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 2024-11-04.

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 5% 4 6% 8% 42%	101185
	Night	14% 6% 2 6% 4 4 4 4 6% 7% 43%	97789
Transparency	Day	38% 7% 12% 20% 18% 5%	104875
	Night	38% 7% 11% 18% 19% 7%	102238
Seeing	Day	16% 9% 24% 25% 19% 8%	15336
	Night	16% 8% 24% 31% 19% 2	25404

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	17% 5% 2 5% 3 3 3 2 3 6% 51%	2836
	11th to 20th	Day	13% 7% 2 5% 3 3 3 3 4 5% 52%	2840
	21st to end	Day	20% 9% 2 6% 3 3 4 3 4 4 42%	3124
February	1st to 10th	Day	15% 7% 2 7% 4 3 5% 3 6% 6% 42%	1710
	11th to 20th	Day	20% 6% 2 4 3 3 4 3 4 5% 46%	1803
	21st to end	Day	18% 7% 3 6% 5% 4 4 3 5% 5% 40%	1639
March	1st to 10th	Day	23% 8% 2 7% 3 3 3 3 5% 6% 38%	3435
	11th to 20th	Day	11% 5% 1 5% 3 4 4 3 5% 6% 53%	3480
	21st to end	Day	19% 7% 3 5% 3 3 4 3 4 6% 43%	3815
April	1st to 10th	Day	10% 5% 1 4 3 4 4 4 6% 8% 51%	2099
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 47%	2254
	21st to end	Day	12% 5% 2 5% 4 4 4 4 5% 7% 50%	2389
May	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 5% 7% 9% 36%	4082
	11th to 20th	Day	16% 7% 3 8% 5% 4 4 4 5% 8% 36%	4081
	21st to end	Day	6% 4 2 6% 5% 4 4 4 6% 9% 49%	4476
June	1st to 10th	Day	9% 5% 2 6% 6% 4 5% 5% 8% 11% 38%	2470
	11th to 20th	Day	8% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	2470
	21st to end	Day	6% 5% 1 6% 6% 6% 6% 6% 8% 10% 39%	2470
July	1st to 10th	Day	8% 6% 3 8% 6% 6% 7% 5% 8% 11% 33%	4120
	11th to 20th	Day	8% 6% 2 8% 6% 6% 6% 5% 9% 12% 32%	4120
	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% 56%	1887
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 6% 6% 53%	1790
	21st to end	Day	15% 5% 2 5% 3 4 3 3 5% 6% 50%	1710
December	1st to 10th	Day	18% 8% 2 7% 3 4 4 3 6% 7% 39%	1710
	11th to 20th	Day	16% 5% 2 4 3 2 3 2 4 7% 52%	2153
	21st to end	Day	17% 7% 3 5% 4 4 3 4 4 6% 43%	3124
January	1st to 10th	Night	17% 6% 2 5% 3 4 3 3 4 5% 49%	4109
	11th to 20th	Night	11% 7% 3 6% 4 4 4 4 5% 6% 47%	4120
	21st to end	Night	21% 8% 2 5% 3 3 3 2 3 4 45%	4532
February	1st to 10th	Night	16% 6% 2 7% 4 3 4 3 5% 5% 45%	2470
	11th to 20th	Night	20% 7% 2 5% 4 3 3 3 5% 5% 43%	2260
	21st to end	Night	15% 8% 2 5% 3 3 3 3 6% 6% 45%	1813
March	1st to 10th	Night	22% 10% 3 7% 4 3 3 3 4 5% 35%	3418
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 7% 51%	3480
	21st to end	Night	19% 5% 2 5% 3 5% 4 4 4 6% 44%	3817
April	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 5% 7% 50%	2080
	11th to 20th	Night	11% 5% 2 4 4 4 4 3 5% 6% 51%	1875
	21st to end	Night	11% 4 1 6% 4 4 4 3 6% 7% 50%	1805
May	1st to 10th	Night	13% 8% 2 6% 4 4 5% 3 5% 9% 42%	2813
	11th to 20th	Night	15% 8% 3 7% 5% 4 4 4 5% 7% 39%	2812
	21st to end	Night	7% 5% 2 6% 4 5% 5% 5% 8% 11% 43%	3079
June	1st to 10th	Night	10% 6% 2 9% 6% 5% 6% 4 7% 9% 36%	1710
	11th to 20th	Night	10% 6% 2 7% 6% 5% 6% 6% 8% 10% 35%	1710
	21st to end	Night	7% 5% 2 7% 5% 6% 6% 5% 9% 12% 35%	1710
July	1st to 10th	Night	9% 7% 3 10% 6% 6% 6% 5% 7% 10% 30%	2840
	11th to 20th	Night	9% 7% 2 7% 5% 5% 6% 6% 10% 12% 31%	2840
	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	13% 6% 2 5% 3 3 3 2 4 6% 54%	2264
	11th to 20th	Night	13% 6% 1 5% 4 3 4 3 4 6% 51%	2388
	21st to end	Night	14% 5% 2 4 4 4 4 3 5% 6% 49%	2470
December	1st to 10th	Night	17% 8% 3 6% 4 4 3 3 4 6% 41%	2470
	11th to 20th	Night	17% 6% 2 5% 3 3 2 2 3 5% 51%	3042
	21st to end	Night	19% 5% 2 5% 4 4 4 3 5% 6% 44%	4532

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	36% 3 6% 25% 23% 6%	2837
	11th to 20th	Day	35% 5% 6% 16% 31% 7%	2840
	21st to end	Day	44% 5% 6% 16% 24% 6%	3124
February	1st to 10th	Day	40% 5% 10% 21% 19% 5%	1710
	11th to 20th	Day	41% 4 8% 18% 23% 6%	1813
	21st to end	Day	46% 5% 7% 16% 19% 7%	1639
March	1st to 10th	Day	47% 5% 9% 16% 15% 9%	3435
	11th to 20th	Day	31% 5% 9% 15% 28% 12%	3480
	21st to end	Day	43% 5% 7% 19% 22% 3	3815
April	1st to 10th	Day	31% 6% 10% 20% 27% 6%	2099
	11th to 20th	Day	35% 6% 9% 19% 22% 9%	2254
	21st to end	Day	34% 6% 11% 22% 22% 4	2389
May	1st to 10th	Day	41% 10% 15% 20% 13% 2	4120
	11th to 20th	Day	44% 8% 12% 19% 15% 1	4085
	21st to end	Day	29% 10% 15% 21% 22% 4	4489
June	1st to 10th	Day	37% 11% 14% 24% 12% 1	2470
	11th to 20th	Day	35% 10% 17% 22% 15% 1	2470
	21st to end	Day	35% 12% 15% 21% 15% 2	2470
July	1st to 10th	Day	41% 12% 18% 18% 10% 1	4055
	11th to 20th	Day	39% 12% 26% 17% 4 1	4051
	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	31% 4 7% 20% 29% 9%	1887
	11th to 20th	Day	31% 4 10% 20% 27% 8%	1790
	21st to end	Day	36% 4 9% 21% 22% 8%	1710
December	1st to 10th	Day	44% 6% 9% 19% 18% 4	1710
	11th to 20th	Day	33% 4 6% 21% 28% 8%	2150
	21st to end	Day	40% 6% 9% 22% 20% 3	3124
January	1st to 10th	Night	37% 5% 8% 18% 26% 7%	4109
	11th to 20th	Night	36% 6% 7% 16% 27% 7%	4120
	21st to end	Night	45% 3 5% 16% 25% 7%	4532
February	1st to 10th	Night	40% 5% 9% 19% 21% 7%	2470
	11th to 20th	Night	44% 4 7% 16% 22% 7%	2261
	21st to end	Night	39% 5% 10% 19% 20% 7%	1813
March	1st to 10th	Night	50% 6% 9% 13% 15% 7%	3418
	11th to 20th	Night	32% 5% 9% 14% 25% 15%	3480
	21st to end	Night	41% 5% 8% 18% 20% 7%	3817
April	1st to 10th	Night	34% 5% 10% 19% 24% 7%	2080
	11th to 20th	Night	32% 6% 11% 20% 22% 9%	1875
	21st to end	Night	32% 8% 11% 24% 21% 4	1805
May	1st to 10th	Night	38% 8% 14% 23% 14% 2	2840
	11th to 20th	Night	44% 8% 12% 21% 15% 1	2813
	21st to end	Night	28% 12% 17% 29% 13% 2	3087
June	1st to 10th	Night	41% 11% 15% 20% 11% 2	1710
	11th to 20th	Night	39% 10% 17% 20% 12% 1	1710
	21st to end	Night	36% 14% 18% 18% 11% 3	1710
July	1st to 10th	Night	46% 11% 18% 15% 9% 2	2795
	11th to 20th	Night	40% 13% 26% 16% 4 1	2786
	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	33% 4 7% 17% 26% 12%	2264
	11th to 20th	Night	34% 4 7% 17% 25% 13%	2388
	21st to end	Night	35% 5% 6% 20% 27% 8%	2470
December	1st to 10th	Night	45% 5% 9% 17% 19% 6%	2470
	11th to 20th	Night	37% 4 8% 17% 24% 9%	3044
	21st to end	Night	40% 5% 9% 20% 23% 3	4532

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% 19% 39% 17% 3	270
	11th to 20th	Day	24% 22% 32% 17% 5%	270
	21st to end	Day	20% 23% 38% 15% 3	297
February	1st to 10th	Day	21% 18% 40% 14% 7%	270
	11th to 20th	Day	34% 15% 39% 10% 2	270
	21st to end	Day	22% 26% 31% 17% 3	294
March	1st to 10th	Day	20% 17% 33% 21% 8% 1	460
	11th to 20th	Day	19% 17% 40% 18% 5% 1	460
	21st to end	Day	21% 15% 36% 20% 7% 1	506
April	1st to 10th	Day	19% 18% 31% 22% 9% 2	460
	11th to 20th	Day	14% 18% 29% 23% 14% 3	452
	21st to end	Day	15% 10% 31% 26% 15% 4	460
May	1st to 10th	Day	11% 13% 33% 23% 17% 3	463
	11th to 20th	Day	15% 6% 22% 29% 22% 7%	487
	21st to end	Day	12% 5% 20% 26% 25% 12%	529
June	1st to 10th	Day	10% 3 19% 32% 27% 8%	490
	11th to 20th	Day	11% 5% 18% 24% 30% 12%	490
	21st to end	Day	8% 2 15% 33% 30% 11%	490
July	1st to 10th	Day	8% 2 11% 22% 38% 19%	485
	11th to 20th	Day	10% 1 8% 29% 34% 18%	484
	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% 26% 31% 15% 4	467
	11th to 20th	Day	17% 14% 32% 25% 11% 1	350
	21st to end	Day	13% 12% 37% 24% 13% 2	270
December	1st to 10th	Day	23% 14% 37% 19% 7%	270
	11th to 20th	Day	21% 11% 34% 24% 9% 1	270
	21st to end	Day	24% 17% 35% 16% 6% 3	297
January	1st to 10th	Night	21% 14% 33% 27% 6%	760
	11th to 20th	Night	19% 14% 33% 28% 6%	760
	21st to end	Night	20% 11% 37% 29% 3	836
February	1st to 10th	Night	18% 11% 38% 29% 4	760
	11th to 20th	Night	25% 12% 39% 22% 2	743
	21st to end	Night	20% 17% 35% 24% 4	628
March	1st to 10th	Night	18% 10% 36% 31% 5%	760
	11th to 20th	Night	18% 14% 36% 24% 8% 1	760
	21st to end	Night	18% 12% 34% 28% 7%	836
April	1st to 10th	Night	15% 14% 34% 28% 8% 1	760
	11th to 20th	Night	14% 13% 29% 29% 14%	751
	21st to end	Night	13% 6% 29% 38% 13%	665
May	1st to 10th	Night	12% 10% 28% 37% 13%	570
	11th to 20th	Night	16% 6% 22% 35% 20% 1	561
	21st to end	Night	12% 3 14% 39% 30% 2	617
June	1st to 10th	Night	11% 3 16% 40% 25% 4	570
	11th to 20th	Night	12% 3 14% 35% 29% 6%	570
	21st to end	Night	9% 2 16% 36% 33% 5%	570
July	1st to 10th	Night	8% 3 8% 35% 38% 7%	555
	11th to 20th	Night	9% 2 7% 34% 40% 8%	552
	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	15% 6% 25% 32% 18% 3	755
	11th to 20th	Night	15% 11% 30% 32% 11% 1	760
	21st to end	Night	16% 9% 27% 34% 14% 1	760
December	1st to 10th	Night	19% 12% 29% 33% 7%	760
	11th to 20th	Night	20% 8% 27% 35% 10%	760
	21st to end	Night	24% 13% 24% 30% 9%	836

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