Putney Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Putney Observatory clear sky chart from 2005-11-06 to 2024-11-05.

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% 36%	101317
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 38%	97727
Transparency	Day	45% 8% 13% 20% 12% 2	104842
	Night	44% 8% 13% 21% 12% 2	102291
Seeing	Day	23% 18% 29% 19% 8% 2	14890
	Night	21% 15% 30% 26% 8% 1	23840

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% 7% 3 8% 5% 5% 5% 5% 6% 6% 37%	2517
	11th to 20th	Day	14% 8% 4 12% 5% 4 5% 4 6% 6% 32%	2520
	21st to end	Day	16% 11% 5% 11% 5% 5% 4 4 5% 5% 29%	2772
February	1st to 10th	Day	14% 11% 3 8% 6% 6% 5% 5% 6% 7% 30%	1725
	11th to 20th	Day	10% 9% 3 9% 6% 5% 4 4 7% 7% 35%	1848
	21st to end	Day	17% 10% 3 10% 5% 5% 4 3 5% 5% 33%	1570
March	1st to 10th	Day	20% 14% 4 10% 5% 4 4 2 4 5% 28%	3116
	11th to 20th	Day	12% 8% 2 7% 6% 6% 5% 5% 7% 6% 36%	3246
	21st to end	Day	19% 8% 3 8% 4 4 5% 3 4 6% 36%	4163
April	1st to 10th	Day	17% 12% 2 8% 5% 4 4 3 5% 6% 36%	2280
	11th to 20th	Day	17% 8% 3 5% 4 5% 5% 5% 6% 7% 37%	2254
	21st to end	Day	19% 8% 2 7% 6% 5% 4 4 5% 6% 34%	2435
May	1st to 10th	Day	22% 10% 3 9% 4 4 5% 4 5% 6% 29%	4106
	11th to 20th	Day	21% 8% 3 7% 5% 4 4 4 6% 7% 32%	4353
	21st to end	Day	12% 5% 2 6% 4 4 4 4 6% 9% 43%	4837
June	1st to 10th	Day	17% 6% 2 6% 6% 5% 5% 4 6% 9% 33%	2660
	11th to 20th	Day	15% 6% 3 6% 5% 5% 5% 3 6% 8% 37%	2660
	21st to end	Day	16% 7% 2 7% 5% 5% 6% 4 7% 8% 32%	2660
July	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 5% 8% 10% 36%	4440
	11th to 20th	Day	10% 5% 2 6% 5% 5% 6% 5% 8% 11% 36%	4440
	21st to end	Day	16% 7% 3 6% 5% 5% 5% 4 6% 7% 36%	4814
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 35%	4308
	11th to 20th	Day	13% 6% 3 8% 5% 5% 5% 4 7% 9% 35%	3869
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 41%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 44%	2280
	11th to 20th	Day	13% 5% 3 5% 4 4 4 3 4 7% 49%	2142
	21st to end	Day	16% 6% 2 6% 5% 4 5% 3 5% 7% 42%	2090
October	1st to 10th	Day	21% 6% 2 7% 4 5% 4 3 5% 5% 38%	2062
	11th to 20th	Day	14% 6% 2 7% 5% 4 5% 4 5% 8% 42%	1843
	21st to end	Day	20% 9% 2 7% 5% 5% 5% 4 4 6% 34%	1881
November	1st to 10th	Day	12% 7% 3 6% 4 4 4 3 5% 6% 45%	1708
	11th to 20th	Day	17% 6% 3 6% 5% 4 4 3 5% 6% 40%	1710
	21st to end	Day	18% 8% 2 6% 4 4 4 3 6% 7% 38%	1630
December	1st to 10th	Day	16% 10% 3 8% 6% 5% 5% 4 5% 6% 31%	1520
	11th to 20th	Day	20% 9% 2 7% 5% 4 3 3 4 4 37%	1906
	21st to end	Day	20% 12% 3 10% 5% 5% 5% 4 4 5% 28%	2772
January	1st to 10th	Night	15% 9% 3 6% 5% 4 4 3 5% 7% 39%	4429
	11th to 20th	Night	11% 11% 4 11% 6% 5% 5% 4 5% 6% 31%	4440
	21st to end	Night	18% 13% 5% 8% 5% 5% 4 3 5% 5% 28%	4884
February	1st to 10th	Night	12% 10% 3 8% 6% 6% 5% 5% 6% 7% 34%	2450
	11th to 20th	Night	10% 9% 3 7% 6% 6% 6% 4 6% 7% 37%	2218
	21st to end	Night	15% 8% 3 8% 6% 4 5% 3 5% 7% 37%	1884
March	1st to 10th	Night	24% 11% 2 7% 5% 4 3 3 4 5% 31%	3724
	11th to 20th	Night	11% 9% 3 8% 6% 5% 5% 4 6% 6% 37%	3713
	21st to end	Night	19% 10% 4 7% 4 4 3 3 5% 6% 35%	3466
April	1st to 10th	Night	17% 7% 3 6% 4 4 5% 3 5% 7% 38%	1900
	11th to 20th	Night	17% 9% 2 5% 4 3 4 3 6% 7% 39%	1875
	21st to end	Night	19% 8% 3 6% 4 4 4 3 4 7% 38%	1743
May	1st to 10th	Night	24% 9% 3 6% 4 4 5% 4 5% 7% 28%	2834
	11th to 20th	Night	16% 8% 3 10% 6% 6% 6% 5% 6% 7% 27%	2569
	21st to end	Night	15% 5% 1 5% 5% 4 4 4 6% 9% 41%	2738
June	1st to 10th	Night	17% 7% 2 5% 5% 4 5% 4 5% 7% 39%	1520
	11th to 20th	Night	18% 5% 2 5% 5% 4 4 3 6% 8% 40%	1520
	21st to end	Night	15% 6% 2 6% 5% 5% 5% 4 7% 6% 39%	1520
July	1st to 10th	Night	10% 5% 2 6% 6% 5% 5% 5% 7% 10% 40%	2520
	11th to 20th	Night	14% 6% 2 6% 4 5% 4 4 6% 8% 41%	2520
	21st to end	Night	17% 6% 2 5% 4 4 5% 3 4 7% 42%	2739
August	1st to 10th	Night	20% 6% 2 7% 5% 4 5% 3 4 7% 36%	2589
	11th to 20th	Night	14% 7% 2 7% 5% 5% 4 4 5% 8% 38%	2941
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 9% 55%	3476
September	1st to 10th	Night	11% 5% 2 5% 3 4 3 4 5% 8% 49%	1900
	11th to 20th	Night	13% 5% 1 4 3 3 4 3 4 6% 53%	2038
	21st to end	Night	15% 4 2 5% 4 3 4 4 6% 6% 47%	2090
October	1st to 10th	Night	21% 6% 2 5% 4 4 4 4 5% 7% 38%	2114
	11th to 20th	Night	14% 6% 2 5% 4 4 5% 5% 6% 7% 42%	2335
	21st to end	Night	21% 8% 2 6% 4 3 5% 4 5% 5% 36%	2717
November	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 6% 6% 45%	2470
	11th to 20th	Night	16% 7% 2 5% 5% 4 4 4 5% 7% 41%	2470
	21st to end	Night	14% 8% 2 6% 4 4 5% 4 6% 6% 42%	2549
December	1st to 10th	Night	16% 10% 3 8% 5% 5% 5% 4 5% 5% 34%	2660
	11th to 20th	Night	19% 9% 3 6% 5% 4 5% 3 5% 5% 37%	3288
	21st to end	Night	21% 12% 5% 9% 4 4 5% 3 5% 5% 28%	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	42% 9% 13% 28% 8% 1	2518
	11th to 20th	Day	49% 10% 13% 18% 8% 1	2520
	21st to end	Day	55% 7% 9% 19% 8% 1	2772
February	1st to 10th	Day	50% 9% 13% 17% 10% 1	1725
	11th to 20th	Day	48% 6% 11% 19% 13% 2	1857
	21st to end	Day	53% 6% 10% 17% 12% 2	1570
March	1st to 10th	Day	59% 6% 11% 12% 11% 2	3119
	11th to 20th	Day	38% 14% 17% 18% 10% 3	3246
	21st to end	Day	47% 6% 8% 16% 18% 4	4163
April	1st to 10th	Day	49% 6% 10% 19% 13% 3	2280
	11th to 20th	Day	44% 9% 11% 17% 16% 2	2254
	21st to end	Day	50% 7% 9% 16% 14% 3	2435
May	1st to 10th	Day	54% 9% 12% 15% 9% 1	4120
	11th to 20th	Day	49% 9% 15% 16% 9% 2	4357
	21st to end	Day	33% 10% 16% 24% 15% 1	4837
June	1st to 10th	Day	46% 9% 14% 19% 11% 1	2660
	11th to 20th	Day	44% 8% 13% 19% 14% 2	2660
	21st to end	Day	47% 10% 14% 18% 11% 1	2660
July	1st to 10th	Day	41% 9% 13% 22% 14% 1	4370
	11th to 20th	Day	39% 11% 17% 23% 11% 1	4365
	21st to end	Day	45% 8% 13% 18% 14% 1	4661
August	1st to 10th	Day	45% 9% 15% 20% 10%	4294
	11th to 20th	Day	43% 9% 15% 26% 7% 1	3908
	21st to end	Day	35% 9% 16% 27% 13% 1	4180
September	1st to 10th	Day	36% 8% 13% 25% 16% 1	2280
	11th to 20th	Day	35% 7% 11% 24% 21% 2	2142
	21st to end	Day	40% 7% 12% 23% 16% 3	2090
October	1st to 10th	Day	46% 7% 8% 21% 16% 2	3426
	11th to 20th	Day	36% 6% 11% 28% 17% 2	3046
	21st to end	Day	37% 5% 10% 30% 14% 3	3081
November	1st to 10th	Day	39% 7% 15% 23% 15% 2	1708
	11th to 20th	Day	43% 8% 11% 20% 16% 2	1710
	21st to end	Day	46% 6% 17% 21% 9% 2	1630
December	1st to 10th	Day	53% 7% 14% 14% 10% 1	1520
	11th to 20th	Day	50% 6% 13% 17% 13% 1	1906
	21st to end	Day	55% 9% 11% 16% 9%	2772
January	1st to 10th	Night	42% 8% 15% 23% 10% 2	4429
	11th to 20th	Night	50% 10% 14% 15% 8% 2	4440
	21st to end	Night	56% 8% 11% 15% 9% 1	4884
February	1st to 10th	Night	48% 8% 14% 18% 10% 2	2450
	11th to 20th	Night	43% 8% 15% 19% 13% 2	2220
	21st to end	Night	46% 6% 13% 22% 9% 4	1884
March	1st to 10th	Night	52% 8% 11% 18% 10% 1	3734
	11th to 20th	Night	42% 11% 15% 19% 11% 2	3714
	21st to end	Night	47% 7% 9% 13% 20% 4	3466
April	1st to 10th	Night	46% 7% 12% 19% 14% 2	1900
	11th to 20th	Night	43% 9% 11% 20% 16% 2	1875
	21st to end	Night	46% 7% 11% 17% 15% 4	1743
May	1st to 10th	Night	52% 10% 13% 15% 9% 1	2840
	11th to 20th	Night	51% 12% 14% 13% 8% 2	2570
	21st to end	Night	37% 10% 15% 22% 14% 1	2739
June	1st to 10th	Night	44% 8% 14% 22% 11% 1	1520
	11th to 20th	Night	43% 9% 16% 19% 12% 1	1520
	21st to end	Night	40% 11% 15% 20% 13% 1	1520
July	1st to 10th	Night	40% 10% 14% 21% 15% 1	2480
	11th to 20th	Night	41% 9% 13% 22% 14%	2472
	21st to end	Night	43% 7% 12% 24% 14% 1	2651
August	1st to 10th	Night	49% 8% 14% 20% 9%	2579
	11th to 20th	Night	44% 8% 17% 21% 9%	2968
	21st to end	Night	28% 6% 17% 33% 14% 1	3476
September	1st to 10th	Night	35% 7% 14% 25% 18% 1	1900
	11th to 20th	Night	32% 7% 13% 26% 20% 2	2038
	21st to end	Night	37% 8% 11% 26% 16% 2	2090
October	1st to 10th	Night	42% 8% 14% 22% 13% 1	3530
	11th to 20th	Night	31% 7% 11% 26% 22% 2	3871
	21st to end	Night	37% 6% 12% 33% 11% 1	4467
November	1st to 10th	Night	39% 7% 12% 23% 16% 3	2470
	11th to 20th	Night	43% 7% 12% 21% 14% 3	2470
	21st to end	Night	43% 7% 17% 23% 10% 1	2549
December	1st to 10th	Night	50% 8% 14% 20% 9% 1	2660
	11th to 20th	Night	47% 9% 15% 14% 14% 2	3288
	21st to end	Night	54% 10% 10% 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	28% 23% 27% 17% 4	180
	11th to 20th	Day	31% 23% 34% 9% 2 1	180
	21st to end	Day	24% 26% 34% 12% 5%	198
February	1st to 10th	Day	32% 30% 26% 11% 1	297
	11th to 20th	Day	30% 26% 30% 12% 1	456
	21st to end	Day	32% 22% 30% 14% 2	382
March	1st to 10th	Day	29% 21% 33% 14% 3	460
	11th to 20th	Day	31% 25% 27% 13% 4 1	460
	21st to end	Day	30% 26% 26% 11% 4 3	506
April	1st to 10th	Day	33% 24% 24% 12% 6% 1	460
	11th to 20th	Day	30% 25% 25% 14% 6% 1	452
	21st to end	Day	31% 19% 31% 12% 6% 2	460
May	1st to 10th	Day	32% 22% 26% 13% 7% 2	463
	11th to 20th	Day	22% 19% 35% 17% 6% 1	487
	21st to end	Day	19% 16% 31% 25% 8% 2	529
June	1st to 10th	Day	17% 18% 34% 20% 10% 1	490
	11th to 20th	Day	18% 15% 33% 22% 9% 2	490
	21st to end	Day	18% 18% 33% 19% 9% 3	490
July	1st to 10th	Day	15% 12% 31% 26% 13% 3	485
	11th to 20th	Day	15% 7% 34% 30% 12% 2	484
	21st to end	Day	15% 11% 35% 26% 10% 3	532
August	1st to 10th	Day	12% 11% 28% 30% 15% 4	486
	11th to 20th	Day	15% 10% 27% 27% 16% 5%	479
	21st to end	Day	13% 11% 28% 29% 14% 6%	539
September	1st to 10th	Day	18% 10% 25% 24% 16% 6%	490
	11th to 20th	Day	17% 13% 25% 21% 13% 11%	490
	21st to end	Day	18% 14% 29% 22% 12% 6%	490
October	1st to 10th	Day	26% 16% 23% 19% 12% 4	490
	11th to 20th	Day	20% 19% 26% 23% 10% 2	460
	21st to end	Day	32% 13% 27% 20% 7% 1	429
November	1st to 10th	Day	24% 16% 26% 22% 10% 1	378
	11th to 20th	Day	29% 19% 28% 19% 4 1	370
	21st to end	Day	31% 18% 27% 20% 4 1	290
December	1st to 10th	Day	24% 27% 31% 13% 4 1	180
	11th to 20th	Day	38% 19% 31% 9% 2	180
	21st to end	Day	31% 20% 33% 13% 4	198
January	1st to 10th	Night	20% 23% 34% 20% 3	760
	11th to 20th	Night	26% 19% 33% 20% 2	760
	21st to end	Night	19% 19% 37% 22% 3	836
February	1st to 10th	Night	21% 19% 37% 21% 2	760
	11th to 20th	Night	20% 24% 38% 17% 2	743
	21st to end	Night	25% 23% 31% 18% 2	628
March	1st to 10th	Night	24% 21% 33% 20% 2	760
	11th to 20th	Night	22% 24% 31% 19% 4	760
	21st to end	Night	26% 18% 30% 20% 6%	836
April	1st to 10th	Night	26% 18% 30% 21% 4	760
	11th to 20th	Night	24% 20% 28% 22% 6%	751
	21st to end	Night	27% 18% 28% 22% 5%	603
May	1st to 10th	Night	26% 15% 30% 23% 5% 1	570
	11th to 20th	Night	19% 16% 32% 28% 6%	422
	21st to end	Night	17% 10% 27% 36% 10% 1	411
June	1st to 10th	Night	20% 8% 30% 31% 11%	380
	11th to 20th	Night	17% 9% 32% 32% 8% 1	380
	21st to end	Night	17% 8% 26% 40% 9% 1	380
July	1st to 10th	Night	13% 6% 27% 44% 10% 1	370
	11th to 20th	Night	14% 4 26% 39% 16% 2	368
	21st to end	Night	13% 3 27% 41% 15% 1	416
August	1st to 10th	Night	14% 5% 22% 41% 18%	422
	11th to 20th	Night	14% 6% 26% 34% 18% 2	656
	21st to end	Night	14% 7% 22% 36% 19% 2	836
September	1st to 10th	Night	14% 6% 29% 33% 17% 2	760
	11th to 20th	Night	15% 8% 22% 32% 19% 3	760
	21st to end	Night	18% 8% 25% 31% 15% 2	760
October	1st to 10th	Night	25% 12% 20% 29% 13% 2	760
	11th to 20th	Night	19% 12% 30% 27% 11% 1	760
	21st to end	Night	28% 15% 28% 24% 5%	836
November	1st to 10th	Night	19% 16% 30% 26% 8%	760
	11th to 20th	Night	22% 18% 30% 22% 7% 1	760
	21st to end	Night	22% 18% 32% 22% 6%	760
December	1st to 10th	Night	24% 19% 34% 19% 4	760
	11th to 20th	Night	25% 18% 34% 19% 3	760
	21st to end	Night	26% 18% 35% 18% 2	836

Page generated at 2024-11-03 02:11:17. Version ac07

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