808.5 Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 808.5 Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 6% 2 6% 4 4 4 4 6% 8% 43%	120449
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	118688
Transparency	Day	38% 7% 11% 20% 19% 6%	124137
	Night	37% 6% 10% 18% 21% 7%	123137
Seeing	Day	16% 10% 24% 24% 19% 7%	16270
	Night	16% 8% 24% 31% 18% 2	26351

By Month

Percentage Cloud Cover SE (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	14% 5% 2 5% 2 3 3 2 3 5% 56%	4056
	11th to 20th	Day	11% 6% 2 5% 3 3 3 2 3 4 58%	4060
	21st to end	Day	15% 9% 3 7% 4 3 4 3 4 4 44%	4466
February	1st to 10th	Day	15% 7% 2 7% 4 3 5% 3 6% 6% 43%	1800
	11th to 20th	Day	21% 6% 2 4 3 3 4 3 4 5% 45%	1901
	21st to end	Day	18% 7% 3 6% 5% 4 4 3 4 5% 41%	1723
March	1st to 10th	Day	20% 7% 2 7% 3 3 3 3 5% 6% 40%	4935
	11th to 20th	Day	9% 5% 2 6% 3 3 3 3 5% 6% 54%	4980
	21st to end	Day	15% 7% 3 6% 4 3 3 3 4 5% 47%	5465
April	1st to 10th	Day	11% 5% 1 4 3 4 4 4 6% 8% 50%	2210
	11th to 20th	Day	12% 5% 2 6% 3 4 4 3 5% 7% 48%	2374
	21st to end	Day	12% 5% 2 5% 4 4 5% 4 5% 7% 49%	2515
May	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 7% 11% 35%	5862
	11th to 20th	Day	18% 7% 3 8% 5% 5% 5% 4 6% 9% 30%	5861
	21st to end	Day	11% 5% 2 6% 4 4 4 4 6% 8% 44%	6433
June	1st to 10th	Day	9% 5% 2 6% 5% 4 5% 5% 8% 11% 38%	2600
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	2600
	21st to end	Day	6% 5% 1 6% 5% 6% 6% 6% 8% 11% 40%	2600
July	1st to 10th	Day	8% 6% 2 8% 6% 6% 7% 5% 8% 11% 33%	4250
	11th to 20th	Day	8% 6% 2 8% 6% 6% 6% 5% 9% 12% 32%	4250
	21st to end	Day	11% 8% 3 8% 7% 7% 7% 6% 8% 10% 25%	4466
August	1st to 10th	Day	20% 8% 3 7% 5% 5% 6% 5% 8% 10% 24%	4104
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 4 8% 11% 35%	4044
	21st to end	Day	6% 4 2 4 3 5% 5% 4 7% 9% 50%	4212
September	1st to 10th	Day	8% 5% 2 7% 4 5% 5% 5% 7% 9% 43%	2145
	11th to 20th	Day	13% 6% 2 6% 4 5% 4 4 6% 8% 41%	2090
	21st to end	Day	19% 6% 2 6% 4 5% 4 4 5% 6% 40%	2090
October	1st to 10th	Day	14% 6% 2 6% 3 2 2 3 4 6% 53%	1957
	11th to 20th	Day	12% 4 2 5% 3 3 3 3 4 5% 57%	1900
	21st to end	Day	13% 4 2 4 4 3 3 2 4 5% 57%	2090
November	1st to 10th	Day	14% 4 2 5% 3 3 2 3 4 6% 54%	1957
	11th to 20th	Day	13% 6% 2 4 3 3 2 3 6% 6% 53%	1884
	21st to end	Day	14% 5% 2 5% 3 4 3 3 5% 6% 50%	1800
December	1st to 10th	Day	12% 6% 3 6% 3 3 3 2 4 5% 52%	2930
	11th to 20th	Day	16% 7% 3 7% 4 4 4 3 5% 6% 42%	3373
	21st to end	Day	20% 6% 2 5% 3 3 3 3 4 6% 46%	4466
January	1st to 10th	Night	13% 5% 2 4 3 3 3 2 3 4 58%	5889
	11th to 20th	Night	11% 6% 2 5% 4 3 3 3 4 5% 54%	5900
	21st to end	Night	16% 9% 2 6% 3 3 3 3 3 6% 46%	6490
February	1st to 10th	Night	15% 6% 3 7% 4 3 4 3 5% 5% 46%	2600
	11th to 20th	Night	21% 7% 2 5% 4 3 3 3 5% 5% 42%	2382
	21st to end	Night	15% 7% 2 5% 3 3 3 3 6% 6% 47%	1905
March	1st to 10th	Night	19% 8% 2 6% 3 3 3 3 4 5% 43%	4918
	11th to 20th	Night	10% 4 2 4 3 3 4 4 5% 7% 54%	4980
	21st to end	Night	15% 5% 2 5% 4 4 4 3 5% 6% 48%	5467
April	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 5% 7% 49%	2189
	11th to 20th	Night	11% 5% 2 4 4 4 4 3 5% 6% 52%	1975
	21st to end	Night	11% 4 1 6% 4 4 4 3 6% 7% 49%	1900
May	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 6% 9% 41%	4033
	11th to 20th	Night	19% 7% 3 7% 5% 4 5% 4 6% 9% 32%	4032
	21st to end	Night	13% 6% 2 5% 4 5% 5% 5% 7% 9% 40%	4421
June	1st to 10th	Night	10% 6% 2 8% 6% 5% 6% 5% 7% 9% 36%	1800
	11th to 20th	Night	10% 5% 2 7% 6% 5% 5% 6% 8% 11% 35%	1800
	21st to end	Night	7% 5% 2 7% 5% 6% 6% 5% 9% 13% 36%	1800
July	1st to 10th	Night	9% 7% 3 10% 6% 6% 6% 5% 8% 10% 30%	2930
	11th to 20th	Night	9% 7% 2 7% 5% 5% 6% 6% 10% 12% 30%	2930
	21st to end	Night	14% 10% 3 10% 7% 6% 6% 6% 7% 9% 23%	3087
August	1st to 10th	Night	23% 8% 3 7% 5% 4 5% 5% 8% 9% 23%	2825
	11th to 20th	Night	9% 7% 2 8% 5% 6% 6% 5% 8% 11% 32%	2785
	21st to end	Night	5% 4 1 6% 4 5% 5% 4 7% 9% 50%	3412
September	1st to 10th	Night	9% 5% 2 6% 5% 4 4 4 7% 8% 45%	2030
	11th to 20th	Night	14% 7% 3 6% 4 4 4 4 5% 7% 41%	2090
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 6% 5% 39%	2090
October	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 4 6% 52%	2223
	11th to 20th	Night	14% 5% 2 4 3 3 2 2 5% 5% 55%	2280
	21st to end	Night	11% 6% 2 3 3 3 3 2 4 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 5% 3 3 3 2 4 6% 52%	2341
	11th to 20th	Night	13% 6% 2 5% 4 3 4 3 4 6% 51%	2514
	21st to end	Night	13% 5% 2 4 4 4 4 3 5% 6% 51%	2600
December	1st to 10th	Night	13% 7% 2 4 3 3 3 3 4 6% 52%	4250
	11th to 20th	Night	18% 7% 3 6% 4 3 3 3 4 6% 43%	4822
	21st to end	Night	19% 5% 2 5% 3 3 4 3 4 5% 47%	6490

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	33% 3 5% 20% 32% 7%	4057
	11th to 20th	Day	31% 4 4 15% 36% 10%	4060
	21st to end	Day	42% 4 5% 17% 23% 9%	4466
February	1st to 10th	Day	39% 5% 10% 22% 19% 5%	1800
	11th to 20th	Day	42% 4 7% 18% 23% 6%	1911
	21st to end	Day	45% 4 7% 16% 20% 7%	1723
March	1st to 10th	Day	45% 4 7% 14% 19% 11%	4935
	11th to 20th	Day	31% 4 7% 14% 26% 18%	4980
	21st to end	Day	41% 4 6% 20% 22% 6%	5465
April	1st to 10th	Day	32% 6% 11% 21% 26% 6%	2210
	11th to 20th	Day	35% 5% 9% 20% 22% 9%	2374
	21st to end	Day	34% 6% 12% 22% 21% 4	2515
May	1st to 10th	Day	40% 9% 17% 22% 11% 1	5897
	11th to 20th	Day	49% 8% 13% 18% 11% 1	5865
	21st to end	Day	36% 8% 12% 20% 19% 3	6447
June	1st to 10th	Day	37% 11% 14% 25% 13% 1	2600
	11th to 20th	Day	34% 10% 18% 21% 15% 1	2600
	21st to end	Day	34% 12% 15% 22% 15% 2	2600
July	1st to 10th	Day	41% 12% 18% 19% 10% 1	4185
	11th to 20th	Day	39% 12% 27% 17% 4 1	4181
	21st to end	Day	49% 13% 15% 17% 5%	4324
August	1st to 10th	Day	53% 9% 18% 15% 5%	4091
	11th to 20th	Day	40% 10% 17% 20% 11% 1	4083
	21st to end	Day	27% 9% 15% 32% 16% 2	4212
September	1st to 10th	Day	35% 9% 14% 19% 18% 4	2145
	11th to 20th	Day	40% 8% 12% 17% 20% 4	2090
	21st to end	Day	44% 6% 10% 19% 17% 5%	2090
October	1st to 10th	Day	30% 4 6% 22% 28% 9%	3256
	11th to 20th	Day	20% 3 5% 19% 27% 26%	3140
	21st to end	Day	26% 3 5% 25% 25% 17%	3428
November	1st to 10th	Day	32% 4 7% 20% 28% 9%	1957
	11th to 20th	Day	31% 4 9% 21% 27% 8%	1884
	21st to end	Day	36% 4 9% 21% 23% 8%	1800
December	1st to 10th	Day	35% 4 6% 18% 28% 9%	2930
	11th to 20th	Day	43% 5% 7% 18% 22% 5%	3370
	21st to end	Day	40% 4 9% 25% 20% 2	4466
January	1st to 10th	Night	31% 3 6% 16% 37% 8%	5889
	11th to 20th	Night	32% 4 5% 15% 35% 8%	5900
	21st to end	Night	41% 3 4 16% 27% 9%	6490
February	1st to 10th	Night	40% 4 9% 19% 20% 6%	2600
	11th to 20th	Night	44% 4 7% 16% 22% 7%	2383
	21st to end	Night	37% 5% 9% 18% 22% 8%	1905
March	1st to 10th	Night	42% 5% 7% 12% 26% 8%	4918
	11th to 20th	Night	30% 4 7% 13% 28% 18%	4980
	21st to end	Night	38% 4 7% 22% 22% 8%	5467
April	1st to 10th	Night	34% 5% 10% 20% 23% 7%	2189
	11th to 20th	Night	31% 6% 11% 21% 22% 9%	1975
	21st to end	Night	33% 8% 11% 24% 20% 4	1900
May	1st to 10th	Night	39% 7% 17% 23% 14% 1	4060
	11th to 20th	Night	46% 8% 15% 19% 11% 1	4033
	21st to end	Night	35% 9% 14% 25% 14% 2	4429
June	1st to 10th	Night	41% 11% 15% 20% 12% 2	1800
	11th to 20th	Night	39% 11% 18% 20% 12% 1	1800
	21st to end	Night	35% 13% 19% 19% 12% 3	1800
July	1st to 10th	Night	46% 11% 18% 15% 8% 2	2885
	11th to 20th	Night	41% 13% 26% 15% 4 1	2876
	21st to end	Night	54% 11% 15% 14% 5%	2988
August	1st to 10th	Night	54% 9% 18% 15% 3	2814
	11th to 20th	Night	42% 10% 23% 17% 8% 1	2812
	21st to end	Night	28% 9% 12% 28% 19% 4	3412
September	1st to 10th	Night	35% 10% 14% 20% 16% 6%	2030
	11th to 20th	Night	40% 9% 14% 16% 19% 2	2090
	21st to end	Night	43% 8% 11% 16% 17% 5%	2090
October	1st to 10th	Night	35% 5% 8% 19% 26% 6%	3704
	11th to 20th	Night	22% 3 5% 18% 28% 24%	3779
	21st to end	Night	27% 3 6% 21% 25% 17%	4120
November	1st to 10th	Night	34% 4 7% 17% 25% 12%	2341
	11th to 20th	Night	34% 4 7% 17% 26% 12%	2514
	21st to end	Night	34% 5% 6% 21% 27% 8%	2600
December	1st to 10th	Night	36% 3 6% 18% 28% 9%	4250
	11th to 20th	Night	44% 4 10% 15% 18% 9%	4824
	21st to end	Night	39% 4 10% 22% 22% 2	6490

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	22% 22% 38% 16% 2	300
	11th to 20th	Day	23% 23% 32% 16% 5%	300
	21st to end	Day	21% 24% 37% 16% 3	330
February	1st to 10th	Day	20% 18% 42% 14% 6%	300
	11th to 20th	Day	37% 15% 37% 9% 2	300
	21st to end	Day	21% 25% 32% 17% 4	322
March	1st to 10th	Day	22% 17% 32% 20% 8% 1	500
	11th to 20th	Day	20% 17% 39% 19% 5% 1	500
	21st to end	Day	20% 16% 36% 21% 6% 1	550
April	1st to 10th	Day	19% 18% 32% 21% 8% 1	500
	11th to 20th	Day	14% 17% 30% 23% 13% 3	492
	21st to end	Day	14% 10% 30% 27% 16% 3	500
May	1st to 10th	Day	11% 13% 34% 23% 16% 3	503
	11th to 20th	Day	16% 6% 22% 29% 20% 7%	527
	21st to end	Day	13% 7% 20% 25% 23% 11%	573
June	1st to 10th	Day	10% 4 21% 31% 26% 8%	530
	11th to 20th	Day	11% 5% 18% 26% 28% 12%	530
	21st to end	Day	7% 2 14% 33% 32% 11%	530
July	1st to 10th	Day	8% 2 11% 22% 38% 19%	525
	11th to 20th	Day	9% 1 7% 27% 35% 19%	524
	21st to end	Day	7% 2 10% 31% 36% 14%	532
August	1st to 10th	Day	16% 4 14% 24% 26% 17%	486
	11th to 20th	Day	11% 4 8% 30% 28% 18%	479
	21st to end	Day	12% 2 10% 27% 29% 20%	539
September	1st to 10th	Day	10% 2 11% 35% 25% 17%	490
	11th to 20th	Day	16% 5% 14% 26% 27% 13%	490
	21st to end	Day	21% 4 19% 25% 24% 5%	490
October	1st to 10th	Day	17% 5% 20% 25% 25% 8%	490
	11th to 20th	Day	12% 8% 24% 27% 22% 7%	490
	21st to end	Day	15% 5% 26% 28% 19% 6%	539
November	1st to 10th	Day	18% 6% 27% 31% 15% 3	495
	11th to 20th	Day	17% 15% 32% 24% 11% 1	384
	21st to end	Day	13% 12% 37% 24% 12% 2	300
December	1st to 10th	Day	22% 15% 39% 18% 6%	300
	11th to 20th	Day	21% 11% 34% 24% 9% 1	300
	21st to end	Day	24% 17% 34% 18% 6% 3	330
January	1st to 10th	Night	20% 14% 34% 26% 6%	800
	11th to 20th	Night	19% 14% 34% 27% 6%	800
	21st to end	Night	20% 11% 38% 28% 3	880
February	1st to 10th	Night	18% 11% 38% 29% 4	800
	11th to 20th	Night	26% 12% 39% 22% 2	783
	21st to end	Night	19% 17% 35% 25% 4	660
March	1st to 10th	Night	18% 12% 36% 30% 5%	800
	11th to 20th	Night	18% 14% 37% 23% 8% 1	800
	21st to end	Night	18% 13% 35% 28% 6%	880
April	1st to 10th	Night	15% 14% 35% 28% 8% 1	800
	11th to 20th	Night	14% 14% 30% 29% 14%	791
	21st to end	Night	13% 6% 29% 39% 13%	700
May	1st to 10th	Night	12% 10% 29% 37% 12%	600
	11th to 20th	Night	16% 6% 22% 36% 19% 1	591
	21st to end	Night	14% 3 14% 38% 29% 2	650
June	1st to 10th	Night	11% 4 17% 40% 25% 4	600
	11th to 20th	Night	12% 3 14% 36% 29% 6%	600
	21st to end	Night	8% 2 15% 36% 34% 5%	600
July	1st to 10th	Night	8% 2 8% 36% 38% 7%	585
	11th to 20th	Night	8% 2 7% 35% 40% 8%	582
	21st to end	Night	8% 2 10% 34% 41% 5%	624
August	1st to 10th	Night	13% 2 9% 29% 41% 6%	567
	11th to 20th	Night	12% 3 10% 31% 37% 6%	564
	21st to end	Night	10% 2 10% 29% 41% 8%	798
September	1st to 10th	Night	11% 2 10% 31% 40% 6%	760
	11th to 20th	Night	17% 3 12% 28% 35% 5%	760
	21st to end	Night	20% 4 15% 34% 24% 3	760
October	1st to 10th	Night	16% 4 19% 27% 29% 6%	760
	11th to 20th	Night	16% 6% 23% 31% 22% 2	760
	21st to end	Night	14% 7% 23% 31% 22% 3	836
November	1st to 10th	Night	16% 7% 24% 33% 18% 3	780
	11th to 20th	Night	15% 11% 30% 32% 12% 1	800
	21st to end	Night	15% 10% 28% 33% 13% 1	800
December	1st to 10th	Night	19% 13% 29% 33% 6%	800
	11th to 20th	Night	20% 7% 27% 35% 10%	800
	21st to end	Night	24% 13% 24% 30% 9%	880

Page generated at 2025-07-18 01:10:00. Version ac07

Copyright © 2025 Attilla Danko.