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 2025-05-26.

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% 10% 3 8% 6% 5% 5% 4 5% 6% 30%	118091
	Night	19% 10% 3 8% 5% 4 4 3 5% 6% 30%	119873
Transparency	Day	52% 7% 14% 17% 8% 1	117847
	Night	53% 7% 14% 17% 8% 1	119763
Seeing	Day	22% 12% 20% 24% 17% 5%	15796
	Night	23% 10% 16% 28% 20% 3	24280

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	32% 13% 4 9% 6% 6% 5% 4 4 4 11%	3179
	11th to 20th	Day	18% 9% 5% 14% 8% 6% 5% 3 5% 6% 21%	3588
	21st to end	Day	30% 12% 4 9% 6% 4 4 3 4 4 21%	4188
February	1st to 10th	Day	24% 13% 5% 9% 6% 6% 6% 4 7% 6% 15%	1800
	11th to 20th	Day	25% 15% 4 9% 5% 5% 4 3 5% 4 22%	1755
	21st to end	Day	22% 13% 3 8% 6% 5% 6% 4 6% 5% 22%	1535
March	1st to 10th	Day	18% 14% 5% 11% 7% 7% 6% 4 6% 6% 15%	4684
	11th to 20th	Day	20% 13% 4 9% 5% 5% 4 3 5% 5% 27%	5000
	21st to end	Day	21% 12% 4 11% 7% 7% 6% 4 6% 6% 15%	5632
April	1st to 10th	Day	23% 11% 4 9% 7% 5% 6% 4 6% 7% 19%	2400
	11th to 20th	Day	19% 11% 3 8% 6% 5% 5% 4 5% 7% 28%	2415
	21st to end	Day	16% 10% 4 9% 7% 5% 5% 4 6% 7% 26%	2620
May	1st to 10th	Day	14% 8% 3 7% 5% 4 5% 4 7% 6% 36%	4560
	11th to 20th	Day	13% 9% 3 7% 5% 5% 5% 4 6% 8% 35%	4517
	21st to end	Day	18% 12% 4 10% 6% 6% 6% 5% 6% 7% 21%	4886
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	12% 7% 3 6% 4 5% 5% 4 8% 11% 35%	3180
	11th to 20th	Day	25% 9% 3 8% 4 3 4 4 6% 8% 26%	3161
	21st to end	Day	26% 11% 4 9% 5% 4 4 3 6% 7% 21%	3293
November	1st to 10th	Day	29% 12% 4 8% 5% 5% 5% 4 6% 6% 18%	1597
	11th to 20th	Day	29% 11% 3 9% 6% 5% 4 3 4 6% 20%	1600
	21st to end	Day	26% 12% 4 10% 6% 5% 4 4 5% 6% 18%	1600
December	1st to 10th	Day	19% 8% 3 11% 10% 7% 6% 4 6% 7% 18%	2620
	11th to 20th	Day	33% 13% 4 13% 7% 5% 4 4 4 4 9%	3025
	21st to end	Day	30% 14% 5% 13% 7% 5% 5% 3 5% 3 10%	3525
January	1st to 10th	Night	31% 14% 5% 11% 6% 5% 5% 3 4 4 11%	6766
	11th to 20th	Night	18% 8% 4 10% 5% 4 4 3 5% 7% 31%	6305
	21st to end	Night	26% 14% 4 7% 5% 4 4 3 4 5% 24%	6761
February	1st to 10th	Night	23% 15% 4 9% 6% 4 4 3 5% 6% 20%	2600
	11th to 20th	Night	22% 13% 3 8% 5% 6% 4 3 5% 6% 25%	2532
	21st to end	Night	22% 13% 3 8% 6% 5% 5% 4 6% 6% 24%	2093
March	1st to 10th	Night	16% 14% 4 11% 6% 5% 6% 5% 6% 8% 19%	5206
	11th to 20th	Night	22% 11% 3 7% 4 4 4 3 5% 7% 31%	4999
	21st to end	Night	26% 14% 4 9% 6% 5% 5% 4 6% 6% 16%	5342
April	1st to 10th	Night	20% 9% 3 10% 7% 5% 5% 4 7% 6% 24%	2000
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 5% 6% 7% 32%	1933
	21st to end	Night	16% 8% 3 9% 6% 6% 4 4 6% 8% 31%	1780
May	1st to 10th	Night	14% 8% 3 6% 4 4 5% 4 5% 5% 41%	2620
	11th to 20th	Night	10% 7% 2 8% 4 4 4 5% 6% 8% 41%	2596
	21st to end	Night	7% 8% 2 9% 7% 6% 6% 5% 8% 9% 31%	2805
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	12% 7% 2 6% 4 4 5% 3 5% 9% 42%	3800
	11th to 20th	Night	22% 8% 3 6% 4 4 5% 4 6% 7% 31%	3777
	21st to end	Night	25% 10% 3 9% 5% 4 4 3 5% 6% 26%	4269
November	1st to 10th	Night	30% 10% 3 7% 5% 5% 5% 3 5% 5% 24%	2701
	11th to 20th	Night	29% 10% 3 8% 6% 5% 4 3 4 5% 24%	2800
	21st to end	Night	25% 11% 4 8% 7% 5% 5% 3 5% 6% 22%	2800
December	1st to 10th	Night	19% 9% 3 11% 6% 6% 6% 5% 5% 6% 24%	4560
	11th to 20th	Night	33% 14% 4 12% 7% 4 4 3 4 4 10%	5180
	21st to end	Night	32% 16% 6% 10% 6% 4 4 3 4 4 11%	7412

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	73% 5% 9% 9% 3	3179
	11th to 20th	Day	61% 6% 13% 15% 4 1	3612
	21st to end	Day	65% 4 10% 13% 7%	4188
February	1st to 10th	Day	67% 7% 13% 7% 5% 1	1800
	11th to 20th	Day	65% 7% 10% 10% 7% 1	1764
	21st to end	Day	60% 8% 11% 15% 5% 1	1535
March	1st to 10th	Day	61% 11% 12% 13% 3	4684
	11th to 20th	Day	57% 6% 11% 22% 4	5000
	21st to end	Day	66% 7% 15% 11% 1	5632
April	1st to 10th	Day	61% 8% 17% 12% 2	2400
	11th to 20th	Day	54% 8% 13% 17% 6% 2	2415
	21st to end	Day	55% 8% 16% 14% 6% 1	2620
May	1st to 10th	Day	43% 9% 16% 20% 10% 1	4560
	11th to 20th	Day	42% 13% 22% 18% 5%	4520
	21st to end	Day	59% 12% 17% 10% 2	4887
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	40% 5% 20% 28% 6%	3180
	11th to 20th	Day	57% 7% 14% 14% 7%	3161
	21st to end	Day	63% 6% 12% 12% 7% 1	3293
November	1st to 10th	Day	65% 7% 11% 11% 5% 1	1597
	11th to 20th	Day	66% 7% 9% 11% 6% 1	1600
	21st to end	Day	65% 6% 10% 12% 6%	1600
December	1st to 10th	Day	61% 6% 10% 15% 7%	2620
	11th to 20th	Day	76% 7% 7% 7% 1	3028
	21st to end	Day	74% 7% 9% 7% 2	3525
January	1st to 10th	Night	75% 6% 10% 7% 2	6766
	11th to 20th	Night	53% 5% 15% 21% 6% 1	6343
	21st to end	Night	63% 5% 9% 16% 6%	6761
February	1st to 10th	Night	63% 9% 12% 10% 6%	2600
	11th to 20th	Night	59% 9% 12% 12% 7% 1	2534
	21st to end	Night	58% 8% 12% 15% 6% 1	2093
March	1st to 10th	Night	58% 10% 14% 13% 5%	5206
	11th to 20th	Night	52% 7% 13% 24% 4 1	5000
	21st to end	Night	67% 8% 14% 9% 2	5342
April	1st to 10th	Night	57% 9% 16% 15% 2	2000
	11th to 20th	Night	48% 10% 16% 16% 10% 2	1933
	21st to end	Night	51% 8% 17% 17% 7% 1	1780
May	1st to 10th	Night	41% 10% 14% 24% 11%	2620
	11th to 20th	Night	38% 10% 24% 19% 7% 1	2596
	21st to end	Night	46% 17% 21% 13% 2	2805
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	40% 5% 17% 32% 7%	3800
	11th to 20th	Night	53% 5% 14% 21% 6%	3777
	21st to end	Night	59% 6% 11% 14% 9% 1	4269
November	1st to 10th	Night	61% 7% 13% 14% 4 1	2701
	11th to 20th	Night	63% 7% 12% 10% 7% 2	2800
	21st to end	Night	61% 7% 13% 10% 7% 2	2800
December	1st to 10th	Night	59% 6% 12% 16% 7%	4560
	11th to 20th	Night	77% 6% 9% 5% 2	5176
	21st to end	Night	74% 8% 7% 8% 2	7412

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	37% 15% 19% 15% 11% 5%	200
	11th to 20th	Day	36% 11% 16% 17% 19% 3	200
	21st to end	Day	32% 16% 19% 18% 13% 2	300
February	1st to 10th	Day	35% 16% 19% 20% 10% 2	400
	11th to 20th	Day	35% 18% 20% 19% 6% 1	395
	21st to end	Day	29% 19% 24% 19% 8% 1	357
March	1st to 10th	Day	28% 18% 26% 18% 8% 2	500
	11th to 20th	Day	27% 18% 22% 16% 11% 5%	500
	21st to end	Day	28% 18% 25% 19% 8% 2	550
April	1st to 10th	Day	32% 14% 22% 18% 10% 4	500
	11th to 20th	Day	23% 17% 26% 19% 10% 5%	492
	21st to end	Day	21% 15% 25% 26% 9% 3	500
May	1st to 10th	Day	16% 11% 25% 30% 16% 3	503
	11th to 20th	Day	24% 10% 21% 21% 18% 7%	528
	21st to end	Day	21% 8% 20% 27% 18% 6%	550
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% 16% 21% 18% 11% 2	215
	11th to 20th	Day	34% 17% 19% 16% 11% 4	200
	21st to end	Day	31% 14% 18% 17% 16% 6%	200
December	1st to 10th	Day	35% 16% 10% 20% 16% 5%	200
	11th to 20th	Day	37% 20% 19% 15% 10% 1	200
	21st to end	Day	39% 15% 18% 18% 9% 1	220
January	1st to 10th	Night	40% 13% 13% 20% 13% 2	900
	11th to 20th	Night	31% 11% 13% 25% 16% 2	805
	21st to end	Night	28% 11% 15% 26% 17% 3	880
February	1st to 10th	Night	30% 16% 15% 25% 12% 3	800
	11th to 20th	Night	27% 13% 18% 26% 14% 2	784
	21st to end	Night	27% 13% 21% 25% 12% 2	660
March	1st to 10th	Night	25% 13% 20% 27% 13% 1	800
	11th to 20th	Night	26% 15% 20% 25% 12% 3	800
	21st to end	Night	27% 16% 19% 25% 11% 2	880
April	1st to 10th	Night	24% 13% 20% 25% 15% 3	800
	11th to 20th	Night	17% 13% 23% 31% 15% 2	745
	21st to end	Night	19% 10% 17% 34% 17% 3	580
May	1st to 10th	Night	16% 11% 15% 39% 19% 2	400
	11th to 20th	Night	14% 6% 15% 33% 28% 3	394
	21st to end	Night	13% 6% 13% 37% 26% 5%	422
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	780
	11th to 20th	Night	33% 13% 18% 22% 13% 1	800
	21st to end	Night	28% 11% 18% 25% 15% 2	800
December	1st to 10th	Night	28% 10% 13% 30% 17% 2	800
	11th to 20th	Night	36% 13% 16% 22% 12% 2	800
	21st to end	Night	38% 13% 17% 22% 9% 1	980

Page generated at 2025-05-23 06:21:00. Version ac07

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