Evans Above Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Evans Above Observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	20% 9% 3 7% 5% 4 4 3 5% 6% 34%	118111
	Night	22% 9% 3 7% 5% 4 4 3 4 5% 33%	119603
Transparency	Day	51% 5% 12% 17% 12% 4	117854
	Night	52% 6% 12% 18% 10% 3	119449
Seeing	Day	23% 6% 20% 27% 18% 6%	15264
	Night	26% 4 18% 35% 16% 1	24246

By Month

Percentage Cloud Cover SW (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	42% 14% 3 9% 5% 4 4 3 3 4 9%	3639
	11th to 20th	Day	28% 11% 3 10% 6% 6% 4 3 4 4 20%	3640
	21st to end	Day	28% 10% 4 7% 4 4 4 3 4 5% 28%	4349
February	1st to 10th	Day	32% 13% 4 9% 6% 5% 3 4 5% 5% 16%	1800
	11th to 20th	Day	32% 13% 5% 9% 6% 5% 3 3 3 4 17%	1755
	21st to end	Day	33% 15% 4 7% 5% 4 3 3 4 5% 19%	1564
March	1st to 10th	Day	19% 13% 4 10% 6% 6% 7% 5% 6% 6% 19%	4726
	11th to 20th	Day	25% 14% 4 9% 5% 4 4 3 4 4 25%	5000
	21st to end	Day	25% 13% 4 11% 7% 6% 5% 4 6% 7% 12%	5612
April	1st to 10th	Day	27% 13% 4 10% 6% 5% 4 3 5% 6% 17%	2400
	11th to 20th	Day	23% 12% 3 8% 5% 5% 4 3 5% 5% 27%	2369
	21st to end	Day	21% 10% 3 10% 6% 5% 5% 5% 5% 6% 25%	2595
May	1st to 10th	Day	19% 8% 3 7% 5% 4 4 3 4 5% 35%	4515
	11th to 20th	Day	11% 6% 2 7% 4 5% 5% 4 6% 8% 41%	4380
	21st to end	Day	23% 8% 3 9% 6% 6% 6% 4 6% 6% 23%	4813
June	1st to 10th	Day	22% 8% 3 7% 5% 5% 4 3 6% 6% 30%	2660
	11th to 20th	Day	19% 9% 3 7% 5% 6% 6% 4 6% 7% 27%	2660
	21st to end	Day	8% 5% 2 6% 4 4 4 3 4 7% 50%	2753
July	1st to 10th	Day	5% 3 1 3 2 3 3 2 4 5% 69%	4420
	11th to 20th	Day	4 3 1 3 2 2 3 2 3 4 74%	4420
	21st to end	Day	6% 3 1 3 3 2 3 2 4 6% 67%	4790
August	1st to 10th	Day	3 4 2 4 3 2 3 2 5% 6% 67%	4157
	11th to 20th	Day	6% 3 2 6% 4 5% 4 4 7% 8% 50%	4026
	21st to end	Day	10% 6% 2 5% 4 4 4 3 4 6% 52%	4306
September	1st to 10th	Day	7% 4 2 5% 3 3 4 3 4 6% 59%	2280
	11th to 20th	Day	13% 7% 3 7% 5% 4 5% 3 5% 7% 42%	2280
	21st to end	Day	16% 6% 2 7% 5% 4 5% 3 5% 6% 44%	2000
October	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 5% 6% 44%	3180
	11th to 20th	Day	20% 8% 3 9% 5% 5% 5% 4 6% 7% 27%	3161
	21st to end	Day	28% 9% 3 8% 5% 5% 5% 3 5% 6% 23%	3358
November	1st to 10th	Day	32% 12% 5% 10% 6% 4 4 3 4 5% 15%	1651
	11th to 20th	Day	34% 12% 3 8% 4 5% 4 3 4 4 19%	1600
	21st to end	Day	30% 13% 4 8% 5% 4 3 3 4 4 21%	1600
December	1st to 10th	Day	24% 9% 2 7% 5% 6% 4 4 5% 6% 27%	2620
	11th to 20th	Day	33% 14% 5% 9% 6% 5% 5% 3 5% 4 11%	3028
	21st to end	Day	34% 14% 5% 12% 6% 5% 5% 3 4 3 9%	4004
January	1st to 10th	Night	38% 13% 4 9% 6% 5% 4 3 4 4 11%	6306
	11th to 20th	Night	28% 12% 3 7% 6% 6% 6% 3 4 4 21%	6320
	21st to end	Night	28% 10% 3 7% 4 3 3 3 5% 6% 29%	6618
February	1st to 10th	Night	29% 12% 3 8% 7% 5% 4 3 4 5% 18%	2600
	11th to 20th	Night	31% 12% 3 7% 5% 5% 3 3 4 4 23%	2532
	21st to end	Night	29% 13% 4 8% 5% 4 4 4 4 6% 19%	2065
March	1st to 10th	Night	21% 13% 4 8% 6% 6% 7% 5% 6% 6% 20%	5166
	11th to 20th	Night	24% 12% 3 9% 6% 4 4 4 4 4 27%	5000
	21st to end	Night	29% 13% 4 10% 6% 6% 5% 3 5% 5% 14%	5363
April	1st to 10th	Night	26% 13% 4 9% 7% 4 4 4 4 5% 21%	2000
	11th to 20th	Night	20% 10% 3 7% 5% 5% 4 3 5% 6% 32%	1979
	21st to end	Night	25% 9% 3 7% 5% 4 3 3 4 7% 31%	1801
May	1st to 10th	Night	21% 9% 3 6% 4 3 3 3 4 4 40%	2664
	11th to 20th	Night	10% 4 2 5% 3 3 3 3 5% 7% 56%	2519
	21st to end	Night	14% 8% 3 8% 5% 5% 5% 4 6% 8% 34%	2762
June	1st to 10th	Night	19% 6% 3 8% 5% 5% 4 3 3 6% 38%	1520
	11th to 20th	Night	16% 9% 2 8% 5% 4 3 3 4 6% 40%	1520
	21st to end	Night	6% 5% 2 6% 3 3 3 2 3 6% 60%	1379
July	1st to 10th	Night	3 2 1 3 2 2 2 1 3 3 80%	2540
	11th to 20th	Night	3 2 1 4 2 2 2 1 2 3 77%	2540
	21st to end	Night	4 2 1 2 1 1 2 2 2 4 79%	2761
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 76%	2777
	11th to 20th	Night	7% 4 1 5% 4 3 4 3 4 7% 59%	2812
	21st to end	Night	11% 4 2 4 3 3 3 3 3 5% 60%	3367
September	1st to 10th	Night	7% 3 1 4 2 3 3 2 4 5% 66%	1900
	11th to 20th	Night	12% 5% 2 7% 5% 4 4 3 5% 5% 48%	1900
	21st to end	Night	12% 4 2 6% 3 4 4 4 4 7% 51%	2176
October	1st to 10th	Night	13% 4 2 5% 4 4 4 3 5% 7% 48%	3800
	11th to 20th	Night	19% 6% 2 6% 4 4 4 4 6% 9% 35%	3777
	21st to end	Night	25% 10% 3 8% 5% 5% 5% 3 5% 6% 26%	4207
November	1st to 10th	Night	31% 10% 3 7% 5% 5% 5% 3 4 6% 20%	2644
	11th to 20th	Night	36% 10% 3 7% 5% 3 4 3 3 4 21%	2800
	21st to end	Night	29% 8% 3 8% 6% 5% 5% 4 3 4 26%	2800
December	1st to 10th	Night	21% 8% 3 6% 6% 8% 6% 5% 6% 6% 24%	4560
	11th to 20th	Night	37% 12% 5% 10% 6% 4 4 3 3 4 13%	5176
	21st to end	Night	34% 13% 4 10% 6% 5% 5% 3 4 4 11%	6952

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	80% 5% 6% 6% 4	3639
	11th to 20th	Day	67% 4 7% 15% 6% 1	3640
	21st to end	Day	59% 5% 6% 13% 14% 2	4349
February	1st to 10th	Day	70% 6% 9% 12% 4	1800
	11th to 20th	Day	72% 5% 7% 10% 5% 1	1764
	21st to end	Day	69% 5% 9% 10% 6% 1	1564
March	1st to 10th	Day	63% 6% 15% 13% 4	4726
	11th to 20th	Day	63% 5% 6% 20% 6%	5000
	21st to end	Day	70% 7% 13% 8% 2	5612
April	1st to 10th	Day	68% 6% 11% 13% 2	2400
	11th to 20th	Day	59% 5% 10% 15% 8% 2	2372
	21st to end	Day	59% 6% 12% 14% 7% 1	2595
May	1st to 10th	Day	48% 5% 13% 20% 12%	4515
	11th to 20th	Day	36% 7% 23% 25% 8% 1	4383
	21st to end	Day	59% 10% 17% 13% 2	4814
June	1st to 10th	Day	53% 5% 17% 17% 7%	2660
	11th to 20th	Day	54% 7% 18% 15% 6%	2660
	21st to end	Day	34% 5% 19% 27% 10% 4	2753
July	1st to 10th	Day	18% 4 9% 18% 22% 28%	4351
	11th to 20th	Day	16% 2 7% 21% 40% 14%	4345
	21st to end	Day	19% 4 12% 28% 33% 5%	4639
August	1st to 10th	Day	19% 3 9% 30% 28% 11%	4141
	11th to 20th	Day	31% 5% 20% 28% 12% 4	4065
	21st to end	Day	33% 4 12% 25% 18% 8%	4305
September	1st to 10th	Day	26% 4 14% 26% 19% 11%	2280
	11th to 20th	Day	41% 6% 14% 18% 14% 7%	2280
	21st to end	Day	42% 5% 14% 20% 15% 5%	2000
October	1st to 10th	Day	40% 3 8% 28% 19% 2	3180
	11th to 20th	Day	54% 5% 13% 16% 10% 1	3161
	21st to end	Day	61% 4 8% 18% 9% 1	3358
November	1st to 10th	Day	72% 5% 8% 12% 4	1651
	11th to 20th	Day	68% 5% 11% 9% 5% 2	1600
	21st to end	Day	67% 5% 7% 10% 11% 1	1600
December	1st to 10th	Day	54% 6% 9% 17% 12% 1	2620
	11th to 20th	Day	76% 4 9% 10% 1	3028
	21st to end	Day	80% 4 8% 6% 2	4004
January	1st to 10th	Night	77% 4 8% 6% 4 1	6306
	11th to 20th	Night	63% 7% 7% 16% 7% 1	6320
	21st to end	Night	57% 6% 7% 14% 14% 2	6618
February	1st to 10th	Night	68% 6% 9% 12% 4	2600
	11th to 20th	Night	65% 4 11% 11% 7% 2	2534
	21st to end	Night	65% 5% 11% 11% 6% 1	2065
March	1st to 10th	Night	61% 7% 13% 14% 5%	5166
	11th to 20th	Night	58% 7% 10% 19% 6% 1	5000
	21st to end	Night	71% 6% 12% 10% 1	5363
April	1st to 10th	Night	64% 5% 14% 13% 4	2000
	11th to 20th	Night	53% 6% 14% 16% 9% 3	1979
	21st to end	Night	55% 6% 15% 17% 5% 2	1801
May	1st to 10th	Night	45% 5% 14% 25% 10% 1	2664
	11th to 20th	Night	27% 5% 23% 33% 9% 2	2519
	21st to end	Night	45% 11% 28% 12% 3	2762
June	1st to 10th	Night	49% 4 16% 25% 6% 1	1520
	11th to 20th	Night	44% 4 23% 23% 6%	1520
	21st to end	Night	27% 5% 20% 33% 11% 4	1379
July	1st to 10th	Night	13% 2 9% 25% 24% 28%	2500
	11th to 20th	Night	15% 2 8% 22% 39% 15%	2492
	21st to end	Night	12% 2 12% 33% 35% 6%	2674
August	1st to 10th	Night	12% 3 11% 36% 32% 7%	2769
	11th to 20th	Night	25% 4 28% 28% 10% 5%	2839
	21st to end	Night	29% 3 15% 27% 21% 5%	3367
September	1st to 10th	Night	21% 4 15% 30% 20% 10%	1900
	11th to 20th	Night	38% 5% 14% 23% 12% 9%	1900
	21st to end	Night	33% 5% 17% 21% 17% 6%	2176
October	1st to 10th	Night	34% 5% 10% 32% 18% 2	3800
	11th to 20th	Night	46% 4 18% 19% 11% 1	3777
	21st to end	Night	57% 5% 9% 18% 9% 1	4207
November	1st to 10th	Night	65% 6% 8% 15% 6%	2644
	11th to 20th	Night	67% 5% 10% 10% 6% 1	2800
	21st to end	Night	62% 5% 9% 11% 12% 1	2800
December	1st to 10th	Night	51% 14% 9% 16% 9%	4560
	11th to 20th	Night	76% 5% 8% 8% 3	5176
	21st to end	Night	76% 6% 8% 8% 2	6952

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	49% 4 15% 16% 14% 4	200
	11th to 20th	Day	41% 5% 14% 24% 10% 8%	200
	21st to end	Day	35% 6% 15% 22% 15% 8%	370
February	1st to 10th	Day	42% 6% 17% 19% 13% 4	400
	11th to 20th	Day	40% 5% 23% 21% 10% 2	395
	21st to end	Day	41% 8% 18% 20% 10% 2	371
March	1st to 10th	Day	32% 8% 23% 23% 12% 2	500
	11th to 20th	Day	30% 7% 24% 21% 13% 5%	500
	21st to end	Day	34% 8% 21% 22% 13% 3	550
April	1st to 10th	Day	30% 8% 20% 25% 12% 5%	500
	11th to 20th	Day	26% 10% 27% 20% 12% 4	492
	21st to end	Day	24% 6% 26% 23% 16% 4	500
May	1st to 10th	Day	17% 10% 24% 27% 17% 4	503
	11th to 20th	Day	25% 8% 20% 22% 17% 7%	489
	21st to end	Day	20% 7% 20% 25% 21% 7%	529
June	1st to 10th	Day	22% 5% 21% 28% 19% 4	490
	11th to 20th	Day	17% 7% 26% 33% 13% 3	490
	21st to end	Day	7% 5% 26% 35% 21% 5%	583
July	1st to 10th	Day	9% 4 20% 34% 24% 8%	486
	11th to 20th	Day	6% 5% 21% 37% 24% 7%	484
	21st to end	Day	5% 3 16% 39% 30% 7%	531
August	1st to 10th	Day	7% 2 16% 37% 28% 10%	487
	11th to 20th	Day	4 2 15% 33% 32% 14%	477
	21st to end	Day	9% 4 20% 33% 23% 10%	539
September	1st to 10th	Day	8% 4 20% 37% 23% 9%	490
	11th to 20th	Day	16% 3 17% 30% 24% 10%	490
	21st to end	Day	17% 4 21% 32% 22% 5%	490
October	1st to 10th	Day	21% 6% 16% 29% 21% 7%	490
	11th to 20th	Day	25% 4 18% 27% 19% 7%	490
	21st to end	Day	32% 6% 21% 22% 13% 6%	485
November	1st to 10th	Day	32% 5% 23% 23% 14% 5%	243
	11th to 20th	Day	37% 6% 22% 17% 13% 7%	200
	21st to end	Day	36% 4 19% 20% 16% 7%	200
December	1st to 10th	Day	37% 2 19% 16% 19% 8%	200
	11th to 20th	Day	44% 7% 20% 19% 8% 3	200
	21st to end	Day	38% 8% 15% 24% 12% 3	220
January	1st to 10th	Night	42% 6% 14% 24% 13% 2	800
	11th to 20th	Night	38% 4 12% 29% 15% 1	800
	21st to end	Night	33% 3 13% 31% 17% 2	880
February	1st to 10th	Night	34% 5% 17% 30% 14% 1	800
	11th to 20th	Night	37% 4 17% 27% 14% 1	784
	21st to end	Night	37% 4 16% 32% 11%	660
March	1st to 10th	Night	32% 6% 15% 32% 14% 1	800
	11th to 20th	Night	30% 4 18% 35% 13% 1	800
	21st to end	Night	33% 6% 17% 29% 14%	880
April	1st to 10th	Night	30% 6% 17% 29% 17% 1	800
	11th to 20th	Night	25% 4 24% 33% 12% 2	791
	21st to end	Night	29% 2 21% 33% 14% 1	601
May	1st to 10th	Night	17% 5% 28% 36% 13% 1	431
	11th to 20th	Night	17% 3 21% 45% 13% 1	375
	21st to end	Night	18% 4 22% 38% 18%	411
June	1st to 10th	Night	22% 2 26% 38% 12%	380
	11th to 20th	Night	17% 4 28% 43% 8% 1	380
	21st to end	Night	7% 2 33% 48% 9%	380
July	1st to 10th	Night	6% 2 27% 53% 12% 1	370
	11th to 20th	Night	6% 3 35% 43% 13%	368
	21st to end	Night	2 1 31% 53% 12% 1	416
August	1st to 10th	Night	4 2 20% 47% 25% 2	522
	11th to 20th	Night	5% 1 12% 51% 29% 3	564
	21st to end	Night	8% 3 23% 43% 22% 2	757
September	1st to 10th	Night	8% 3 21% 46% 20% 2	760
	11th to 20th	Night	14% 3 14% 36% 29% 4	760
	21st to end	Night	13% 2 19% 43% 21% 2	760
October	1st to 10th	Night	21% 3 17% 35% 22% 3	760
	11th to 20th	Night	23% 3 15% 33% 24% 3	760
	21st to end	Night	30% 2 18% 33% 15% 2	836
November	1st to 10th	Night	35% 3 15% 34% 13% 1	780
	11th to 20th	Night	40% 5% 15% 26% 14% 1	800
	21st to end	Night	31% 3 15% 33% 17% 1	800
December	1st to 10th	Night	33% 4 14% 31% 17% 2	800
	11th to 20th	Night	43% 4 16% 25% 11% 1	800
	21st to end	Night	43% 5% 18% 24% 9% 1	880

Page generated at 2025-05-08 06:33:59. Version ac07

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