% 2 7% 4 4 4 4 6% 8% 32%	3600
	21st to end	Night	16% 7% 2 6% 4 4 5% 3 5% 6% 43%	3926
August	1st to 10th	Night	25% 7% 3 7% 4 4 5% 4 6% 8% 28%	2812
	11th to 20th	Night	10% 7% 2 8% 5% 5% 6% 5% 6% 8% 37%	3059
	21st to end	Night	9% 4 2 4 4 4 4 4 5% 8% 53%	3586
September	1st to 10th	Night	14% 6% 3 6% 4 4 4 3 6% 6% 43%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 6% 52%	2180
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 5% 39%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 5% 3 3 4 6% 43%	2200
	11th to 20th	Night	16% 5% 2 6% 3 3 4 3 5% 6% 47%	2430
	21st to end	Night	20% 7% 2 5% 3 3 4 3 5% 6% 41%	2860
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 6% 49%	2564
	11th to 20th	Night	18% 7% 2 4 3 3 3 3 4 6% 46%	2600
	21st to end	Night	15% 8% 2 5% 4 3 4 3 4 7% 46%	2600
December	1st to 10th	Night	18% 7% 2 5% 3 3 3 2 3 4 48%	4530
	11th to 20th	Night	25% 8% 2 4 3 3 3 2 3 4 42%	5208
	21st to end	Night	19% 9% 3 6% 4 3 3 2 4 5% 42%	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	39% 4 7% 27% 20% 3	3598
	11th to 20th	Day	39% 4 11% 21% 16% 8%	3600
	21st to end	Day	46% 3 10% 22% 18% 2	3960
February	1st to 10th	Day	45% 6% 14% 19% 14% 2	1920
	11th to 20th	Day	43% 5% 11% 22% 15% 3	1957
	21st to end	Day	47% 5% 12% 19% 15% 3	1650
March	1st to 10th	Day	46% 4 11% 16% 13% 10%	4479
	11th to 20th	Day	39% 6% 14% 28% 12% 1	4700
	21st to end	Day	50% 4 8% 15% 17% 6%	5873
April	1st to 10th	Day	45% 6% 11% 19% 15% 3	2400
	11th to 20th	Day	42% 7% 10% 19% 19% 2	2374
	21st to end	Day	44% 7% 10% 20% 15% 4	2560
May	1st to 10th	Day	56% 8% 15% 12% 8% 1	5897
	11th to 20th	Day	50% 7% 13% 17% 11% 1	5990
	21st to end	Day	46% 7% 12% 20% 13% 1	6949
June	1st to 10th	Day	44% 9% 13% 22% 12% 1	2800
	11th to 20th	Day	42% 8% 15% 20% 13% 1	2800
	21st to end	Day	41% 9% 14% 22% 14% 1	2800
July	1st to 10th	Day	48% 8% 11% 19% 13% 1	6290
	11th to 20th	Day	43% 5% 18% 23% 9% 1	6285
	21st to end	Day	42% 5% 11% 24% 16% 1	6728
August	1st to 10th	Day	50% 7% 16% 19% 8%	4289
	11th to 20th	Day	40% 6% 15% 29% 10% 1	4025
	21st to end	Day	33% 7% 15% 25% 18% 2	4312
September	1st to 10th	Day	40% 7% 12% 21% 19% 2	2400
	11th to 20th	Day	33% 6% 11% 23% 24% 3	2220
	21st to end	Day	44% 6% 9% 19% 20% 2	2200
October	1st to 10th	Day	37% 5% 10% 21% 22% 5%	3590
	11th to 20th	Day	29% 4 9% 23% 31% 3	3169
	21st to end	Day	32% 4 14% 26% 20% 3	3180
November	1st to 10th	Day	37% 6% 14% 20% 21% 2	1779
	11th to 20th	Day	36% 5% 12% 21% 22% 5%	1800
	21st to end	Day	41% 5% 11% 27% 13% 3	1800
December	1st to 10th	Day	40% 3 13% 28% 15% 1	2648
	11th to 20th	Day	41% 3 14% 18% 20% 5%	2986
	21st to end	Day	48% 5% 8% 19% 16% 4	3960
January	1st to 10th	Night	35% 4 9% 31% 19% 3	6349
	11th to 20th	Night	43% 4 11% 20% 16% 5%	6360
	21st to end	Night	48% 5% 9% 19% 15% 4	6996
February	1st to 10th	Night	46% 5% 12% 23% 12% 2	2475
	11th to 20th	Night	44% 5% 10% 21% 17% 5%	2340
	21st to end	Night	46% 5% 7% 22% 15% 4	1980
March	1st to 10th	Night	47% 4 10% 18% 14% 7%	5374
	11th to 20th	Night	45% 6% 15% 20% 12% 3	5258
	21st to end	Night	50% 4 6% 12% 23% 5%	5055
April	1st to 10th	Night	47% 5% 11% 19% 15% 4	2000
	11th to 20th	Night	45% 6% 11% 18% 19% 2	1975
	21st to end	Night	47% 5% 9% 19% 16% 4	1838
May	1st to 10th	Night	59% 5% 15% 12% 9% 1	4060
	11th to 20th	Night	54% 7% 13% 14% 8% 3	3951
	21st to end	Night	51% 8% 13% 18% 9%	3927
June	1st to 10th	Night	45% 7% 12% 19% 15% 1	1600
	11th to 20th	Night	43% 8% 13% 24% 12% 1	1600
	21st to end	Night	41% 7% 15% 24% 13% 1	1600
July	1st to 10th	Night	52% 6% 11% 20% 10% 1	3560
	11th to 20th	Night	51% 5% 15% 20% 8% 1	3552
	21st to end	Night	43% 5% 13% 23% 16% 1	3809
August	1st to 10th	Night	53% 8% 13% 16% 8% 1	2802
	11th to 20th	Night	42% 7% 15% 27% 8%	3086
	21st to end	Night	28% 6% 15% 30% 19% 1	3586
September	1st to 10th	Night	42% 7% 9% 23% 17% 2	2000
	11th to 20th	Night	34% 5% 12% 22% 25% 2	2180
	21st to end	Night	48% 6% 8% 17% 19% 3	2200
October	1st to 10th	Night	39% 4 11% 24% 19% 3	3590
	11th to 20th	Night	26% 4 9% 25% 32% 4	3966
	21st to end	Night	33% 4 12% 27% 21% 2	4610
November	1st to 10th	Night	37% 6% 13% 21% 20% 4	2564
	11th to 20th	Night	40% 4 9% 22% 20% 5%	2600
	21st to end	Night	39% 5% 13% 28% 13% 2	2600
December	1st to 10th	Night	41% 4 11% 28% 15% 1	4530
	11th to 20th	Night	47% 3 12% 16% 20% 2	5208
	21st to end	Night	46% 4 9% 19% 18% 4	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	28% 25% 29% 16% 4	200
	11th to 20th	Day	24% 35% 32% 8% 3	200
	21st to end	Day	24% 28% 34% 10% 3 1	220
February	1st to 10th	Day	29% 26% 32% 12% 1	425
	11th to 20th	Day	28% 30% 32% 9% 1	496
	21st to end	Day	26% 27% 34% 10% 3	414
March	1st to 10th	Day	28% 22% 34% 14% 2	500
	11th to 20th	Day	24% 25% 35% 12% 3 1	500
	21st to end	Day	31% 20% 33% 12% 3 1	550
April	1st to 10th	Day	30% 24% 29% 11% 5% 1	500
	11th to 20th	Day	27% 24% 30% 14% 4	492
	21st to end	Day	30% 19% 33% 13% 4 2	500
May	1st to 10th	Day	30% 21% 30% 14% 4 1	503
	11th to 20th	Day	20% 20% 39% 17% 4 1	527
	21st to end	Day	21% 12% 32% 25% 8% 2	573
June	1st to 10th	Day	19% 12% 34% 22% 10% 2	530
	11th to 20th	Day	19% 13% 36% 25% 6% 1	530
	21st to end	Day	17% 14% 32% 25% 9% 2	530
July	1st to 10th	Day	18% 8% 31% 28% 13% 2	525
	11th to 20th	Day	14% 9% 29% 32% 13% 3	524
	21st to end	Day	14% 11% 36% 30% 7% 2	576
August	1st to 10th	Day	13% 10% 33% 30% 10% 4	526
	11th to 20th	Day	15% 6% 31% 29% 13% 5%	519
	21st to end	Day	10% 12% 32% 27% 14% 6%	583
September	1st to 10th	Day	18% 10% 29% 25% 13% 4	530
	11th to 20th	Day	13% 8% 34% 25% 12% 7%	530
	21st to end	Day	23% 9% 36% 21% 9% 2	530
October	1st to 10th	Day	24% 12% 26% 23% 9% 5%	530
	11th to 20th	Day	21% 17% 29% 21% 11% 1	494
	21st to end	Day	25% 16% 33% 18% 6% 2	462
November	1st to 10th	Day	17% 17% 32% 23% 8% 2	401
	11th to 20th	Day	24% 24% 28% 18% 6%	400
	21st to end	Day	22% 22% 30% 19% 7% 1	400
December	1st to 10th	Day	23% 23% 34% 15% 4	235
	11th to 20th	Day	33% 26% 30% 9% 4	200
	21st to end	Day	25% 25% 32% 13% 4 2	220
January	1st to 10th	Night	22% 23% 36% 18% 3	800
	11th to 20th	Night	29% 20% 32% 17% 3	800
	21st to end	Night	20% 24% 35% 19% 2	880
February	1st to 10th	Night	22% 20% 35% 22% 1	800
	11th to 20th	Night	27% 19% 39% 14% 1	783
	21st to end	Night	25% 22% 34% 16% 3	660
March	1st to 10th	Night	26% 18% 35% 18% 3	800
	11th to 20th	Night	22% 17% 38% 20% 4	800
	21st to end	Night	24% 18% 36% 18% 4 1	880
April	1st to 10th	Night	28% 15% 34% 20% 3	800
	11th to 20th	Night	24% 14% 37% 21% 4	791
	21st to end	Night	24% 12% 38% 20% 6%	638
May	1st to 10th	Night	27% 11% 32% 24% 7%	600
	11th to 20th	Night	22% 13% 39% 22% 4	561
	21st to end	Night	22% 5% 28% 34% 10% 1	433
June	1st to 10th	Night	21% 6% 31% 32% 11%	400
	11th to 20th	Night	23% 7% 25% 38% 8%	400
	21st to end	Night	17% 6% 28% 38% 11% 1	400
July	1st to 10th	Night	18% 5% 24% 35% 18% 1	390
	11th to 20th	Night	16% 4 21% 43% 15% 1	388
	21st to end	Night	16% 3 24% 44% 12% 1	438
August	1st to 10th	Night	13% 4 23% 44% 14% 1	533
	11th to 20th	Night	15% 3 22% 40% 18% 2	701
	21st to end	Night	14% 4 22% 38% 19% 3	880
September	1st to 10th	Night	18% 4 27% 33% 17% 2	800
	11th to 20th	Night	16% 5% 28% 29% 18% 4	800
	21st to end	Night	22% 7% 28% 28% 14% 2	800
October	1st to 10th	Night	22% 10% 29% 23% 14% 2	800
	11th to 20th	Night	20% 12% 33% 24% 11% 1	800
	21st to end	Night	24% 14% 34% 20% 8%	880
November	1st to 10th	Night	16% 14% 35% 23% 11% 1	789
	11th to 20th	Night	23% 18% 34% 19% 6% 1	800
	21st to end	Night	21% 19% 34% 21% 6%	800
December	1st to 10th	Night	26% 17% 32% 20% 4 1	800
	11th to 20th	Night	26% 19% 33% 19% 4	800
	21st to end	Night	25% 18% 35% 19% 3	880

Page generated at 2025-10-31 02:02:39. Version ac07

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