The Glenelg Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Glenelg Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	16% 12% 5% 10% 5% 4 4 3 5% 6% 29%	127465
	Night	15% 14% 6% 12% 6% 5% 4 3 4 5% 27%	123563
Transparency	Day	55% 6% 10% 18% 10% 1	131027
	Night	58% 6% 10% 16% 9% 1	128089
Seeing	Day	21% 16% 30% 21% 9% 3	16456
	Night	21% 15% 31% 24% 9% 1	25610

By Month

Percentage Cloud Cover NE (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	11% 17% 10% 22% 11% 6% 3 3 3 3 12%	3181
	11th to 20th	Day	8% 21% 11% 27% 11% 9% 3 1 2 2 5%	3959
	21st to end	Day	12% 19% 9% 24% 7% 6% 5% 3 3 3 8%	4466
February	1st to 10th	Day	13% 21% 7% 21% 10% 6% 3 2 3 3 11%	1800
	11th to 20th	Day	13% 16% 8% 19% 12% 5% 3 4 4 4 12%	1753
	21st to end	Day	14% 21% 7% 13% 8% 5% 5% 3 4 4 15%	1590
March	1st to 10th	Day	16% 12% 5% 16% 8% 5% 3 3 4 5% 24%	4840
	11th to 20th	Day	12% 15% 8% 12% 5% 4 4 3 5% 6% 25%	4980
	21st to end	Day	17% 15% 7% 11% 5% 3 3 3 5% 5% 25%	5465
April	1st to 10th	Day	18% 17% 5% 11% 4 4 3 3 5% 5% 26%	2310
	11th to 20th	Day	19% 15% 3 5% 4 3 4 3 4 6% 34%	2542
	21st to end	Day	20% 13% 4 8% 4 4 3 3 5% 6% 31%	2600
May	1st to 10th	Day	18% 10% 4 7% 4 4 4 3 5% 7% 34%	5888
	11th to 20th	Day	16% 11% 3 8% 5% 4 4 3 5% 6% 35%	6129
	21st to end	Day	21% 9% 2 6% 4 4 4 3 5% 6% 36%	6949
June	1st to 10th	Day	16% 8% 3 5% 5% 4 5% 4 6% 7% 36%	2844
	11th to 20th	Day	13% 6% 2 7% 4 4 4 4 6% 9% 40%	3000
	21st to end	Day	18% 7% 2 6% 4 4 4 4 6% 7% 38%	3000
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 4 6% 9% 36%	6819
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 5% 8% 43%	6468
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 6% 8% 46%	6424
August	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 5% 6% 8% 35%	4234
	11th to 20th	Day	15% 8% 3 7% 5% 5% 6% 4 7% 9% 31%	4174
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	4592
September	1st to 10th	Day	16% 8% 3 6% 5% 4 5% 4 5% 7% 37%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 45%	2200
	21st to end	Day	17% 10% 2 8% 4 4 5% 4 5% 8% 33%	2200
October	1st to 10th	Day	21% 10% 4 7% 4 4 4 3 4 7% 34%	2010
	11th to 20th	Day	24% 15% 3 7% 5% 5% 5% 4 5% 6% 22%	2000
	21st to end	Day	26% 19% 3 8% 5% 4 5% 3 4 5% 18%	2163
November	1st to 10th	Day	17% 21% 4 8% 5% 4 4 3 4 6% 25%	1795
	11th to 20th	Day	16% 21% 6% 10% 6% 4 4 3 5% 7% 18%	1680
	21st to end	Day	17% 26% 8% 10% 5% 4 4 3 4 6% 12%	1680
December	1st to 10th	Day	14% 27% 13% 16% 5% 3 3 3 3 4 8%	2680
	11th to 20th	Day	18% 24% 8% 16% 5% 3 3 3 3 4 11%	3042
	21st to end	Day	24% 21% 8% 14% 6% 4 3 2 3 4 11%	3608
January	1st to 10th	Night	11% 17% 10% 24% 10% 5% 3 2 3 4 10%	6782
	11th to 20th	Night	11% 16% 10% 26% 12% 9% 4 2 3 3 6%	5908
	21st to end	Night	11% 17% 10% 21% 10% 6% 5% 3 4 4 11%	6490
February	1st to 10th	Night	13% 20% 8% 16% 10% 6% 4 3 4 3 12%	2600
	11th to 20th	Night	12% 17% 8% 16% 8% 5% 5% 3 4 5% 17%	2533
	21st to end	Night	14% 15% 6% 14% 7% 5% 5% 4 5% 5% 20%	2036
March	1st to 10th	Night	14% 13% 7% 12% 6% 5% 5% 3 4 6% 26%	4999
	11th to 20th	Night	9% 13% 7% 11% 5% 4 5% 4 6% 7% 29%	4979
	21st to end	Night	20% 12% 5% 11% 5% 4 4 3 4 4 28%	5467
April	1st to 10th	Night	18% 15% 4 10% 4 4 4 3 4 6% 27%	2089
	11th to 20th	Night	17% 13% 4 6% 4 4 4 3 4 6% 35%	1825
	21st to end	Night	16% 14% 4 8% 5% 5% 4 4 4 5% 31%	1800
May	1st to 10th	Night	17% 9% 3 7% 5% 4 3 3 5% 7% 37%	4053
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 3 5% 7% 41%	3763
	21st to end	Night	19% 7% 2 5% 3 3 3 3 4 6% 43%	3926
June	1st to 10th	Night	15% 7% 2 7% 4 3 5% 3 6% 8% 39%	1550
	11th to 20th	Night	13% 5% 3 6% 5% 4 5% 5% 6% 10% 39%	1400
	21st to end	Night	16% 6% 3 6% 3 5% 3 4 6% 7% 42%	1400
July	1st to 10th	Night	11% 8% 3 6% 4 4 4 3 5% 8% 45%	3160
	11th to 20th	Night	15% 8% 3 4 3 3 3 3 4 6% 48%	3483
	21st to end	Night	10% 6% 2 6% 4 4 4 3 6% 7% 48%	4428
August	1st to 10th	Night	12% 6% 2 6% 4 4 4 3 4 6% 46%	2915
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 7% 39%	2875
	21st to end	Night	14% 6% 2 6% 4 4 4 3 4 7% 46%	3272
September	1st to 10th	Night	12% 6% 2 7% 4 5% 4 3 5% 7% 45%	2000
	11th to 20th	Night	12% 6% 2 6% 3 3 3 3 4 7% 51%	2197
	21st to end	Night	17% 9% 3 8% 4 4 4 3 4 7% 37%	2200
October	1st to 10th	Night	18% 9% 3 7% 4 4 4 3 5% 7% 37%	2390
	11th to 20th	Night	24% 12% 4 8% 5% 5% 5% 3 5% 5% 26%	2400
	21st to end	Night	23% 15% 4 8% 5% 4 5% 3 4 6% 23%	2675
November	1st to 10th	Night	18% 18% 4 7% 5% 4 4 3 4 6% 26%	2720
	11th to 20th	Night	16% 19% 6% 10% 5% 5% 5% 3 5% 5% 21%	2940
	21st to end	Night	20% 22% 7% 11% 5% 4 4 3 4 6% 15%	2940
December	1st to 10th	Night	15% 27% 10% 14% 6% 5% 4 3 4 4 9%	4720
	11th to 20th	Night	12% 23% 10% 19% 7% 4 4 3 4 3 12%	5307
	21st to end	Night	19% 23% 9% 16% 6% 5% 4 3 3 4 9%	7341

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	76% 5% 8% 6% 5% 1	3182
	11th to 20th	Day	89% 3 3 3 2	3993
	21st to end	Day	80% 5% 6% 5% 3	4466
February	1st to 10th	Day	80% 4 4 9% 2 1	1800
	11th to 20th	Day	75% 4 7% 9% 5% 1	1762
	21st to end	Day	72% 5% 7% 11% 5%	1590
March	1st to 10th	Day	63% 5% 12% 11% 9% 1	4843
	11th to 20th	Day	59% 5% 11% 17% 5% 2	4980
	21st to end	Day	61% 5% 9% 16% 8% 1	5465
April	1st to 10th	Day	61% 5% 8% 17% 9%	2310
	11th to 20th	Day	51% 5% 9% 20% 13% 1	2542
	21st to end	Day	54% 5% 10% 17% 11% 2	2600
May	1st to 10th	Day	47% 7% 11% 18% 11% 6%	5897
	11th to 20th	Day	49% 7% 10% 17% 16% 1	6133
	21st to end	Day	47% 5% 10% 19% 14% 4	6949
June	1st to 10th	Day	45% 7% 11% 21% 15% 1	2844
	11th to 20th	Day	40% 7% 12% 24% 15% 1	3000
	21st to end	Day	44% 7% 13% 23% 12% 1	3000
July	1st to 10th	Day	44% 7% 11% 21% 16% 1	6746
	11th to 20th	Day	38% 6% 10% 31% 15% 1	6387
	21st to end	Day	35% 6% 11% 29% 18% 1	6240
August	1st to 10th	Day	45% 7% 13% 21% 13% 1	4221
	11th to 20th	Day	47% 8% 13% 23% 9%	4213
	21st to end	Day	38% 8% 12% 31% 10% 1	4592
September	1st to 10th	Day	45% 7% 12% 23% 10% 2	2400
	11th to 20th	Day	38% 6% 14% 30% 13% 1	2200
	21st to end	Day	49% 7% 12% 23% 9% 1	2200
October	1st to 10th	Day	48% 6% 10% 23% 12% 1	3270
	11th to 20th	Day	52% 5% 10% 19% 12%	3240
	21st to end	Day	61% 5% 11% 17% 6%	3477
November	1st to 10th	Day	62% 6% 10% 15% 7%	1795
	11th to 20th	Day	66% 6% 10% 11% 6% 1	1680
	21st to end	Day	74% 6% 9% 9% 1	1680
December	1st to 10th	Day	81% 4 8% 6% 1	2680
	11th to 20th	Day	76% 5% 7% 8% 3	3042
	21st to end	Day	80% 5% 5% 4 5%	3608
January	1st to 10th	Night	77% 6% 6% 6% 4 1	6782
	11th to 20th	Night	85% 4 4 4 3	5949
	21st to end	Night	77% 5% 8% 4 5%	6490
February	1st to 10th	Night	75% 5% 8% 10% 2	2600
	11th to 20th	Night	69% 5% 10% 11% 5%	2535
	21st to end	Night	65% 6% 11% 11% 7%	2036
March	1st to 10th	Night	57% 6% 11% 15% 9% 1	5009
	11th to 20th	Night	52% 6% 15% 17% 8% 1	4980
	21st to end	Night	57% 6% 9% 14% 12% 2	5467
April	1st to 10th	Night	58% 6% 8% 16% 12% 1	2089
	11th to 20th	Night	51% 5% 7% 20% 16% 1	1825
	21st to end	Night	54% 6% 9% 15% 15% 2	1800
May	1st to 10th	Night	47% 5% 10% 17% 13% 8%	4060
	11th to 20th	Night	41% 9% 11% 22% 16% 1	3764
	21st to end	Night	42% 5% 11% 24% 15% 2	3927
June	1st to 10th	Night	42% 5% 12% 24% 16% 2	1550
	11th to 20th	Night	38% 8% 13% 24% 16% 1	1400
	21st to end	Night	40% 6% 12% 26% 15% 1	1400
July	1st to 10th	Night	37% 5% 13% 23% 18% 3	3125
	11th to 20th	Night	38% 4 9% 29% 18% 1	3441
	21st to end	Night	35% 6% 9% 32% 16% 1	4297
August	1st to 10th	Night	38% 5% 11% 28% 16% 1	2904
	11th to 20th	Night	44% 5% 15% 24% 12% 1	2902
	21st to end	Night	38% 7% 11% 31% 12% 1	3272
September	1st to 10th	Night	36% 7% 12% 29% 13% 2	2000
	11th to 20th	Night	34% 5% 13% 30% 17% 1	2197
	21st to end	Night	47% 7% 11% 24% 11% 1	2200
October	1st to 10th	Night	40% 6% 9% 24% 19% 1	3910
	11th to 20th	Night	51% 4 11% 18% 14%	3899
	21st to end	Night	53% 6% 14% 19% 7%	4311
November	1st to 10th	Night	59% 5% 11% 17% 7% 1	2720
	11th to 20th	Night	64% 6% 12% 12% 5% 2	2940
	21st to end	Night	71% 6% 12% 9% 2 1	2940
December	1st to 10th	Night	78% 6% 10% 5% 1	4720
	11th to 20th	Night	77% 5% 7% 9% 2	5307
	21st to end	Night	81% 5% 5% 7% 2	7341

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% 25% 36% 14% 2	205
	11th to 20th	Day	20% 30% 34% 11% 4	300
	21st to end	Day	23% 26% 33% 15% 3	330
February	1st to 10th	Day	27% 29% 33% 8% 2	300
	11th to 20th	Day	27% 27% 37% 9% 1	300
	21st to end	Day	31% 31% 29% 8%	249
March	1st to 10th	Day	26% 20% 36% 13% 5%	456
	11th to 20th	Day	25% 19% 36% 16% 3 1	500
	21st to end	Day	28% 18% 33% 16% 5%	550
April	1st to 10th	Day	25% 22% 33% 16% 4 1	500
	11th to 20th	Day	30% 21% 24% 13% 9% 3	492
	21st to end	Day	29% 15% 31% 19% 5% 2	500
May	1st to 10th	Day	22% 15% 33% 22% 7% 1	503
	11th to 20th	Day	20% 13% 29% 21% 13% 4	527
	21st to end	Day	21% 11% 27% 25% 12% 3	573
June	1st to 10th	Day	18% 16% 28% 28% 9% 1	574
	11th to 20th	Day	15% 12% 33% 29% 9% 2	730
	21st to end	Day	19% 10% 36% 24% 9% 2	730
July	1st to 10th	Day	12% 11% 29% 29% 15% 4	721
	11th to 20th	Day	14% 10% 29% 27% 13% 6%	622
	21st to end	Day	9% 8% 34% 30% 14% 5%	576
August	1st to 10th	Day	12% 10% 22% 33% 16% 7%	526
	11th to 20th	Day	17% 7% 21% 27% 20% 9%	519
	21st to end	Day	15% 7% 29% 26% 16% 7%	583
September	1st to 10th	Day	17% 13% 23% 25% 14% 8%	530
	11th to 20th	Day	11% 9% 20% 25% 22% 12%	530
	21st to end	Day	17% 15% 26% 24% 14% 4	530
October	1st to 10th	Day	24% 13% 27% 21% 13% 2	530
	11th to 20th	Day	25% 20% 33% 17% 4 1	530
	21st to end	Day	34% 19% 27% 16% 4	546
November	1st to 10th	Day	32% 23% 26% 14% 5%	300
	11th to 20th	Day	27% 25% 33% 10% 3 2	220
	21st to end	Day	29% 25% 35% 10%	220
December	1st to 10th	Day	28% 29% 30% 13%	220
	11th to 20th	Day	32% 25% 33% 7% 2	214
	21st to end	Day	34% 24% 31% 8% 2 1	220
January	1st to 10th	Night	21% 24% 36% 16% 4	1000
	11th to 20th	Night	26% 22% 33% 17% 2	823
	21st to end	Night	19% 18% 39% 19% 5%	880
February	1st to 10th	Night	21% 26% 35% 16% 2	800
	11th to 20th	Night	22% 21% 35% 18% 3	783
	21st to end	Night	22% 24% 35% 16% 3	660
March	1st to 10th	Night	19% 20% 39% 18% 4	800
	11th to 20th	Night	19% 22% 37% 17% 4 1	800
	21st to end	Night	23% 15% 31% 24% 7% 1	880
April	1st to 10th	Night	25% 17% 33% 22% 3	800
	11th to 20th	Night	22% 18% 29% 21% 8% 2	641
	21st to end	Night	25% 14% 28% 24% 8%	600
May	1st to 10th	Night	19% 9% 32% 30% 10% 1	600
	11th to 20th	Night	17% 9% 30% 33% 11%	478
	21st to end	Night	14% 5% 31% 36% 13%	433
June	1st to 10th	Night	17% 8% 33% 33% 10%	400
	11th to 20th	Night	13% 8% 31% 37% 12%	400
	21st to end	Night	15% 7% 32% 32% 13% 1	400
July	1st to 10th	Night	9% 5% 20% 47% 18% 1	390
	11th to 20th	Night	14% 6% 24% 36% 19% 1	432
	21st to end	Night	11% 5% 23% 42% 19% 1	657
August	1st to 10th	Night	11% 3 22% 38% 24% 1	597
	11th to 20th	Night	12% 3 20% 37% 25% 3	594
	21st to end	Night	15% 6% 26% 28% 24% 2	683
September	1st to 10th	Night	14% 9% 25% 31% 18% 5%	800
	11th to 20th	Night	14% 5% 23% 34% 19% 5%	800
	21st to end	Night	20% 9% 24% 30% 14% 4	800
October	1st to 10th	Night	22% 10% 28% 26% 13% 1	800
	11th to 20th	Night	28% 14% 28% 22% 7% 1	800
	21st to end	Night	29% 16% 32% 19% 4	880
November	1st to 10th	Night	28% 17% 30% 19% 6%	820
	11th to 20th	Night	25% 23% 30% 17% 5%	840
	21st to end	Night	27% 21% 34% 16% 2	840
December	1st to 10th	Night	27% 21% 34% 14% 4	840
	11th to 20th	Night	20% 27% 38% 13% 2	829
	21st to end	Night	30% 21% 33% 14% 2	1030

Page generated at 2025-12-15 01:13:53. Version ac07

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