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 2026-09-29.

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	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	154071
	Night	18% 9% 3 8% 5% 4 4 3 5% 6% 33%	146066
Transparency	Day	49% 6% 15% 19% 9% 1	153786
	Night	51% 6% 15% 18% 8% 1	145933
Seeing	Day	22% 12% 21% 24% 16% 5%	17628
	Night	23% 10% 16% 28% 20% 3	25868

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 10% 6% 6% 6% 4 5% 4 10%	4125
	11th to 20th	Day	17% 10% 4 13% 7% 6% 4 3 5% 6% 25%	4688
	21st to end	Day	29% 11% 4 9% 6% 4 4 3 4 5% 21%	5425
February	1st to 10th	Day	24% 13% 5% 9% 7% 6% 6% 4 7% 6% 15%	1890
	11th to 20th	Day	24% 15% 4 9% 5% 5% 4 3 4 4 22%	1845
	21st to end	Day	22% 13% 3 9% 6% 5% 6% 4 6% 5% 22%	1610
March	1st to 10th	Day	18% 14% 5% 11% 7% 7% 6% 4 6% 6% 16%	4787
	11th to 20th	Day	20% 13% 4 9% 5% 4 4 3 5% 5% 27%	5110
	21st to end	Day	21% 12% 4 11% 7% 7% 6% 5% 6% 6% 16%	5756
April	1st to 10th	Day	22% 10% 4 8% 7% 6% 6% 5% 6% 7% 21%	2520
	11th to 20th	Day	18% 11% 3 8% 6% 5% 5% 4 5% 7% 27%	2537
	21st to end	Day	16% 9% 4 9% 7% 5% 6% 4 6% 7% 26%	2751
May	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	8195
	11th to 20th	Day	14% 10% 3 9% 6% 5% 6% 4 7% 9% 27%	8160
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 4 7% 9% 32%	8993
June	1st to 10th	Day	15% 8% 4 7% 6% 5% 5% 5% 7% 8% 29%	3013
	11th to 20th	Day	12% 10% 4 11% 7% 7% 7% 5% 7% 8% 24%	3150
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 5% 6% 7% 37%	3150
July	1st to 10th	Day	6% 7% 3 6% 3 4 4 3 4 6% 53%	8820
	11th to 20th	Day	4 4 1 4 3 3 3 3 4 6% 65%	8502
	21st to end	Day	7% 6% 2 5% 4 4 4 3 5% 7% 51%	8916
August	1st to 10th	Day	3 4 2 5% 4 3 3 3 5% 6% 62%	6262
	11th to 20th	Day	8% 6% 3 6% 5% 5% 5% 4 6% 9% 44%	5923
	21st to end	Day	10% 8% 3 7% 4 4 5% 4 6% 8% 41%	6327
September	1st to 10th	Day	9% 6% 2 7% 5% 4 4 4 5% 8% 47%	2520
	11th to 20th	Day	11% 8% 3 7% 5% 5% 5% 3 6% 7% 40%	2520
	21st to end	Day	15% 8% 2 7% 5% 4 5% 3 6% 8% 38%	2192
October	1st to 10th	Day	12% 7% 3 6% 4 5% 5% 5% 8% 11% 35%	3280
	11th to 20th	Day	24% 9% 3 8% 4 3 4 4 6% 8% 26%	3261
	21st to end	Day	26% 11% 4 9% 5% 4 4 3 6% 7% 21%	3398
November	1st to 10th	Day	29% 12% 4 8% 5% 4 5% 4 6% 6% 18%	1678
	11th to 20th	Day	30% 11% 3 9% 5% 5% 4 3 4 6% 20%	1680
	21st to end	Day	27% 12% 4 10% 6% 5% 4 4 5% 6% 18%	1680
December	1st to 10th	Day	20% 9% 3 11% 10% 7% 6% 4 6% 7% 18%	2700
	11th to 20th	Day	34% 12% 4 13% 7% 5% 4 4 4 4 9%	3105
	21st to end	Day	29% 14% 5% 13% 7% 5% 5% 3 5% 4 10%	3602
January	1st to 10th	Night	31% 15% 5% 11% 6% 5% 5% 3 4 4 12%	8770
	11th to 20th	Night	18% 9% 4 9% 5% 3 4 3 4 6% 35%	8205
	21st to end	Night	25% 12% 3 8% 5% 5% 4 3 4 5% 25%	8752
February	1st to 10th	Night	23% 15% 4 9% 6% 4 4 3 5% 6% 21%	2730
	11th to 20th	Night	21% 13% 3 9% 5% 6% 4 3 5% 6% 25%	2662
	21st to end	Night	21% 13% 3 8% 6% 5% 5% 4 6% 6% 24%	2194
March	1st to 10th	Night	16% 13% 4 11% 6% 5% 6% 5% 7% 8% 19%	5322
	11th to 20th	Night	22% 11% 3 7% 4 4 4 3 5% 6% 30%	5109
	21st to end	Night	26% 14% 4 9% 6% 5% 5% 4 6% 6% 16%	5460
April	1st to 10th	Night	19% 9% 3 10% 7% 5% 5% 4 7% 6% 25%	2100
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 5% 6% 8% 31%	2031
	21st to end	Night	15% 8% 3 8% 6% 6% 4 4 6% 8% 33%	1869
May	1st to 10th	Night	10% 7% 3 6% 4 4 5% 4 5% 5% 47%	4724
	11th to 20th	Night	11% 8% 3 10% 5% 6% 5% 5% 7% 8% 34%	4700
	21st to end	Night	7% 7% 3 7% 6% 6% 6% 6% 8% 10% 36%	5178
June	1st to 10th	Night	12% 7% 3 7% 4 5% 5% 4 8% 7% 38%	1601
	11th to 20th	Night	10% 7% 2 7% 6% 6% 7% 4 7% 9% 35%	1470
	21st to end	Night	5% 5% 3 7% 6% 5% 4 4 6% 7% 48%	1470
July	1st to 10th	Night	4 4 2 5% 4 4 4 4 5% 6% 58%	4140
	11th to 20th	Night	2 2 1 2 2 3 3 2 4 6% 73%	4440
	21st to end	Night	3 3 2 5% 4 4 4 3 5% 7% 60%	5152
August	1st to 10th	Night	1 2 1 2 2 2 2 2 4 5% 76%	3893
	11th to 20th	Night	6% 4 2 5% 3 4 4 4 5% 9% 53%	4142
	21st to end	Night	11% 6% 2 5% 3 3 3 3 5% 7% 51%	4831
September	1st to 10th	Night	6% 4 2 4 4 3 4 3 5% 7% 58%	2100
	11th to 20th	Night	12% 6% 3 6% 4 4 4 3 5% 7% 44%	2100
	21st to end	Night	13% 7% 3 7% 4 4 5% 3 5% 7% 41%	2382
October	1st to 10th	Night	13% 7% 2 6% 4 4 5% 3 5% 9% 42%	3920
	11th to 20th	Night	22% 8% 3 6% 4 4 5% 4 6% 7% 31%	3897
	21st to end	Night	25% 10% 3 9% 6% 4 4 3 5% 6% 26%	4406
November	1st to 10th	Night	30% 10% 3 7% 5% 5% 5% 3 5% 5% 23%	2840
	11th to 20th	Night	29% 10% 3 8% 6% 5% 4 3 4 5% 23%	2940
	21st to end	Night	25% 11% 4 8% 6% 5% 5% 3 5% 6% 21%	2940
December	1st to 10th	Night	20% 10% 3 11% 6% 5% 6% 5% 5% 6% 23%	4700
	11th to 20th	Night	33% 14% 4 12% 7% 4 4 3 4 4 10%	5320
	21st to end	Night	32% 16% 6% 11% 6% 4 4 3 4 5% 11%	7576

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	75% 5% 9% 9% 3	4125
	11th to 20th	Day	58% 4 12% 16% 9% 1	4712
	21st to end	Day	65% 4 10% 14% 8%	5426
February	1st to 10th	Day	67% 7% 12% 8% 5% 1	1890
	11th to 20th	Day	64% 7% 10% 11% 7% 1	1854
	21st to end	Day	60% 8% 12% 14% 4 1	1610
March	1st to 10th	Day	61% 11% 12% 13% 3	4787
	11th to 20th	Day	57% 6% 11% 22% 4	5110
	21st to end	Day	66% 7% 15% 11% 1	5756
April	1st to 10th	Day	60% 8% 18% 12% 2	2520
	11th to 20th	Day	55% 8% 13% 17% 6% 2	2537
	21st to end	Day	55% 7% 16% 15% 6% 1	2751
May	1st to 10th	Day	41% 6% 17% 26% 9% 1	8195
	11th to 20th	Day	52% 9% 19% 17% 4	8163
	21st to end	Day	48% 8% 24% 17% 3	8994
June	1st to 10th	Day	51% 8% 19% 15% 6% 1	3013
	11th to 20th	Day	57% 9% 15% 15% 4	3150
	21st to end	Day	45% 8% 19% 19% 7% 2	3150
July	1st to 10th	Day	36% 4 16% 19% 19% 6%	8746
	11th to 20th	Day	23% 5% 14% 28% 28% 3	8422
	21st to end	Day	33% 4 20% 32% 10% 1	8723
August	1st to 10th	Day	23% 6% 13% 31% 24% 4	6245
	11th to 20th	Day	39% 5% 25% 24% 6% 1	5962
	21st to end	Day	43% 5% 15% 28% 9% 1	6326
September	1st to 10th	Day	35% 6% 18% 23% 13% 5%	2520
	11th to 20th	Day	43% 7% 15% 18% 15% 2	2520
	21st to end	Day	45% 7% 18% 18% 10% 2	2192
October	1st to 10th	Day	41% 5% 20% 28% 6%	3280
	11th to 20th	Day	57% 7% 14% 14% 8%	3261
	21st to end	Day	63% 6% 12% 12% 7% 1	3398
November	1st to 10th	Day	64% 7% 13% 11% 4 1	1678
	11th to 20th	Day	67% 6% 9% 11% 5% 1	1680
	21st to end	Day	66% 5% 10% 12% 6%	1680
December	1st to 10th	Day	61% 6% 10% 15% 7%	2700
	11th to 20th	Day	77% 7% 7% 8% 2	3108
	21st to end	Day	74% 7% 9% 8% 2	3602
January	1st to 10th	Night	75% 5% 9% 9% 2	8770
	11th to 20th	Night	52% 4 12% 18% 12% 1	8243
	21st to end	Night	62% 4 10% 17% 7%	8752
February	1st to 10th	Night	63% 8% 12% 10% 7%	2730
	11th to 20th	Night	59% 8% 12% 12% 7% 1	2664
	21st to end	Night	59% 8% 13% 15% 5% 1	2194
March	1st to 10th	Night	58% 9% 14% 13% 6%	5322
	11th to 20th	Night	53% 7% 13% 23% 4 1	5110
	21st to end	Night	67% 8% 14% 10% 2	5460
April	1st to 10th	Night	56% 9% 17% 15% 3	2100
	11th to 20th	Night	49% 10% 16% 15% 9% 1	2031
	21st to end	Night	50% 7% 17% 17% 7% 1	1869
May	1st to 10th	Night	37% 7% 12% 31% 12% 1	4721
	11th to 20th	Night	45% 8% 20% 21% 5% 1	4700
	21st to end	Night	42% 12% 25% 19% 1	5182
June	1st to 10th	Night	43% 7% 21% 22% 5% 1	1601
	11th to 20th	Night	44% 10% 21% 20% 4	1470
	21st to end	Night	35% 7% 26% 21% 8% 3	1470
July	1st to 10th	Night	30% 4 22% 20% 14% 9%	4105
	11th to 20th	Night	16% 4 14% 31% 33% 3	4397
	21st to end	Night	26% 4 24% 32% 12% 3	5040
August	1st to 10th	Night	14% 3 15% 37% 27% 5%	3885
	11th to 20th	Night	31% 4 31% 25% 7% 2	4169
	21st to end	Night	37% 4 20% 26% 11% 1	4831
September	1st to 10th	Night	27% 5% 21% 27% 15% 5%	2100
	11th to 20th	Night	41% 7% 14% 18% 16% 5%	2100
	21st to end	Night	43% 7% 14% 22% 12% 2	2382
October	1st to 10th	Night	40% 5% 17% 31% 6%	3920
	11th to 20th	Night	53% 5% 14% 21% 6%	3897
	21st to end	Night	59% 6% 11% 14% 9% 1	4406
November	1st to 10th	Night	62% 7% 13% 14% 4 1	2840
	11th to 20th	Night	64% 7% 11% 10% 6% 2	2940
	21st to end	Night	61% 7% 13% 10% 6% 1	2940
December	1st to 10th	Night	59% 6% 12% 16% 7%	4700
	11th to 20th	Night	77% 6% 9% 6% 2	5316
	21st to end	Night	74% 8% 8% 8% 2	7576

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	38% 16% 19% 14% 10% 4	220
	11th to 20th	Day	34% 11% 16% 17% 18% 3	220
	21st to end	Day	33% 15% 18% 19% 13% 2	326
February	1st to 10th	Day	35% 15% 19% 20% 9% 2	430
	11th to 20th	Day	35% 18% 21% 19% 6% 1	425
	21st to end	Day	30% 20% 24% 17% 8% 1	384
March	1st to 10th	Day	29% 18% 25% 18% 8% 2	540
	11th to 20th	Day	31% 17% 21% 15% 10% 5%	540
	21st to end	Day	29% 18% 25% 18% 8% 2	594
April	1st to 10th	Day	31% 14% 23% 20% 10% 4	540
	11th to 20th	Day	23% 17% 27% 19% 10% 5%	532
	21st to end	Day	21% 15% 26% 26% 9% 3	540
May	1st to 10th	Day	16% 12% 24% 30% 15% 3	543
	11th to 20th	Day	25% 10% 21% 21% 17% 7%	568
	21st to end	Day	21% 7% 21% 27% 17% 6%	617
June	1st to 10th	Day	20% 12% 24% 24% 16% 4	643
	11th to 20th	Day	21% 11% 23% 29% 12% 3	780
	21st to end	Day	16% 8% 23% 28% 20% 6%	780
July	1st to 10th	Day	12% 6% 19% 31% 25% 7%	771
	11th to 20th	Day	5% 4 21% 34% 27% 9%	659
	21st to end	Day	9% 4 13% 27% 34% 13%	619
August	1st to 10th	Day	9% 7% 14% 28% 31% 11%	567
	11th to 20th	Day	10% 4 13% 31% 32% 11%	557
	21st to end	Day	14% 9% 17% 30% 22% 7%	627
September	1st to 10th	Day	14% 7% 22% 33% 19% 6%	570
	11th to 20th	Day	17% 9% 19% 30% 18% 7%	570
	21st to end	Day	18% 12% 23% 29% 15% 3	563
October	1st to 10th	Day	21% 11% 24% 21% 18% 5%	530
	11th to 20th	Day	27% 12% 18% 24% 15% 5%	530
	21st to end	Day	33% 16% 19% 19% 11% 3	485
November	1st to 10th	Day	33% 17% 20% 18% 10% 2	236
	11th to 20th	Day	35% 16% 20% 15% 10% 4	220
	21st to end	Day	31% 14% 19% 16% 14% 5%	220
December	1st to 10th	Day	39% 14% 9% 19% 14% 5%	220
	11th to 20th	Day	41% 18% 18% 13% 9% 1	220
	21st to end	Day	38% 14% 18% 18% 10% 2	242
January	1st to 10th	Night	40% 13% 14% 19% 12% 2	950
	11th to 20th	Night	31% 11% 14% 26% 16% 2	845
	21st to end	Night	28% 11% 15% 27% 17% 3	924
February	1st to 10th	Night	30% 15% 16% 25% 11% 2	840
	11th to 20th	Night	27% 13% 18% 25% 14% 2	824
	21st to end	Night	27% 14% 21% 25% 11% 2	692
March	1st to 10th	Night	26% 13% 21% 26% 13% 1	840
	11th to 20th	Night	28% 15% 19% 24% 11% 2	840
	21st to end	Night	27% 16% 20% 25% 11% 2	924
April	1st to 10th	Night	23% 13% 20% 26% 16% 3	840
	11th to 20th	Night	17% 12% 22% 31% 14% 2	783
	21st to end	Night	19% 10% 18% 34% 16% 2	609
May	1st to 10th	Night	15% 10% 15% 39% 19% 2	420
	11th to 20th	Night	15% 7% 15% 32% 27% 3	414
	21st to end	Night	13% 7% 13% 37% 26% 5%	455
June	1st to 10th	Night	16% 10% 14% 32% 25% 3	420
	11th to 20th	Night	13% 9% 21% 34% 21% 2	420
	21st to end	Night	10% 6% 15% 37% 28% 3	420
July	1st to 10th	Night	8% 2 11% 36% 39% 3	410
	11th to 20th	Night	7% 3 11% 37% 38% 3	408
	21st to end	Night	5% 3 10% 32% 43% 7%	460
August	1st to 10th	Night	5% 3 7% 30% 42% 12%	498
	11th to 20th	Night	7% 3 6% 26% 46% 11%	624
	21st to end	Night	11% 4 11% 36% 34% 4	802
September	1st to 10th	Night	8% 5% 15% 37% 31% 5%	840
	11th to 20th	Night	15% 5% 14% 28% 32% 6%	840
	21st to end	Night	17% 8% 16% 34% 22% 3	832
October	1st to 10th	Night	18% 8% 16% 29% 25% 4	800
	11th to 20th	Night	25% 9% 15% 27% 22% 3	800
	21st to end	Night	28% 10% 16% 28% 16% 2	880
November	1st to 10th	Night	34% 11% 15% 28% 12% 1	820
	11th to 20th	Night	34% 13% 18% 22% 13% 1	840
	21st to end	Night	29% 11% 18% 25% 15% 2	840
December	1st to 10th	Night	29% 9% 13% 30% 17% 2	840
	11th to 20th	Night	38% 13% 15% 21% 11% 2	840
	21st to end	Night	38% 13% 17% 23% 9% 1	1034

Page generated at 2026-09-26 01:18:39. Version ac07

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