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 2024-05-13.

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	16% 8% 3 7% 5% 5% 5% 4 5% 7% 36%	83357
	Night	16% 8% 3 7% 5% 4 4 4 5% 6% 38%	86141
Transparency	Day	45% 8% 13% 20% 12% 2	83062
	Night	45% 8% 13% 20% 12% 2	85970
Seeing	Day	24% 18% 29% 19% 8% 2	14204
	Night	21% 15% 30% 25% 8% 1	23316

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% 9% 3 8% 5% 4 4 4 5% 6% 32%	2470
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 4 5% 7% 39%	2477
	21st to end	Day	15% 6% 2 6% 5% 4 4 3 5% 8% 42%	2725
June	1st to 10th	Day	17% 6% 2 7% 7% 5% 5% 4 6% 8% 33%	2520
	11th to 20th	Day	16% 6% 2 6% 5% 5% 4 3 6% 8% 37%	2520
	21st to end	Day	16% 7% 2 7% 5% 5% 6% 4 7% 8% 33%	2520
July	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 5% 7% 8% 40%	2520
	11th to 20th	Day	11% 6% 2 6% 6% 5% 5% 4 6% 10% 38%	2520
	21st to end	Day	13% 5% 2 6% 5% 6% 5% 5% 7% 8% 38%	2744
August	1st to 10th	Day	10% 5% 3 7% 5% 5% 6% 4 8% 8% 39%	2446
	11th to 20th	Day	12% 5% 2 7% 5% 5% 5% 4 7% 9% 38%	2212
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 7% 9% 43%	2376
September	1st to 10th	Day	12% 6% 2 5% 4 4 5% 4 6% 8% 43%	2160
	11th to 20th	Day	13% 5% 3 5% 4 4 4 2 4 7% 50%	2030
	21st to end	Day	16% 6% 2 5% 4 4 5% 3 5% 7% 44%	1980
October	1st to 10th	Day	22% 6% 2 7% 4 4 4 3 4 5% 37%	1954
	11th to 20th	Day	14% 5% 2 7% 5% 4 5% 4 5% 8% 41%	1746
	21st to end	Day	21% 9% 3 7% 5% 5% 5% 4 4 6% 31%	1782
November	1st to 10th	Day	12% 7% 3 6% 4 4 4 3 5% 6% 45%	1671
	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	22% 8% 3 6% 5% 5% 4 4 4 6% 33%	1710
	11th to 20th	Night	16% 5% 2 8% 5% 5% 5% 4 6% 7% 36%	1487
	21st to end	Night	15% 5% 1 5% 6% 4 3 5% 5% 9% 40%	1550
June	1st to 10th	Night	18% 7% 2 5% 5% 4 4 3 5% 7% 39%	1440
	11th to 20th	Night	19% 5% 2 5% 5% 4 4 3 6% 8% 39%	1440
	21st to end	Night	14% 5% 2 5% 5% 5% 5% 4 7% 6% 41%	1440
July	1st to 10th	Night	12% 5% 2 5% 5% 4 5% 5% 6% 9% 43%	1440
	11th to 20th	Night	12% 5% 2 5% 4 5% 5% 5% 7% 9% 41%	1440
	21st to end	Night	11% 5% 1 4 4 4 5% 4 5% 10% 46%	1574
August	1st to 10th	Night	9% 5% 2 6% 4 4 5% 3 5% 8% 48%	1473
	11th to 20th	Night	12% 5% 2 6% 5% 5% 5% 4 6% 8% 40%	1685
	21st to end	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	1980
September	1st to 10th	Night	12% 5% 2 5% 3 4 4 4 5% 8% 49%	1800
	11th to 20th	Night	14% 5% 1 4 3 3 4 3 4 6% 53%	1930
	21st to end	Night	14% 4 2 5% 3 3 4 4 5% 6% 48%	1980
October	1st to 10th	Night	21% 7% 2 5% 3 4 4 4 5% 7% 39%	2002
	11th to 20th	Night	15% 6% 2 5% 4 4 5% 5% 6% 7% 41%	2212
	21st to end	Night	22% 8% 2 6% 4 3 5% 4 5% 5% 35%	2574
November	1st to 10th	Night	14% 6% 2 6% 4 4 3 3 6% 6% 44%	2407
	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	53% 7% 9% 17% 12% 2	2470
	11th to 20th	Day	44% 7% 11% 20% 16% 3	2481
	21st to end	Day	40% 8% 16% 22% 12% 2	2725
June	1st to 10th	Day	47% 9% 14% 19% 10% 1	2520
	11th to 20th	Day	45% 8% 12% 19% 15% 2	2520
	21st to end	Day	47% 10% 14% 19% 10% 1	2520
July	1st to 10th	Day	40% 9% 13% 21% 15% 1	2450
	11th to 20th	Day	38% 11% 17% 24% 10% 1	2445
	21st to end	Day	40% 10% 14% 22% 13% 1	2591
August	1st to 10th	Day	38% 11% 16% 23% 11% 1	2432
	11th to 20th	Day	40% 9% 15% 24% 10% 1	2212
	21st to end	Day	36% 9% 15% 25% 13% 2	2376
September	1st to 10th	Day	38% 8% 14% 24% 15% 1	2160
	11th to 20th	Day	36% 7% 11% 23% 22% 2	2030
	21st to end	Day	39% 7% 12% 23% 16% 3	1980
October	1st to 10th	Day	49% 7% 9% 20% 12% 3	1954
	11th to 20th	Day	40% 8% 14% 27% 10% 1	1746
	21st to end	Day	52% 7% 11% 17% 10% 3	1782
November	1st to 10th	Day	39% 7% 15% 23% 15% 2	1671
	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% 7% 10% 17% 12% 2	1710
	11th to 20th	Night	44% 8% 14% 17% 14% 3	1488
	21st to end	Night	43% 9% 13% 23% 12% 1	1551
June	1st to 10th	Night	45% 8% 14% 22% 11% 1	1440
	11th to 20th	Night	44% 8% 16% 19% 12% 1	1440
	21st to end	Night	39% 11% 16% 20% 14% 1	1440
July	1st to 10th	Night	37% 11% 15% 23% 13% 1	1400
	11th to 20th	Night	36% 11% 15% 26% 11%	1392
	21st to end	Night	32% 10% 17% 26% 12% 2	1486
August	1st to 10th	Night	33% 9% 17% 26% 13% 1	1463
	11th to 20th	Night	41% 9% 16% 22% 11%	1685
	21st to end	Night	35% 7% 14% 28% 16% 1	1980
September	1st to 10th	Night	35% 8% 14% 25% 18% 1	1800
	11th to 20th	Night	32% 7% 13% 25% 21% 2	1930
	21st to end	Night	36% 7% 11% 26% 17% 2	1980
October	1st to 10th	Night	46% 7% 13% 21% 11% 2	2002
	11th to 20th	Night	40% 8% 15% 25% 11% 1	2212
	21st to end	Night	50% 8% 13% 18% 10% 1	2574
November	1st to 10th	Night	40% 7% 13% 22% 16% 3	2407
	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	31% 22% 25% 13% 7% 2	463
	11th to 20th	Day	22% 19% 35% 18% 5% 2	456
	21st to end	Day	19% 15% 31% 24% 8% 2	485
June	1st to 10th	Day	18% 18% 34% 19% 10% 1	450
	11th to 20th	Day	19% 15% 34% 22% 8% 2	450
	21st to end	Day	17% 17% 33% 20% 9% 3	450
July	1st to 10th	Day	16% 11% 30% 27% 13% 3	445
	11th to 20th	Day	16% 7% 33% 30% 11% 2	444
	21st to end	Day	14% 11% 36% 27% 10% 3	488
August	1st to 10th	Day	11% 11% 28% 30% 15% 4	446
	11th to 20th	Day	14% 10% 28% 28% 16% 5%	439
	21st to end	Day	13% 12% 28% 27% 13% 6%	495
September	1st to 10th	Day	19% 10% 24% 24% 17% 6%	450
	11th to 20th	Day	18% 14% 27% 19% 12% 9%	450
	21st to end	Day	17% 15% 30% 21% 11% 6%	450
October	1st to 10th	Day	27% 16% 23% 18% 12% 4	450
	11th to 20th	Day	19% 18% 27% 24% 10% 2	423
	21st to end	Day	33% 13% 27% 19% 7% 1	396
November	1st to 10th	Day	24% 16% 25% 23% 10% 1	365
	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% 15% 31% 28% 6%	408
	21st to end	Night	17% 10% 25% 36% 10% 1	389
June	1st to 10th	Night	20% 8% 29% 31% 11%	360
	11th to 20th	Night	18% 9% 33% 31% 9%	360
	21st to end	Night	16% 8% 26% 41% 9% 1	360
July	1st to 10th	Night	13% 6% 26% 43% 11% 1	350
	11th to 20th	Night	14% 3 26% 39% 16% 2	348
	21st to end	Night	11% 3 28% 42% 15% 1	394
August	1st to 10th	Night	13% 5% 22% 42% 19% 1	399
	11th to 20th	Night	14% 6% 26% 35% 18% 2	621
	21st to end	Night	15% 7% 22% 35% 19% 2	792
September	1st to 10th	Night	15% 6% 29% 32% 17% 1	720
	11th to 20th	Night	16% 8% 23% 33% 17% 3	720
	21st to end	Night	18% 8% 26% 31% 15% 2	720
October	1st to 10th	Night	26% 12% 20% 28% 12% 2	720
	11th to 20th	Night	19% 12% 31% 27% 10% 1	720
	21st to end	Night	29% 16% 27% 23% 5%	792
November	1st to 10th	Night	20% 16% 29% 26% 9%	740
	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 2024-05-10 12:39:05. Version ac07

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