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 2024-11-02.

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	17% 9% 3 8% 5% 5% 5% 4 5% 6% 32%	101817
	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 32%	97215
Transparency	Day	50% 8% 14% 18% 9% 1	101573
	Night	51% 8% 14% 17% 8% 1	97105
Seeing	Day	22% 12% 20% 24% 17% 5%	15178
	Night	23% 10% 15% 28% 20% 3	23503

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% 13% 4 8% 7% 5% 5% 4 4 4 12%	2219
	11th to 20th	Day	26% 11% 5% 10% 5% 4 4 3 4 5% 24%	2488
	21st to end	Day	39% 13% 4 10% 5% 4 4 3 4 4 9%	2922
February	1st to 10th	Day	25% 13% 5% 8% 7% 6% 6% 4 6% 6% 15%	1710
	11th to 20th	Day	25% 15% 4 9% 5% 5% 4 3 5% 5% 21%	1665
	21st to end	Day	21% 13% 3 9% 6% 5% 7% 4 6% 5% 22%	1460
March	1st to 10th	Day	17% 13% 5% 12% 8% 8% 6% 4 6% 6% 15%	3252
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 36%	3490
	21st to end	Day	20% 12% 4 10% 7% 6% 6% 4 6% 6% 19%	3932
April	1st to 10th	Day	22% 10% 4 9% 7% 5% 6% 5% 6% 7% 20%	2280
	11th to 20th	Day	20% 12% 3 8% 6% 5% 5% 4 5% 6% 26%	2293
	21st to end	Day	17% 10% 4 9% 7% 5% 6% 4 6% 7% 25%	2489
May	1st to 10th	Day	14% 8% 3 8% 5% 4 5% 4 7% 6% 36%	4420
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	4377
	21st to end	Day	18% 12% 4 10% 6% 6% 6% 5% 6% 7% 21%	4813
June	1st to 10th	Day	16% 9% 4 8% 6% 5% 5% 5% 7% 8% 27%	2726
	11th to 20th	Day	12% 11% 4 11% 7% 7% 7% 5% 7% 8% 21%	2850
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 5% 6% 7% 38%	2850
July	1st to 10th	Day	5% 5% 2 5% 3 4 3 3 4 5% 60%	4740
	11th to 20th	Day	5% 4 1 3 3 3 3 3 4 6% 65%	4562
	21st to end	Day	8% 7% 2 5% 4 4 4 3 5% 7% 51%	4790
August	1st to 10th	Day	4 5% 2 5% 4 4 3 3 5% 7% 58%	4278
	11th to 20th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	4026
	21st to end	Day	13% 9% 3 8% 4 4 4 3 4 7% 42%	4349
September	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 5% 7% 49%	2280
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	2280
	21st to end	Day	15% 8% 2 7% 5% 4 5% 3 6% 8% 38%	2000
October	1st to 10th	Day	17% 9% 3 7% 4 5% 5% 4 7% 9% 31%	1900
	11th to 20th	Day	21% 9% 2 9% 5% 3 5% 4 6% 8% 26%	1900
	21st to end	Day	28% 10% 3 8% 5% 3 4 3 6% 6% 24%	1997
November	1st to 10th	Day	30% 12% 4 8% 5% 4 5% 3 5% 6% 18%	1525
	11th to 20th	Day	29% 11% 3 9% 6% 4 4 3 4 6% 21%	1520
	21st to end	Day	26% 12% 3 10% 6% 5% 4 4 5% 6% 19%	1520
December	1st to 10th	Day	26% 11% 4 8% 7% 5% 5% 3 5% 5% 22%	1520
	11th to 20th	Day	31% 13% 4 14% 7% 5% 4 4 4 4 9%	1925
	21st to end	Day	25% 13% 6% 14% 8% 6% 5% 3 5% 3 11%	2469
January	1st to 10th	Night	32% 14% 4 11% 6% 5% 5% 3 4 4 12%	4726
	11th to 20th	Night	26% 10% 4 8% 4 3 4 3 5% 7% 27%	4405
	21st to end	Night	34% 15% 4 9% 5% 5% 4 3 4 5% 11%	4736
February	1st to 10th	Night	24% 14% 4 9% 6% 4 4 3 5% 6% 20%	2470
	11th to 20th	Night	21% 13% 3 9% 5% 6% 4 3 5% 6% 25%	2402
	21st to end	Night	21% 13% 3 8% 6% 5% 5% 4 6% 6% 24%	1992
March	1st to 10th	Night	19% 15% 5% 11% 6% 5% 6% 4 6% 6% 16%	3618
	11th to 20th	Night	17% 8% 3 6% 3 4 4 3 5% 7% 41%	3489
	21st to end	Night	23% 15% 5% 10% 6% 5% 5% 4 5% 5% 17%	3720
April	1st to 10th	Night	20% 9% 3 9% 6% 5% 5% 4 7% 6% 24%	1900
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 5% 6% 7% 31%	1836
	21st to end	Night	16% 8% 3 9% 6% 6% 4 4 6% 8% 29%	1691
May	1st to 10th	Night	15% 8% 3 7% 4 4 5% 4 5% 5% 41%	2540
	11th to 20th	Night	9% 7% 2 8% 4 4 4 5% 6% 8% 42%	2516
	21st to end	Night	7% 8% 2 9% 7% 7% 6% 5% 8% 9% 31%	2762
June	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 8% 7% 38%	1449
	11th to 20th	Night	11% 8% 2 8% 6% 6% 7% 4 7% 9% 32%	1330
	21st to end	Night	6% 5% 3 7% 6% 5% 4 4 6% 7% 48%	1330
July	1st to 10th	Night	4 4 2 5% 4 3 3 3 4 4 65%	2220
	11th to 20th	Night	3 2 1 3 2 2 2 2 4 6% 73%	2380
	21st to end	Night	4 4 2 6% 3 3 3 3 5% 7% 60%	2762
August	1st to 10th	Night	2 2 1 3 2 3 2 2 4 6% 72%	2653
	11th to 20th	Night	8% 5% 2 6% 4 4 3 3 4 8% 52%	2812
	21st to end	Night	13% 6% 2 5% 3 4 4 3 5% 7% 49%	3323
September	1st to 10th	Night	6% 4 2 4 4 3 4 3 4 7% 60%	1900
	11th to 20th	Night	12% 7% 3 6% 5% 4 4 3 6% 7% 43%	1900
	21st to end	Night	13% 7% 3 7% 4 4 5% 3 5% 7% 42%	2177
October	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 41%	2280
	11th to 20th	Night	22% 7% 3 7% 4 4 4 3 6% 6% 33%	2280
	21st to end	Night	23% 8% 3 8% 6% 4 4 3 5% 6% 31%	2599
November	1st to 10th	Night	31% 10% 3 7% 5% 5% 5% 2 5% 5% 24%	2575
	11th to 20th	Night	27% 9% 4 8% 6% 5% 4 3 4 5% 25%	2660
	21st to end	Night	25% 10% 3 8% 7% 5% 5% 3 5% 6% 22%	2660
December	1st to 10th	Night	25% 12% 4 8% 5% 4 4 4 4 5% 24%	2660
	11th to 20th	Night	27% 13% 4 12% 9% 5% 4 3 4 5% 13%	3280
	21st to end	Night	28% 15% 6% 11% 7% 4 4 3 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	72% 7% 10% 9% 2	2219
	11th to 20th	Day	61% 7% 14% 14% 3 1	2512
	21st to end	Day	77% 6% 9% 6% 2	2922
February	1st to 10th	Day	68% 7% 13% 7% 5%	1710
	11th to 20th	Day	64% 7% 11% 11% 6% 1	1674
	21st to end	Day	60% 9% 12% 14% 5% 1	1460
March	1st to 10th	Day	57% 14% 13% 11% 3	3252
	11th to 20th	Day	48% 6% 13% 29% 3 1	3490
	21st to end	Day	62% 9% 15% 13% 2	3932
April	1st to 10th	Day	61% 8% 17% 12% 2	2280
	11th to 20th	Day	56% 8% 12% 16% 6% 2	2293
	21st to end	Day	56% 8% 15% 14% 6% 1	2489
May	1st to 10th	Day	43% 10% 17% 19% 10% 1	4420
	11th to 20th	Day	41% 13% 23% 18% 5%	4380
	21st to end	Day	59% 12% 17% 10% 2	4814
June	1st to 10th	Day	53% 9% 19% 13% 6% 1	2726
	11th to 20th	Day	59% 10% 16% 12% 3	2850
	21st to end	Day	43% 9% 20% 19% 7% 2	2850
July	1st to 10th	Day	30% 5% 9% 20% 24% 12%	4666
	11th to 20th	Day	20% 6% 9% 30% 32% 2	4482
	21st to end	Day	34% 6% 14% 32% 12% 2	4639
August	1st to 10th	Day	25% 6% 11% 37% 19% 2	4261
	11th to 20th	Day	43% 5% 20% 22% 8% 2	4065
	21st to end	Day	44% 5% 13% 26% 12% 1	4348
September	1st to 10th	Day	34% 6% 17% 23% 14% 5%	2280
	11th to 20th	Day	43% 8% 15% 17% 15% 2	2280
	21st to end	Day	44% 8% 17% 18% 11% 2	2000
October	1st to 10th	Day	47% 7% 16% 21% 8%	1900
	11th to 20th	Day	53% 9% 16% 16% 5% 1	1900
	21st to end	Day	60% 7% 12% 13% 8%	1997
November	1st to 10th	Day	65% 7% 11% 11% 5% 1	1525
	11th to 20th	Day	65% 7% 9% 12% 6% 1	1520
	21st to end	Day	64% 6% 11% 13% 7%	1520
December	1st to 10th	Day	63% 6% 11% 13% 7%	1520
	11th to 20th	Day	75% 9% 7% 6% 2 1	1928
	21st to end	Day	72% 10% 9% 7% 2	2469
January	1st to 10th	Night	73% 8% 10% 7% 2 1	4726
	11th to 20th	Night	57% 6% 14% 18% 5% 1	4443
	21st to end	Night	75% 6% 8% 8% 2	4736
February	1st to 10th	Night	63% 9% 12% 11% 5%	2470
	11th to 20th	Night	59% 9% 12% 12% 7% 1	2404
	21st to end	Night	58% 8% 12% 15% 6% 1	1992
March	1st to 10th	Night	61% 11% 12% 12% 4	3618
	11th to 20th	Night	42% 8% 17% 29% 4 1	3490
	21st to end	Night	66% 9% 14% 10% 2	3720
April	1st to 10th	Night	56% 9% 17% 15% 2	1900
	11th to 20th	Night	49% 10% 15% 15% 9% 2	1836
	21st to end	Night	53% 8% 16% 16% 7% 1	1691
May	1st to 10th	Night	42% 10% 14% 23% 11%	2540
	11th to 20th	Night	38% 10% 24% 19% 7% 1	2516
	21st to end	Night	46% 17% 21% 13% 2	2762
June	1st to 10th	Night	44% 7% 22% 22% 5% 1	1449
	11th to 20th	Night	47% 11% 22% 17% 3	1330
	21st to end	Night	35% 7% 26% 20% 8% 3	1330
July	1st to 10th	Night	25% 5% 12% 23% 19% 16%	2185
	11th to 20th	Night	15% 6% 11% 32% 35% 2	2337
	21st to end	Night	25% 6% 17% 31% 17% 4	2674
August	1st to 10th	Night	17% 4 13% 38% 26% 2	2645
	11th to 20th	Night	32% 4 29% 22% 10% 3	2839
	21st to end	Night	37% 6% 16% 26% 13% 2	3323
September	1st to 10th	Night	26% 5% 19% 27% 17% 6%	1900
	11th to 20th	Night	42% 8% 13% 17% 15% 5%	1900
	21st to end	Night	42% 7% 14% 22% 12% 2	2177
October	1st to 10th	Night	43% 7% 15% 25% 10%	2280
	11th to 20th	Night	51% 7% 14% 19% 8%	2280
	21st to end	Night	53% 6% 13% 17% 10% 1	2599
November	1st to 10th	Night	61% 7% 14% 14% 4 1	2575
	11th to 20th	Night	61% 7% 12% 11% 7% 2	2660
	21st to end	Night	60% 8% 13% 11% 7% 2	2660
December	1st to 10th	Night	62% 7% 10% 13% 7% 1	2660
	11th to 20th	Night	70% 9% 12% 6% 3	3276
	21st to end	Night	71% 10% 8% 9% 3	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	27% 19% 26% 19% 8% 1	460
	11th to 20th	Day	26% 18% 22% 17% 11% 6%	460
	21st to end	Day	27% 18% 25% 19% 8% 3	506
April	1st to 10th	Day	31% 15% 22% 18% 10% 4	460
	11th to 20th	Day	23% 19% 26% 17% 9% 5%	452
	21st to end	Day	23% 15% 26% 25% 8% 2	460
May	1st to 10th	Day	16% 11% 24% 30% 15% 3	463
	11th to 20th	Day	23% 9% 20% 23% 18% 7%	488
	21st to end	Day	22% 7% 20% 26% 18% 6%	529
June	1st to 10th	Day	21% 12% 24% 23% 16% 5%	556
	11th to 20th	Day	22% 12% 23% 27% 12% 3	680
	21st to end	Day	15% 9% 22% 28% 21% 5%	680
July	1st to 10th	Day	12% 6% 18% 31% 25% 7%	671
	11th to 20th	Day	5% 4 18% 36% 28% 10%	569
	21st to end	Day	9% 3 12% 27% 36% 13%	531
August	1st to 10th	Day	8% 7% 15% 27% 32% 12%	487
	11th to 20th	Day	9% 4 13% 31% 32% 12%	477
	21st to end	Day	15% 9% 17% 30% 21% 7%	539
September	1st to 10th	Day	13% 8% 22% 33% 18% 6%	490
	11th to 20th	Day	18% 10% 18% 30% 18% 6%	490
	21st to end	Day	17% 13% 23% 29% 15% 3	490
October	1st to 10th	Day	21% 10% 24% 21% 19% 5%	490
	11th to 20th	Day	26% 11% 17% 25% 15% 5%	490
	21st to end	Day	31% 16% 19% 19% 11% 3	446
November	1st to 10th	Day	32% 17% 19% 19% 10% 2	197
	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% 27% 13% 1	760
	11th to 20th	Night	24% 15% 20% 25% 13% 3	760
	21st to end	Night	26% 16% 19% 25% 12% 2	836
April	1st to 10th	Night	24% 12% 20% 25% 15% 3	760
	11th to 20th	Night	17% 13% 22% 31% 15% 2	708
	21st to end	Night	20% 11% 17% 34% 16% 2	551
May	1st to 10th	Night	16% 11% 15% 37% 19% 2	380
	11th to 20th	Night	14% 6% 14% 34% 30% 3	374
	21st to end	Night	13% 6% 13% 37% 26% 5%	411
June	1st to 10th	Night	17% 9% 15% 32% 24% 3	380
	11th to 20th	Night	14% 9% 21% 32% 22% 2	380
	21st to end	Night	11% 7% 14% 37% 28% 3	380
July	1st to 10th	Night	8% 3 11% 35% 40% 4	370
	11th to 20th	Night	7% 3 11% 36% 39% 4	368
	21st to end	Night	5% 2 8% 34% 44% 7%	416
August	1st to 10th	Night	5% 2 7% 28% 44% 13%	450
	11th to 20th	Night	7% 2 6% 27% 47% 11%	564
	21st to end	Night	11% 4 10% 35% 35% 4	726
September	1st to 10th	Night	8% 5% 14% 37% 31% 5%	760
	11th to 20th	Night	15% 5% 13% 27% 34% 6%	760
	21st to end	Night	18% 7% 16% 33% 23% 3	760
October	1st to 10th	Night	17% 8% 16% 29% 25% 4	760
	11th to 20th	Night	24% 9% 14% 27% 22% 3	760
	21st to end	Night	28% 10% 15% 28% 17% 2	836
November	1st to 10th	Night	33% 11% 16% 28% 12% 1	744
	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 2024-10-29 22:23:00. Version ac07

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