St George Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the St George Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	10% 6% 3 7% 5% 5% 6% 5% 7% 9% 37%	137475
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	135285
Transparency	Day	40% 9% 13% 18% 15% 6%	141122
	Night	34% 7% 15% 19% 17% 8%	139717
Seeing	Day	12% 6% 20% 25% 22% 15%	16912
	Night	11% 5% 18% 31% 26% 8%	26510

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	15% 6% 3 7% 4 4 4 4 6% 9% 38%	5289
	11th to 20th	Day	12% 7% 2 5% 4 4 4 4 5% 7% 47%	5310
	21st to end	Day	18% 9% 3 7% 4 4 4 4 5% 6% 35%	6281
February	1st to 10th	Day	13% 7% 3 7% 6% 4 5% 4 7% 6% 39%	2100
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 6% 7% 41%	2048
	21st to end	Day	16% 9% 3 6% 5% 4 4 4 5% 7% 37%	1730
March	1st to 10th	Day	17% 7% 3 7% 5% 5% 4 4 5% 7% 36%	5001
	11th to 20th	Day	10% 5% 2 6% 4 4 4 4 7% 8% 45%	5090
	21st to end	Day	13% 6% 3 7% 4 4 4 4 5% 7% 43%	5760
April	1st to 10th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	2520
	11th to 20th	Day	13% 6% 3 6% 5% 4 5% 4 6% 9% 40%	2494
	21st to end	Day	7% 5% 2 6% 5% 6% 5% 5% 6% 9% 43%	2520
May	1st to 10th	Day	17% 7% 3 7% 5% 4 5% 5% 6% 9% 33%	7021
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 10% 45%	7031
	21st to end	Day	8% 5% 3 7% 6% 6% 7% 6% 8% 12% 34%	7736
June	1st to 10th	Day	8% 7% 3 10% 7% 8% 7% 6% 8% 8% 28%	2520
	11th to 20th	Day	6% 6% 3 9% 7% 9% 9% 6% 8% 10% 29%	2726
	21st to end	Day	6% 7% 3 9% 9% 9% 8% 7% 9% 9% 24%	2730
July	1st to 10th	Day	6% 6% 3 9% 7% 9% 8% 7% 9% 13% 23%	6030
	11th to 20th	Day	6% 6% 3 10% 8% 8% 9% 7% 9% 11% 22%	5966
	21st to end	Day	6% 7% 3 10% 8% 8% 9% 7% 9% 12% 20%	5931
August	1st to 10th	Day	4 4 2 6% 5% 6% 7% 5% 9% 12% 42%	3905
	11th to 20th	Day	6% 4 2 8% 6% 8% 8% 6% 9% 12% 31%	3849
	21st to end	Day	8% 8% 4 11% 7% 7% 6% 5% 7% 7% 29%	4312
September	1st to 10th	Day	11% 7% 3 9% 7% 7% 7% 5% 7% 9% 29%	2400
	11th to 20th	Day	11% 6% 2 8% 6% 7% 6% 5% 7% 7% 33%	2376
	21st to end	Day	8% 5% 3 6% 5% 6% 5% 5% 7% 8% 43%	2200
October	1st to 10th	Day	7% 4 2 6% 4 5% 5% 4 5% 6% 53%	2040
	11th to 20th	Day	6% 6% 1 5% 4 4 4 3 5% 7% 54%	2000
	21st to end	Day	9% 5% 2 5% 4 4 4 3 5% 7% 52%	2200
November	1st to 10th	Day	8% 5% 2 5% 4 4 5% 3 6% 7% 51%	2058
	11th to 20th	Day	9% 7% 2 5% 4 3 3 4 5% 6% 52%	2100
	21st to end	Day	12% 6% 2 6% 4 4 5% 5% 5% 8% 43%	2100
December	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 39%	3360
	11th to 20th	Day	10% 5% 2 5% 4 3 4 4 6% 7% 49%	3856
	21st to end	Day	13% 7% 2 7% 5% 4 4 3 5% 6% 44%	4885
January	1st to 10th	Night	17% 7% 2 5% 4 3 4 3 4 6% 45%	7634
	11th to 20th	Night	16% 7% 2 5% 3 3 3 3 5% 7% 46%	7680
	21st to end	Night	19% 7% 3 7% 5% 4 4 3 5% 7% 37%	7886
February	1st to 10th	Night	13% 7% 3 7% 5% 3 4 3 5% 6% 43%	2520
	11th to 20th	Night	17% 8% 3 5% 3 3 3 4 5% 6% 43%	2458
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 7% 42%	2076
March	1st to 10th	Night	17% 10% 3 7% 3 4 4 3 4 6% 40%	5103
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 5% 7% 49%	5090
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 6% 46%	5408
April	1st to 10th	Night	11% 6% 2 5% 4 5% 5% 4 5% 7% 46%	2100
	11th to 20th	Night	10% 6% 2 6% 4 4 4 4 5% 7% 48%	2075
	21st to end	Night	6% 4 1 5% 4 4 4 4 6% 8% 52%	2100
May	1st to 10th	Night	16% 9% 3 9% 4 4 4 3 5% 8% 35%	5832
	11th to 20th	Night	12% 5% 2 6% 4 4 4 4 6% 9% 44%	5829
	21st to end	Night	6% 4 2 6% 5% 5% 5% 4 7% 12% 43%	6415
June	1st to 10th	Night	6% 6% 2 5% 3 4 5% 5% 7% 11% 45%	2096
	11th to 20th	Night	4 5% 1 5% 5% 4 6% 6% 8% 10% 46%	1890
	21st to end	Night	6% 5% 2 5% 4 5% 5% 5% 8% 11% 43%	1890
July	1st to 10th	Night	3 4 2 5% 4 5% 6% 5% 9% 14% 43%	4168
	11th to 20th	Night	3 3 1 4 4 5% 6% 5% 8% 12% 48%	4122
	21st to end	Night	3 4 2 6% 5% 6% 6% 6% 10% 15% 38%	4902
August	1st to 10th	Night	2 2 1 4 3 3 4 4 7% 12% 58%	3244
	11th to 20th	Night	3 2 1 3 4 5% 6% 6% 9% 15% 46%	3200
	21st to end	Night	4 5% 2 6% 6% 5% 6% 5% 8% 12% 41%	3586
September	1st to 10th	Night	7% 6% 3 6% 5% 5% 6% 6% 8% 9% 40%	2000
	11th to 20th	Night	9% 4 2 6% 4 4 4 5% 8% 9% 45%	2001
	21st to end	Night	6% 3 2 5% 5% 4 5% 4 6% 9% 51%	2200
October	1st to 10th	Night	7% 3 1 5% 4 3 4 3 6% 7% 58%	2355
	11th to 20th	Night	6% 3 1 4 3 3 3 2 4 7% 64%	2400
	21st to end	Night	10% 4 2 4 3 3 3 3 4 7% 57%	2640
November	1st to 10th	Night	9% 5% 2 5% 3 4 4 3 5% 6% 54%	2460
	11th to 20th	Night	11% 5% 2 4 3 3 3 3 4 6% 56%	2520
	21st to end	Night	9% 6% 2 6% 4 4 4 4 4 7% 49%	2520
December	1st to 10th	Night	12% 7% 3 8% 5% 4 4 4 5% 6% 42%	4040
	11th to 20th	Night	9% 6% 2 6% 4 3 4 3 5% 7% 52%	4559
	21st to end	Night	14% 9% 3 7% 4 3 3 3 4 6% 44%	6286

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	41% 6% 10% 19% 17% 5%	5290
	11th to 20th	Day	36% 5% 6% 19% 25% 9%	5310
	21st to end	Day	49% 5% 7% 13% 16% 10%	6281
February	1st to 10th	Day	44% 6% 9% 14% 19% 9%	2100
	11th to 20th	Day	40% 7% 10% 17% 18% 8%	2058
	21st to end	Day	44% 6% 10% 17% 18% 6%	1730
March	1st to 10th	Day	46% 6% 9% 17% 13% 9%	5001
	11th to 20th	Day	34% 6% 12% 17% 17% 14%	5090
	21st to end	Day	38% 6% 12% 19% 18% 7%	5762
April	1st to 10th	Day	38% 8% 12% 17% 16% 9%	2520
	11th to 20th	Day	39% 5% 12% 21% 14% 9%	2494
	21st to end	Day	33% 8% 12% 21% 20% 5%	2520
May	1st to 10th	Day	45% 8% 14% 20% 11% 2	7056
	11th to 20th	Day	31% 8% 17% 25% 17% 3	7035
	21st to end	Day	37% 12% 19% 24% 7%	7749
June	1st to 10th	Day	48% 11% 12% 18% 10% 1	2520
	11th to 20th	Day	44% 12% 15% 18% 10% 1	2726
	21st to end	Day	49% 13% 15% 17% 5%	2730
July	1st to 10th	Day	44% 15% 25% 14% 1	5966
	11th to 20th	Day	49% 16% 29% 5% 1	5897
	21st to end	Day	49% 16% 18% 15% 2	5761
August	1st to 10th	Day	30% 13% 15% 25% 18%	3893
	11th to 20th	Day	40% 13% 25% 18% 4	3885
	21st to end	Day	52% 12% 12% 14% 10% 1	4311
September	1st to 10th	Day	49% 10% 16% 13% 10% 2	2400
	11th to 20th	Day	44% 11% 14% 14% 13% 4	2376
	21st to end	Day	37% 8% 12% 20% 15% 9%	2200
October	1st to 10th	Day	29% 5% 7% 13% 21% 25%	3326
	11th to 20th	Day	18% 4 6% 22% 30% 20%	3240
	21st to end	Day	22% 5% 7% 12% 41% 13%	3538
November	1st to 10th	Day	31% 5% 9% 21% 24% 10%	2058
	11th to 20th	Day	31% 6% 7% 19% 24% 14%	2100
	21st to end	Day	38% 6% 9% 18% 19% 10%	2100
December	1st to 10th	Day	45% 6% 9% 15% 21% 4	3360
	11th to 20th	Day	31% 8% 9% 21% 21% 10%	3854
	21st to end	Day	40% 6% 8% 16% 24% 6%	4885
January	1st to 10th	Night	41% 5% 10% 17% 20% 8%	7635
	11th to 20th	Night	39% 4 5% 17% 23% 11%	7680
	21st to end	Night	47% 4 10% 13% 17% 9%	7886
February	1st to 10th	Night	41% 6% 7% 16% 20% 10%	2520
	11th to 20th	Night	41% 6% 9% 17% 17% 11%	2459
	21st to end	Night	42% 6% 11% 14% 16% 11%	2076
March	1st to 10th	Night	45% 6% 10% 13% 16% 11%	5103
	11th to 20th	Night	34% 7% 10% 16% 19% 14%	5090
	21st to end	Night	38% 5% 11% 19% 19% 7%	5408
April	1st to 10th	Night	36% 7% 13% 16% 16% 11%	2100
	11th to 20th	Night	35% 6% 11% 21% 17% 10%	2075
	21st to end	Night	28% 7% 15% 23% 22% 6%	2100
May	1st to 10th	Night	47% 8% 13% 19% 11% 3	5862
	11th to 20th	Night	35% 7% 16% 25% 15% 2	5830
	21st to end	Night	31% 11% 22% 27% 8% 1	6427
June	1st to 10th	Night	31% 10% 18% 27% 13%	2096
	11th to 20th	Night	29% 11% 22% 27% 11%	1890
	21st to end	Night	31% 11% 26% 25% 7%	1890
July	1st to 10th	Night	28% 12% 39% 19% 2	4125
	11th to 20th	Night	25% 16% 46% 11% 2	4071
	21st to end	Night	31% 17% 28% 22% 2	4759
August	1st to 10th	Night	16% 11% 25% 31% 17%	3232
	11th to 20th	Night	22% 12% 40% 24% 2	3230
	21st to end	Night	34% 12% 19% 21% 12% 2	3586
September	1st to 10th	Night	34% 13% 22% 16% 13% 3	2000
	11th to 20th	Night	31% 11% 18% 20% 15% 4	2001
	21st to end	Night	27% 8% 16% 26% 15% 8%	2200
October	1st to 10th	Night	28% 4 9% 15% 21% 23%	3849
	11th to 20th	Night	14% 4 6% 23% 30% 24%	3899
	21st to end	Night	19% 4 7% 11% 41% 18%	4252
November	1st to 10th	Night	30% 4 9% 21% 24% 11%	2460
	11th to 20th	Night	30% 5% 7% 18% 27% 13%	2520
	21st to end	Night	35% 5% 9% 19% 21% 11%	2520
December	1st to 10th	Night	43% 5% 7% 19% 20% 6%	4040
	11th to 20th	Night	31% 5% 12% 21% 18% 12%	4560
	21st to end	Night	42% 6% 9% 13% 23% 8%	6286

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	20% 14% 36% 22% 6% 2	330
	11th to 20th	Day	13% 17% 35% 27% 7%	330
	21st to end	Day	22% 15% 39% 19% 5% 1	363
February	1st to 10th	Day	15% 13% 39% 25% 7% 1	330
	11th to 20th	Day	22% 14% 34% 22% 8% 1	330
	21st to end	Day	12% 21% 42% 20% 5%	273
March	1st to 10th	Day	16% 15% 36% 25% 7%	330
	11th to 20th	Day	18% 15% 34% 23% 8% 2	330
	21st to end	Day	14% 11% 33% 28% 12% 1	437
April	1st to 10th	Day	13% 11% 34% 30% 11% 2	540
	11th to 20th	Day	16% 11% 30% 26% 14% 2	532
	21st to end	Day	8% 7% 29% 34% 19% 3	540
May	1st to 10th	Day	13% 7% 24% 30% 20% 5%	543
	11th to 20th	Day	9% 5% 25% 29% 24% 8%	567
	21st to end	Day	6% 3 14% 33% 29% 16%	617
June	1st to 10th	Day	11% 4 10% 26% 28% 21%	570
	11th to 20th	Day	9% 3 12% 31% 27% 19%	776
	21st to end	Day	8% 2 11% 26% 28% 25%	780
July	1st to 10th	Day	10% 1 8% 22% 28% 30%	771
	11th to 20th	Day	10% 1 4 17% 34% 34%	744
	21st to end	Day	8% 2 6% 17% 33% 35%	585
August	1st to 10th	Day	5% 2 5% 15% 34% 40%	526
	11th to 20th	Day	8% 1 4 9% 32% 46%	519
	21st to end	Day	8% 3 9% 17% 35% 28%	583
September	1st to 10th	Day	12% 1 6% 22% 33% 26%	530
	11th to 20th	Day	11% 2 8% 23% 38% 19%	530
	21st to end	Day	8% 2 10% 27% 36% 15%	530
October	1st to 10th	Day	8% 2 16% 40% 25% 10%	370
	11th to 20th	Day	6% 4 18% 37% 27% 8%	330
	21st to end	Day	16% 8% 21% 33% 17% 5%	363
November	1st to 10th	Day	12% 6% 21% 32% 25% 4	330
	11th to 20th	Day	13% 7% 28% 32% 15% 5%	330
	21st to end	Day	15% 10% 34% 28% 11% 3	330
December	1st to 10th	Day	19% 8% 35% 21% 12% 6%	330
	11th to 20th	Day	15% 11% 38% 28% 7% 1	330
	21st to end	Day	20% 12% 29% 25% 12% 2	363
January	1st to 10th	Night	18% 9% 30% 34% 8% 1	1050
	11th to 20th	Night	17% 9% 27% 37% 10%	1050
	21st to end	Night	19% 10% 32% 32% 6%	974
February	1st to 10th	Night	15% 10% 31% 36% 8%	840
	11th to 20th	Night	21% 12% 26% 32% 8% 1	823
	21st to end	Night	18% 13% 36% 29% 4	692
March	1st to 10th	Night	15% 10% 31% 34% 10%	667
	11th to 20th	Night	13% 6% 27% 37% 16% 1	630
	21st to end	Night	10% 8% 26% 37% 18%	693
April	1st to 10th	Night	14% 10% 26% 38% 12% 1	630
	11th to 20th	Night	13% 7% 28% 35% 17% 1	623
	21st to end	Night	7% 6% 26% 40% 20% 1	630
May	1st to 10th	Night	11% 4 19% 44% 20% 2	630
	11th to 20th	Night	7% 5% 14% 44% 28% 2	621
	21st to end	Night	7% 1 10% 37% 36% 9%	683
June	1st to 10th	Night	7% 2 6% 31% 46% 8%	630
	11th to 20th	Night	4 2 5% 22% 53% 13%	630
	21st to end	Night	9% 1 5% 19% 48% 19%	630
July	1st to 10th	Night	5% 4 13% 47% 30%	615
	11th to 20th	Night	5% 1 3 5% 52% 34%	597
	21st to end	Night	4 1 3 13% 51% 28%	657
August	1st to 10th	Night	2 1 9% 46% 41%	597
	11th to 20th	Night	4 1 3 8% 49% 36%	594
	21st to end	Night	7% 2 5% 14% 46% 26%	660
September	1st to 10th	Night	9% 1 4 17% 49% 22%	600
	11th to 20th	Night	9% 1 6% 19% 51% 13%	601
	21st to end	Night	7% 2 7% 30% 45% 9%	800
October	1st to 10th	Night	8% 2 10% 39% 36% 6%	800
	11th to 20th	Night	7% 3 15% 38% 32% 6%	800
	21st to end	Night	13% 4 18% 38% 25% 3	880
November	1st to 10th	Night	12% 3 14% 42% 24% 4	820
	11th to 20th	Night	13% 5% 19% 40% 21% 2	840
	21st to end	Night	11% 6% 26% 38% 18% 1	840
December	1st to 10th	Night	16% 8% 26% 32% 17% 1	840
	11th to 20th	Night	14% 6% 26% 41% 13% 1	840
	21st to end	Night	19% 9% 28% 30% 13% 1	1003

Page generated at 2026-07-13 02:14:54. Version ac07

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