Arcadia
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arcadia clear sky chart from 2005-11-06 to 2025-08-05.

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	12% 7% 2 6% 4 4 4 3 5% 7% 46%	125105
	Night	12% 6% 2 5% 4 3 3 3 5% 7% 50%	123178
Transparency	Day	37% 5% 11% 26% 18% 4	128716
	Night	35% 4 10% 26% 20% 4	127643
Seeing	Day	14% 9% 27% 28% 18% 5%	16619
	Night	14% 8% 31% 32% 13% 1	26413

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	19% 8% 3 7% 4 3 3 3 4 6% 39%	4077
	11th to 20th	Day	9% 11% 4 8% 5% 5% 4 3 4 6% 43%	4080
	21st to end	Day	16% 11% 3 6% 3 3 3 3 4 5% 44%	4488
February	1st to 10th	Day	17% 10% 3 9% 5% 3 3 3 4 4 40%	1800
	11th to 20th	Day	14% 9% 4 7% 4 3 3 3 3 4 45%	1759
	21st to end	Day	13% 9% 3 7% 4 4 3 4 6% 6% 41%	1748
March	1st to 10th	Day	13% 7% 3 6% 3 3 3 2 4 5% 50%	4936
	11th to 20th	Day	11% 7% 2 4 3 3 3 3 5% 6% 55%	4980
	21st to end	Day	13% 9% 3 7% 5% 5% 5% 4 5% 8% 36%	5462
April	1st to 10th	Day	13% 7% 3 5% 4 5% 4 4 5% 6% 44%	2300
	11th to 20th	Day	16% 6% 2 7% 4 4 4 4 5% 6% 42%	2374
	21st to end	Day	21% 7% 2 7% 5% 4 4 3 5% 6% 36%	2491
May	1st to 10th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 38%	5862
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 8% 44%	5862
	21st to end	Day	20% 6% 2 6% 4 3 4 3 6% 8% 39%	6434
June	1st to 10th	Day	7% 4 2 6% 5% 5% 5% 4 6% 9% 48%	2622
	11th to 20th	Day	8% 5% 2 6% 5% 4 5% 5% 6% 9% 46%	2725
	21st to end	Day	5% 2 1 4 4 4 5% 4 6% 9% 56%	2600
July	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 4 6% 8% 44%	5900
	11th to 20th	Day	7% 4 2 6% 4 4 5% 4 6% 10% 48%	5900
	21st to end	Day	6% 4 2 5% 4 5% 6% 5% 8% 11% 45%	6422
August	1st to 10th	Day	5% 3 2 5% 3 4 4 4 7% 9% 54%	4157
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 4 6% 8% 46%	4041
	21st to end	Day	6% 3 2 4 3 4 4 3 5% 8% 57%	4221
September	1st to 10th	Day	11% 6% 2 5% 4 3 3 3 5% 6% 54%	2280
	11th to 20th	Day	10% 4 2 5% 4 5% 4 3 6% 7% 48%	2098
	21st to end	Day	9% 4 2 4 4 4 4 3 5% 6% 55%	2080
October	1st to 10th	Day	11% 4 2 5% 3 3 3 3 4 7% 54%	1900
	11th to 20th	Day	12% 5% 2 4 3 3 3 2 4 6% 58%	1900
	21st to end	Day	18% 6% 2 6% 2 3 2 3 4 5% 49%	2090
November	1st to 10th	Day	10% 6% 2 5% 3 2 3 2 4 5% 57%	1958
	11th to 20th	Day	15% 6% 2 4 3 2 3 3 4 5% 53%	1855
	21st to end	Day	14% 8% 3 6% 3 2 2 2 4 5% 51%	1620
December	1st to 10th	Day	11% 7% 2 5% 3 3 2 3 4 5% 55%	2610
	11th to 20th	Day	12% 7% 3 6% 3 2 3 2 3 6% 53%	3008
	21st to end	Day	22% 19% 5% 8% 3 3 3 3 3 5% 27%	4465
January	1st to 10th	Night	15% 8% 3 9% 5% 4 3 3 4 5% 41%	5888
	11th to 20th	Night	10% 7% 3 8% 6% 5% 4 4 4 5% 45%	5900
	21st to end	Night	17% 9% 3 7% 3 2 2 2 3 5% 46%	6490
February	1st to 10th	Night	13% 11% 3 7% 4 3 4 3 5% 5% 42%	2600
	11th to 20th	Night	14% 7% 3 6% 4 4 4 3 5% 6% 44%	2533
	21st to end	Night	14% 8% 3 6% 3 3 3 3 5% 7% 46%	1890
March	1st to 10th	Night	14% 8% 2 6% 3 3 3 3 3 5% 49%	4919
	11th to 20th	Night	9% 6% 2 4 3 3 3 3 4 6% 57%	4980
	21st to end	Night	12% 6% 3 6% 4 4 4 4 6% 7% 44%	5467
April	1st to 10th	Night	11% 6% 2 6% 4 3 3 4 5% 6% 50%	2100
	11th to 20th	Night	14% 8% 2 5% 3 3 3 3 4 6% 49%	1975
	21st to end	Night	16% 7% 3 6% 5% 3 3 3 5% 6% 43%	1905
May	1st to 10th	Night	17% 7% 2 6% 4 3 3 3 4 5% 45%	4053
	11th to 20th	Night	14% 6% 2 6% 3 3 3 3 6% 8% 46%	4053
	21st to end	Night	18% 5% 2 4 3 3 3 3 5% 6% 49%	4443
June	1st to 10th	Night	6% 3 1 6% 4 4 4 3 6% 10% 53%	1755
	11th to 20th	Night	5% 3 2 4 3 4 5% 4 8% 11% 52%	1650
	21st to end	Night	4 3 1 4 3 3 4 3 7% 8% 59%	1800
July	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 49%	4078
	11th to 20th	Night	5% 3 2 5% 3 3 4 3 6% 10% 55%	4077
	21st to end	Night	5% 3 1 4 4 4 4 4 7% 12% 51%	4449
August	1st to 10th	Night	3 2 1 3 3 3 4 3 6% 9% 63%	2872
	11th to 20th	Night	9% 4 2 6% 4 4 3 3 6% 8% 51%	2798
	21st to end	Night	3 2 1 4 3 3 4 3 5% 8% 63%	3435
September	1st to 10th	Night	10% 5% 2 4 3 3 4 3 4 6% 57%	1900
	11th to 20th	Night	9% 4 2 5% 4 3 5% 3 5% 8% 53%	2078
	21st to end	Night	7% 5% 1 4 4 4 4 3 4 6% 58%	2090
October	1st to 10th	Night	8% 4 1 4 2 3 3 2 5% 7% 61%	2270
	11th to 20th	Night	13% 5% 2 4 3 3 2 3 4 5% 58%	2280
	21st to end	Night	15% 6% 2 4 3 3 3 2 4 6% 53%	2508
November	1st to 10th	Night	11% 5% 1 3 2 2 3 2 3 5% 61%	2340
	11th to 20th	Night	11% 7% 2 6% 4 3 3 3 4 5% 54%	2540
	21st to end	Night	16% 6% 2 4 3 2 3 2 4 5% 53%	2775
December	1st to 10th	Night	9% 7% 2 4 3 3 3 3 5% 5% 56%	4580
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 4 5% 49%	5199
	21st to end	Night	20% 15% 5% 8% 5% 4 3 3 4 4 31%	6508

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	48% 3 9% 24% 14% 1	4078
	11th to 20th	Day	43% 3 10% 24% 16% 3	4080
	21st to end	Day	45% 2 9% 19% 19% 6%	4488
February	1st to 10th	Day	49% 4 9% 25% 10% 3	1800
	11th to 20th	Day	45% 3 12% 21% 15% 3	1768
	21st to end	Day	43% 4 7% 23% 16% 6%	1748
March	1st to 10th	Day	37% 3 7% 22% 21% 9%	4936
	11th to 20th	Day	31% 4 8% 22% 25% 10%	4980
	21st to end	Day	47% 5% 15% 25% 7% 1	5464
April	1st to 10th	Day	40% 6% 12% 23% 16% 3	2300
	11th to 20th	Day	42% 6% 12% 22% 15% 2	2374
	21st to end	Day	49% 5% 11% 23% 11% 1	2491
May	1st to 10th	Day	46% 6% 10% 21% 16% 1	5900
	11th to 20th	Day	36% 7% 12% 25% 17% 3	5866
	21st to end	Day	44% 6% 14% 22% 14% 1	6447
June	1st to 10th	Day	31% 8% 16% 26% 18% 2	2622
	11th to 20th	Day	33% 7% 12% 30% 17% 2	2725
	21st to end	Day	23% 7% 17% 32% 18% 4	2600
July	1st to 10th	Day	36% 6% 11% 24% 21% 2	5836
	11th to 20th	Day	32% 6% 12% 34% 14% 1	5831
	21st to end	Day	29% 8% 19% 32% 11%	6240
August	1st to 10th	Day	25% 6% 17% 24% 24% 3	4143
	11th to 20th	Day	36% 7% 15% 26% 16% 1	4080
	21st to end	Day	25% 6% 12% 40% 13% 4	4220
September	1st to 10th	Day	32% 5% 10% 27% 17% 8%	2280
	11th to 20th	Day	34% 6% 8% 27% 23% 2	2098
	21st to end	Day	30% 6% 9% 25% 26% 4	2080
October	1st to 10th	Day	19% 3 6% 18% 39% 15%	3160
	11th to 20th	Day	18% 2 4 28% 37% 11%	3140
	21st to end	Day	30% 3 8% 27% 26% 4	3428
November	1st to 10th	Day	30% 4 9% 30% 22% 5%	1958
	11th to 20th	Day	35% 3 7% 26% 23% 6%	1855
	21st to end	Day	38% 3 9% 28% 20% 1	1620
December	1st to 10th	Day	33% 3 9% 37% 18%	2610
	11th to 20th	Day	36% 3 7% 31% 21% 2	3005
	21st to end	Day	62% 3 8% 19% 7% 1	4465
January	1st to 10th	Night	47% 3 11% 24% 13% 2	5888
	11th to 20th	Night	41% 3 9% 25% 19% 3	5900
	21st to end	Night	43% 2 9% 20% 20% 5%	6490
February	1st to 10th	Night	44% 4 11% 24% 14% 3	2600
	11th to 20th	Night	42% 4 10% 25% 16% 3	2534
	21st to end	Night	39% 3 9% 23% 20% 7%	1890
March	1st to 10th	Night	39% 3 8% 21% 24% 5%	4919
	11th to 20th	Night	30% 3 7% 21% 31% 8%	4980
	21st to end	Night	38% 5% 14% 27% 13% 2	5467
April	1st to 10th	Night	35% 5% 11% 22% 22% 5%	2100
	11th to 20th	Night	36% 5% 9% 24% 20% 5%	1975
	21st to end	Night	42% 5% 12% 24% 15% 2	1905
May	1st to 10th	Night	41% 6% 11% 25% 14% 3	4080
	11th to 20th	Night	36% 6% 12% 25% 15% 6%	4054
	21st to end	Night	37% 6% 11% 27% 17% 3	4452
June	1st to 10th	Night	26% 8% 16% 28% 18% 3	1755
	11th to 20th	Night	25% 7% 16% 30% 21% 2	1650
	21st to end	Night	22% 6% 14% 33% 22% 4	1800
July	1st to 10th	Night	32% 5% 13% 25% 21% 3	4035
	11th to 20th	Night	25% 5% 10% 41% 17% 2	4026
	21st to end	Night	24% 6% 19% 36% 14%	4320
August	1st to 10th	Night	19% 4 17% 33% 21% 6%	2862
	11th to 20th	Night	32% 5% 14% 29% 19% 2	2825
	21st to end	Night	20% 5% 12% 40% 19% 4	3435
September	1st to 10th	Night	29% 4 11% 26% 22% 9%	1900
	11th to 20th	Night	30% 5% 8% 24% 26% 5%	2078
	21st to end	Night	28% 4 8% 26% 28% 5%	2090
October	1st to 10th	Night	15% 3 6% 18% 43% 16%	3790
	11th to 20th	Night	21% 3 6% 20% 33% 17%	3779
	21st to end	Night	30% 3 6% 30% 25% 5%	4120
November	1st to 10th	Night	27% 4 10% 29% 24% 7%	2340
	11th to 20th	Night	34% 3 7% 24% 27% 5%	2540
	21st to end	Night	35% 3 8% 26% 23% 4	2775
December	1st to 10th	Night	31% 3 11% 37% 17% 1	4580
	11th to 20th	Night	38% 3 9% 26% 21% 3	5201
	21st to end	Night	58% 3 9% 21% 8% 2	6508

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	21% 13% 34% 25% 6%	300
	11th to 20th	Day	22% 19% 38% 20% 2	300
	21st to end	Day	15% 15% 45% 19% 6%	330
February	1st to 10th	Day	25% 18% 34% 18% 4	300
	11th to 20th	Day	21% 19% 40% 16% 3 1	300
	21st to end	Day	21% 18% 41% 17% 3	249
March	1st to 10th	Day	20% 18% 36% 23% 3	300
	11th to 20th	Day	27% 12% 33% 23% 5%	300
	21st to end	Day	27% 16% 38% 14% 4 1	330
April	1st to 10th	Day	18% 20% 37% 19% 6% 1	400
	11th to 20th	Day	18% 16% 37% 23% 6%	492
	21st to end	Day	24% 15% 29% 23% 9% 1	591
May	1st to 10th	Day	18% 11% 35% 25% 10% 1	703
	11th to 20th	Day	16% 11% 29% 28% 14% 3	725
	21st to end	Day	16% 7% 29% 25% 17% 6%	789
June	1st to 10th	Day	7% 4 19% 33% 28% 9%	730
	11th to 20th	Day	9% 2 21% 40% 22% 6%	730
	21st to end	Day	5% 4 29% 36% 22% 3	730
July	1st to 10th	Day	10% 3 15% 29% 31% 12%	721
	11th to 20th	Day	5% 2 15% 38% 32% 8%	718
	21st to end	Day	7% 1 12% 34% 34% 12%	794
August	1st to 10th	Day	8% 2 13% 39% 33% 5%	697
	11th to 20th	Day	8% 2 16% 34% 33% 7%	665
	21st to end	Day	10% 2 9% 27% 40% 12%	567
September	1st to 10th	Day	11% 1 14% 31% 32% 10%	490
	11th to 20th	Day	6% 2 14% 39% 27% 11%	308
	21st to end	Day	13% 5% 18% 34% 22% 8%	300
October	1st to 10th	Day	12% 7% 24% 30% 18% 9%	300
	11th to 20th	Day	11% 14% 37% 27% 8% 2	300
	21st to end	Day	27% 12% 33% 21% 7% 1	330
November	1st to 10th	Day	16% 12% 35% 23% 10% 2	300
	11th to 20th	Day	16% 16% 34% 23% 10% 1	300
	21st to end	Day	19% 18% 32% 21% 9% 1	300
December	1st to 10th	Day	16% 12% 43% 19% 8% 2	300
	11th to 20th	Day	16% 14% 40% 24% 6% 1	300
	21st to end	Day	23% 21% 37% 17% 2	330
January	1st to 10th	Night	16% 13% 39% 26% 6%	1000
	11th to 20th	Night	16% 14% 42% 24% 4	1000
	21st to end	Night	15% 13% 40% 28% 4	1100
February	1st to 10th	Night	19% 12% 42% 24% 3	1000
	11th to 20th	Night	18% 16% 39% 25% 3	979
	21st to end	Night	21% 13% 38% 25% 3	570
March	1st to 10th	Night	17% 11% 39% 28% 5%	600
	11th to 20th	Night	20% 11% 35% 28% 5%	600
	21st to end	Night	21% 13% 39% 23% 4	660
April	1st to 10th	Night	15% 12% 38% 28% 7%	600
	11th to 20th	Night	18% 14% 35% 28% 5%	593
	21st to end	Night	19% 9% 38% 27% 7%	600
May	1st to 10th	Night	16% 6% 36% 33% 9%	600
	11th to 20th	Night	15% 4 29% 35% 16% 1	591
	21st to end	Night	13% 5% 26% 35% 17% 3	650
June	1st to 10th	Night	7% 2 19% 42% 29% 2	600
	11th to 20th	Night	7% 1 23% 47% 21% 1	600
	21st to end	Night	4 2 23% 50% 20% 1	600
July	1st to 10th	Night	9% 1 12% 43% 33% 3	585
	11th to 20th	Night	5% 1 11% 48% 32% 3	582
	21st to end	Night	8% 1 11% 41% 37% 2	657
August	1st to 10th	Night	5% 2 15% 44% 33% 2	579
	11th to 20th	Night	8% 1 13% 47% 28% 2	564
	21st to end	Night	5% 1 10% 38% 43% 3	627
September	1st to 10th	Night	11% 2 13% 42% 30% 2	570
	11th to 20th	Night	10% 2 17% 39% 28% 4	570
	21st to end	Night	8% 2 22% 43% 22% 3	570
October	1st to 10th	Night	9% 3 27% 36% 22% 2	750
	11th to 20th	Night	14% 7% 37% 33% 9%	760
	21st to end	Night	18% 10% 32% 30% 9% 1	836
November	1st to 10th	Night	14% 7% 38% 31% 10% 1	780
	11th to 20th	Night	15% 12% 37% 29% 8%	940
	21st to end	Night	19% 10% 37% 27% 6%	1000
December	1st to 10th	Night	17% 10% 39% 28% 6%	1000
	11th to 20th	Night	21% 11% 36% 29% 4	1000
	21st to end	Night	19% 14% 37% 27% 3	1100

Page generated at 2025-08-02 00:32:28. Version ac07

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