ORAS Dark-Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ORAS Dark-Sky Site clear sky chart from 2005-11-06 to 2024-03-08.

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	18% 11% 5% 8% 6% 3 5% 5% 5% 6% 28%	78586
	Night	18% 12% 5% 8% 6% 3 4 4 4 6% 29%	80977
Transparency	Day	56% 6% 11% 17% 9% 1	78299
	Night	56% 6% 10% 17% 9% 1	80783
Seeing	Day	25% 15% 28% 19% 10% 3	13682
	Night	24% 13% 27% 25% 10% 1	23421

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	20% 21% 10% 11% 6% 3 4 3 2 4 15%	2677
	11th to 20th	Day	12% 21% 10% 15% 10% 4 3 2 3 4 16%	2840
	21st to end	Day	26% 19% 10% 14% 8% 3 3 3 3 3 9%	3124
February	1st to 10th	Day	18% 18% 7% 11% 8% 4 4 4 3 4 19%	1710
	11th to 20th	Day	16% 14% 6% 14% 8% 4 5% 5% 4 5% 20%	1663
	21st to end	Day	21% 14% 5% 10% 6% 3 5% 4 4 5% 23%	1541
March	1st to 10th	Day	17% 11% 7% 9% 6% 3 4 5% 5% 7% 26%	2057
	11th to 20th	Day	19% 13% 5% 7% 7% 3 4 4 4 6% 27%	1980
	21st to end	Day	21% 12% 5% 8% 6% 4 4 4 3 5% 28%	2178
April	1st to 10th	Day	17% 13% 5% 6% 6% 3 4 4 4 7% 31%	2101
	11th to 20th	Day	20% 12% 3 6% 5% 3 4 4 5% 6% 32%	2153
	21st to end	Day	23% 10% 4 6% 5% 3 3 4 5% 5% 32%	2340
May	1st to 10th	Day	18% 11% 4 7% 6% 3 5% 5% 6% 7% 28%	2340
	11th to 20th	Day	22% 8% 3 5% 6% 4 5% 5% 6% 6% 29%	2338
	21st to end	Day	13% 6% 3 6% 6% 3 6% 6% 6% 8% 37%	2725
June	1st to 10th	Day	17% 6% 3 6% 6% 3 6% 6% 5% 8% 34%	2520
	11th to 20th	Day	14% 7% 3 6% 6% 4 6% 5% 6% 9% 34%	2520
	21st to end	Day	16% 6% 2 4 6% 4 5% 5% 7% 10% 35%	2520
July	1st to 10th	Day	11% 7% 3 6% 5% 4 6% 5% 6% 9% 39%	2520
	11th to 20th	Day	12% 6% 2 6% 6% 4 6% 5% 7% 8% 37%	2379
	21st to end	Day	12% 6% 3 7% 6% 4 6% 7% 7% 9% 33%	2547
August	1st to 10th	Day	11% 6% 3 6% 6% 4 6% 6% 7% 10% 35%	2327
	11th to 20th	Day	14% 7% 3 6% 6% 5% 6% 6% 7% 8% 31%	2303
	21st to end	Day	14% 5% 3 7% 6% 4 6% 6% 7% 8% 34%	2446
September	1st to 10th	Day	18% 8% 2 5% 5% 3 4 4 5% 6% 40%	2160
	11th to 20th	Day	13% 8% 3 6% 5% 3 5% 5% 7% 8% 40%	2027
	21st to end	Day	19% 8% 3 7% 5% 4 4 5% 4 7% 35%	1956
October	1st to 10th	Day	18% 8% 3 5% 6% 4 4 6% 5% 8% 35%	1800
	11th to 20th	Day	23% 9% 3 6% 6% 4 5% 3 5% 6% 31%	1800
	21st to end	Day	26% 11% 3 7% 6% 2 4 4 5% 7% 25%	1980
November	1st to 10th	Day	13% 9% 3 7% 6% 3 5% 5% 5% 7% 37%	1772
	11th to 20th	Day	18% 14% 3 8% 6% 3 4 4 6% 6% 28%	1520
	21st to end	Day	21% 18% 5% 8% 5% 3 4 4 4 6% 21%	1520
December	1st to 10th	Day	20% 21% 10% 10% 6% 3 4 4 3 4 14%	1520
	11th to 20th	Day	19% 18% 7% 12% 5% 3 4 4 4 6% 17%	1910
	21st to end	Day	28% 23% 6% 10% 7% 3 3 2 3 3 12%	2772
January	1st to 10th	Night	17% 22% 10% 14% 6% 3 4 3 3 4 14%	4265
	11th to 20th	Night	16% 18% 10% 16% 8% 4 5% 4 4 4 12%	4120
	21st to end	Night	28% 17% 10% 11% 7% 2 3 3 3 3 13%	4532
February	1st to 10th	Night	18% 15% 6% 10% 9% 5% 4 4 4 4 21%	2470
	11th to 20th	Night	18% 14% 6% 11% 7% 4 6% 4 5% 4 21%	2403
	21st to end	Night	22% 11% 6% 12% 8% 3 4 4 4 4 23%	1909
March	1st to 10th	Night	17% 11% 6% 8% 6% 4 5% 5% 4 6% 28%	2064
	11th to 20th	Night	20% 10% 5% 7% 5% 2 4 4 4 6% 32%	1980
	21st to end	Night	18% 13% 4 7% 5% 3 4 4 4 5% 32%	2178
April	1st to 10th	Night	18% 12% 5% 7% 5% 3 5% 4 4 6% 32%	1858
	11th to 20th	Night	18% 9% 2 7% 5% 3 5% 4 5% 6% 36%	1771
	21st to end	Night	15% 11% 3 7% 6% 2 5% 4 5% 8% 34%	1620
May	1st to 10th	Night	18% 7% 2 6% 6% 4 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	16% 8% 4 6% 5% 3 4 5% 5% 7% 36%	1553
	21st to end	Night	12% 5% 3 5% 5% 3 5% 4 6% 10% 43%	1550
June	1st to 10th	Night	10% 6% 2 5% 6% 3 4 6% 6% 10% 43%	1440
	11th to 20th	Night	13% 6% 3 5% 5% 3 5% 5% 5% 9% 41%	1440
	21st to end	Night	12% 5% 2 6% 5% 3 4 5% 6% 10% 42%	1440
July	1st to 10th	Night	8% 6% 3 6% 5% 4 5% 4 5% 8% 46%	1440
	11th to 20th	Night	9% 5% 3 5% 5% 3 4 4 7% 9% 47%	1576
	21st to end	Night	11% 5% 2 5% 6% 3 5% 5% 6% 9% 42%	1771
August	1st to 10th	Night	9% 6% 2 6% 5% 3 4 6% 6% 9% 46%	1611
	11th to 20th	Night	12% 5% 2 5% 5% 3 5% 6% 5% 9% 42%	1598
	21st to end	Night	12% 4 2 4 4 2 5% 5% 5% 9% 46%	1890
September	1st to 10th	Night	14% 6% 2 5% 4 3 4 4 5% 6% 46%	1800
	11th to 20th	Night	11% 5% 2 5% 4 3 3 5% 5% 8% 50%	1930
	21st to end	Night	18% 7% 3 5% 6% 3 5% 3 4 6% 39%	2002
October	1st to 10th	Night	15% 7% 2 5% 5% 2 4 4 5% 7% 41%	2160
	11th to 20th	Night	19% 8% 2 6% 6% 3 5% 5% 5% 7% 36%	2160
	21st to end	Night	26% 10% 3 6% 5% 3 4 4 5% 6% 29%	2376
November	1st to 10th	Night	14% 9% 3 6% 4 3 5% 4 5% 7% 40%	2301
	11th to 20th	Night	19% 15% 4 6% 6% 3 5% 4 4 6% 27%	2660
	21st to end	Night	21% 16% 5% 7% 6% 3 4 4 4 6% 25%	2660
December	1st to 10th	Night	22% 17% 8% 10% 7% 4 4 4 5% 4 15%	2660
	11th to 20th	Night	19% 18% 7% 11% 6% 3 4 3 3 5% 20%	3285
	21st to end	Night	25% 21% 7% 10% 6% 3 5% 4 4 4 11%	4884

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% 3 7% 10% 5%	2678
	11th to 20th	Day	76% 2 8% 11% 3	2840
	21st to end	Day	82% 3 7% 6% 2	3124
February	1st to 10th	Day	70% 4 8% 12% 6%	1710
	11th to 20th	Day	67% 5% 8% 14% 6%	1672
	21st to end	Day	65% 5% 9% 14% 6% 1	1541
March	1st to 10th	Day	57% 5% 12% 20% 6%	2057
	11th to 20th	Day	59% 4 10% 16% 8% 2	1980
	21st to end	Day	60% 3 7% 18% 11% 1	2178
April	1st to 10th	Day	54% 4 12% 17% 12% 1	2101
	11th to 20th	Day	53% 5% 10% 18% 13% 2	2153
	21st to end	Day	53% 5% 10% 18% 11% 2	2340
May	1st to 10th	Day	53% 6% 11% 18% 9% 2	2340
	11th to 20th	Day	55% 6% 9% 16% 12% 2	2342
	21st to end	Day	43% 9% 15% 21% 11% 1	2725
June	1st to 10th	Day	47% 8% 11% 21% 12% 1	2520
	11th to 20th	Day	47% 7% 13% 19% 13% 1	2520
	21st to end	Day	42% 8% 11% 26% 12% 1	2520
July	1st to 10th	Day	42% 8% 11% 23% 14% 2	2450
	11th to 20th	Day	42% 8% 12% 23% 14% 1	2307
	21st to end	Day	43% 9% 12% 20% 15% 1	2405
August	1st to 10th	Day	42% 9% 15% 26% 8% 1	2314
	11th to 20th	Day	46% 9% 15% 19% 9% 1	2303
	21st to end	Day	44% 8% 12% 21% 13% 2	2446
September	1st to 10th	Day	44% 7% 10% 21% 15% 3	2160
	11th to 20th	Day	41% 6% 12% 24% 15% 1	2027
	21st to end	Day	49% 6% 11% 21% 13%	1956
October	1st to 10th	Day	47% 8% 12% 19% 13% 1	1800
	11th to 20th	Day	55% 5% 11% 21% 8%	1800
	21st to end	Day	59% 6% 12% 14% 8% 1	1980
November	1st to 10th	Day	46% 7% 12% 18% 15% 2	1772
	11th to 20th	Day	57% 4 12% 18% 7% 2	1520
	21st to end	Day	64% 6% 11% 14% 5% 1	1520
December	1st to 10th	Day	75% 4 10% 8% 3	1520
	11th to 20th	Day	70% 5% 9% 13% 3	1906
	21st to end	Day	80% 3 8% 8% 1	2772
January	1st to 10th	Night	75% 4 7% 11% 3	4265
	11th to 20th	Night	77% 5% 8% 9% 2	4120
	21st to end	Night	78% 3 6% 9% 3	4532
February	1st to 10th	Night	68% 4 8% 15% 5%	2470
	11th to 20th	Night	67% 5% 8% 14% 6%	2405
	21st to end	Night	65% 4 9% 14% 8% 1	1909
March	1st to 10th	Night	57% 5% 8% 20% 9%	2064
	11th to 20th	Night	54% 5% 9% 21% 9% 2	1980
	21st to end	Night	55% 5% 6% 18% 14% 2	2178
April	1st to 10th	Night	54% 5% 10% 18% 12% 1	1858
	11th to 20th	Night	47% 6% 10% 19% 14% 2	1771
	21st to end	Night	48% 6% 11% 22% 11% 3	1620
May	1st to 10th	Night	49% 7% 10% 18% 13% 3	1620
	11th to 20th	Night	46% 7% 10% 19% 15% 2	1554
	21st to end	Night	37% 8% 16% 24% 14% 2	1551
June	1st to 10th	Night	37% 8% 13% 25% 14% 1	1440
	11th to 20th	Night	41% 8% 13% 23% 14% 2	1440
	21st to end	Night	38% 8% 14% 23% 17%	1440
July	1st to 10th	Night	38% 8% 11% 22% 18% 3	1400
	11th to 20th	Night	32% 6% 15% 24% 19% 2	1525
	21st to end	Night	37% 9% 14% 21% 18% 2	1672
August	1st to 10th	Night	33% 8% 16% 28% 14% 1	1600
	11th to 20th	Night	38% 8% 14% 26% 12% 1	1598
	21st to end	Night	33% 9% 15% 25% 15% 3	1890
September	1st to 10th	Night	39% 7% 11% 24% 16% 4	1800
	11th to 20th	Night	31% 6% 13% 28% 20% 1	1930
	21st to end	Night	47% 6% 11% 20% 15% 1	2002
October	1st to 10th	Night	42% 6% 14% 21% 15% 1	2160
	11th to 20th	Night	48% 6% 13% 21% 11% 1	2160
	21st to end	Night	56% 5% 11% 17% 9% 1	2376
November	1st to 10th	Night	44% 6% 11% 23% 13% 3	2301
	11th to 20th	Night	58% 5% 9% 17% 8% 3	2660
	21st to end	Night	61% 5% 10% 18% 4 1	2660
December	1st to 10th	Night	72% 5% 10% 10% 3	2660
	11th to 20th	Night	69% 3 11% 13% 3	3288
	21st to end	Night	77% 5% 9% 8% 1	4884

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	32% 31% 25% 9% 3	225
	11th to 20th	Day	36% 29% 25% 7% 2 1	270
	21st to end	Day	33% 27% 31% 7% 2	297
February	1st to 10th	Day	33% 26% 33% 7%	270
	11th to 20th	Day	37% 27% 27% 7% 1	270
	21st to end	Day	41% 23% 28% 8%	268
March	1st to 10th	Day	30% 22% 31% 14% 2	448
	11th to 20th	Day	32% 18% 34% 10% 3 2	420
	21st to end	Day	35% 19% 27% 14% 4 1	462
April	1st to 10th	Day	30% 23% 25% 17% 5% 1	420
	11th to 20th	Day	28% 20% 25% 15% 8% 3	412
	21st to end	Day	30% 19% 28% 16% 5% 1	420
May	1st to 10th	Day	30% 18% 27% 16% 7% 3	423
	11th to 20th	Day	26% 13% 29% 20% 10% 2	447
	21st to end	Day	16% 11% 27% 24% 16% 6%	485
June	1st to 10th	Day	19% 13% 30% 26% 10% 2	450
	11th to 20th	Day	20% 14% 24% 27% 13% 2	450
	21st to end	Day	18% 14% 30% 24% 11% 2	450
July	1st to 10th	Day	13% 5% 24% 29% 20% 8%	445
	11th to 20th	Day	12% 7% 28% 28% 18% 7%	444
	21st to end	Day	14% 5% 32% 26% 17% 6%	488
August	1st to 10th	Day	13% 5% 24% 28% 22% 7%	446
	11th to 20th	Day	16% 4 26% 28% 15% 10%	439
	21st to end	Day	16% 7% 24% 29% 16% 8%	495
September	1st to 10th	Day	21% 7% 20% 26% 17% 8%	450
	11th to 20th	Day	13% 6% 23% 28% 19% 10%	450
	21st to end	Day	23% 12% 25% 22% 14% 4	450
October	1st to 10th	Day	20% 9% 28% 23% 16% 4	450
	11th to 20th	Day	28% 17% 30% 17% 7% 1	450
	21st to end	Day	36% 15% 27% 18% 4	495
November	1st to 10th	Day	24% 16% 30% 19% 10% 2	375
	11th to 20th	Day	33% 22% 26% 13% 4 1	180
	21st to end	Day	24% 30% 36% 9% 1	180
December	1st to 10th	Day	37% 26% 25% 7% 2 2	180
	11th to 20th	Day	36% 18% 34% 8% 3	180
	21st to end	Day	33% 25% 29% 11% 2	198
January	1st to 10th	Night	30% 23% 33% 11% 3	760
	11th to 20th	Night	33% 22% 28% 15% 2	760
	21st to end	Night	26% 22% 32% 17% 2	836
February	1st to 10th	Night	32% 18% 29% 19% 1	760
	11th to 20th	Night	30% 22% 33% 14% 1	743
	21st to end	Night	30% 22% 29% 16% 2	628
March	1st to 10th	Night	25% 18% 35% 19% 3	751
	11th to 20th	Night	29% 18% 28% 19% 6%	720
	21st to end	Night	27% 15% 35% 18% 5% 1	792
April	1st to 10th	Night	27% 15% 31% 20% 6%	720
	11th to 20th	Night	26% 13% 31% 21% 9% 1	707
	21st to end	Night	21% 10% 37% 25% 5% 1	540
May	1st to 10th	Night	24% 10% 22% 33% 11% 1	540
	11th to 20th	Night	20% 10% 28% 33% 9%	492
	21st to end	Night	12% 4 29% 32% 20% 2	389
June	1st to 10th	Night	10% 5% 30% 40% 15%	360
	11th to 20th	Night	14% 6% 25% 39% 16%	360
	21st to end	Night	14% 3 29% 36% 17% 1	360
July	1st to 10th	Night	11% 2 19% 43% 20% 4	350
	11th to 20th	Night	11% 4 19% 38% 26% 4	481
	21st to end	Night	13% 3 16% 43% 24% 1	591
August	1st to 10th	Night	11% 1 16% 44% 25% 2	537
	11th to 20th	Night	14% 3 12% 39% 28% 4	534
	21st to end	Night	12% 4 21% 32% 26% 4	702
September	1st to 10th	Night	16% 5% 21% 31% 23% 3	720
	11th to 20th	Night	13% 5% 19% 33% 26% 4	720
	21st to end	Night	20% 9% 23% 26% 18% 3	720
October	1st to 10th	Night	20% 7% 25% 28% 18% 3	720
	11th to 20th	Night	24% 11% 31% 25% 8% 1	720
	21st to end	Night	31% 13% 27% 24% 4	792
November	1st to 10th	Night	21% 12% 29% 28% 11%	740
	11th to 20th	Night	29% 16% 27% 20% 6% 1	760
	21st to end	Night	31% 17% 29% 19% 4	760
December	1st to 10th	Night	34% 20% 27% 15% 3	760
	11th to 20th	Night	29% 23% 28% 16% 4	760
	21st to end	Night	31% 19% 30% 17% 2	836

Page generated at 2024-03-05 07:24:04. Version ac07

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