SWAOG Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the SWAOG Dark Site clear sky chart from 2005-11-06 to 2024-04-10.

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	14% 9% 4 7% 6% 3 5% 5% 5% 7% 36%	83067
	Night	15% 9% 4 7% 5% 3 4 4 5% 6% 38%	85779
Transparency	Day	47% 7% 12% 21% 12% 2	82768
	Night	46% 6% 11% 21% 13% 2	85599
Seeing	Day	16% 10% 28% 26% 15% 5%	14913
	Night	18% 9% 30% 30% 12% 1	24960

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% 11% 7% 11% 7% 4 5% 5% 4 5% 26%	2527
	11th to 20th	Day	13% 12% 6% 14% 7% 4 6% 7% 8% 4 19%	2530
	21st to end	Day	19% 19% 9% 11% 7% 3 3 3 3 5% 17%	2916
February	1st to 10th	Day	17% 12% 5% 7% 7% 4 5% 5% 5% 6% 27%	1729
	11th to 20th	Day	14% 10% 4 8% 6% 4 4 4 5% 6% 35%	1848
	21st to end	Day	15% 12% 5% 10% 5% 3 3 4 5% 6% 31%	1570
March	1st to 10th	Day	16% 13% 4 6% 6% 3 4 4 4 6% 35%	3120
	11th to 20th	Day	11% 6% 3 6% 6% 3 5% 5% 6% 8% 41%	3293
	21st to end	Day	20% 11% 7% 11% 8% 4 5% 5% 5% 6% 20%	4165
April	1st to 10th	Day	17% 11% 5% 8% 5% 3 4 5% 5% 6% 31%	2269
	11th to 20th	Day	16% 10% 3 6% 5% 3 5% 5% 5% 6% 36%	2134
	21st to end	Day	21% 8% 3 7% 6% 3 4 5% 5% 7% 33%	2229
May	1st to 10th	Day	17% 8% 3 6% 6% 3 5% 5% 6% 9% 30%	2340
	11th to 20th	Day	19% 8% 3 6% 5% 4 5% 4 5% 8% 34%	2440
	21st to end	Day	14% 6% 3 6% 6% 4 5% 5% 7% 9% 36%	2725
June	1st to 10th	Day	10% 5% 3 4 6% 3 6% 4 6% 8% 46%	2520
	11th to 20th	Day	9% 6% 3 5% 6% 3 5% 5% 6% 7% 46%	2520
	21st to end	Day	11% 5% 3 6% 6% 4 6% 6% 7% 8% 38%	2520
July	1st to 10th	Day	8% 4 2 4 5% 3 5% 5% 7% 9% 47%	2520
	11th to 20th	Day	9% 5% 2 4 4 3 5% 5% 7% 9% 48%	2520
	21st to end	Day	7% 4 2 5% 6% 4 5% 6% 7% 9% 44%	2743
August	1st to 10th	Day	7% 5% 3 5% 5% 3 5% 7% 8% 9% 44%	2450
	11th to 20th	Day	9% 4 3 5% 6% 3 5% 6% 7% 9% 43%	2125
	21st to end	Day	9% 3 2 4 4 3 5% 5% 7% 10% 48%	2376
September	1st to 10th	Day	11% 6% 2 4 5% 4 4 4 5% 8% 46%	2160
	11th to 20th	Day	11% 4 2 4 4 3 3 4 4 7% 54%	2062
	21st to end	Day	9% 5% 3 6% 5% 3 5% 4 6% 8% 46%	1980
October	1st to 10th	Day	12% 7% 3 5% 5% 2 3 5% 5% 7% 46%	1907
	11th to 20th	Day	18% 8% 3 6% 5% 3 4 5% 5% 8% 36%	1800
	21st to end	Day	19% 9% 4 7% 4 2 3 4 4 6% 37%	1796
November	1st to 10th	Day	12% 7% 3 5% 4 3 3 5% 5% 7% 45%	1672
	11th to 20th	Day	16% 11% 5% 7% 5% 3 5% 5% 4 7% 33%	1710
	21st to end	Day	19% 10% 4 9% 4 3 4 3 4 5% 35%	1630
December	1st to 10th	Day	15% 12% 4 8% 6% 4 5% 4 5% 6% 32%	1520
	11th to 20th	Day	14% 13% 6% 7% 5% 4 6% 6% 5% 6% 29%	1918
	21st to end	Day	23% 20% 9% 12% 7% 3 4 3 2 4 14%	2783
January	1st to 10th	Night	14% 14% 8% 9% 6% 3 3 4 3 5% 29%	4428
	11th to 20th	Night	14% 17% 7% 10% 8% 3 4 6% 7% 4 20%	4440
	21st to end	Night	21% 17% 7% 9% 6% 3 3 3 4 4 22%	4747
February	1st to 10th	Night	16% 14% 5% 7% 7% 3 3 3 4 6% 32%	2450
	11th to 20th	Night	13% 10% 4 7% 6% 3 5% 4 4 6% 38%	2218
	21st to end	Night	18% 9% 4 9% 5% 3 3 4 4 5% 36%	1884
March	1st to 10th	Night	19% 11% 3 5% 3 3 3 3 4 5% 41%	3733
	11th to 20th	Night	10% 7% 3 6% 5% 3 4 4 5% 5% 48%	3696
	21st to end	Night	21% 10% 5% 10% 6% 3 4 4 5% 5% 27%	3466
April	1st to 10th	Night	16% 9% 4 6% 5% 2 4 4 5% 6% 39%	1900
	11th to 20th	Night	19% 9% 2 5% 5% 3 3 4 5% 5% 40%	1775
	21st to end	Night	19% 7% 3 6% 6% 3 5% 5% 5% 7% 35%	1729
May	1st to 10th	Night	17% 6% 3 6% 6% 4 4 5% 6% 9% 36%	1620
	11th to 20th	Night	16% 6% 2 5% 4 3 5% 5% 5% 8% 41%	1452
	21st to end	Night	12% 5% 2 6% 5% 4 6% 4 6% 9% 40%	1551
June	1st to 10th	Night	10% 4 2 5% 5% 3 4 5% 5% 8% 51%	1440
	11th to 20th	Night	9% 5% 2 4 5% 3 4 5% 7% 10% 46%	1440
	21st to end	Night	9% 5% 3 6% 6% 3 5% 6% 6% 7% 44%	1440
July	1st to 10th	Night	6% 4 2 5% 4 2 4 5% 6% 10% 50%	1440
	11th to 20th	Night	6% 4 2 3 5% 4 4 6% 6% 8% 53%	1440
	21st to end	Night	4 3 2 4 5% 3 5% 5% 7% 10% 52%	1575
August	1st to 10th	Night	5% 5% 2 5% 5% 3 5% 6% 6% 9% 51%	1483
	11th to 20th	Night	8% 3 1 4 4 2 4 5% 5% 9% 53%	1776
	21st to end	Night	9% 3 2 4 3 3 4 3 6% 8% 55%	1980
September	1st to 10th	Night	10% 5% 3 5% 3 2 3 3 5% 8% 54%	1800
	11th to 20th	Night	9% 4 2 3 4 3 3 4 5% 7% 55%	1878
	21st to end	Night	9% 5% 2 5% 5% 3 4 5% 6% 7% 49%	1980
October	1st to 10th	Night	10% 6% 2 5% 4 2 4 4 5% 6% 51%	2051
	11th to 20th	Night	18% 7% 2 4 5% 2 4 3 4 6% 45%	2160
	21st to end	Night	20% 9% 3 6% 4 3 3 4 4 6% 38%	2560
November	1st to 10th	Night	12% 7% 3 5% 4 2 3 4 5% 6% 50%	2406
	11th to 20th	Night	15% 8% 4 7% 6% 3 4 4 5% 6% 38%	2470
	21st to end	Night	18% 8% 3 7% 6% 3 4 4 4 5% 39%	2548
December	1st to 10th	Night	17% 9% 4 7% 6% 4 5% 4 4 6% 33%	2660
	11th to 20th	Night	16% 12% 5% 9% 5% 3 4 4 4 6% 33%	3279
	21st to end	Night	24% 16% 7% 9% 6% 4 5% 4 4 5% 16%	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	60% 5% 12% 17% 6%	2528
	11th to 20th	Day	62% 6% 10% 12% 9% 1	2530
	21st to end	Day	70% 5% 8% 12% 5%	2916
February	1st to 10th	Day	58% 6% 11% 15% 8% 2	1729
	11th to 20th	Day	51% 6% 10% 23% 9% 2	1858
	21st to end	Day	54% 6% 11% 20% 7% 2	1570
March	1st to 10th	Day	50% 6% 11% 20% 11% 1	3120
	11th to 20th	Day	40% 6% 13% 29% 11% 1	3293
	21st to end	Day	62% 8% 13% 13% 3	4165
April	1st to 10th	Day	52% 6% 10% 20% 10%	2269
	11th to 20th	Day	46% 7% 11% 21% 11% 3	2134
	21st to end	Day	51% 6% 12% 17% 12% 2	2229
May	1st to 10th	Day	50% 8% 12% 21% 8%	2340
	11th to 20th	Day	48% 7% 10% 23% 11%	2444
	21st to end	Day	43% 7% 13% 19% 15% 2	2725
June	1st to 10th	Day	35% 6% 12% 27% 17% 2	2520
	11th to 20th	Day	35% 8% 11% 24% 20% 3	2520
	21st to end	Day	41% 9% 13% 22% 14% 2	2520
July	1st to 10th	Day	31% 8% 15% 28% 15% 3	2451
	11th to 20th	Day	31% 8% 16% 27% 16% 2	2445
	21st to end	Day	33% 9% 17% 23% 15% 2	2591
August	1st to 10th	Day	33% 8% 17% 29% 12% 2	2435
	11th to 20th	Day	35% 8% 15% 25% 16% 1	2125
	21st to end	Day	30% 8% 13% 27% 19% 3	2376
September	1st to 10th	Day	35% 7% 14% 23% 16% 5%	2160
	11th to 20th	Day	30% 6% 12% 28% 20% 3	2062
	21st to end	Day	35% 6% 9% 27% 20% 2	1980
October	1st to 10th	Day	38% 6% 11% 26% 18% 2	1907
	11th to 20th	Day	47% 5% 12% 22% 13% 1	1800
	21st to end	Day	48% 5% 12% 22% 10% 3	1796
November	1st to 10th	Day	38% 6% 13% 24% 16% 2	1672
	11th to 20th	Day	51% 4 11% 22% 10% 1	1710
	21st to end	Day	53% 5% 11% 23% 9%	1630
December	1st to 10th	Day	54% 5% 13% 20% 8% 1	1520
	11th to 20th	Day	54% 6% 14% 18% 8% 1	1915
	21st to end	Day	76% 4 8% 8% 3	2783
January	1st to 10th	Night	57% 6% 10% 17% 9%	4428
	11th to 20th	Night	65% 5% 11% 11% 8%	4440
	21st to end	Night	66% 5% 10% 14% 6%	4747
February	1st to 10th	Night	55% 4 10% 18% 11% 2	2450
	11th to 20th	Night	47% 5% 10% 23% 13% 1	2219
	21st to end	Night	51% 5% 11% 19% 12% 3	1884
March	1st to 10th	Night	45% 6% 13% 22% 10% 3	3733
	11th to 20th	Night	37% 6% 10% 28% 18% 2	3696
	21st to end	Night	58% 7% 11% 18% 6% 1	3466
April	1st to 10th	Night	45% 5% 11% 23% 13% 2	1900
	11th to 20th	Night	46% 5% 8% 22% 15% 4	1775
	21st to end	Night	47% 7% 11% 22% 11% 3	1729
May	1st to 10th	Night	45% 6% 13% 24% 11%	1620
	11th to 20th	Night	40% 8% 13% 24% 14% 1	1453
	21st to end	Night	39% 8% 14% 19% 17% 2	1552
June	1st to 10th	Night	31% 7% 13% 25% 20% 4	1440
	11th to 20th	Night	32% 8% 12% 24% 22% 3	1440
	21st to end	Night	37% 9% 14% 21% 16% 3	1440
July	1st to 10th	Night	30% 9% 14% 26% 17% 5%	1400
	11th to 20th	Night	27% 7% 14% 27% 22% 3	1392
	21st to end	Night	25% 7% 17% 27% 19% 5%	1487
August	1st to 10th	Night	27% 9% 18% 27% 16% 2	1474
	11th to 20th	Night	26% 7% 16% 28% 20% 2	1776
	21st to end	Night	26% 7% 12% 29% 20% 5%	1980
September	1st to 10th	Night	30% 6% 13% 25% 22% 4	1800
	11th to 20th	Night	28% 6% 8% 32% 24% 2	1878
	21st to end	Night	33% 7% 11% 28% 17% 3	1980
October	1st to 10th	Night	34% 6% 10% 27% 21% 3	2051
	11th to 20th	Night	40% 4 10% 24% 20% 2	2160
	21st to end	Night	48% 4 9% 24% 13% 1	2560
November	1st to 10th	Night	36% 5% 10% 26% 19% 4	2406
	11th to 20th	Night	46% 5% 13% 22% 13% 2	2470
	21st to end	Night	48% 4 11% 24% 12% 1	2548
December	1st to 10th	Night	53% 5% 11% 22% 8% 1	2660
	11th to 20th	Night	53% 5% 12% 18% 10% 1	3281
	21st to end	Night	70% 6% 8% 11% 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	21% 12% 33% 27% 7%	270
	11th to 20th	Day	20% 15% 39% 21% 5% 1	270
	21st to end	Day	23% 16% 30% 24% 6% 1	297
February	1st to 10th	Day	23% 16% 37% 19% 5%	270
	11th to 20th	Day	23% 15% 36% 20% 5%	270
	21st to end	Day	24% 15% 40% 18% 3	225
March	1st to 10th	Day	25% 19% 36% 18% 2	270
	11th to 20th	Day	21% 20% 35% 17% 7% 1	275
	21st to end	Day	26% 15% 32% 19% 6% 2	506
April	1st to 10th	Day	25% 12% 33% 23% 7%	457
	11th to 20th	Day	19% 20% 27% 19% 12% 3	412
	21st to end	Day	24% 13% 30% 24% 8% 2	420
May	1st to 10th	Day	23% 11% 29% 26% 9% 2	423
	11th to 20th	Day	21% 9% 28% 25% 12% 4	586
	21st to end	Day	15% 7% 29% 22% 18% 8%	679
June	1st to 10th	Day	9% 7% 28% 29% 20% 6%	630
	11th to 20th	Day	11% 7% 28% 30% 19% 5%	630
	21st to end	Day	12% 7% 30% 30% 15% 5%	630
July	1st to 10th	Day	10% 3 16% 30% 24% 18%	621
	11th to 20th	Day	11% 5% 22% 31% 21% 10%	618
	21st to end	Day	7% 2 25% 35% 23% 8%	684
August	1st to 10th	Day	7% 2 16% 37% 25% 13%	595
	11th to 20th	Day	11% 5% 15% 26% 28% 15%	439
	21st to end	Day	7% 3 19% 31% 27% 13%	495
September	1st to 10th	Day	13% 7% 20% 28% 25% 7%	450
	11th to 20th	Day	12% 2 17% 30% 29% 11%	450
	21st to end	Day	11% 11% 24% 26% 21% 7%	450
October	1st to 10th	Day	14% 7% 27% 29% 18% 5%	377
	11th to 20th	Day	18% 23% 31% 21% 7%	270
	21st to end	Day	30% 14% 33% 16% 6% 1	297
November	1st to 10th	Day	13% 16% 36% 22% 8% 4	270
	11th to 20th	Day	24% 18% 31% 19% 6% 2	270
	21st to end	Day	26% 11% 33% 23% 7%	270
December	1st to 10th	Day	17% 17% 38% 19% 7% 1	270
	11th to 20th	Day	17% 15% 38% 23% 7%	270
	21st to end	Day	27% 20% 35% 15% 3	297
January	1st to 10th	Night	20% 13% 38% 23% 5%	950
	11th to 20th	Night	22% 13% 38% 21% 5% 1	950
	21st to end	Night	21% 12% 35% 29% 3	1045
February	1st to 10th	Night	24% 11% 37% 24% 4	930
	11th to 20th	Night	20% 12% 36% 27% 4	743
	21st to end	Night	22% 13% 38% 22% 4	628
March	1st to 10th	Night	21% 13% 35% 24% 6%	760
	11th to 20th	Night	17% 11% 37% 28% 7% 1	713
	21st to end	Night	25% 11% 33% 24% 6%	627
April	1st to 10th	Night	18% 14% 36% 25% 6%	570
	11th to 20th	Night	22% 11% 32% 27% 8%	533
	21st to end	Night	21% 9% 35% 27% 8%	540
May	1st to 10th	Night	19% 7% 34% 31% 7% 1	540
	11th to 20th	Night	18% 7% 28% 34% 12% 1	531
	21st to end	Night	13% 5% 26% 38% 19%	584
June	1st to 10th	Night	11% 4 24% 44% 16%	540
	11th to 20th	Night	11% 3 26% 46% 13% 1	540
	21st to end	Night	11% 4 25% 41% 18% 1	540
July	1st to 10th	Night	8% 2 16% 42% 30% 3	525
	11th to 20th	Night	8% 1 21% 39% 28% 3	522
	21st to end	Night	5% 2 17% 47% 28% 2	591
August	1st to 10th	Night	5% 3 12% 46% 30% 3	537
	11th to 20th	Night	6% 3 12% 41% 35% 3	534
	21st to end	Night	9% 2 17% 37% 33% 3	594
September	1st to 10th	Night	12% 2 19% 37% 27% 3	540
	11th to 20th	Night	9% 4 18% 36% 30% 4	618
	21st to end	Night	10% 7% 24% 36% 21% 2	720
October	1st to 10th	Night	13% 6% 26% 34% 19% 2	720
	11th to 20th	Night	21% 12% 28% 29% 10%	720
	21st to end	Night	25% 11% 32% 24% 8%	792
November	1st to 10th	Night	16% 9% 39% 25% 11% 1	740
	11th to 20th	Night	19% 15% 32% 27% 7%	760
	21st to end	Night	21% 11% 35% 26% 7%	838
December	1st to 10th	Night	22% 12% 36% 25% 5%	950
	11th to 20th	Night	23% 11% 33% 28% 5%	950
	21st to end	Night	28% 14% 33% 22% 3	1045

Page generated at 2024-04-07 08:28:10. Version ac07

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