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 2024-11-06.

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%	101194
	Night	19% 8% 2 6% 4 4 4 3 5% 6% 40%	97885
Transparency	Day	43% 6% 13% 21% 15% 2	104745
	Night	44% 6% 12% 21% 15% 2	102423
Seeing	Day	21% 17% 32% 20% 8% 2	15095
	Night	21% 13% 32% 25% 8% 1	24051

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	16% 8% 3 5% 4 4 4 3 4 4 46%	2517
	11th to 20th	Day	18% 8% 3 6% 4 5% 5% 3 5% 6% 38%	2520
	21st to end	Day	23% 13% 3 5% 3 3 4 3 4 4 35%	2772
February	1st to 10th	Day	19% 8% 2 5% 4 4 4 4 6% 6% 37%	1823
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 4 6% 41%	1848
	21st to end	Day	21% 10% 3 6% 4 4 3 3 4 5% 38%	1570
March	1st to 10th	Day	26% 11% 3 6% 4 3 2 2 4 4 35%	3116
	11th to 20th	Day	12% 6% 2 5% 5% 4 5% 4 6% 8% 44%	3284
	21st to end	Day	22% 9% 2 6% 4 3 4 4 6% 6% 36%	4097
April	1st to 10th	Day	20% 8% 3 6% 4 4 4 3 5% 7% 38%	2280
	11th to 20th	Day	19% 6% 2 6% 3 4 4 4 4 7% 41%	2254
	21st to end	Day	22% 5% 2 6% 3 4 4 3 4 7% 38%	2432
May	1st to 10th	Day	29% 9% 2 7% 3 3 3 4 5% 7% 28%	4106
	11th to 20th	Day	24% 9% 2 7% 4 4 4 3 4 6% 33%	4164
	21st to end	Day	16% 5% 1 5% 4 3 4 4 6% 8% 45%	4837
June	1st to 10th	Day	18% 6% 2 6% 5% 5% 5% 3 6% 8% 35%	2660
	11th to 20th	Day	16% 7% 2 5% 5% 4 4 4 7% 9% 36%	2660
	21st to end	Day	14% 7% 2 6% 5% 4 5% 4 6% 9% 37%	2660
July	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 4 6% 8% 35%	4440
	11th to 20th	Day	15% 5% 2 6% 4 5% 5% 4 6% 9% 38%	4440
	21st to end	Day	21% 8% 3 7% 4 4 5% 4 5% 7% 33%	4814
August	1st to 10th	Day	20% 7% 3 7% 5% 4 5% 4 6% 8% 30%	4171
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 3 6% 8% 40%	3862
	21st to end	Day	7% 5% 3 7% 5% 5% 6% 4 7% 9% 44%	4180
September	1st to 10th	Day	15% 7% 2 5% 4 4 4 4 4 5% 46%	2280
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 7% 50%	2109
	21st to end	Day	20% 7% 2 5% 4 4 4 3 4 6% 41%	2090
October	1st to 10th	Day	21% 8% 2 5% 3 3 3 3 4 5% 42%	2090
	11th to 20th	Day	15% 6% 2 5% 4 3 4 4 4 6% 49%	1868
	21st to end	Day	18% 6% 3 7% 4 4 3 3 5% 6% 44%	1881
November	1st to 10th	Day	15% 7% 1 6% 4 4 4 3 4 4 48%	1717
	11th to 20th	Day	18% 5% 2 4 3 3 4 3 4 6% 46%	1710
	21st to end	Day	17% 6% 2 6% 5% 4 4 3 5% 5% 45%	1710
December	1st to 10th	Day	21% 8% 2 5% 4 3 4 3 6% 6% 37%	1554
	11th to 20th	Day	22% 7% 2 5% 3 3 2 2 4 5% 46%	1906
	21st to end	Day	21% 11% 3 7% 4 3 4 2 4 5% 36%	2772
January	1st to 10th	Night	17% 6% 2 4 3 3 3 3 4 7% 48%	4429
	11th to 20th	Night	17% 10% 3 7% 4 3 3 3 3 4 43%	4440
	21st to end	Night	23% 12% 3 7% 5% 3 4 3 4 4 32%	4884
February	1st to 10th	Night	17% 9% 2 7% 4 4 3 3 5% 5% 41%	2352
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 5% 5% 41%	2218
	21st to end	Night	20% 9% 2 5% 4 4 3 3 4 5% 40%	1884
March	1st to 10th	Night	28% 10% 2 5% 4 3 3 2 5% 5% 34%	3724
	11th to 20th	Night	11% 7% 3 6% 5% 5% 4 3 5% 6% 44%	3673
	21st to end	Night	24% 8% 2 5% 3 3 4 4 5% 6% 36%	3531
April	1st to 10th	Night	19% 8% 3 6% 4 4 4 4 5% 6% 38%	1900
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 39%	1875
	21st to end	Night	22% 7% 2 5% 5% 4 3 3 4 5% 40%	1746
May	1st to 10th	Night	25% 10% 3 7% 4 4 4 3 4 6% 29%	2834
	11th to 20th	Night	23% 9% 3 9% 5% 5% 5% 3 5% 7% 27%	2758
	21st to end	Night	18% 7% 2 5% 4 4 4 4 5% 7% 40%	2738
June	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 6% 7% 38%	1520
	11th to 20th	Night	19% 7% 2 5% 4 3 3 4 5% 8% 39%	1520
	21st to end	Night	14% 8% 2 5% 5% 3 5% 3 6% 8% 39%	1520
July	1st to 10th	Night	18% 6% 3 8% 6% 5% 5% 4 6% 8% 32%	2520
	11th to 20th	Night	19% 7% 2 5% 4 3 4 4 7% 9% 36%	2520
	21st to end	Night	21% 9% 2 7% 5% 4 5% 4 5% 6% 33%	2739
August	1st to 10th	Night	25% 7% 3 7% 4 4 5% 4 6% 8% 27%	2725
	11th to 20th	Night	10% 7% 2 8% 5% 5% 6% 5% 6% 8% 36%	2963
	21st to end	Night	9% 4 2 5% 4 4 4 3 5% 8% 53%	3476
September	1st to 10th	Night	15% 6% 3 6% 4 4 4 3 6% 6% 42%	1900
	11th to 20th	Night	14% 5% 2 5% 3 3 3 3 4 6% 52%	2071
	21st to end	Night	19% 8% 3 7% 4 4 4 3 4 5% 39%	2090
October	1st to 10th	Night	18% 7% 2 6% 4 5% 3 3 4 6% 41%	2090
	11th to 20th	Night	15% 5% 2 6% 3 3 4 3 5% 6% 48%	2309
	21st to end	Night	21% 8% 2 6% 3 3 4 3 5% 6% 41%	2717
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 6% 48%	2483
	11th to 20th	Night	19% 7% 2 5% 3 3 3 3 4 6% 45%	2470
	21st to end	Night	15% 8% 2 5% 4 3 4 3 4 6% 46%	2470
December	1st to 10th	Night	21% 8% 2 5% 4 4 4 2 4 6% 40%	2624
	11th to 20th	Night	21% 7% 2 4 3 4 4 3 4 6% 42%	3288
	21st to end	Night	22% 10% 3 6% 4 3 3 2 3 5% 38%	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	41% 6% 9% 29% 14% 2	2518
	11th to 20th	Day	46% 5% 11% 23% 12% 2	2520
	21st to end	Day	52% 4 9% 22% 12% 2	2772
February	1st to 10th	Day	45% 6% 14% 19% 13% 2	1823
	11th to 20th	Day	42% 5% 11% 23% 16% 3	1857
	21st to end	Day	48% 5% 12% 18% 14% 3	1570
March	1st to 10th	Day	52% 5% 12% 17% 12% 3	3119
	11th to 20th	Day	32% 8% 15% 29% 14% 2	3284
	21st to end	Day	48% 5% 7% 18% 17% 5%	4097
April	1st to 10th	Day	44% 6% 11% 19% 15% 3	2280
	11th to 20th	Day	42% 7% 10% 20% 19% 2	2254
	21st to end	Day	45% 7% 10% 19% 15% 4	2432
May	1st to 10th	Day	54% 9% 13% 13% 10% 1	4120
	11th to 20th	Day	50% 9% 14% 17% 9% 2	4168
	21st to end	Day	36% 8% 14% 22% 18% 2	4837
June	1st to 10th	Day	43% 9% 13% 22% 12% 1	2660
	11th to 20th	Day	42% 8% 15% 21% 12% 1	2660
	21st to end	Day	41% 10% 14% 22% 13% 1	2660
July	1st to 10th	Day	46% 8% 11% 20% 14% 1	4370
	11th to 20th	Day	41% 6% 18% 23% 11% 1	4365
	21st to end	Day	50% 6% 13% 17% 13% 2	4661
August	1st to 10th	Day	50% 7% 17% 18% 8%	4157
	11th to 20th	Day	40% 6% 15% 29% 10% 1	3901
	21st to end	Day	33% 7% 15% 25% 17% 2	4180
September	1st to 10th	Day	40% 7% 12% 20% 20% 2	2280
	11th to 20th	Day	33% 6% 12% 23% 24% 3	2109
	21st to end	Day	44% 6% 8% 19% 21% 2	2090
October	1st to 10th	Day	38% 5% 10% 21% 21% 5%	3480
	11th to 20th	Day	28% 4 9% 23% 32% 3	3071
	21st to end	Day	32% 4 14% 26% 20% 3	3081
November	1st to 10th	Day	38% 6% 13% 20% 20% 3	1717
	11th to 20th	Day	37% 5% 12% 21% 21% 4	1710
	21st to end	Day	41% 5% 12% 26% 13% 3	1710
December	1st to 10th	Day	47% 5% 13% 18% 16% 1	1554
	11th to 20th	Day	42% 4 14% 17% 18% 4	1906
	21st to end	Day	50% 6% 11% 22% 10% 1	2772
January	1st to 10th	Night	38% 4 11% 31% 12% 4	4429
	11th to 20th	Night	46% 5% 11% 23% 13% 3	4440
	21st to end	Night	55% 6% 9% 19% 11% 2	4884
February	1st to 10th	Night	45% 5% 12% 23% 12% 2	2352
	11th to 20th	Night	44% 5% 10% 20% 16% 5%	2220
	21st to end	Night	46% 5% 7% 23% 15% 4	1884
March	1st to 10th	Night	52% 5% 9% 21% 12% 2	3734
	11th to 20th	Night	38% 8% 15% 22% 16% 2	3674
	21st to end	Night	48% 5% 7% 13% 22% 5%	3531
April	1st to 10th	Night	46% 5% 11% 19% 15% 3	1900
	11th to 20th	Night	45% 6% 10% 18% 19% 2	1875
	21st to end	Night	47% 5% 10% 19% 15% 4	1746
May	1st to 10th	Night	57% 6% 13% 14% 8% 1	2840
	11th to 20th	Night	55% 9% 13% 13% 8% 2	2759
	21st to end	Night	42% 10% 15% 21% 13% 1	2739
June	1st to 10th	Night	44% 8% 13% 20% 14% 1	1520
	11th to 20th	Night	42% 8% 14% 24% 12% 1	1520
	21st to end	Night	41% 7% 15% 24% 12% 1	1520
July	1st to 10th	Night	52% 7% 11% 18% 12% 1	2480
	11th to 20th	Night	44% 7% 17% 23% 8% 1	2472
	21st to end	Night	51% 6% 13% 14% 14% 1	2651
August	1st to 10th	Night	54% 9% 13% 15% 8% 1	2715
	11th to 20th	Night	42% 7% 15% 27% 8%	2990
	21st to end	Night	28% 6% 16% 30% 19% 1	3476
September	1st to 10th	Night	43% 7% 9% 22% 17% 2	1900
	11th to 20th	Night	34% 5% 12% 22% 24% 2	2071
	21st to end	Night	48% 6% 8% 16% 20% 3	2090
October	1st to 10th	Night	40% 4 12% 23% 18% 3	3480
	11th to 20th	Night	26% 4 9% 25% 33% 4	3845
	21st to end	Night	33% 4 13% 27% 21% 2	4467
November	1st to 10th	Night	37% 6% 12% 21% 19% 4	2483
	11th to 20th	Night	41% 4 10% 22% 19% 4	2470
	21st to end	Night	39% 5% 13% 28% 13% 2	2470
December	1st to 10th	Night	46% 5% 11% 21% 15% 1	2624
	11th to 20th	Night	43% 5% 14% 19% 18% 2	3288
	21st to end	Night	50% 5% 10% 22% 13% 1	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	26% 25% 28% 17% 4	180
	11th to 20th	Day	22% 37% 31% 8% 3	180
	21st to end	Day	21% 28% 36% 11% 4 1	198
February	1st to 10th	Day	28% 25% 33% 13% 1	388
	11th to 20th	Day	25% 30% 34% 10% 1	456
	21st to end	Day	26% 27% 34% 10% 4	382
March	1st to 10th	Day	26% 21% 36% 15% 2	460
	11th to 20th	Day	23% 25% 36% 12% 3 1	460
	21st to end	Day	30% 20% 34% 12% 3 1	506
April	1st to 10th	Day	27% 24% 30% 12% 6% 1	460
	11th to 20th	Day	25% 24% 31% 15% 4	452
	21st to end	Day	30% 18% 32% 13% 4 2	460
May	1st to 10th	Day	30% 21% 30% 15% 4 1	463
	11th to 20th	Day	20% 19% 39% 18% 4 1	487
	21st to end	Day	19% 11% 33% 27% 9% 2	529
June	1st to 10th	Day	18% 13% 34% 23% 11% 2	490
	11th to 20th	Day	17% 13% 37% 26% 7% 1	490
	21st to end	Day	17% 14% 33% 25% 9% 2	490
July	1st to 10th	Day	18% 8% 31% 28% 14% 2	485
	11th to 20th	Day	14% 9% 29% 32% 13% 3	484
	21st to end	Day	14% 10% 36% 30% 8% 2	532
August	1st to 10th	Day	14% 10% 32% 31% 10% 4	486
	11th to 20th	Day	16% 6% 30% 29% 13% 5%	479
	21st to end	Day	10% 10% 30% 28% 15% 6%	539
September	1st to 10th	Day	19% 10% 28% 26% 13% 4	490
	11th to 20th	Day	14% 8% 33% 24% 13% 8%	490
	21st to end	Day	23% 9% 36% 21% 9% 3	490
October	1st to 10th	Day	25% 12% 26% 23% 9% 4	490
	11th to 20th	Day	19% 17% 29% 21% 11% 2	457
	21st to end	Day	24% 16% 33% 19% 7% 2	429
November	1st to 10th	Day	17% 17% 32% 23% 9% 2	381
	11th to 20th	Day	25% 23% 28% 19% 6%	370
	21st to end	Day	21% 21% 30% 20% 8% 1	370
December	1st to 10th	Day	24% 25% 32% 14% 5%	214
	11th to 20th	Day	29% 27% 31% 9% 3	180
	21st to end	Day	24% 26% 32% 12% 4 2	198
January	1st to 10th	Night	21% 21% 36% 19% 3	760
	11th to 20th	Night	29% 20% 32% 17% 3	760
	21st to end	Night	19% 24% 35% 19% 2	836
February	1st to 10th	Night	22% 19% 36% 22% 1	760
	11th to 20th	Night	24% 20% 40% 15% 1	743
	21st to end	Night	26% 21% 34% 15% 3	628
March	1st to 10th	Night	25% 18% 36% 18% 3	760
	11th to 20th	Night	21% 18% 38% 20% 3	760
	21st to end	Night	25% 18% 35% 18% 4 1	836
April	1st to 10th	Night	27% 14% 34% 21% 3	760
	11th to 20th	Night	23% 13% 38% 22% 4	751
	21st to end	Night	24% 11% 38% 20% 6%	606
May	1st to 10th	Night	26% 12% 31% 24% 7%	570
	11th to 20th	Night	22% 13% 38% 23% 4	533
	21st to end	Night	21% 5% 28% 35% 10% 1	411
June	1st to 10th	Night	20% 6% 31% 32% 11%	380
	11th to 20th	Night	22% 7% 24% 38% 8%	380
	21st to end	Night	18% 6% 28% 37% 11% 1	380
July	1st to 10th	Night	17% 4 25% 34% 19% 1	370
	11th to 20th	Night	15% 4 21% 44% 15% 1	368
	21st to end	Night	16% 3 24% 43% 12% 1	416
August	1st to 10th	Night	13% 5% 24% 44% 13% 1	506
	11th to 20th	Night	15% 3 23% 39% 18% 2	665
	21st to end	Night	14% 4 21% 39% 19% 3	836
September	1st to 10th	Night	18% 4 27% 33% 16% 2	760
	11th to 20th	Night	16% 6% 27% 28% 18% 4	760
	21st to end	Night	21% 7% 29% 27% 14% 2	760
October	1st to 10th	Night	23% 9% 29% 24% 13% 3	760
	11th to 20th	Night	18% 12% 33% 25% 11% 1	760
	21st to end	Night	24% 13% 35% 20% 8%	836
November	1st to 10th	Night	16% 13% 35% 24% 12% 1	764
	11th to 20th	Night	24% 17% 34% 19% 6% 1	760
	21st to end	Night	19% 19% 34% 21% 6%	760
December	1st to 10th	Night	26% 17% 32% 20% 4 1	760
	11th to 20th	Night	25% 19% 33% 19% 4	760
	21st to end	Night	25% 18% 36% 18% 3	836

Page generated at 2024-11-03 13:19:00. Version ac07

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