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 2025-09-19.

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 6% 7% 35%	126189
	Night	16% 8% 3 7% 5% 4 4 4 5% 7% 37%	122865
Transparency	Day	46% 8% 12% 20% 13% 2	129696
	Night	45% 8% 13% 20% 13% 2	127445
Seeing	Day	24% 18% 29% 19% 8% 3	15997
	Night	21% 15% 30% 26% 8% 1	24907

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	11% 6% 4 15% 7% 6% 6% 4 5% 5% 31%	3597
	11th to 20th	Day	12% 8% 4 11% 5% 5% 6% 4 5% 5% 35%	3570
	21st to end	Day	13% 10% 4 9% 4 4 4 4 5% 5% 39%	3960
February	1st to 10th	Day	14% 11% 3 8% 6% 6% 5% 5% 7% 7% 30%	1816
	11th to 20th	Day	10% 8% 3 10% 6% 5% 4 4 7% 7% 35%	1948
	21st to end	Day	17% 9% 3 10% 5% 5% 4 3 5% 5% 33%	1650
March	1st to 10th	Day	18% 12% 5% 10% 5% 4 4 3 4 5% 31%	4476
	11th to 20th	Day	15% 8% 2 7% 5% 5% 4 4 6% 6% 38%	4648
	21st to end	Day	23% 9% 2 7% 4 4 4 3 4 6% 34%	5967
April	1st to 10th	Day	17% 12% 2 8% 5% 4 4 3 5% 6% 35%	2400
	11th to 20th	Day	17% 8% 2 6% 4 4 5% 5% 6% 7% 37%	2374
	21st to end	Day	18% 8% 2 7% 6% 5% 4 4 5% 6% 35%	2563
May	1st to 10th	Day	30% 9% 3 8% 5% 4 5% 3 4 6% 23%	5886
	11th to 20th	Day	20% 11% 4 8% 5% 4 4 3 5% 6% 29%	6259
	21st to end	Day	18% 7% 3 7% 4 5% 5% 4 6% 9% 34%	6949
June	1st to 10th	Day	17% 6% 2 6% 6% 5% 5% 4 6% 9% 33%	2800
	11th to 20th	Day	16% 6% 3 6% 5% 5% 5% 3 6% 8% 37%	2800
	21st to end	Day	16% 7% 2 7% 5% 5% 5% 4 7% 8% 33%	2800
July	1st to 10th	Day	12% 6% 2 7% 5% 6% 6% 5% 8% 10% 33%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 6% 6% 5% 8% 11% 36%	6360
	21st to end	Day	14% 6% 3 5% 4 4 5% 4 6% 8% 42%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 4 7% 8% 35%	4444
	11th to 20th	Day	13% 6% 3 8% 5% 5% 5% 4 7% 9% 36%	3994
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 11% 42%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 5% 4 6% 8% 45%	2400
	11th to 20th	Day	12% 5% 3 5% 4 4 4 3 4 7% 50%	2232
	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% 46%	1761
	11th to 20th	Day	16% 6% 3 6% 5% 4 4 3 5% 6% 42%	1800
	21st to end	Day	19% 9% 2 6% 4 4 4 3 6% 7% 37%	1715
December	1st to 10th	Day	14% 11% 5% 8% 5% 5% 4 4 5% 6% 32%	2600
	11th to 20th	Day	21% 9% 2 6% 4 4 3 3 4 5% 37%	2986
	21st to end	Day	17% 10% 3 9% 5% 5% 4 4 4 5% 34%	3960
January	1st to 10th	Night	12% 7% 5% 12% 7% 4 4 3 5% 6% 34%	6349
	11th to 20th	Night	10% 11% 4 12% 6% 5% 6% 4 5% 6% 32%	6315
	21st to end	Night	13% 10% 4 7% 5% 5% 5% 4 5% 6% 36%	6996
February	1st to 10th	Night	12% 10% 3 8% 6% 5% 5% 5% 6% 7% 34%	2578
	11th to 20th	Night	10% 9% 3 7% 6% 6% 6% 4 6% 7% 37%	2338
	21st to end	Night	14% 8% 3 7% 6% 4 5% 3 6% 7% 37%	1980
March	1st to 10th	Night	20% 9% 2 7% 5% 4 4 4 5% 6% 33%	5364
	11th to 20th	Night	14% 9% 3 7% 5% 4 5% 3 5% 7% 37%	5311
	21st to end	Night	22% 9% 3 7% 4 4 3 3 5% 6% 33%	4962
April	1st to 10th	Night	18% 8% 3 6% 4 4 5% 3 5% 7% 37%	2000
	11th to 20th	Night	18% 9% 2 6% 4 3 4 3 6% 7% 38%	1975
	21st to end	Night	19% 8% 3 6% 4 4 4 3 5% 7% 39%	1835
May	1st to 10th	Night	33% 8% 3 5% 4 4 4 4 5% 6% 24%	4054
	11th to 20th	Night	16% 10% 4 10% 5% 5% 5% 4 5% 6% 29%	3677
	21st to end	Night	20% 7% 3 7% 5% 5% 4 4 5% 8% 32%	3926
June	1st to 10th	Night	18% 7% 2 5% 4 4 4 4 5% 7% 40%	1600
	11th to 20th	Night	18% 5% 2 6% 5% 4 4 4 6% 8% 39%	1600
	21st to end	Night	15% 6% 2 6% 5% 4 5% 4 7% 6% 40%	1600
July	1st to 10th	Night	11% 5% 2 6% 5% 4 4 4 7% 10% 42%	3600
	11th to 20th	Night	12% 7% 3 7% 5% 6% 5% 5% 7% 9% 37%	3600
	21st to end	Night	12% 5% 2 5% 3 4 4 3 4 6% 51%	3926
August	1st to 10th	Night	20% 7% 2 7% 5% 4 5% 3 4 7% 36%	2672
	11th to 20th	Night	14% 7% 2 7% 5% 5% 4 4 5% 8% 39%	3036
	21st to end	Night	8% 4 2 5% 3 3 3 3 5% 9% 55%	3586
September	1st to 10th	Night	11% 5% 2 5% 3 3 3 4 5% 8% 50%	2000
	11th to 20th	Night	13% 5% 2 4 3 3 4 3 4 6% 53%	2124
	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%	2537
	11th to 20th	Night	16% 7% 2 5% 5% 4 4 3 5% 7% 42%	2600
	21st to end	Night	14% 9% 2 6% 4 4 5% 4 6% 6% 42%	2684
December	1st to 10th	Night	15% 11% 4 9% 5% 5% 4 4 5% 6% 32%	4580
	11th to 20th	Night	23% 9% 3 6% 4 3 4 2 4 5% 37%	5208
	21st to end	Night	19% 12% 5% 8% 4 4 4 3 4 5% 33%	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	52% 7% 10% 22% 9% 1	3598
	11th to 20th	Day	46% 9% 12% 17% 14% 3	3600
	21st to end	Day	47% 6% 8% 20% 17% 3	3960
February	1st to 10th	Day	50% 9% 13% 17% 10% 1	1816
	11th to 20th	Day	49% 6% 11% 19% 12% 3	1957
	21st to end	Day	52% 6% 10% 18% 13% 2	1650
March	1st to 10th	Day	56% 5% 10% 11% 10% 8%	4479
	11th to 20th	Day	41% 11% 17% 20% 9% 3	4648
	21st to end	Day	50% 5% 8% 15% 18% 5%	5967
April	1st to 10th	Day	50% 6% 10% 19% 13% 3	2400
	11th to 20th	Day	45% 9% 11% 17% 16% 2	2374
	21st to end	Day	50% 7% 9% 17% 15% 3	2563
May	1st to 10th	Day	61% 8% 10% 13% 7% 1	5897
	11th to 20th	Day	54% 8% 13% 15% 9% 1	6263
	21st to end	Day	44% 9% 14% 21% 11% 1	6949
June	1st to 10th	Day	46% 9% 13% 20% 11% 1	2800
	11th to 20th	Day	45% 8% 13% 19% 14% 2	2800
	21st to end	Day	46% 9% 14% 19% 11% 1	2800
July	1st to 10th	Day	44% 9% 13% 20% 13% 1	6290
	11th to 20th	Day	39% 10% 16% 22% 12% 1	6285
	21st to end	Day	40% 7% 12% 26% 14% 1	6728
August	1st to 10th	Day	45% 9% 15% 20% 10% 1	4430
	11th to 20th	Day	43% 8% 15% 26% 7% 1	4033
	21st to end	Day	35% 8% 15% 26% 14% 1	4312
September	1st to 10th	Day	36% 8% 13% 25% 16% 1	2400
	11th to 20th	Day	35% 7% 11% 24% 21% 2	2232
	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	38% 7% 15% 23% 15% 1	1761
	11th to 20th	Day	42% 8% 10% 19% 19% 2	1800
	21st to end	Day	46% 6% 16% 21% 9% 2	1715
December	1st to 10th	Day	50% 7% 13% 23% 6% 1	2600
	11th to 20th	Day	49% 5% 11% 18% 15% 2	2986
	21st to end	Day	50% 7% 8% 17% 13% 4	3960
January	1st to 10th	Night	49% 8% 12% 20% 10% 1	6349
	11th to 20th	Night	49% 9% 14% 15% 11% 2	6360
	21st to end	Night	47% 7% 10% 16% 15% 4	6996
February	1st to 10th	Night	48% 8% 14% 18% 10% 1	2578
	11th to 20th	Night	44% 8% 15% 19% 13% 2	2340
	21st to end	Night	46% 6% 13% 22% 10% 4	1980
March	1st to 10th	Night	49% 7% 12% 15% 14% 3	5374
	11th to 20th	Night	45% 9% 18% 17% 9% 2	5312
	21st to end	Night	51% 6% 7% 13% 20% 3	4962
April	1st to 10th	Night	47% 7% 11% 19% 14% 2	2000
	11th to 20th	Night	44% 9% 11% 19% 16% 2	1975
	21st to end	Night	46% 7% 10% 17% 16% 4	1835
May	1st to 10th	Night	58% 10% 12% 12% 8% 1	4060
	11th to 20th	Night	52% 10% 12% 15% 9% 1	3678
	21st to end	Night	49% 9% 13% 18% 10% 1	3927
June	1st to 10th	Night	44% 8% 14% 23% 11% 1	1600
	11th to 20th	Night	43% 8% 16% 20% 12% 1	1600
	21st to end	Night	40% 10% 15% 21% 13% 1	1600
July	1st to 10th	Night	39% 8% 15% 24% 13% 1	3560
	11th to 20th	Night	43% 9% 15% 18% 14%	3552
	21st to end	Night	35% 6% 13% 31% 14% 1	3809
August	1st to 10th	Night	48% 7% 14% 20% 10% 1	2662
	11th to 20th	Night	44% 8% 17% 22% 9%	3063
	21st to end	Night	28% 6% 17% 34% 14% 1	3586
September	1st to 10th	Night	34% 7% 13% 25% 19% 1	2000
	11th to 20th	Night	32% 7% 13% 26% 20% 2	2124
	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% 17% 3	2537
	11th to 20th	Night	42% 7% 12% 21% 16% 3	2600
	21st to end	Night	43% 7% 17% 23% 10% 1	2684
December	1st to 10th	Night	51% 7% 11% 22% 8%	4580
	11th to 20th	Night	49% 6% 13% 13% 17% 2	5208
	21st to end	Night	51% 8% 9% 18% 11% 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	30% 25% 26% 16% 4	200
	11th to 20th	Day	31% 22% 35% 10% 2 1	200
	21st to end	Day	25% 27% 32% 12% 4	220
February	1st to 10th	Day	33% 29% 27% 10% 1	328
	11th to 20th	Day	32% 26% 29% 12% 1	496
	21st to end	Day	31% 23% 31% 13% 2	414
March	1st to 10th	Day	31% 22% 32% 13% 2	500
	11th to 20th	Day	31% 25% 27% 12% 4 1	500
	21st to end	Day	31% 25% 26% 10% 4 3	550
April	1st to 10th	Day	35% 24% 23% 11% 6% 1	500
	11th to 20th	Day	32% 25% 23% 13% 5% 1	492
	21st to end	Day	30% 19% 31% 12% 6% 1	500
May	1st to 10th	Day	33% 22% 24% 13% 7% 1	503
	11th to 20th	Day	23% 19% 35% 16% 6% 1	527
	21st to end	Day	20% 16% 30% 25% 7% 2	573
June	1st to 10th	Day	18% 18% 32% 21% 9% 1	530
	11th to 20th	Day	20% 15% 33% 22% 9% 2	530
	21st to end	Day	18% 17% 32% 20% 9% 3	530
July	1st to 10th	Day	15% 12% 32% 26% 12% 3	525
	11th to 20th	Day	14% 8% 33% 30% 12% 3	524
	21st to end	Day	14% 12% 36% 26% 10% 3	576
August	1st to 10th	Day	12% 10% 27% 30% 15% 5%	526
	11th to 20th	Day	15% 9% 27% 27% 16% 6%	519
	21st to end	Day	12% 12% 28% 29% 13% 6%	583
September	1st to 10th	Day	18% 10% 25% 24% 17% 6%	530
	11th to 20th	Day	16% 13% 24% 21% 14% 12%	522
	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% 27% 22% 10% 1	395
	11th to 20th	Day	29% 20% 28% 18% 5% 1	400
	21st to end	Day	32% 19% 26% 18% 4 1	315
December	1st to 10th	Day	26% 26% 30% 15% 4 1	200
	11th to 20th	Day	39% 20% 30% 9% 3	200
	21st to end	Day	29% 19% 33% 15% 5%	220
January	1st to 10th	Night	21% 24% 34% 20% 2	800
	11th to 20th	Night	25% 19% 33% 21% 2	800
	21st to end	Night	19% 19% 38% 21% 3	880
February	1st to 10th	Night	21% 20% 36% 21% 2	800
	11th to 20th	Night	21% 23% 38% 16% 2	783
	21st to end	Night	24% 23% 33% 18% 2	660
March	1st to 10th	Night	24% 22% 32% 20% 2	800
	11th to 20th	Night	23% 23% 31% 20% 4	800
	21st to end	Night	26% 18% 31% 19% 5%	880
April	1st to 10th	Night	27% 18% 30% 21% 4	800
	11th to 20th	Night	26% 20% 28% 21% 6%	791
	21st to end	Night	26% 19% 29% 22% 5%	635
May	1st to 10th	Night	28% 15% 29% 23% 5% 1	600
	11th to 20th	Night	19% 16% 31% 29% 6%	444
	21st to end	Night	18% 9% 26% 35% 10% 1	433
June	1st to 10th	Night	20% 8% 30% 31% 11%	400
	11th to 20th	Night	18% 9% 32% 32% 8% 1	400
	21st to end	Night	17% 8% 26% 40% 10% 1	400
July	1st to 10th	Night	13% 6% 27% 44% 10% 1	390
	11th to 20th	Night	14% 4 25% 39% 16% 2	388
	21st to end	Night	12% 3 29% 41% 14% 1	438
August	1st to 10th	Night	14% 5% 21% 41% 19%	445
	11th to 20th	Night	14% 6% 26% 34% 19% 1	691
	21st to end	Night	14% 7% 22% 36% 19% 2	880
September	1st to 10th	Night	14% 6% 28% 33% 17% 2	800
	11th to 20th	Night	15% 8% 22% 33% 20% 3	793
	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% 31% 26% 8%	780
	11th to 20th	Night	22% 19% 30% 22% 7% 1	800
	21st to end	Night	23% 17% 33% 22% 6%	800
December	1st to 10th	Night	25% 18% 34% 19% 4	800
	11th to 20th	Night	26% 18% 34% 20% 4	800
	21st to end	Night	26% 18% 34% 19% 3	880

Page generated at 2025-09-16 01:49:10. Version ac07

Copyright © 2025 Attilla Danko.