The Naked Astronomer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Naked Astronomer Observatory clear sky chart from 2005-11-06 to 2024-03-06.

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	19% 9% 4 7% 6% 4 5% 5% 5% 7% 29%	78420
	Night	21% 9% 4 7% 6% 3 5% 5% 5% 6% 30%	81042
Transparency	Day	36% 40% 9% 10% 4 1	78137
	Night	37% 38% 9% 10% 4 1	80905
Seeing	Day	22% 12% 20% 24% 17% 5%	14183
	Night	23% 10% 15% 28% 21% 3	22770

By Month

Percentage Cloud Cover NW (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	34% 12% 5% 7% 8% 4 5% 5% 4 4 12%	2219
	11th to 20th	Day	26% 11% 5% 9% 6% 3 4 3 4 5% 24%	2488
	21st to end	Day	39% 13% 5% 9% 6% 3 4 4 4 4 10%	2922
February	1st to 10th	Day	26% 12% 5% 8% 7% 4 7% 4 6% 6% 15%	1710
	11th to 20th	Day	25% 14% 4 8% 6% 4 4 4 4 5% 22%	1665
	21st to end	Day	21% 12% 4 7% 8% 3 7% 5% 5% 5% 22%	1460
March	1st to 10th	Day	23% 11% 5% 8% 7% 5% 6% 5% 5% 7% 18%	1913
	11th to 20th	Day	27% 11% 4 7% 7% 4 5% 4 5% 6% 19%	1980
	21st to end	Day	23% 11% 4 8% 8% 5% 6% 6% 6% 6% 17%	2234
April	1st to 10th	Day	23% 9% 5% 8% 8% 4 6% 6% 6% 7% 20%	2160
	11th to 20th	Day	21% 10% 5% 7% 7% 4 6% 5% 5% 7% 24%	2170
	21st to end	Day	17% 9% 5% 8% 7% 4 6% 6% 6% 7% 25%	2358
May	1st to 10th	Day	13% 8% 3 8% 7% 4 6% 5% 7% 8% 32%	2520
	11th to 20th	Day	17% 8% 3 7% 6% 4 7% 5% 7% 8% 28%	2477
	21st to end	Day	17% 10% 4 8% 7% 4 6% 6% 5% 6% 26%	2723
June	1st to 10th	Day	16% 8% 4 7% 7% 4 5% 6% 7% 8% 27%	2582
	11th to 20th	Day	13% 10% 5% 10% 8% 5% 7% 6% 7% 8% 21%	2700
	21st to end	Day	11% 7% 3 6% 6% 3 5% 6% 6% 8% 39%	2700
July	1st to 10th	Day	8% 7% 3 6% 5% 3 4 5% 5% 7% 48%	2700
	11th to 20th	Day	8% 5% 2 4 5% 3 5% 5% 6% 8% 49%	2606
	21st to end	Day	6% 4 2 4 4 2 4 4 4 7% 59%	2741
August	1st to 10th	Day	6% 5% 3 5% 6% 3 4 4 5% 7% 51%	2436
	11th to 20th	Day	6% 5% 2 4 5% 2 4 4 5% 8% 55%	2298
	21st to end	Day	9% 7% 3 6% 6% 3 4 4 5% 8% 44%	2480
September	1st to 10th	Day	9% 6% 2 5% 5% 3 4 4 5% 8% 49%	2160
	11th to 20th	Day	11% 7% 3 6% 6% 3 5% 5% 6% 8% 40%	2160
	21st to end	Day	15% 7% 3 6% 6% 3 5% 4 5% 8% 38%	1896
October	1st to 10th	Day	18% 9% 4 6% 5% 4 4 5% 7% 8% 31%	1800
	11th to 20th	Day	21% 9% 3 7% 7% 3 5% 5% 6% 8% 27%	1800
	21st to end	Day	28% 10% 3 7% 5% 2 4 4 6% 6% 24%	1892
November	1st to 10th	Day	29% 12% 4 7% 5% 3 4 5% 5% 6% 18%	1516
	11th to 20th	Day	29% 10% 4 8% 6% 4 4 4 3 6% 21%	1520
	21st to end	Day	27% 11% 3 8% 7% 4 4 4 5% 6% 19%	1520
December	1st to 10th	Day	27% 10% 4 7% 7% 4 5% 4 4 5% 22%	1520
	11th to 20th	Day	31% 12% 4 13% 9% 4 5% 5% 4 4 9%	1925
	21st to end	Day	26% 13% 6% 13% 9% 5% 6% 4 5% 4 11%	2469
January	1st to 10th	Night	32% 13% 5% 9% 8% 4 5% 4 4 4 12%	4726
	11th to 20th	Night	26% 9% 4 7% 4 2 4 4 5% 7% 27%	4405
	21st to end	Night	34% 15% 4 8% 6% 4 4 4 4 5% 12%	4736
February	1st to 10th	Night	25% 13% 4 8% 6% 3 4 4 5% 7% 21%	2470
	11th to 20th	Night	21% 13% 3 7% 6% 5% 5% 4 5% 6% 25%	2402
	21st to end	Night	21% 12% 4 7% 7% 4 5% 5% 6% 6% 24%	1992
March	1st to 10th	Night	23% 10% 5% 7% 6% 3 6% 6% 5% 7% 24%	2159
	11th to 20th	Night	25% 8% 4 7% 6% 4 5% 5% 6% 7% 25%	1979
	21st to end	Night	23% 11% 5% 8% 7% 4 5% 5% 7% 6% 19%	2121
April	1st to 10th	Night	20% 8% 3 8% 7% 4 5% 6% 7% 7% 25%	1800
	11th to 20th	Night	16% 8% 4 8% 7% 4 5% 6% 6% 8% 30%	1739
	21st to end	Night	16% 7% 3 8% 7% 4 5% 5% 6% 8% 30%	1602
May	1st to 10th	Night	12% 7% 4 7% 6% 3 6% 5% 5% 7% 39%	1440
	11th to 20th	Night	13% 6% 3 6% 6% 4 6% 6% 6% 9% 34%	1416
	21st to end	Night	9% 8% 2 8% 8% 4 6% 6% 6% 9% 34%	1552
June	1st to 10th	Night	12% 6% 3 7% 5% 4 5% 4 8% 7% 38%	1373
	11th to 20th	Night	11% 7% 3 7% 6% 4 8% 6% 7% 9% 32%	1260
	21st to end	Night	6% 4 3 6% 6% 3 4 5% 6% 7% 50%	1260
July	1st to 10th	Night	6% 5% 2 5% 5% 3 3 5% 5% 6% 56%	1260
	11th to 20th	Night	4 3 3 4 3 2 4 4 4 6% 63%	1350
	21st to end	Night	4 2 1 4 2 2 2 3 4 6% 70%	1576
August	1st to 10th	Night	3 3 2 4 3 3 3 4 5% 8% 62%	1500
	11th to 20th	Night	4 3 1 3 3 2 1 3 3 9% 67%	1602
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 5% 7% 53%	1875
September	1st to 10th	Night	6% 4 2 3 5% 2 4 4 4 7% 60%	1800
	11th to 20th	Night	11% 5% 3 5% 6% 3 4 4 6% 8% 45%	1800
	21st to end	Night	14% 7% 3 6% 5% 3 4 5% 5% 7% 42%	2061
October	1st to 10th	Night	17% 7% 2 6% 4 3 4 4 5% 6% 40%	2160
	11th to 20th	Night	22% 7% 3 7% 5% 3 5% 4 5% 6% 33%	2160
	21st to end	Night	23% 8% 3 7% 6% 3 4 4 5% 7% 32%	2462
November	1st to 10th	Night	30% 9% 4 6% 5% 3 5% 3 5% 5% 24%	2562
	11th to 20th	Night	27% 8% 4 7% 6% 3 5% 4 4 5% 25%	2660
	21st to end	Night	25% 10% 4 7% 7% 4 5% 5% 4 7% 23%	2660
December	1st to 10th	Night	25% 10% 4 7% 6% 3 5% 5% 4 5% 25%	2660
	11th to 20th	Night	27% 12% 5% 11% 10% 4 5% 4 4 5% 14%	3280
	21st to end	Night	29% 14% 6% 9% 8% 3 4 5% 4 4 13%	5182

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	55% 34% 4 6%	2219
	11th to 20th	Day	47% 29% 8% 12% 2 1	2512
	21st to end	Day	63% 27% 5% 3 1	2922
February	1st to 10th	Day	45% 40% 8% 5% 2	1710
	11th to 20th	Day	41% 40% 8% 7% 4	1674
	21st to end	Day	43% 39% 9% 6% 3	1460
March	1st to 10th	Day	43% 41% 10% 5% 1	1913
	11th to 20th	Day	46% 39% 6% 7% 2	1980
	21st to end	Day	44% 41% 8% 7%	2234
April	1st to 10th	Day	39% 41% 11% 7% 1	2160
	11th to 20th	Day	38% 39% 7% 10% 4 1	2170
	21st to end	Day	36% 42% 11% 8% 3 1	2358
May	1st to 10th	Day	33% 42% 11% 10% 3	2520
	11th to 20th	Day	30% 42% 13% 10% 4	2480
	21st to end	Day	40% 41% 9% 8% 1	2724
June	1st to 10th	Day	37% 41% 12% 6% 3 1	2582
	11th to 20th	Day	38% 44% 10% 7% 1	2700
	21st to end	Day	26% 42% 14% 11% 5% 2	2700
July	1st to 10th	Day	20% 42% 11% 18% 8% 2	2626
	11th to 20th	Day	20% 41% 10% 19% 10%	2526
	21st to end	Day	16% 40% 10% 21% 11% 2	2589
August	1st to 10th	Day	22% 40% 13% 18% 7%	2419
	11th to 20th	Day	17% 39% 14% 18% 9% 3	2298
	21st to end	Day	23% 40% 11% 19% 7% 1	2480
September	1st to 10th	Day	18% 39% 11% 18% 9% 4	2160
	11th to 20th	Day	23% 42% 11% 13% 9% 2	2160
	21st to end	Day	30% 40% 11% 11% 7% 2	1896
October	1st to 10th	Day	32% 41% 9% 12% 5%	1800
	11th to 20th	Day	36% 41% 10% 11% 2 1	1800
	21st to end	Day	38% 41% 8% 9% 5%	1892
November	1st to 10th	Day	42% 43% 5% 6% 4	1516
	11th to 20th	Day	43% 43% 5% 6% 3	1520
	21st to end	Day	43% 41% 7% 7% 2	1520
December	1st to 10th	Day	42% 41% 7% 7% 3	1520
	11th to 20th	Day	54% 37% 4 4 1	1928
	21st to end	Day	56% 33% 6% 5% 1	2469
January	1st to 10th	Night	58% 32% 6% 4	4726
	11th to 20th	Night	42% 29% 9% 17% 2 1	4443
	21st to end	Night	61% 27% 5% 4 2	4736
February	1st to 10th	Night	43% 39% 10% 7% 2	2470
	11th to 20th	Night	38% 41% 9% 7% 5%	2404
	21st to end	Night	43% 38% 8% 7% 3	1992
March	1st to 10th	Night	41% 40% 9% 8% 2	2159
	11th to 20th	Night	41% 41% 7% 8% 3	1980
	21st to end	Night	43% 42% 9% 5% 1	2121
April	1st to 10th	Night	38% 42% 10% 8% 2	1800
	11th to 20th	Night	33% 42% 10% 9% 5% 1	1739
	21st to end	Night	33% 42% 11% 9% 4 1	1602
May	1st to 10th	Night	30% 44% 12% 10% 5%	1440
	11th to 20th	Night	27% 41% 13% 12% 5% 1	1416
	21st to end	Night	34% 43% 12% 10% 2	1552
June	1st to 10th	Night	32% 40% 12% 11% 4 1	1373
	11th to 20th	Night	31% 46% 13% 9% 1	1260
	21st to end	Night	21% 40% 18% 12% 6% 3	1260
July	1st to 10th	Night	15% 40% 13% 21% 6% 4	1225
	11th to 20th	Night	14% 40% 10% 25% 10% 1	1307
	21st to end	Night	9% 41% 9% 23% 14% 5%	1488
August	1st to 10th	Night	15% 39% 14% 20% 11% 1	1492
	11th to 20th	Night	11% 37% 15% 20% 13% 3	1602
	21st to end	Night	20% 41% 10% 18% 9% 1	1875
September	1st to 10th	Night	13% 39% 12% 20% 12% 5%	1800
	11th to 20th	Night	23% 40% 11% 11% 12% 3	1800
	21st to end	Night	28% 41% 8% 14% 8% 2	2061
October	1st to 10th	Night	29% 39% 10% 14% 8%	2160
	11th to 20th	Night	33% 40% 10% 13% 4	2160
	21st to end	Night	33% 40% 9% 11% 7%	2462
November	1st to 10th	Night	39% 43% 8% 6% 3 1	2562
	11th to 20th	Night	40% 43% 8% 6% 3	2660
	21st to end	Night	39% 44% 8% 6% 2	2660
December	1st to 10th	Night	39% 43% 8% 7% 4	2660
	11th to 20th	Night	48% 40% 7% 3 1	3276
	21st to end	Night	55% 32% 7% 5% 1	5182

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	36% 16% 17% 14% 12% 5%	180
	11th to 20th	Day	39% 12% 13% 16% 17% 3	180
	21st to end	Day	34% 16% 19% 18% 11% 2	274
February	1st to 10th	Day	34% 16% 18% 21% 10% 2	370
	11th to 20th	Day	34% 19% 20% 19% 7% 1	365
	21st to end	Day	28% 19% 25% 19% 8% 1	330
March	1st to 10th	Day	28% 18% 26% 19% 8% 1	441
	11th to 20th	Day	27% 19% 24% 15% 10% 5%	420
	21st to end	Day	28% 19% 24% 18% 9% 3	462
April	1st to 10th	Day	31% 15% 21% 19% 9% 4	420
	11th to 20th	Day	25% 17% 26% 16% 10% 6%	412
	21st to end	Day	22% 14% 27% 26% 8% 2	420
May	1st to 10th	Day	16% 12% 24% 30% 15% 3	423
	11th to 20th	Day	24% 8% 20% 23% 17% 7%	448
	21st to end	Day	21% 7% 20% 26% 20% 7%	485
June	1st to 10th	Day	20% 13% 25% 22% 16% 5%	512
	11th to 20th	Day	22% 13% 22% 27% 13% 3	630
	21st to end	Day	15% 8% 21% 29% 21% 5%	630
July	1st to 10th	Day	13% 7% 17% 32% 25% 7%	621
	11th to 20th	Day	5% 4 19% 35% 28% 9%	525
	21st to end	Day	8% 3 11% 26% 38% 14%	487
August	1st to 10th	Day	9% 7% 15% 24% 33% 13%	447
	11th to 20th	Day	7% 3 12% 32% 34% 11%	437
	21st to end	Day	13% 9% 18% 31% 22% 8%	495
September	1st to 10th	Day	14% 7% 23% 33% 18% 5%	450
	11th to 20th	Day	16% 9% 18% 31% 19% 7%	450
	21st to end	Day	16% 13% 22% 29% 16% 4	450
October	1st to 10th	Day	22% 10% 22% 21% 19% 6%	450
	11th to 20th	Day	25% 11% 18% 25% 16% 5%	450
	21st to end	Day	32% 16% 18% 19% 12% 3	407
November	1st to 10th	Day	31% 18% 20% 20% 10% 2	194
	11th to 20th	Day	34% 16% 18% 17% 12% 4	180
	21st to end	Day	32% 14% 18% 14% 15% 7%	180
December	1st to 10th	Day	36% 17% 11% 18% 14% 4	180
	11th to 20th	Day	33% 21% 19% 16% 11% 1	180
	21st to end	Day	35% 16% 19% 19% 10% 2	198
January	1st to 10th	Night	40% 13% 13% 19% 13% 2	850
	11th to 20th	Night	33% 12% 13% 24% 16% 2	765
	21st to end	Night	29% 11% 15% 26% 16% 3	836
February	1st to 10th	Night	30% 15% 15% 25% 12% 3	760
	11th to 20th	Night	26% 13% 17% 26% 15% 2	744
	21st to end	Night	26% 13% 21% 25% 12% 2	628
March	1st to 10th	Night	26% 13% 20% 25% 14% 1	742
	11th to 20th	Night	26% 15% 21% 25% 12% 3	720
	21st to end	Night	25% 16% 18% 26% 12% 2	792
April	1st to 10th	Night	24% 12% 20% 25% 16% 3	720
	11th to 20th	Night	18% 12% 21% 31% 15% 2	671
	21st to end	Night	20% 9% 17% 34% 17% 2	522
May	1st to 10th	Night	16% 11% 16% 36% 19% 2	360
	11th to 20th	Night	14% 6% 14% 34% 29% 3	354
	21st to end	Night	13% 5% 12% 38% 26% 5%	389
June	1st to 10th	Night	17% 9% 15% 31% 25% 3	360
	11th to 20th	Night	15% 9% 20% 32% 22% 2	360
	21st to end	Night	11% 7% 14% 37% 29% 3	360
July	1st to 10th	Night	8% 3 11% 34% 41% 4	350
	11th to 20th	Night	7% 3 10% 37% 39% 4	348
	21st to end	Night	4 3 7% 34% 46% 7%	394
August	1st to 10th	Night	5% 2 8% 27% 45% 14%	426
	11th to 20th	Night	6% 2 6% 26% 48% 11%	534
	21st to end	Night	10% 4 10% 35% 36% 5%	687
September	1st to 10th	Night	8% 4 14% 37% 31% 5%	720
	11th to 20th	Night	14% 4 13% 27% 35% 6%	720
	21st to end	Night	18% 7% 15% 34% 23% 3	720
October	1st to 10th	Night	18% 7% 15% 29% 26% 5%	720
	11th to 20th	Night	23% 9% 14% 28% 23% 4	720
	21st to end	Night	28% 10% 14% 28% 18% 2	792
November	1st to 10th	Night	33% 11% 16% 28% 12% 1	740
	11th to 20th	Night	30% 13% 18% 23% 14% 1	760
	21st to end	Night	27% 11% 19% 25% 15% 3	760
December	1st to 10th	Night	28% 9% 14% 30% 16% 2	760
	11th to 20th	Night	36% 13% 15% 22% 12% 2	760
	21st to end	Night	37% 13% 17% 23% 9% 1	926

Page generated at 2024-03-03 03:29:00. Version ac07

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