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 2024-01-13.

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	17% 12% 5% 8% 6% 3 4 4 5% 7% 28%	74840
	Night	16% 13% 6% 9% 7% 3 4 4 4 6% 27%	74863
Transparency	Day	56% 6% 11% 17% 9% 1	74547
	Night	58% 6% 10% 16% 9% 1	74691
Seeing	Day	21% 16% 30% 21% 9% 3	13887
	Night	21% 15% 31% 24% 9% 1	23182

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	15% 17% 11% 18% 10% 4 4 3 3 4 10%	1337
	11th to 20th	Day	15% 20% 11% 16% 10% 4 4 3 3 3 11%	1608
	21st to end	Day	11% 18% 10% 17% 10% 4 6% 5% 4 4 11%	1782
February	1st to 10th	Day	14% 19% 9% 18% 12% 5% 4 3 3 3 10%	1620
	11th to 20th	Day	14% 14% 9% 16% 13% 4 3 6% 4 5% 13%	1573
	21st to end	Day	15% 20% 8% 11% 9% 4 5% 4 3 4 16%	1426
March	1st to 10th	Day	15% 13% 5% 11% 10% 5% 3 4 4 5% 25%	1937
	11th to 20th	Day	13% 13% 7% 10% 8% 4 5% 4 3 5% 27%	1980
	21st to end	Day	18% 12% 5% 8% 6% 3 4 5% 5% 5% 29%	2178
April	1st to 10th	Day	18% 15% 7% 10% 5% 3 4 3 5% 6% 25%	2078
	11th to 20th	Day	19% 14% 4 5% 4 3 3 5% 4 6% 34%	2284
	21st to end	Day	21% 12% 5% 7% 5% 3 3 4 4 7% 31%	2340
May	1st to 10th	Day	19% 8% 3 7% 5% 3 4 4 5% 8% 33%	2340
	11th to 20th	Day	17% 10% 3 6% 5% 3 4 4 6% 5% 37%	2401
	21st to end	Day	15% 7% 3 5% 4 3 5% 5% 6% 8% 41%	2725
June	1st to 10th	Day	16% 7% 4 5% 5% 3 5% 5% 5% 8% 37%	2559
	11th to 20th	Day	14% 6% 2 5% 5% 3 4 5% 7% 9% 41%	2700
	21st to end	Day	17% 7% 2 5% 5% 3 5% 4 6% 8% 39%	2700
July	1st to 10th	Day	12% 5% 2 4 5% 3 5% 6% 6% 9% 43%	2700
	11th to 20th	Day	14% 6% 3 6% 6% 3 6% 6% 7% 8% 37%	2572
	21st to end	Day	12% 6% 3 6% 6% 4 5% 6% 7% 10% 35%	2547
August	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 6% 6% 11% 36%	2327
	11th to 20th	Day	14% 6% 2 6% 7% 4 6% 5% 6% 9% 35%	2303
	21st to end	Day	14% 7% 3 6% 5% 5% 5% 5% 5% 9% 36%	2535
September	1st to 10th	Day	16% 8% 3 5% 6% 3 4 5% 5% 7% 36%	2160
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 4 6% 8% 41%	1980
	21st to end	Day	18% 9% 3 7% 5% 3 5% 5% 5% 8% 33%	1980
October	1st to 10th	Day	22% 10% 4 6% 5% 3 4 4 4 6% 31%	1810
	11th to 20th	Day	27% 15% 4 6% 6% 4 5% 5% 4 6% 20%	1800
	21st to end	Day	28% 19% 3 7% 6% 3 5% 3 4 5% 16%	1947
November	1st to 10th	Day	17% 20% 5% 6% 6% 2 4 4 4 6% 25%	1621
	11th to 20th	Day	17% 19% 7% 9% 7% 3 5% 4 5% 6% 17%	1520
	21st to end	Day	19% 23% 9% 9% 5% 3 4 4 4 6% 13%	1520
December	1st to 10th	Day	15% 25% 14% 16% 6% 3 4 4 3 4 7%	1520
	11th to 20th	Day	19% 20% 9% 14% 8% 3 4 4 4 4 11%	1906
	21st to end	Day	25% 21% 10% 14% 6% 3 3 3 3 4 10%	2524
January	1st to 10th	Night	15% 18% 10% 16% 10% 4 5% 4 3 4 10%	2826
	11th to 20th	Night	19% 19% 8% 14% 10% 4 5% 4 3 4 10%	2400
	21st to end	Night	11% 17% 9% 14% 12% 6% 7% 5% 5% 4 11%	2574
February	1st to 10th	Night	14% 19% 9% 14% 13% 5% 6% 4 4 3 11%	2340
	11th to 20th	Night	13% 15% 9% 13% 10% 4 5% 3 4 5% 17%	2273
	21st to end	Night	15% 14% 8% 12% 8% 3 5% 5% 4 5% 20%	1827
March	1st to 10th	Night	14% 12% 7% 11% 8% 3 5% 4 4 4 28%	2009
	11th to 20th	Night	13% 15% 6% 9% 7% 3 4 4 4 5% 30%	1979
	21st to end	Night	16% 12% 5% 7% 6% 4 5% 4 4 5% 32%	2178
April	1st to 10th	Night	19% 13% 5% 8% 4 3 5% 4 4 6% 27%	1881
	11th to 20th	Night	17% 12% 4 5% 5% 3 4 4 4 6% 36%	1641
	21st to end	Night	16% 14% 4 7% 5% 4 5% 5% 4 5% 31%	1620
May	1st to 10th	Night	16% 9% 3 5% 6% 3 4 5% 5% 8% 36%	1620
	11th to 20th	Night	15% 8% 3 5% 5% 2 5% 5% 5% 6% 42%	1491
	21st to end	Night	10% 5% 2 5% 5% 3 5% 5% 5% 8% 48%	1550
June	1st to 10th	Night	14% 7% 2 7% 4 2 5% 4 6% 8% 40%	1396
	11th to 20th	Night	13% 4 2 5% 4 3 5% 6% 6% 9% 42%	1260
	21st to end	Night	15% 5% 3 5% 4 3 3 4 5% 8% 43%	1260
July	1st to 10th	Night	9% 5% 2 4 3 2 4 4 5% 9% 54%	1260
	11th to 20th	Night	11% 5% 3 4 4 3 3 5% 6% 9% 45%	1383
	21st to end	Night	10% 5% 3 6% 6% 3 5% 5% 6% 7% 46%	1771
August	1st to 10th	Night	11% 6% 2 4 4 3 5% 4 4 8% 48%	1611
	11th to 20th	Night	10% 4 2 5% 5% 3 4 4 6% 10% 47%	1598
	21st to end	Night	13% 6% 3 6% 6% 3 4 4 4 7% 42%	1802
September	1st to 10th	Night	13% 5% 3 6% 5% 4 5% 4 5% 8% 45%	1800
	11th to 20th	Night	14% 6% 2 5% 5% 2 4 4 5% 7% 47%	1977
	21st to end	Night	17% 8% 3 7% 5% 3 4 4 4 7% 38%	1980
October	1st to 10th	Night	19% 9% 3 6% 6% 3 4 3 5% 7% 35%	2150
	11th to 20th	Night	25% 11% 4 6% 6% 3 5% 4 5% 5% 24%	2160
	21st to end	Night	25% 15% 4 7% 7% 3 5% 5% 4 6% 21%	2407
November	1st to 10th	Night	18% 17% 4 7% 6% 3 5% 4 4 6% 27%	2454
	11th to 20th	Night	18% 17% 6% 9% 6% 4 5% 5% 5% 5% 21%	2660
	21st to end	Night	20% 19% 7% 10% 6% 3 4 4 4 6% 16%	2660
December	1st to 10th	Night	20% 23% 11% 13% 6% 3 4 3 3 4 10%	2660
	11th to 20th	Night	14% 21% 10% 15% 8% 3 4 4 4 4 13%	3288
	21st to end	Night	22% 22% 10% 16% 7% 3 3 3 3 3 7%	5117

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	79% 5% 8% 6% 2	1338
	11th to 20th	Day	80% 4 6% 5% 4 1	1608
	21st to end	Day	76% 5% 7% 6% 5% 1	1782
February	1st to 10th	Day	81% 3 4 8% 2 1	1620
	11th to 20th	Day	74% 4 7% 9% 5% 1	1582
	21st to end	Day	73% 5% 7% 10% 5%	1426
March	1st to 10th	Day	63% 5% 11% 11% 8% 1	1940
	11th to 20th	Day	59% 6% 9% 15% 9% 2	1980
	21st to end	Day	57% 5% 10% 14% 11% 2	2178
April	1st to 10th	Day	61% 6% 8% 16% 9%	2078
	11th to 20th	Day	51% 5% 10% 20% 14% 1	2284
	21st to end	Day	54% 6% 10% 17% 11% 2	2340
May	1st to 10th	Day	50% 6% 9% 21% 13% 1	2340
	11th to 20th	Day	48% 6% 9% 23% 12% 1	2405
	21st to end	Day	41% 6% 15% 23% 14% 2	2725
June	1st to 10th	Day	44% 7% 11% 22% 14% 1	2559
	11th to 20th	Day	40% 7% 12% 24% 16% 1	2700
	21st to end	Day	43% 7% 13% 23% 13% 1	2700
July	1st to 10th	Day	37% 7% 13% 23% 18% 2	2626
	11th to 20th	Day	41% 8% 13% 26% 11% 1	2491
	21st to end	Day	43% 8% 12% 22% 13% 2	2405
August	1st to 10th	Day	42% 9% 16% 22% 10% 2	2314
	11th to 20th	Day	44% 9% 15% 22% 10%	2303
	21st to end	Day	44% 9% 14% 21% 11% 1	2535
September	1st to 10th	Day	46% 7% 12% 22% 10% 3	2160
	11th to 20th	Day	41% 6% 14% 29% 11%	1980
	21st to end	Day	49% 7% 12% 23% 7% 1	1980
October	1st to 10th	Day	55% 6% 10% 16% 12% 1	1810
	11th to 20th	Day	64% 7% 10% 13% 5% 1	1800
	21st to end	Day	70% 6% 8% 10% 5%	1947
November	1st to 10th	Day	61% 6% 11% 14% 7%	1621
	11th to 20th	Day	67% 7% 10% 10% 6% 1	1520
	21st to end	Day	73% 7% 9% 10% 1	1520
December	1st to 10th	Day	83% 5% 5% 6% 1	1520
	11th to 20th	Day	75% 7% 9% 8% 1	1906
	21st to end	Day	83% 5% 7% 5% 1	2524
January	1st to 10th	Night	78% 5% 8% 6% 3	2826
	11th to 20th	Night	79% 5% 7% 4 4 1	2400
	21st to end	Night	76% 5% 9% 5% 4 1	2574
February	1st to 10th	Night	77% 5% 7% 9% 1	2340
	11th to 20th	Night	69% 5% 10% 11% 5%	2275
	21st to end	Night	66% 6% 10% 10% 7%	1827
March	1st to 10th	Night	58% 6% 10% 16% 9% 1	2019
	11th to 20th	Night	56% 6% 8% 17% 11% 2	1980
	21st to end	Night	53% 5% 10% 17% 13% 2	2178
April	1st to 10th	Night	57% 6% 8% 16% 12% 1	1881
	11th to 20th	Night	49% 5% 8% 20% 16% 2	1641
	21st to end	Night	54% 6% 9% 15% 14% 2	1620
May	1st to 10th	Night	46% 5% 9% 22% 15% 2	1620
	11th to 20th	Night	42% 6% 9% 24% 16% 3	1492
	21st to end	Night	34% 7% 15% 26% 15% 4	1551
June	1st to 10th	Night	41% 5% 12% 24% 16% 2	1396
	11th to 20th	Night	37% 8% 13% 25% 17% 1	1260
	21st to end	Night	39% 6% 12% 27% 14% 2	1260
July	1st to 10th	Night	27% 7% 13% 28% 23% 3	1225
	11th to 20th	Night	35% 6% 13% 29% 16% 1	1341
	21st to end	Night	36% 8% 12% 26% 16% 2	1672
August	1st to 10th	Night	34% 7% 15% 28% 14% 1	1600
	11th to 20th	Night	32% 7% 15% 28% 15% 2	1598
	21st to end	Night	41% 7% 11% 26% 12% 2	1802
September	1st to 10th	Night	37% 8% 13% 28% 13% 2	1800
	11th to 20th	Night	37% 5% 14% 29% 15% 1	1977
	21st to end	Night	46% 7% 12% 24% 10% 1	1980
October	1st to 10th	Night	49% 7% 11% 18% 13% 1	2150
	11th to 20th	Night	60% 6% 12% 16% 6%	2160
	21st to end	Night	65% 7% 10% 11% 6%	2407
November	1st to 10th	Night	59% 6% 12% 17% 7% 1	2454
	11th to 20th	Night	64% 6% 12% 12% 4 2	2660
	21st to end	Night	69% 7% 13% 8% 2 1	2660
December	1st to 10th	Night	79% 6% 8% 6% 1	2660
	11th to 20th	Night	75% 6% 10% 8% 1	3288
	21st to end	Night	84% 5% 6% 4 1	5117

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% 24% 34% 15% 3 1	184
	11th to 20th	Day	20% 27% 39% 10% 4	246
	21st to end	Day	23% 25% 34% 15% 3	264
February	1st to 10th	Day	29% 31% 32% 7% 2	240
	11th to 20th	Day	25% 28% 39% 8% 1	240
	21st to end	Day	30% 35% 28% 6% 1	198
March	1st to 10th	Day	25% 21% 35% 13% 5%	380
	11th to 20th	Day	24% 17% 39% 15% 3 1	420
	21st to end	Day	28% 16% 34% 16% 5%	462
April	1st to 10th	Day	24% 21% 33% 17% 5% 1	420
	11th to 20th	Day	29% 19% 25% 14% 9% 4	412
	21st to end	Day	28% 15% 30% 20% 5% 2	420
May	1st to 10th	Day	23% 15% 31% 23% 8% 1	423
	11th to 20th	Day	20% 13% 29% 22% 12% 4	447
	21st to end	Day	20% 10% 26% 27% 13% 4	485
June	1st to 10th	Day	17% 14% 27% 31% 9% 2	489
	11th to 20th	Day	16% 12% 31% 29% 10% 3	630
	21st to end	Day	17% 10% 35% 26% 10% 2	630
July	1st to 10th	Day	12% 9% 28% 30% 17% 5%	621
	11th to 20th	Day	14% 11% 27% 27% 13% 7%	532
	21st to end	Day	9% 7% 34% 32% 13% 4	488
August	1st to 10th	Day	12% 9% 23% 32% 16% 8%	446
	11th to 20th	Day	16% 5% 21% 28% 21% 9%	439
	21st to end	Day	16% 7% 30% 25% 17% 6%	495
September	1st to 10th	Day	17% 12% 25% 25% 14% 7%	450
	11th to 20th	Day	12% 10% 23% 26% 21% 8%	450
	21st to end	Day	17% 16% 27% 22% 14% 4	450
October	1st to 10th	Day	26% 14% 26% 19% 13% 3	450
	11th to 20th	Day	28% 21% 32% 16% 4	450
	21st to end	Day	37% 20% 25% 15% 4	462
November	1st to 10th	Day	30% 22% 27% 15% 6%	246
	11th to 20th	Day	27% 24% 34% 9% 3 2	180
	21st to end	Day	27% 24% 38% 9% 1	180
December	1st to 10th	Day	28% 29% 31% 12%	180
	11th to 20th	Day	29% 28% 33% 8% 2	180
	21st to end	Day	34% 25% 33% 7% 1	198
January	1st to 10th	Night	21% 24% 34% 16% 4	950
	11th to 20th	Night	27% 21% 32% 17% 3 1	754
	21st to end	Night	18% 16% 40% 20% 5%	792
February	1st to 10th	Night	22% 25% 36% 15% 1	720
	11th to 20th	Night	22% 20% 36% 18% 4	703
	21st to end	Night	23% 25% 34% 16% 3	592
March	1st to 10th	Night	18% 20% 38% 19% 4	720
	11th to 20th	Night	20% 21% 37% 17% 5% 1	720
	21st to end	Night	22% 15% 30% 25% 7% 1	792
April	1st to 10th	Night	24% 17% 33% 23% 3	720
	11th to 20th	Night	21% 18% 29% 22% 8% 2	577
	21st to end	Night	25% 13% 28% 25% 9%	540
May	1st to 10th	Night	19% 9% 31% 29% 10% 1	540
	11th to 20th	Night	18% 8% 30% 33% 11%	430
	21st to end	Night	13% 4 32% 36% 14% 1	389
June	1st to 10th	Night	16% 7% 34% 33% 10%	360
	11th to 20th	Night	13% 8% 31% 36% 13%	360
	21st to end	Night	15% 6% 31% 33% 15% 1	360
July	1st to 10th	Night	9% 5% 19% 48% 19% 1	350
	11th to 20th	Night	14% 5% 25% 36% 18% 1	387
	21st to end	Night	11% 5% 23% 42% 18% 1	591
August	1st to 10th	Night	12% 3 24% 36% 25% 1	537
	11th to 20th	Night	11% 3 20% 38% 25% 3	534
	21st to end	Night	15% 6% 27% 26% 24% 2	614
September	1st to 10th	Night	14% 8% 25% 31% 17% 5%	720
	11th to 20th	Night	15% 6% 25% 34% 16% 5%	720
	21st to end	Night	19% 10% 24% 30% 14% 4	720
October	1st to 10th	Night	23% 11% 28% 24% 13% 1	720
	11th to 20th	Night	29% 14% 28% 22% 6% 1	720
	21st to end	Night	31% 16% 31% 18% 3	792
November	1st to 10th	Night	27% 15% 31% 20% 6%	740
	11th to 20th	Night	25% 23% 30% 17% 4 1	760
	21st to end	Night	26% 20% 34% 17% 2	760
December	1st to 10th	Night	29% 21% 33% 13% 3	760
	11th to 20th	Night	20% 26% 38% 13% 3	760
	21st to end	Night	31% 22% 33% 14% 1	978

Page generated at 2024-01-10 10:16:12. Version ac07

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