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 2024-11-05.

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	13% 9% 3 7% 5% 4 4 4 6% 7% 37%	101337
	Night	14% 9% 4 7% 5% 4 4 3 5% 6% 39%	97786
Transparency	Day	43% 7% 12% 23% 14% 2	104890
	Night	44% 6% 11% 22% 15% 2	102361
Seeing	Day	16% 10% 28% 26% 15% 5%	15806
	Night	17% 9% 30% 31% 12% 1	25638

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% 12% 7% 12% 6% 5% 5% 4 4 5% 26%	2527
	11th to 20th	Day	13% 13% 5% 15% 6% 5% 6% 6% 8% 4 19%	2530
	21st to end	Day	18% 20% 9% 12% 6% 4 3 3 3 5% 17%	2916
February	1st to 10th	Day	17% 13% 5% 8% 7% 5% 5% 4 5% 5% 27%	1729
	11th to 20th	Day	14% 11% 4 8% 5% 6% 3 3 5% 6% 35%	1848
	21st to end	Day	15% 13% 5% 10% 5% 4 3 3 5% 5% 31%	1570
March	1st to 10th	Day	15% 14% 3 7% 5% 3 4 3 4 6% 35%	3120
	11th to 20th	Day	11% 6% 3 7% 6% 5% 5% 4 6% 8% 40%	3293
	21st to end	Day	19% 12% 6% 12% 6% 5% 5% 4 5% 6% 20%	4165
April	1st to 10th	Day	16% 14% 3 8% 5% 4 4 4 5% 6% 31%	2280
	11th to 20th	Day	16% 10% 3 7% 4 4 4 4 5% 6% 36%	2254
	21st to end	Day	20% 8% 2 8% 5% 5% 4 3 5% 6% 33%	2353
May	1st to 10th	Day	17% 7% 3 6% 5% 4 5% 4 6% 8% 35%	4082
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 5% 7% 42%	4332
	21st to end	Day	11% 6% 2 6% 4 4 4 4 7% 10% 42%	4824
June	1st to 10th	Day	10% 5% 2 5% 5% 5% 5% 4 6% 8% 46%	2660
	11th to 20th	Day	8% 6% 2 5% 5% 5% 5% 4 7% 7% 46%	2660
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 7% 38%	2660
July	1st to 10th	Day	10% 8% 2 7% 6% 6% 6% 5% 8% 9% 33%	4440
	11th to 20th	Day	7% 4 2 5% 4 3 4 4 7% 11% 49%	4440
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 40%	4813
August	1st to 10th	Day	8% 5% 2 6% 5% 4 5% 5% 7% 9% 44%	4310
	11th to 20th	Day	10% 5% 2 6% 4 4 5% 5% 8% 10% 42%	3729
	21st to end	Day	7% 5% 2 6% 4 4 4 3 7% 9% 48%	4180
September	1st to 10th	Day	11% 6% 2 5% 4 5% 4 3 5% 8% 48%	2280
	11th to 20th	Day	10% 4 1 4 4 3 4 4 5% 7% 53%	2176
	21st to end	Day	10% 6% 3 6% 4 4 4 3 6% 8% 45%	2090
October	1st to 10th	Day	12% 7% 3 6% 4 3 3 4 5% 7% 47%	2012
	11th to 20th	Day	17% 8% 3 7% 4 4 4 4 5% 7% 38%	1900
	21st to end	Day	18% 9% 3 8% 4 3 4 4 4 6% 37%	1895
November	1st to 10th	Day	12% 8% 2 6% 4 3 4 4 4 7% 45%	1708
	11th to 20th	Day	16% 12% 4 8% 5% 4 5% 4 4 7% 33%	1710
	21st to end	Day	18% 11% 4 9% 4 4 3 3 4 5% 35%	1630
December	1st to 10th	Day	15% 13% 4 8% 6% 5% 4 3 5% 6% 32%	1520
	11th to 20th	Day	13% 14% 5% 8% 5% 5% 5% 4 5% 6% 29%	1918
	21st to end	Day	23% 21% 9% 13% 5% 4 3 3 3 4 14%	2783
January	1st to 10th	Night	14% 15% 8% 11% 5% 4 3 3 4 5% 29%	4428
	11th to 20th	Night	14% 18% 7% 12% 6% 4 4 5% 7% 4 20%	4440
	21st to end	Night	21% 18% 7% 10% 6% 4 3 3 4 4 21%	4747
February	1st to 10th	Night	15% 15% 4 8% 6% 4 3 3 4 5% 32%	2450
	11th to 20th	Night	13% 10% 4 8% 5% 4 4 4 3 6% 38%	2218
	21st to end	Night	18% 10% 4 9% 4 3 3 4 4 5% 36%	1884
March	1st to 10th	Night	18% 11% 2 5% 3 3 4 2 4 5% 41%	3733
	11th to 20th	Night	10% 7% 3 6% 4 4 4 3 5% 5% 47%	3696
	21st to end	Night	21% 10% 5% 11% 5% 4 4 3 5% 5% 27%	3466
April	1st to 10th	Night	16% 10% 3 6% 5% 4 3 3 5% 6% 39%	1900
	11th to 20th	Night	19% 10% 2 5% 5% 4 3 3 4 5% 41%	1875
	21st to end	Night	18% 7% 3 7% 5% 4 5% 4 6% 7% 35%	1825
May	1st to 10th	Night	16% 7% 3 7% 6% 4 4 5% 6% 8% 34%	2813
	11th to 20th	Night	13% 6% 2 6% 4 4 4 5% 7% 8% 41%	2568
	21st to end	Night	10% 7% 2 6% 4 4 5% 4 6% 8% 44%	2743
June	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 5% 8% 50%	1520
	11th to 20th	Night	8% 5% 2 5% 4 5% 4 4 7% 9% 47%	1520
	21st to end	Night	8% 7% 2 7% 6% 4 5% 4 6% 7% 43%	1520
July	1st to 10th	Night	12% 6% 3 8% 5% 5% 5% 4 6% 8% 39%	2530
	11th to 20th	Night	6% 4 2 5% 4 4 4 4 6% 8% 54%	2530
	21st to end	Night	4 4 2 7% 6% 5% 6% 5% 7% 9% 45%	2750
August	1st to 10th	Night	5% 4 2 5% 4 5% 5% 5% 7% 9% 49%	2622
	11th to 20th	Night	8% 4 2 5% 4 4 4 4 6% 8% 51%	3100
	21st to end	Night	8% 4 2 5% 3 3 3 3 6% 9% 53%	3476
September	1st to 10th	Night	9% 5% 2 5% 3 3 2 3 4 7% 56%	1900
	11th to 20th	Night	8% 4 2 4 3 3 3 4 5% 8% 56%	1983
	21st to end	Night	10% 5% 2 6% 4 5% 4 4 6% 7% 48%	2090
October	1st to 10th	Night	10% 5% 2 5% 4 3 3 3 5% 6% 52%	2165
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 6% 46%	2280
	21st to end	Night	20% 9% 3 6% 3 4 3 3 4 7% 38%	2703
November	1st to 10th	Night	13% 7% 3 6% 3 3 3 3 5% 6% 49%	2470
	11th to 20th	Night	15% 9% 3 8% 5% 4 4 3 6% 6% 37%	2470
	21st to end	Night	17% 9% 3 7% 5% 4 4 3 5% 5% 39%	2548
December	1st to 10th	Night	17% 10% 4 8% 5% 5% 5% 3 4 6% 33%	2660
	11th to 20th	Night	16% 13% 4 9% 5% 4 3 3 4 6% 33%	3279
	21st to end	Night	24% 17% 6% 10% 5% 5% 5% 3 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% 21% 10%	2280
	11th to 20th	Day	46% 7% 11% 21% 12% 3	2254
	21st to end	Day	51% 6% 12% 17% 12% 2	2353
May	1st to 10th	Day	44% 9% 11% 24% 11%	4120
	11th to 20th	Day	40% 7% 10% 28% 14%	4336
	21st to end	Day	35% 8% 13% 22% 18% 5%	4837
June	1st to 10th	Day	35% 6% 12% 27% 18% 2	2660
	11th to 20th	Day	34% 7% 10% 25% 20% 3	2660
	21st to end	Day	40% 9% 13% 22% 14% 2	2660
July	1st to 10th	Day	43% 8% 13% 23% 10% 2	4371
	11th to 20th	Day	27% 7% 18% 30% 16% 1	4365
	21st to end	Day	38% 9% 17% 16% 18% 3	4662
August	1st to 10th	Day	33% 8% 17% 29% 12% 1	4295
	11th to 20th	Day	33% 8% 12% 31% 15%	3765
	21st to end	Day	31% 7% 12% 30% 17% 2	4179
September	1st to 10th	Day	34% 6% 14% 24% 18% 5%	2280
	11th to 20th	Day	29% 6% 11% 28% 22% 3	2176
	21st to end	Day	36% 6% 9% 27% 20% 2	2090
October	1st to 10th	Day	25% 4 7% 29% 33% 2	3337
	11th to 20th	Day	30% 4 12% 26% 27% 1	3140
	21st to end	Day	41% 4 10% 27% 15% 3	3095
November	1st to 10th	Day	38% 6% 13% 24% 16% 2	1708
	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	45% 5% 8% 22% 16% 4	1875
	21st to end	Night	46% 7% 10% 22% 11% 3	1825
May	1st to 10th	Night	45% 8% 13% 21% 12% 2	2840
	11th to 20th	Night	36% 11% 15% 27% 11% 1	2569
	21st to end	Night	34% 9% 12% 20% 21% 4	2751
June	1st to 10th	Night	31% 7% 13% 25% 19% 4	1520
	11th to 20th	Night	31% 8% 12% 25% 22% 3	1520
	21st to end	Night	38% 9% 14% 20% 16% 3	1520
July	1st to 10th	Night	42% 7% 15% 19% 13% 4	2490
	11th to 20th	Night	26% 7% 13% 31% 21% 2	2482
	21st to end	Night	32% 10% 16% 19% 21% 3	2662
August	1st to 10th	Night	28% 10% 17% 26% 18% 1	2613
	11th to 20th	Night	29% 8% 14% 30% 18% 1	3130
	21st to end	Night	27% 6% 12% 33% 18% 3	3476
September	1st to 10th	Night	29% 6% 12% 26% 24% 4	1900
	11th to 20th	Night	27% 5% 8% 32% 25% 2	1983
	21st to end	Night	34% 7% 11% 27% 17% 3	2090
October	1st to 10th	Night	22% 4 7% 36% 29% 2	3607
	11th to 20th	Night	33% 5% 7% 23% 30% 2	3779
	21st to end	Night	40% 4 9% 25% 19% 3	4453
November	1st to 10th	Night	36% 5% 9% 26% 19% 4	2470
	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	24% 12% 33% 23% 7%	460
	11th to 20th	Day	19% 21% 27% 20% 11% 2	452
	21st to end	Day	24% 13% 31% 23% 8% 2	460
May	1st to 10th	Day	22% 11% 29% 27% 9% 2	463
	11th to 20th	Day	20% 10% 28% 26% 13% 3	634
	21st to end	Day	14% 8% 30% 22% 18% 8%	734
June	1st to 10th	Day	10% 9% 28% 29% 19% 6%	680
	11th to 20th	Day	10% 6% 30% 30% 18% 5%	680
	21st to end	Day	12% 8% 30% 30% 15% 5%	680
July	1st to 10th	Day	10% 3 18% 30% 22% 16%	671
	11th to 20th	Day	10% 6% 22% 31% 20% 11%	668
	21st to end	Day	8% 2 24% 34% 24% 8%	739
August	1st to 10th	Day	7% 3 17% 37% 24% 12%	643
	11th to 20th	Day	11% 5% 18% 26% 26% 13%	479
	21st to end	Day	6% 3 18% 31% 29% 13%	539
September	1st to 10th	Day	12% 6% 20% 29% 26% 7%	490
	11th to 20th	Day	11% 2 16% 29% 31% 11%	490
	21st to end	Day	13% 11% 24% 25% 20% 7%	490
October	1st to 10th	Day	13% 8% 27% 28% 19% 5%	412
	11th to 20th	Day	16% 22% 31% 21% 9% 1	300
	21st to end	Day	28% 14% 33% 17% 6% 2	330
November	1st to 10th	Day	13% 16% 35% 23% 8% 4	282
	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% 37% 25% 6%	570
	11th to 20th	Night	22% 12% 32% 26% 7%	563
	21st to end	Night	20% 10% 35% 26% 8%	570
May	1st to 10th	Night	18% 8% 35% 31% 7% 1	570
	11th to 20th	Night	17% 7% 28% 35% 12% 1	561
	21st to end	Night	12% 6% 26% 38% 18%	617
June	1st to 10th	Night	11% 4 26% 44% 15%	570
	11th to 20th	Night	11% 3 28% 45% 12% 1	570
	21st to end	Night	11% 4 26% 41% 17% 1	570
July	1st to 10th	Night	9% 2 16% 41% 28% 3	555
	11th to 20th	Night	8% 1 20% 39% 28% 3	552
	21st to end	Night	5% 1 16% 47% 28% 2	624
August	1st to 10th	Night	5% 3 13% 47% 30% 3	567
	11th to 20th	Night	6% 3 12% 42% 34% 3	564
	21st to end	Night	9% 1 16% 38% 33% 2	627
September	1st to 10th	Night	11% 2 18% 37% 28% 3	570
	11th to 20th	Night	9% 4 17% 37% 30% 4	653
	21st to end	Night	11% 7% 25% 36% 20% 2	760
October	1st to 10th	Night	13% 6% 26% 35% 19% 2	760
	11th to 20th	Night	21% 12% 28% 29% 10%	760
	21st to end	Night	24% 11% 31% 25% 8%	836
November	1st to 10th	Night	17% 9% 38% 25% 10% 1	760
	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 2024-11-03 01:11:26. Version ac07

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