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 2025-03-12.

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	11% 6% 3 7% 6% 6% 6% 5% 7% 8% 37%	109605
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	109454
Transparency	Day	41% 9% 12% 17% 16% 6%	113288
	Night	34% 8% 14% 19% 18% 8%	113911
Seeing	Day	12% 6% 20% 25% 23% 15%	15064
	Night	12% 5% 18% 31% 26% 8%	24850

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	16% 6% 4 7% 5% 4 4 3 6% 9% 37%	4077
	11th to 20th	Day	14% 8% 3 5% 4 3 3 3 4 6% 47%	4080
	21st to end	Day	19% 9% 3 7% 5% 4 5% 4 6% 6% 33%	4856
February	1st to 10th	Day	14% 8% 3 7% 6% 4 5% 4 7% 6% 37%	2000
	11th to 20th	Day	15% 7% 2 6% 4 4 4 4 6% 7% 42%	1948
	21st to end	Day	16% 9% 3 6% 5% 4 4 3 5% 7% 36%	1650
March	1st to 10th	Day	19% 8% 4 9% 6% 5% 5% 4 5% 7% 28%	3515
	11th to 20th	Day	13% 6% 3 7% 5% 5% 5% 5% 8% 9% 32%	3491
	21st to end	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	3931
April	1st to 10th	Day	10% 5% 2 6% 5% 4 5% 5% 7% 8% 41%	2280
	11th to 20th	Day	14% 6% 3 6% 5% 4 5% 4 6% 10% 38%	2254
	21st to end	Day	8% 5% 2 6% 5% 6% 4 5% 6% 9% 45%	2280
May	1st to 10th	Day	9% 6% 3 7% 5% 5% 6% 6% 8% 10% 36%	3765
	11th to 20th	Day	9% 6% 2 7% 5% 5% 5% 4 6% 8% 42%	3764
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 7% 10% 43%	4129
June	1st to 10th	Day	9% 7% 3 10% 7% 7% 7% 6% 8% 8% 28%	2280
	11th to 20th	Day	6% 6% 2 9% 7% 8% 9% 6% 8% 10% 28%	2466
	21st to end	Day	7% 8% 3 9% 9% 9% 8% 7% 9% 8% 22%	2470
July	1st to 10th	Day	7% 7% 3 10% 8% 9% 9% 7% 9% 12% 18%	4120
	11th to 20th	Day	6% 7% 3 12% 9% 9% 9% 7% 8% 9% 20%	4120
	21st to end	Day	8% 8% 4 11% 9% 9% 9% 6% 8% 10% 17%	4127
August	1st to 10th	Day	4 4 2 6% 5% 5% 6% 5% 9% 12% 43%	3785
	11th to 20th	Day	6% 4 2 9% 6% 8% 8% 6% 9% 11% 31%	3729
	21st to end	Day	8% 8% 4 11% 8% 7% 6% 5% 7% 7% 29%	4180
September	1st to 10th	Day	12% 7% 3 9% 7% 7% 7% 5% 7% 9% 27%	2280
	11th to 20th	Day	11% 7% 2 8% 7% 7% 7% 5% 7% 7% 32%	2257
	21st to end	Day	8% 5% 2 6% 6% 6% 5% 5% 6% 7% 43%	2090
October	1st to 10th	Day	7% 4 1 6% 4 5% 5% 4 5% 6% 53%	1938
	11th to 20th	Day	6% 6% 1 5% 4 5% 4 3 5% 7% 53%	1900
	21st to end	Day	10% 5% 2 5% 4 4 4 3 5% 7% 51%	2090
November	1st to 10th	Day	9% 6% 2 6% 5% 4 5% 3 6% 8% 50%	1958
	11th to 20th	Day	9% 7% 2 6% 4 3 3 4 5% 6% 51%	2000
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 5% 7% 43%	2000
December	1st to 10th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 39%	3260
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 6% 7% 49%	3756
	21st to end	Day	14% 7% 2 7% 5% 4 4 3 5% 6% 44%	4779
January	1st to 10th	Night	17% 7% 3 5% 4 3 3 3 4 5% 46%	5888
	11th to 20th	Night	19% 8% 2 4 3 3 3 2 4 6% 46%	5900
	21st to end	Night	20% 7% 2 6% 4 4 4 3 5% 8% 37%	6097
February	1st to 10th	Night	13% 7% 3 7% 5% 3 4 3 5% 6% 42%	2400
	11th to 20th	Night	17% 7% 3 5% 3 4 3 4 5% 5% 44%	2338
	21st to end	Night	16% 8% 2 6% 4 4 3 3 5% 7% 42%	1980
March	1st to 10th	Night	22% 11% 3 8% 3 4 4 3 4 6% 32%	3575
	11th to 20th	Night	13% 8% 3 7% 4 5% 5% 4 6% 8% 38%	3497
	21st to end	Night	14% 6% 2 5% 4 3 4 4 6% 7% 46%	3697
April	1st to 10th	Night	11% 6% 2 5% 4 4 5% 4 5% 7% 46%	1900
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 5% 7% 46%	1875
	21st to end	Night	6% 4 1 5% 4 4 4 4 6% 7% 54%	1900
May	1st to 10th	Night	11% 8% 2 8% 4 4 4 4 6% 9% 40%	3130
	11th to 20th	Night	15% 5% 2 6% 4 4 4 4 7% 7% 43%	3129
	21st to end	Night	6% 3 1 4 4 4 4 4 7% 12% 51%	3426
June	1st to 10th	Night	6% 6% 2 5% 3 4 5% 5% 7% 11% 46%	1896
	11th to 20th	Night	3 5% 1 5% 4 4 6% 6% 9% 11% 46%	1710
	21st to end	Night	6% 6% 2 5% 4 5% 5% 5% 8% 11% 42%	1710
July	1st to 10th	Night	4 5% 2 5% 5% 5% 6% 6% 10% 13% 40%	2850
	11th to 20th	Night	3 3 1 4 4 5% 6% 5% 9% 13% 47%	2850
	21st to end	Night	4 4 2 7% 5% 6% 7% 6% 10% 15% 34%	3421
August	1st to 10th	Night	2 2 1 4 3 3 4 4 7% 12% 59%	3144
	11th to 20th	Night	3 2 1 3 4 5% 6% 6% 9% 15% 46%	3100
	21st to end	Night	4 5% 2 6% 6% 5% 6% 5% 8% 11% 41%	3476
September	1st to 10th	Night	7% 6% 3 6% 5% 5% 6% 6% 8% 9% 40%	1900
	11th to 20th	Night	10% 4 2 6% 4 5% 4 5% 8% 9% 43%	1901
	21st to end	Night	6% 3 2 5% 4 4 4 4 6% 9% 51%	2090
October	1st to 10th	Night	7% 3 1 5% 4 3 4 3 5% 7% 59%	2237
	11th to 20th	Night	6% 3 1 4 3 3 3 2 4 7% 63%	2280
	21st to end	Night	10% 5% 2 4 3 3 3 3 3 7% 57%	2508
November	1st to 10th	Night	9% 5% 2 5% 3 4 4 3 5% 6% 53%	2340
	11th to 20th	Night	11% 6% 2 4 3 3 4 3 4 6% 54%	2400
	21st to end	Night	9% 6% 2 7% 4 4 4 4 4 7% 49%	2400
December	1st to 10th	Night	11% 7% 3 8% 5% 5% 4 4 5% 6% 43%	3920
	11th to 20th	Night	9% 6% 2 5% 4 3 4 3 5% 7% 52%	4439
	21st to end	Night	15% 9% 3 7% 4 3 3 3 4 5% 44%	6150

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	43% 6% 8% 18% 18% 7%	4078
	11th to 20th	Day	39% 5% 6% 16% 27% 7%	4080
	21st to end	Day	51% 5% 8% 13% 16% 8%	4856
February	1st to 10th	Day	45% 6% 9% 14% 18% 9%	2000
	11th to 20th	Day	41% 7% 10% 16% 19% 8%	1958
	21st to end	Day	45% 6% 10% 16% 17% 6%	1650
March	1st to 10th	Day	52% 8% 11% 15% 11% 4	3515
	11th to 20th	Day	45% 8% 14% 17% 12% 4	3491
	21st to end	Day	34% 6% 15% 16% 21% 8%	3931
April	1st to 10th	Day	37% 8% 12% 17% 16% 9%	2280
	11th to 20th	Day	41% 5% 12% 20% 14% 8%	2254
	21st to end	Day	33% 7% 11% 20% 22% 6%	2280
May	1st to 10th	Day	38% 10% 16% 23% 10% 4	3800
	11th to 20th	Day	37% 9% 12% 18% 20% 4	3768
	21st to end	Day	31% 10% 15% 32% 11% 1	4141
June	1st to 10th	Day	48% 11% 13% 19% 9% 1	2280
	11th to 20th	Day	44% 12% 14% 18% 11% 1	2466
	21st to end	Day	51% 14% 15% 14% 6%	2470
July	1st to 10th	Day	50% 16% 20% 13% 1	4056
	11th to 20th	Day	53% 16% 24% 6% 1	4051
	21st to end	Day	55% 16% 16% 11% 2	3996
August	1st to 10th	Day	29% 12% 15% 25% 18%	3773
	11th to 20th	Day	40% 13% 24% 18% 4 1	3765
	21st to end	Day	52% 11% 11% 14% 10% 1	4179
September	1st to 10th	Day	50% 10% 15% 12% 10% 2	2280
	11th to 20th	Day	45% 12% 14% 13% 12% 4	2257
	21st to end	Day	38% 8% 11% 19% 15% 9%	2090
October	1st to 10th	Day	29% 4 7% 13% 21% 26%	3224
	11th to 20th	Day	18% 4 6% 22% 29% 20%	3140
	21st to end	Day	22% 5% 7% 11% 41% 14%	3428
November	1st to 10th	Day	32% 5% 9% 21% 25% 8%	1958
	11th to 20th	Day	32% 6% 7% 18% 24% 14%	2000
	21st to end	Day	38% 6% 8% 18% 20% 10%	2000
December	1st to 10th	Day	44% 6% 9% 15% 21% 5%	3260
	11th to 20th	Day	32% 8% 9% 21% 21% 10%	3754
	21st to end	Day	40% 6% 8% 15% 25% 5%	4779
January	1st to 10th	Night	41% 4 7% 18% 22% 8%	5888
	11th to 20th	Night	41% 3 5% 16% 26% 8%	5900
	21st to end	Night	46% 4 10% 14% 16% 10%	6097
February	1st to 10th	Night	42% 6% 7% 16% 19% 10%	2400
	11th to 20th	Night	40% 6% 8% 17% 17% 12%	2339
	21st to end	Night	42% 6% 12% 14% 16% 11%	1980
March	1st to 10th	Night	52% 7% 11% 13% 12% 6%	3575
	11th to 20th	Night	41% 9% 13% 18% 14% 5%	3497
	21st to end	Night	36% 6% 13% 20% 20% 6%	3697
April	1st to 10th	Night	36% 8% 13% 17% 16% 11%	1900
	11th to 20th	Night	37% 7% 11% 19% 17% 10%	1875
	21st to end	Night	27% 7% 12% 23% 24% 7%	1900
May	1st to 10th	Night	38% 10% 15% 21% 12% 4	3160
	11th to 20th	Night	38% 7% 13% 19% 21% 2	3130
	21st to end	Night	23% 9% 20% 33% 14% 1	3435
June	1st to 10th	Night	31% 10% 18% 28% 13%	1896
	11th to 20th	Night	28% 11% 21% 28% 12%	1710
	21st to end	Night	33% 11% 25% 24% 7%	1710
July	1st to 10th	Night	31% 14% 38% 16% 2	2805
	11th to 20th	Night	25% 17% 45% 11% 3	2796
	21st to end	Night	33% 19% 29% 16% 2	3312
August	1st to 10th	Night	16% 11% 24% 31% 18%	3132
	11th to 20th	Night	22% 13% 39% 24% 2	3130
	21st to end	Night	34% 12% 19% 21% 12% 2	3476
September	1st to 10th	Night	35% 13% 22% 16% 12% 3	1900
	11th to 20th	Night	32% 11% 18% 19% 14% 5%	1901
	21st to end	Night	27% 8% 15% 26% 16% 8%	2090
October	1st to 10th	Night	28% 4 9% 15% 21% 24%	3731
	11th to 20th	Night	14% 4 6% 23% 29% 24%	3779
	21st to end	Night	19% 4 7% 11% 40% 19%	4120
November	1st to 10th	Night	31% 4 10% 22% 24% 10%	2340
	11th to 20th	Night	31% 5% 7% 16% 27% 13%	2400
	21st to end	Night	35% 5% 9% 19% 21% 11%	2400
December	1st to 10th	Night	43% 5% 7% 20% 20% 6%	3920
	11th to 20th	Night	31% 5% 13% 21% 18% 12%	4440
	21st to end	Night	42% 6% 9% 12% 23% 8%	6150

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% 13% 36% 22% 6% 2	300
	11th to 20th	Day	14% 18% 34% 27% 7%	300
	21st to end	Day	20% 15% 39% 20% 5% 1	330
February	1st to 10th	Day	17% 13% 37% 25% 8% 1	300
	11th to 20th	Day	23% 12% 33% 22% 9% 1	300
	21st to end	Day	13% 18% 44% 20% 5%	249
March	1st to 10th	Day	18% 15% 36% 23% 7%	300
	11th to 20th	Day	16% 14% 36% 25% 7% 1	273
	21st to end	Day	14% 13% 35% 26% 11% 1	363
April	1st to 10th	Day	13% 10% 34% 30% 12% 2	460
	11th to 20th	Day	18% 12% 28% 25% 15% 2	452
	21st to end	Day	9% 6% 28% 34% 21% 3	460
May	1st to 10th	Day	10% 6% 25% 31% 23% 6%	463
	11th to 20th	Day	10% 4 21% 30% 25% 10%	487
	21st to end	Day	5% 2 12% 33% 30% 18%	529
June	1st to 10th	Day	11% 3 8% 24% 30% 23%	490
	11th to 20th	Day	8% 2 11% 32% 27% 19%	676
	21st to end	Day	9% 2 11% 25% 28% 25%	680
July	1st to 10th	Day	11% 1 8% 21% 27% 31%	671
	11th to 20th	Day	10% 1 4 17% 34% 35%	668
	21st to end	Day	8% 2 6% 16% 33% 36%	541
August	1st to 10th	Day	5% 1 5% 14% 33% 42%	486
	11th to 20th	Day	9% 1 5% 9% 31% 46%	479
	21st to end	Day	9% 3 9% 17% 35% 28%	539
September	1st to 10th	Day	13% 1 6% 19% 32% 29%	490
	11th to 20th	Day	12% 2 9% 22% 37% 19%	490
	21st to end	Day	9% 2 10% 25% 37% 16%	490
October	1st to 10th	Day	8% 2 14% 40% 25% 10%	338
	11th to 20th	Day	7% 3 18% 35% 28% 8%	300
	21st to end	Day	17% 8% 18% 34% 18% 5%	330
November	1st to 10th	Day	13% 5% 21% 32% 25% 4	300
	11th to 20th	Day	14% 8% 30% 32% 13% 3	300
	21st to end	Day	15% 10% 33% 27% 12% 4	300
December	1st to 10th	Day	17% 7% 35% 22% 13% 6%	300
	11th to 20th	Day	15% 12% 38% 27% 7% 1	300
	21st to end	Day	22% 11% 30% 23% 12% 1	330
January	1st to 10th	Night	18% 9% 31% 34% 8% 1	1000
	11th to 20th	Night	18% 9% 27% 37% 10%	1000
	21st to end	Night	19% 10% 32% 33% 6%	928
February	1st to 10th	Night	15% 10% 31% 36% 8% 1	800
	11th to 20th	Night	21% 11% 26% 32% 9% 1	783
	21st to end	Night	18% 12% 36% 29% 4	660
March	1st to 10th	Night	14% 11% 31% 34% 10%	635
	11th to 20th	Night	13% 5% 27% 38% 16% 1	575
	21st to end	Night	11% 8% 27% 36% 17%	627
April	1st to 10th	Night	14% 8% 26% 37% 13% 1	570
	11th to 20th	Night	13% 7% 29% 34% 16% 1	563
	21st to end	Night	7% 6% 26% 40% 21% 1	570
May	1st to 10th	Night	9% 4 18% 44% 22% 2	570
	11th to 20th	Night	7% 4 11% 46% 29% 3	561
	21st to end	Night	6% 1 10% 37% 36% 10%	617
June	1st to 10th	Night	7% 2 5% 31% 46% 9%	570
	11th to 20th	Night	4 2 5% 23% 54% 13%	570
	21st to end	Night	9% 1 5% 20% 45% 20%	570
July	1st to 10th	Night	5% 1 4 13% 46% 31%	555
	11th to 20th	Night	5% 1 3 5% 53% 34%	552
	21st to end	Night	4 1 3 13% 50% 28%	624
August	1st to 10th	Night	2 1 9% 46% 41%	567
	11th to 20th	Night	5% 1 3 8% 48% 36%	564
	21st to end	Night	7% 2 5% 14% 45% 26%	627
September	1st to 10th	Night	9% 1 4 15% 49% 23%	570
	11th to 20th	Night	10% 1 7% 19% 50% 13%	571
	21st to end	Night	7% 2 7% 30% 45% 9%	760
October	1st to 10th	Night	8% 1 9% 40% 35% 6%	760
	11th to 20th	Night	7% 3 15% 38% 32% 6%	760
	21st to end	Night	13% 4 17% 37% 25% 3	836
November	1st to 10th	Night	13% 3 15% 42% 24% 4	780
	11th to 20th	Night	14% 5% 19% 40% 20% 2	800
	21st to end	Night	11% 7% 25% 38% 18% 1	800
December	1st to 10th	Night	15% 8% 26% 33% 18% 1	800
	11th to 20th	Night	15% 6% 26% 41% 12% 1	800
	21st to end	Night	20% 9% 29% 30% 12% 1	955

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