Ed Majden Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ed Majden Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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% 10% 3 8% 5% 5% 5% 4 5% 6% 33%	108778
	Night	19% 11% 3 8% 5% 5% 4 4 5% 6% 30%	108854
Transparency	Day	50% 5% 16% 20% 8% 1	108519
	Night	55% 4 14% 18% 7% 1	108759
Seeing	Day	20% 7% 21% 26% 19% 7%	15863
	Night	25% 4 15% 29% 22% 6%	23942

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	26% 21% 5% 8% 6% 5% 4 3 4 5% 13%	2261
	11th to 20th	Day	21% 13% 4 9% 6% 5% 5% 3 5% 8% 22%	2199
	21st to end	Day	39% 19% 4 7% 5% 4 3 3 4 4 10%	2668
February	1st to 10th	Day	21% 16% 5% 9% 6% 5% 5% 5% 6% 6% 17%	1710
	11th to 20th	Day	17% 15% 4 9% 6% 5% 5% 4 4 5% 27%	1665
	21st to end	Day	16% 16% 4 8% 8% 6% 6% 5% 6% 5% 20%	1413
March	1st to 10th	Day	16% 16% 4 9% 6% 6% 6% 6% 6% 6% 19%	3283
	11th to 20th	Day	14% 12% 3 8% 7% 7% 5% 4 5% 6% 30%	3490
	21st to end	Day	18% 14% 4 10% 6% 5% 6% 5% 6% 6% 21%	3825
April	1st to 10th	Day	18% 13% 5% 9% 7% 7% 4 5% 5% 7% 20%	2286
	11th to 20th	Day	15% 13% 4 9% 6% 6% 6% 5% 6% 6% 24%	2438
	21st to end	Day	18% 12% 4 10% 6% 5% 5% 4 6% 5% 26%	2470
May	1st to 10th	Day	13% 9% 3 8% 6% 4 5% 4 5% 6% 37%	4389
	11th to 20th	Day	10% 8% 3 8% 5% 6% 5% 4 6% 9% 35%	4394
	21st to end	Day	20% 13% 3 9% 6% 5% 6% 4 5% 6% 23%	5161
June	1st to 10th	Day	17% 10% 3 9% 8% 6% 6% 4 5% 7% 25%	2850
	11th to 20th	Day	13% 9% 4 10% 8% 8% 6% 5% 7% 7% 23%	2850
	21st to end	Day	14% 7% 3 10% 5% 6% 5% 4 6% 6% 34%	2850
July	1st to 10th	Day	6% 4 2 5% 4 3 3 3 4 5% 62%	4740
	11th to 20th	Day	4 3 1 3 3 3 3 3 4 5% 67%	4740
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 48%	5112
August	1st to 10th	Day	5% 3 1 3 2 2 3 3 5% 7% 67%	4218
	11th to 20th	Day	10% 6% 2 6% 4 5% 6% 5% 8% 9% 39%	4026
	21st to end	Day	12% 6% 2 6% 4 4 4 3 6% 7% 45%	4521
September	1st to 10th	Day	9% 4 2 6% 4 4 4 4 6% 7% 51%	2301
	11th to 20th	Day	17% 7% 2 7% 4 4 4 3 5% 6% 40%	2176
	21st to end	Day	17% 8% 3 7% 5% 4 6% 3 5% 8% 35%	2068
October	1st to 10th	Day	19% 10% 4 9% 5% 5% 6% 4 6% 5% 28%	3180
	11th to 20th	Day	23% 9% 3 8% 5% 5% 5% 3 4 6% 28%	3161
	21st to end	Day	21% 10% 3 10% 7% 6% 6% 4 6% 6% 22%	3125
November	1st to 10th	Day	27% 11% 3 8% 6% 7% 6% 4 5% 5% 18%	1568
	11th to 20th	Day	21% 15% 4 9% 5% 5% 5% 5% 5% 5% 22%	1600
	21st to end	Day	24% 12% 4 8% 6% 5% 5% 4 5% 6% 21%	1600
December	1st to 10th	Day	15% 16% 5% 9% 7% 5% 5% 3 4 5% 24%	2297
	11th to 20th	Day	33% 16% 4 9% 4 4 4 3 4 3 16%	2645
	21st to end	Day	33% 17% 4 9% 5% 4 4 3 4 4 12%	3498
January	1st to 10th	Night	25% 18% 4 9% 5% 5% 4 3 4 5% 16%	4816
	11th to 20th	Night	21% 15% 3 8% 6% 5% 4 3 5% 6% 24%	4699
	21st to end	Night	36% 18% 3 7% 5% 4 3 3 4 3 12%	4990
February	1st to 10th	Night	19% 16% 4 9% 6% 5% 5% 4 4 6% 21%	2470
	11th to 20th	Night	17% 13% 3 7% 5% 4 5% 3 5% 7% 30%	2402
	21st to end	Night	15% 15% 5% 8% 5% 6% 5% 4 5% 5% 27%	2041
March	1st to 10th	Night	15% 17% 3 8% 6% 6% 5% 4 5% 7% 23%	3574
	11th to 20th	Night	10% 8% 3 7% 6% 6% 7% 5% 7% 7% 36%	3489
	21st to end	Night	18% 15% 4 9% 5% 5% 5% 4 5% 5% 24%	3828
April	1st to 10th	Night	16% 12% 3 9% 6% 6% 6% 4 7% 6% 26%	1891
	11th to 20th	Night	14% 11% 3 7% 7% 6% 4 4 6% 8% 29%	1691
	21st to end	Night	16% 9% 4 8% 7% 6% 5% 3 6% 7% 30%	1710
May	1st to 10th	Night	12% 7% 3 5% 4 5% 5% 4 4 7% 46%	2572
	11th to 20th	Night	12% 6% 2 6% 5% 4 4 4 6% 9% 42%	2497
	21st to end	Night	17% 13% 5% 8% 5% 5% 5% 4 6% 6% 26%	2414
June	1st to 10th	Night	15% 8% 3 8% 7% 5% 5% 4 7% 8% 31%	1330
	11th to 20th	Night	14% 8% 3 10% 6% 7% 5% 4 5% 7% 30%	1330
	21st to end	Night	14% 7% 3 8% 5% 5% 4 4 6% 8% 38%	1330
July	1st to 10th	Night	6% 5% 1 4 3 3 3 3 4 6% 62%	2220
	11th to 20th	Night	4 3 1 2 1 2 3 3 5% 6% 71%	2220
	21st to end	Night	8% 4 2 6% 3 3 5% 4 5% 6% 53%	2438
August	1st to 10th	Night	6% 3 1 3 3 2 2 2 3 4 71%	2715
	11th to 20th	Night	11% 5% 2 6% 4 5% 5% 5% 7% 8% 41%	2812
	21st to end	Night	12% 7% 3 7% 4 4 4 4 5% 5% 45%	3146
September	1st to 10th	Night	8% 5% 2 5% 4 4 3 3 4 8% 54%	1878
	11th to 20th	Night	14% 8% 2 6% 5% 4 4 4 5% 6% 41%	2003
	21st to end	Night	16% 8% 2 7% 5% 4 4 3 4 7% 40%	2111
October	1st to 10th	Night	17% 9% 3 9% 6% 5% 5% 3 6% 6% 30%	3800
	11th to 20th	Night	21% 10% 4 9% 5% 4 4 4 5% 6% 28%	3777
	21st to end	Night	20% 10% 3 9% 5% 5% 5% 4 5% 5% 29%	4432
November	1st to 10th	Night	27% 13% 3 8% 5% 5% 5% 4 5% 6% 20%	2730
	11th to 20th	Night	21% 12% 3 8% 6% 6% 6% 4 6% 6% 23%	2800
	21st to end	Night	23% 11% 4 7% 6% 5% 5% 4 6% 5% 24%	2800
December	1st to 10th	Night	18% 15% 5% 10% 8% 6% 5% 3 5% 6% 20%	4880
	11th to 20th	Night	32% 18% 4 9% 5% 5% 4 3 4 4 12%	5560
	21st to end	Night	34% 14% 4 8% 4 4 4 3 4 5% 16%	7458

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	76% 2 5% 13% 3 1	2261
	11th to 20th	Day	66% 3 12% 11% 6% 1	2220
	21st to end	Day	82% 2 7% 7% 2	2668
February	1st to 10th	Day	67% 5% 10% 11% 5% 1	1710
	11th to 20th	Day	61% 3 13% 16% 5%	1674
	21st to end	Day	62% 6% 12% 11% 7% 2	1413
March	1st to 10th	Day	63% 3 11% 16% 7%	3283
	11th to 20th	Day	54% 4 12% 24% 6%	3490
	21st to end	Day	61% 4 16% 15% 3	3825
April	1st to 10th	Day	61% 6% 13% 16% 4	2286
	11th to 20th	Day	56% 5% 12% 17% 8% 2	2438
	21st to end	Day	57% 3 11% 19% 9% 1	2470
May	1st to 10th	Day	45% 4 17% 26% 7% 1	4389
	11th to 20th	Day	45% 5% 21% 24% 4	4397
	21st to end	Day	58% 4 16% 15% 6% 1	5162
June	1st to 10th	Day	53% 6% 16% 18% 6% 2	2850
	11th to 20th	Day	51% 8% 19% 17% 4 1	2850
	21st to end	Day	46% 5% 21% 21% 7% 1	2850
July	1st to 10th	Day	23% 3 14% 29% 24% 7%	4666
	11th to 20th	Day	17% 3 18% 45% 17% 1	4659
	21st to end	Day	28% 6% 25% 27% 14% 1	4950
August	1st to 10th	Day	17% 3 15% 48% 17%	4201
	11th to 20th	Day	35% 8% 31% 17% 7% 1	4065
	21st to end	Day	37% 5% 27% 23% 7%	4520
September	1st to 10th	Day	30% 6% 24% 27% 12% 2	2301
	11th to 20th	Day	43% 4 18% 21% 13% 1	2176
	21st to end	Day	48% 5% 18% 20% 9% 1	2068
October	1st to 10th	Day	54% 5% 18% 17% 6%	3180
	11th to 20th	Day	57% 5% 14% 20% 4	3161
	21st to end	Day	57% 8% 15% 14% 5%	3125
November	1st to 10th	Day	67% 4 10% 11% 6% 1	1568
	11th to 20th	Day	64% 5% 9% 14% 5% 1	1600
	21st to end	Day	66% 4 13% 11% 5% 1	1600
December	1st to 10th	Day	64% 4 13% 13% 5%	2297
	11th to 20th	Day	74% 4 9% 9% 3	2648
	21st to end	Day	76% 3 9% 8% 2 1	3498
January	1st to 10th	Night	73% 2 8% 12% 5%	4816
	11th to 20th	Night	64% 3 13% 10% 8% 2	4740
	21st to end	Night	78% 2 7% 10% 2	4990
February	1st to 10th	Night	65% 4 11% 12% 7% 1	2470
	11th to 20th	Night	56% 4 14% 18% 8% 1	2404
	21st to end	Night	60% 5% 10% 16% 8% 2	2041
March	1st to 10th	Night	62% 3 10% 18% 7%	3574
	11th to 20th	Night	46% 3 14% 31% 6%	3490
	21st to end	Night	62% 3 15% 16% 5%	3828
April	1st to 10th	Night	54% 5% 15% 19% 7% 1	1891
	11th to 20th	Night	52% 4 12% 20% 10% 2	1691
	21st to end	Night	53% 3 12% 22% 9% 1	1710
May	1st to 10th	Night	41% 3 14% 31% 11%	2572
	11th to 20th	Night	41% 4 26% 21% 6% 2	2497
	21st to end	Night	58% 4 13% 19% 5% 1	2414
June	1st to 10th	Night	51% 5% 19% 18% 6% 2	1330
	11th to 20th	Night	52% 4 20% 17% 7%	1330
	21st to end	Night	43% 5% 22% 21% 7% 2	1330
July	1st to 10th	Night	24% 3 14% 27% 25% 7%	2185
	11th to 20th	Night	15% 3 18% 45% 18% 1	2178
	21st to end	Night	28% 5% 24% 30% 12% 1	2361
August	1st to 10th	Night	18% 3 11% 43% 24% 1	2707
	11th to 20th	Night	35% 8% 29% 17% 9% 2	2839
	21st to end	Night	39% 5% 24% 24% 7%	3146
September	1st to 10th	Night	29% 4 19% 32% 14% 2	1878
	11th to 20th	Night	44% 4 15% 19% 15% 2	2003
	21st to end	Night	45% 3 19% 20% 10% 2	2111
October	1st to 10th	Night	52% 7% 17% 17% 7%	3800
	11th to 20th	Night	56% 5% 16% 20% 4	3777
	21st to end	Night	54% 6% 17% 18% 5%	4432
November	1st to 10th	Night	66% 4 12% 12% 5% 1	2730
	11th to 20th	Night	63% 5% 14% 12% 6% 1	2800
	21st to end	Night	63% 5% 12% 14% 6% 1	2800
December	1st to 10th	Night	65% 5% 13% 13% 3	4880
	11th to 20th	Night	77% 3 9% 7% 3	5556
	21st to end	Night	70% 4 10% 12% 4	7458

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	40% 8% 20% 20% 6% 6%	192
	11th to 20th	Day	39% 6% 19% 18% 15% 2	180
	21st to end	Day	32% 7% 22% 20% 16% 4	295
February	1st to 10th	Day	31% 8% 25% 23% 11% 3	370
	11th to 20th	Day	25% 9% 26% 23% 14% 2	365
	21st to end	Day	29% 10% 25% 21% 10% 5%	307
March	1st to 10th	Day	24% 12% 22% 24% 14% 4	430
	11th to 20th	Day	28% 7% 24% 20% 16% 4	460
	21st to end	Day	27% 10% 23% 22% 14% 4	506
April	1st to 10th	Day	23% 13% 28% 21% 11% 5%	460
	11th to 20th	Day	21% 11% 25% 24% 14% 5%	452
	21st to end	Day	25% 11% 24% 22% 12% 6%	460
May	1st to 10th	Day	17% 8% 23% 28% 19% 6%	463
	11th to 20th	Day	17% 5% 20% 28% 22% 8%	492
	21st to end	Day	17% 5% 20% 31% 21% 6%	735
June	1st to 10th	Day	20% 8% 22% 22% 23% 5%	680
	11th to 20th	Day	16% 8% 25% 26% 20% 5%	680
	21st to end	Day	14% 6% 19% 30% 23% 7%	680
July	1st to 10th	Day	11% 3 19% 32% 28% 8%	671
	11th to 20th	Day	7% 4 20% 33% 26% 10%	668
	21st to end	Day	6% 5% 17% 29% 29% 15%	725
August	1st to 10th	Day	11% 1 12% 29% 28% 19%	487
	11th to 20th	Day	7% 4 14% 30% 30% 15%	477
	21st to end	Day	11% 5% 20% 33% 21% 10%	539
September	1st to 10th	Day	11% 5% 19% 31% 24% 10%	490
	11th to 20th	Day	20% 6% 19% 25% 21% 10%	490
	21st to end	Day	21% 9% 22% 22% 20% 6%	490
October	1st to 10th	Day	24% 8% 18% 22% 19% 8%	490
	11th to 20th	Day	28% 6% 23% 20% 17% 7%	490
	21st to end	Day	28% 9% 21% 25% 11% 5%	419
November	1st to 10th	Day	28% 10% 22% 24% 14% 3	200
	11th to 20th	Day	30% 8% 15% 19% 18% 11%	200
	21st to end	Day	36% 4 21% 18% 10% 12%	200
December	1st to 10th	Day	32% 4 17% 22% 14% 12%	200
	11th to 20th	Day	33% 5% 15% 27% 15% 6%	200
	21st to end	Day	38% 5% 19% 24% 12% 2	220
January	1st to 10th	Night	40% 7% 18% 21% 10% 4	880
	11th to 20th	Night	36% 6% 11% 28% 16% 3	850
	21st to end	Night	36% 3 15% 24% 17% 5%	916
February	1st to 10th	Night	29% 5% 18% 32% 14% 2	760
	11th to 20th	Night	27% 4 17% 32% 17% 2	744
	21st to end	Night	26% 6% 19% 29% 16% 4	628
March	1st to 10th	Night	24% 5% 22% 27% 17% 5%	760
	11th to 20th	Night	24% 7% 21% 27% 17% 3	760
	21st to end	Night	25% 6% 17% 27% 20% 6%	836
April	1st to 10th	Night	22% 7% 15% 32% 19% 5%	741
	11th to 20th	Night	19% 5% 20% 36% 18% 2	563
	21st to end	Night	22% 4 18% 33% 19% 4	570
May	1st to 10th	Night	19% 3 12% 37% 24% 5%	399
	11th to 20th	Night	15% 3 13% 34% 25% 9%	374
	21st to end	Night	19% 3 14% 30% 28% 5%	411
June	1st to 10th	Night	17% 3 13% 38% 25% 4	380
	11th to 20th	Night	19% 3 15% 33% 25% 6%	380
	21st to end	Night	18% 4 11% 34% 27% 7%	380
July	1st to 10th	Night	9% 2 9% 31% 41% 10%	370
	11th to 20th	Night	9% 2 10% 32% 38% 9%	368
	21st to end	Night	7% 2 4 30% 43% 13%	416
August	1st to 10th	Night	12% 2 4 23% 44% 16%	486
	11th to 20th	Night	7% 1 5% 27% 50% 10%	564
	21st to end	Night	11% 3 10% 30% 39% 7%	627
September	1st to 10th	Night	11% 2 6% 35% 36% 10%	738
	11th to 20th	Night	17% 3 11% 21% 31% 16%	760
	21st to end	Night	23% 5% 13% 28% 24% 7%	760
October	1st to 10th	Night	21% 3 12% 30% 28% 6%	760
	11th to 20th	Night	27% 3 13% 28% 21% 8%	760
	21st to end	Night	28% 5% 18% 29% 16% 3	836
November	1st to 10th	Night	36% 5% 17% 29% 13% 1	780
	11th to 20th	Night	30% 4 18% 30% 15% 3	800
	21st to end	Night	31% 4 18% 26% 16% 5%	800
December	1st to 10th	Night	30% 6% 14% 26% 19% 4	895
	11th to 20th	Night	39% 4 16% 26% 12% 3	900
	21st to end	Night	38% 5% 21% 25% 11% 1	990

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