ARGUS Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ARGUS Dark Sky Site clear sky chart from 2005-11-06 to 2024-04-03.

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	15% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	78827
	Night	15% 10% 4 7% 5% 3 4 4 5% 6% 35%	81346
Transparency	Day	49% 6% 11% 20% 12% 2	78533
	Night	50% 5% 11% 20% 12% 2	81161
Seeing	Day	17% 10% 28% 23% 16% 6%	14739
	Night	19% 9% 29% 29% 13% 1	24758

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% 14% 10% 12% 7% 3 4 4 5% 5% 20%	2527
	11th to 20th	Day	16% 14% 10% 13% 6% 5% 6% 9% 5% 4 13%	2530
	21st to end	Day	29% 19% 9% 11% 4 2 3 2 2 3 15%	2874
February	1st to 10th	Day	18% 14% 5% 9% 9% 4 5% 3 5% 5% 23%	1855
	11th to 20th	Day	17% 11% 4 10% 6% 4 5% 3 5% 5% 29%	1848
	21st to end	Day	17% 14% 6% 10% 5% 2 4 4 5% 5% 28%	1570
March	1st to 10th	Day	18% 13% 6% 8% 7% 3 4 4 4 6% 28%	1900
	11th to 20th	Day	16% 10% 4 7% 6% 3 4 5% 5% 9% 31%	2026
	21st to end	Day	21% 12% 5% 8% 7% 3 4 4 4 6% 27%	2383
April	1st to 10th	Day	17% 11% 4 7% 6% 3 4 5% 5% 6% 32%	2184
	11th to 20th	Day	18% 10% 3 6% 5% 3 5% 4 4 7% 35%	2138
	21st to end	Day	20% 8% 3 6% 6% 4 4 4 5% 8% 32%	2340
May	1st to 10th	Day	17% 9% 3 8% 6% 3 5% 4 5% 8% 32%	2340
	11th to 20th	Day	19% 8% 3 6% 6% 3 5% 4 5% 7% 34%	2301
	21st to end	Day	11% 5% 3 6% 6% 4 6% 7% 6% 8% 37%	2614
June	1st to 10th	Day	11% 5% 3 7% 6% 3 5% 6% 6% 7% 42%	2520
	11th to 20th	Day	9% 5% 2 5% 6% 3 5% 6% 7% 9% 44%	2520
	21st to end	Day	10% 5% 2 5% 6% 4 7% 6% 7% 9% 38%	2520
July	1st to 10th	Day	9% 5% 3 5% 5% 3 5% 6% 6% 10% 43%	2520
	11th to 20th	Day	8% 4 2 5% 4 4 5% 6% 6% 10% 46%	2520
	21st to end	Day	8% 5% 2 5% 6% 3 6% 7% 8% 8% 42%	2687
August	1st to 10th	Day	6% 4 2 5% 6% 4 5% 6% 8% 10% 44%	2327
	11th to 20th	Day	9% 4 2 5% 6% 4 5% 6% 7% 8% 43%	2254
	21st to end	Day	9% 3 2 5% 5% 4 5% 5% 7% 8% 47%	2376
September	1st to 10th	Day	11% 6% 2 5% 5% 3 5% 5% 6% 7% 45%	2124
	11th to 20th	Day	11% 4 2 4 5% 3 5% 6% 6% 8% 46%	1980
	21st to end	Day	12% 6% 2 5% 5% 2 4 4 6% 8% 46%	1980
October	1st to 10th	Day	13% 6% 2 6% 4 3 4 5% 6% 6% 44%	1980
	11th to 20th	Day	16% 6% 3 6% 5% 3 4 5% 6% 6% 40%	1818
	21st to end	Day	20% 10% 3 7% 6% 3 4 4 5% 7% 30%	1782
November	1st to 10th	Day	11% 6% 2 4 4 3 4 4 5% 7% 49%	1672
	11th to 20th	Day	18% 11% 4 7% 6% 3 4 4 5% 6% 33%	1710
	21st to end	Day	19% 11% 4 8% 5% 3 4 4 4 5% 32%	1710
December	1st to 10th	Day	18% 16% 6% 9% 8% 4 4 4 4 5% 24%	1696
	11th to 20th	Day	15% 16% 7% 10% 7% 3 5% 3 3 6% 24%	1918
	21st to end	Day	25% 21% 9% 10% 6% 2 4 3 3 3 12%	2783
January	1st to 10th	Night	17% 16% 9% 11% 6% 3 4 3 4 4 22%	4428
	11th to 20th	Night	15% 20% 8% 13% 7% 4 5% 5% 4 3 17%	4440
	21st to end	Night	26% 17% 9% 7% 5% 3 3 3 3 5% 20%	4788
February	1st to 10th	Night	19% 14% 6% 9% 6% 3 3 4 4 5% 27%	2323
	11th to 20th	Night	13% 13% 4 8% 8% 4 5% 4 5% 6% 32%	2218
	21st to end	Night	19% 12% 5% 9% 5% 2 4 3 4 6% 30%	1884
March	1st to 10th	Night	16% 11% 4 8% 7% 4 5% 4 4 5% 34%	2280
	11th to 20th	Night	14% 8% 4 6% 4 3 5% 5% 6% 7% 38%	2152
	21st to end	Night	20% 10% 4 6% 5% 3 5% 4 4 6% 33%	2232
April	1st to 10th	Night	14% 8% 3 6% 6% 3 5% 5% 5% 8% 37%	1820
	11th to 20th	Night	18% 8% 2 5% 5% 3 5% 4 5% 7% 38%	1771
	21st to end	Night	19% 8% 3 5% 4 3 4 5% 5% 7% 35%	1620
May	1st to 10th	Night	19% 7% 4 7% 7% 3 4 5% 6% 8% 31%	1620
	11th to 20th	Night	19% 7% 3 5% 6% 3 4 4 5% 7% 39%	1592
	21st to end	Night	8% 4 2 5% 6% 3 6% 6% 6% 9% 43%	1657
June	1st to 10th	Night	7% 5% 3 5% 5% 3 5% 6% 6% 7% 49%	1440
	11th to 20th	Night	8% 5% 3 5% 5% 2 4 5% 5% 9% 48%	1440
	21st to end	Night	9% 4 2 6% 6% 3 5% 5% 6% 10% 45%	1440
July	1st to 10th	Night	7% 6% 1 5% 5% 3 5% 7% 5% 9% 46%	1440
	11th to 20th	Night	6% 4 3 4 4 2 4 5% 5% 10% 53%	1440
	21st to end	Night	7% 5% 3 5% 4 3 5% 5% 6% 9% 49%	1629
August	1st to 10th	Night	6% 4 2 4 4 3 5% 7% 6% 10% 49%	1611
	11th to 20th	Night	7% 5% 3 4 4 4 4 5% 6% 8% 50%	1642
	21st to end	Night	8% 4 3 5% 5% 3 4 5% 5% 8% 52%	1980
September	1st to 10th	Night	11% 4 3 5% 6% 2 4 4 5% 8% 48%	1817
	11th to 20th	Night	9% 3 2 4 3 2 4 4 5% 7% 57%	1980
	21st to end	Night	10% 5% 2 4 5% 3 4 5% 5% 7% 50%	1980
October	1st to 10th	Night	10% 5% 2 5% 4 3 4 3 5% 8% 49%	1980
	11th to 20th	Night	18% 7% 2 6% 5% 2 3 4 4 5% 43%	2140
	21st to end	Night	21% 8% 2 5% 5% 3 4 5% 4 6% 37%	2574
November	1st to 10th	Night	12% 8% 3 5% 4 3 3 5% 5% 6% 46%	2406
	11th to 20th	Night	15% 9% 3 7% 6% 2 5% 4 5% 7% 37%	2470
	21st to end	Night	19% 10% 4 7% 7% 4 4 4 3 7% 30%	2470
December	1st to 10th	Night	18% 13% 5% 8% 6% 2 4 4 5% 6% 28%	2479
	11th to 20th	Night	18% 14% 7% 10% 5% 3 4 5% 4 5% 25%	3279
	21st to end	Night	25% 19% 7% 9% 6% 3 4 4 4 4 15%	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	67% 4 8% 16% 5%	2528
	11th to 20th	Day	73% 4 10% 8% 5%	2530
	21st to end	Day	77% 3 6% 11% 3	2874
February	1st to 10th	Day	64% 4 9% 13% 8% 1	1855
	11th to 20th	Day	58% 4 10% 21% 7% 1	1858
	21st to end	Day	58% 5% 11% 18% 7% 2	1570
March	1st to 10th	Day	58% 4 9% 20% 8%	1900
	11th to 20th	Day	51% 6% 12% 20% 10% 1	2026
	21st to end	Day	60% 4 13% 18% 4 1	2383
April	1st to 10th	Day	51% 6% 11% 19% 12%	2184
	11th to 20th	Day	49% 6% 9% 21% 11% 3	2138
	21st to end	Day	51% 5% 11% 19% 10% 4	2340
May	1st to 10th	Day	52% 6% 12% 19% 10%	2340
	11th to 20th	Day	50% 7% 10% 19% 13% 1	2305
	21st to end	Day	41% 9% 12% 22% 14% 2	2614
June	1st to 10th	Day	39% 8% 11% 24% 16% 3	2520
	11th to 20th	Day	35% 7% 11% 24% 20% 3	2520
	21st to end	Day	39% 9% 15% 18% 15% 3	2520
July	1st to 10th	Day	36% 8% 13% 23% 16% 4	2451
	11th to 20th	Day	31% 8% 15% 27% 18% 2	2445
	21st to end	Day	35% 8% 15% 23% 15% 3	2538
August	1st to 10th	Day	31% 9% 16% 28% 14% 2	2314
	11th to 20th	Day	35% 9% 13% 24% 16% 2	2254
	21st to end	Day	33% 8% 13% 22% 19% 6%	2376
September	1st to 10th	Day	38% 7% 12% 22% 17% 4	2124
	11th to 20th	Day	33% 8% 11% 26% 20% 2	1980
	21st to end	Day	36% 6% 11% 24% 21% 2	1980
October	1st to 10th	Day	38% 7% 13% 23% 18% 2	1980
	11th to 20th	Day	42% 6% 10% 23% 18% 1	1818
	21st to end	Day	53% 5% 11% 20% 11% 1	1782
November	1st to 10th	Day	35% 6% 14% 26% 17% 2	1672
	11th to 20th	Day	53% 4 11% 19% 11% 2	1710
	21st to end	Day	55% 4 11% 21% 8% 1	1710
December	1st to 10th	Day	64% 4 11% 15% 6%	1696
	11th to 20th	Day	63% 4 9% 21% 3	1915
	21st to end	Day	79% 4 7% 7% 4	2783
January	1st to 10th	Night	65% 4 10% 14% 7%	4428
	11th to 20th	Night	73% 3 8% 12% 4	4440
	21st to end	Night	70% 3 9% 13% 5%	4788
February	1st to 10th	Night	60% 4 12% 15% 8% 1	2323
	11th to 20th	Night	55% 4 11% 19% 11% 1	2219
	21st to end	Night	57% 5% 10% 17% 9% 2	1884
March	1st to 10th	Night	53% 5% 10% 21% 11% 1	2280
	11th to 20th	Night	44% 6% 11% 24% 12% 3	2152
	21st to end	Night	53% 5% 11% 22% 8% 1	2232
April	1st to 10th	Night	45% 7% 11% 22% 13% 1	1820
	11th to 20th	Night	45% 5% 10% 21% 15% 4	1771
	21st to end	Night	46% 7% 10% 22% 12% 4	1620
May	1st to 10th	Night	50% 8% 11% 19% 10% 2	1620
	11th to 20th	Night	45% 7% 11% 22% 14% 2	1593
	21st to end	Night	36% 10% 14% 22% 14% 4	1658
June	1st to 10th	Night	33% 7% 11% 26% 20% 3	1440
	11th to 20th	Night	33% 7% 11% 24% 21% 4	1440
	21st to end	Night	34% 9% 16% 22% 15% 4	1440
July	1st to 10th	Night	33% 9% 13% 23% 17% 6%	1400
	11th to 20th	Night	26% 7% 15% 28% 21% 3	1392
	21st to end	Night	30% 8% 15% 24% 17% 6%	1538
August	1st to 10th	Night	29% 8% 15% 27% 20% 2	1600
	11th to 20th	Night	30% 9% 14% 24% 21% 2	1642
	21st to end	Night	30% 7% 12% 24% 22% 6%	1980
September	1st to 10th	Night	35% 7% 13% 22% 17% 6%	1817
	11th to 20th	Night	27% 6% 11% 28% 26% 2	1980
	21st to end	Night	33% 7% 10% 24% 23% 3	1980
October	1st to 10th	Night	35% 5% 10% 27% 21% 2	1980
	11th to 20th	Night	44% 4 8% 25% 18% 2	2140
	21st to end	Night	48% 5% 9% 22% 14% 2	2574
November	1st to 10th	Night	38% 5% 11% 24% 20% 2	2406
	11th to 20th	Night	47% 5% 11% 21% 14% 2	2470
	21st to end	Night	56% 5% 10% 21% 8%	2470
December	1st to 10th	Night	57% 6% 11% 17% 8% 1	2479
	11th to 20th	Night	61% 4 11% 17% 6% 1	3281
	21st to end	Night	73% 4 10% 8% 4	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	23% 18% 36% 16% 6% 1	270
	11th to 20th	Day	26% 17% 38% 13% 5%	270
	21st to end	Day	31% 15% 31% 18% 5%	297
February	1st to 10th	Day	25% 19% 36% 15% 5%	270
	11th to 20th	Day	26% 19% 34% 16% 4 1	270
	21st to end	Day	27% 20% 38% 12% 3	225
March	1st to 10th	Day	22% 21% 37% 16% 4	270
	11th to 20th	Day	24% 13% 35% 20% 6% 1	396
	21st to end	Day	30% 15% 29% 18% 6% 1	506
April	1st to 10th	Day	22% 12% 36% 19% 10% 2	428
	11th to 20th	Day	21% 18% 28% 20% 10% 3	412
	21st to end	Day	25% 16% 30% 18% 10% 2	420
May	1st to 10th	Day	23% 12% 33% 20% 8% 3	423
	11th to 20th	Day	22% 7% 30% 23% 13% 6%	447
	21st to end	Day	13% 5% 29% 25% 18% 10%	568
June	1st to 10th	Day	11% 7% 29% 26% 21% 5%	630
	11th to 20th	Day	12% 6% 27% 27% 21% 7%	630
	21st to end	Day	13% 6% 28% 27% 18% 7%	630
July	1st to 10th	Day	12% 3 17% 27% 25% 16%	621
	11th to 20th	Day	7% 2 23% 30% 24% 13%	618
	21st to end	Day	10% 3 18% 30% 28% 11%	628
August	1st to 10th	Day	6% 2 13% 30% 33% 15%	446
	11th to 20th	Day	10% 3 19% 29% 24% 15%	439
	21st to end	Day	8% 4 16% 27% 32% 13%	495
September	1st to 10th	Day	14% 5% 17% 27% 27% 11%	450
	11th to 20th	Day	11% 2 17% 24% 30% 16%	450
	21st to end	Day	15% 9% 20% 25% 23% 8%	450
October	1st to 10th	Day	13% 6% 26% 30% 18% 8%	450
	11th to 20th	Day	17% 19% 30% 24% 10% 1	386
	21st to end	Day	27% 15% 37% 19% 1 1	297
November	1st to 10th	Day	11% 15% 34% 25% 13% 3	270
	11th to 20th	Day	25% 17% 31% 17% 9% 1	270
	21st to end	Day	28% 14% 37% 18% 4	270
December	1st to 10th	Day	23% 21% 35% 13% 8%	270
	11th to 20th	Day	21% 16% 40% 17% 6%	270
	21st to end	Day	30% 19% 33% 16% 2	297
January	1st to 10th	Night	23% 16% 34% 21% 5%	950
	11th to 20th	Night	26% 14% 36% 20% 4	950
	21st to end	Night	23% 15% 35% 23% 4	988
February	1st to 10th	Night	28% 10% 35% 25% 3	760
	11th to 20th	Night	20% 13% 38% 26% 3	743
	21st to end	Night	27% 15% 37% 18% 3	628
March	1st to 10th	Night	20% 15% 34% 26% 6%	760
	11th to 20th	Night	19% 12% 37% 26% 6% 1	760
	21st to end	Night	25% 11% 33% 26% 5%	769
April	1st to 10th	Night	18% 11% 36% 28% 7% 1	546
	11th to 20th	Night	21% 11% 33% 26% 8% 1	533
	21st to end	Night	23% 8% 36% 24% 9% 1	540
May	1st to 10th	Night	22% 7% 32% 30% 9%	540
	11th to 20th	Night	20% 5% 25% 32% 16% 2	531
	21st to end	Night	10% 2 23% 43% 19% 1	584
June	1st to 10th	Night	8% 3 25% 44% 19% 1	540
	11th to 20th	Night	10% 4 24% 40% 21% 1	540
	21st to end	Night	10% 3 22% 39% 26% 1	540
July	1st to 10th	Night	10% 2 16% 36% 31% 5%	525
	11th to 20th	Night	7% 2 16% 40% 31% 4	522
	21st to end	Night	8% 2 14% 42% 32% 2	591
August	1st to 10th	Night	7% 2 13% 42% 32% 4	537
	11th to 20th	Night	8% 1 16% 38% 31% 4	534
	21st to end	Night	8% 2 18% 33% 35% 5%	594
September	1st to 10th	Night	13% 3 14% 36% 29% 4	557
	11th to 20th	Night	11% 3 15% 35% 32% 4	720
	21st to end	Night	12% 5% 23% 35% 23% 2	720
October	1st to 10th	Night	13% 6% 24% 34% 20% 4	720
	11th to 20th	Night	21% 12% 28% 28% 10% 1	720
	21st to end	Night	25% 11% 32% 25% 7%	792
November	1st to 10th	Night	16% 10% 36% 26% 11% 1	740
	11th to 20th	Night	20% 15% 33% 25% 7%	760
	21st to end	Night	25% 13% 31% 23% 8%	760
December	1st to 10th	Night	25% 13% 35% 21% 5%	769
	11th to 20th	Night	25% 13% 31% 24% 6%	950
	21st to end	Night	31% 13% 33% 20% 2	1045

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