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 2024-11-11.

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% 5% 4 4 5% 6% 29%	101733
	Night	15% 14% 5% 11% 6% 5% 4 3 4 5% 28%	97426
Transparency	Day	54% 6% 11% 18% 10% 1	105305
	Night	56% 6% 11% 17% 9% 1	101943
Seeing	Day	21% 16% 30% 21% 9% 3	15072
	Night	21% 15% 31% 24% 9% 1	24190

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	14% 21% 12% 19% 7% 5% 3 2 3 4 10%	2217
	11th to 20th	Day	9% 21% 11% 23% 12% 10% 3 1 2 2 6%	2786
	21st to end	Day	16% 22% 9% 18% 7% 5% 4 3 3 3 9%	3124
February	1st to 10th	Day	13% 21% 7% 20% 10% 6% 4 2 3 3 11%	1710
	11th to 20th	Day	13% 16% 9% 19% 10% 5% 3 4 4 5% 13%	1663
	21st to end	Day	14% 21% 7% 13% 8% 5% 5% 3 4 4 16%	1513
March	1st to 10th	Day	16% 15% 5% 14% 6% 4 3 3 4 4 26%	3368
	11th to 20th	Day	11% 16% 9% 14% 6% 5% 4 3 3 5% 24%	3480
	21st to end	Day	16% 14% 6% 10% 6% 4 4 3 5% 5% 27%	3815
April	1st to 10th	Day	18% 16% 5% 11% 4 4 3 3 5% 5% 25%	2194
	11th to 20th	Day	19% 15% 3 5% 4 3 4 3 4 6% 34%	2413
	21st to end	Day	21% 13% 4 7% 4 4 3 3 4 6% 32%	2470
May	1st to 10th	Day	19% 9% 3 7% 4 4 4 3 5% 7% 33%	4108
	11th to 20th	Day	17% 10% 3 7% 5% 5% 4 3 6% 7% 33%	4265
	21st to end	Day	16% 6% 2 5% 3 4 4 3 5% 7% 45%	4837
June	1st to 10th	Day	16% 8% 4 5% 4 4 5% 4 6% 7% 36%	2702
	11th to 20th	Day	14% 6% 2 7% 4 4 4 4 6% 9% 40%	2850
	21st to end	Day	18% 8% 1 6% 4 4 4 4 6% 7% 38%	2850
July	1st to 10th	Day	14% 7% 2 7% 6% 5% 6% 5% 7% 8% 34%	4760
	11th to 20th	Day	13% 6% 2 6% 5% 4 5% 4 6% 8% 41%	4520
	21st to end	Day	10% 6% 3 6% 5% 5% 5% 4 7% 9% 40%	4466
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 5% 6% 8% 34%	4104
	11th to 20th	Day	16% 8% 3 7% 5% 5% 6% 4 7% 9% 31%	4044
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 6% 9% 41%	4451
September	1st to 10th	Day	16% 8% 3 6% 5% 4 4 4 5% 7% 37%	2280
	11th to 20th	Day	14% 6% 2 6% 4 4 4 3 6% 8% 43%	2090
	21st to end	Day	17% 10% 2 8% 4 4 5% 4 5% 8% 33%	2090
October	1st to 10th	Day	21% 10% 4 7% 4 4 4 3 4 6% 32%	1910
	11th to 20th	Day	25% 15% 3 7% 5% 5% 5% 4 4 6% 22%	1900
	21st to end	Day	27% 19% 3 8% 5% 4 5% 3 4 5% 17%	2055
November	1st to 10th	Day	17% 21% 4 8% 5% 4 4 3 4 6% 25%	1708
	11th to 20th	Day	17% 21% 6% 11% 6% 4 4 3 5% 6% 17%	1520
	21st to end	Day	18% 25% 8% 9% 5% 4 5% 3 4 6% 13%	1520
December	1st to 10th	Day	14% 28% 11% 17% 5% 4 4 3 3 4 6%	1520
	11th to 20th	Day	19% 22% 8% 15% 6% 4 3 4 4 4 11%	1906
	21st to end	Day	25% 22% 9% 16% 4 4 2 2 3 3 10%	2524
January	1st to 10th	Night	13% 22% 12% 23% 8% 4 4 3 3 3 7%	4736
	11th to 20th	Night	13% 15% 9% 25% 12% 9% 4 2 2 3 6%	4155
	21st to end	Night	16% 19% 10% 18% 8% 6% 5% 3 4 3 10%	4532
February	1st to 10th	Night	13% 21% 8% 15% 11% 6% 4 3 4 3 12%	2470
	11th to 20th	Night	12% 17% 8% 15% 8% 5% 5% 3 4 5% 18%	2403
	21st to end	Night	14% 15% 6% 13% 7% 5% 5% 4 5% 5% 20%	1938
March	1st to 10th	Night	14% 13% 6% 10% 5% 4 4 4 4 5% 31%	3471
	11th to 20th	Night	10% 16% 7% 11% 5% 4 4 3 4 5% 30%	3479
	21st to end	Night	15% 10% 5% 12% 5% 5% 5% 3 5% 4 31%	3817
April	1st to 10th	Night	19% 16% 4 9% 4 4 4 3 4 6% 27%	1985
	11th to 20th	Night	18% 12% 4 6% 4 4 4 3 4 6% 35%	1733
	21st to end	Night	16% 14% 4 7% 5% 5% 4 4 4 5% 31%	1710
May	1st to 10th	Night	15% 9% 2 6% 4 4 4 4 5% 8% 40%	2833
	11th to 20th	Night	16% 8% 3 6% 5% 4 5% 4 5% 6% 39%	2627
	21st to end	Night	12% 6% 2 6% 4 4 4 3 5% 7% 49%	2738
June	1st to 10th	Night	15% 7% 2 7% 4 3 5% 3 6% 8% 39%	1473
	11th to 20th	Night	12% 5% 2 5% 5% 4 5% 5% 6% 9% 40%	1330
	21st to end	Night	16% 6% 3 6% 3 5% 3 4 6% 7% 43%	1330
July	1st to 10th	Night	9% 6% 2 7% 4 3 4 4 5% 7% 49%	2210
	11th to 20th	Night	14% 6% 3 4 3 3 3 3 5% 7% 50%	2440
	21st to end	Night	8% 7% 2 6% 4 4 5% 4 6% 7% 46%	3087
August	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 4 6% 46%	2825
	11th to 20th	Night	17% 8% 3 7% 4 4 4 3 5% 8% 39%	2785
	21st to end	Night	14% 5% 2 6% 4 4 4 3 4 7% 46%	3172
September	1st to 10th	Night	12% 6% 2 6% 4 5% 4 3 4 7% 46%	1900
	11th to 20th	Night	13% 6% 2 6% 4 3 3 3 4 7% 49%	2087
	21st to end	Night	17% 8% 3 8% 4 4 4 3 4 7% 37%	2090
October	1st to 10th	Night	19% 9% 3 7% 4 4 4 3 5% 7% 36%	2270
	11th to 20th	Night	24% 12% 4 8% 5% 5% 5% 3 5% 5% 26%	2280
	21st to end	Night	24% 15% 4 8% 5% 4 5% 4 4 6% 21%	2541
November	1st to 10th	Night	18% 17% 4 7% 5% 4 4 3 4 6% 26%	2587
	11th to 20th	Night	17% 19% 5% 10% 5% 5% 5% 3 5% 5% 21%	2667
	21st to end	Night	20% 21% 7% 11% 5% 4 4 3 4 6% 16%	2660
December	1st to 10th	Night	19% 26% 10% 14% 5% 4 3 3 4 3 10%	2660
	11th to 20th	Night	14% 23% 9% 16% 7% 3 4 3 4 4 13%	3288
	21st to end	Night	22% 23% 9% 17% 6% 4 3 2 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	76% 6% 10% 7% 1	2218
	11th to 20th	Day	88% 3 3 3 2	2786
	21st to end	Day	79% 6% 7% 5% 3	3124
February	1st to 10th	Day	79% 4 4 9% 2 1	1710
	11th to 20th	Day	74% 4 7% 9% 5% 1	1672
	21st to end	Day	72% 5% 7% 11% 5%	1513
March	1st to 10th	Day	62% 6% 11% 13% 8% 1	3371
	11th to 20th	Day	63% 5% 9% 16% 5% 1	3480
	21st to end	Day	56% 7% 11% 15% 9% 2	3815
April	1st to 10th	Day	61% 6% 8% 16% 9%	2194
	11th to 20th	Day	51% 5% 9% 20% 14% 1	2413
	21st to end	Day	54% 6% 10% 17% 11% 2	2470
May	1st to 10th	Day	47% 8% 12% 19% 14% 1	4120
	11th to 20th	Day	48% 8% 13% 22% 8% 1	4269
	21st to end	Day	38% 6% 12% 21% 18% 5%	4837
June	1st to 10th	Day	46% 7% 11% 21% 14% 1	2702
	11th to 20th	Day	40% 7% 12% 24% 16% 1	2850
	21st to end	Day	44% 7% 13% 23% 12% 1	2850
July	1st to 10th	Day	46% 7% 12% 18% 15% 1	4686
	11th to 20th	Day	38% 7% 11% 31% 12% 1	4439
	21st to end	Day	39% 7% 11% 24% 18% 2	4324
August	1st to 10th	Day	46% 7% 13% 21% 13% 1	4091
	11th to 20th	Day	47% 8% 14% 22% 9%	4083
	21st to end	Day	38% 8% 12% 31% 10% 1	4451
September	1st to 10th	Day	46% 7% 12% 23% 11% 2	2280
	11th to 20th	Day	39% 6% 14% 28% 13% 1	2090
	21st to end	Day	49% 8% 12% 23% 8% 1	2090
October	1st to 10th	Day	49% 6% 10% 22% 12% 1	3170
	11th to 20th	Day	53% 5% 10% 19% 12% 1	3140
	21st to end	Day	62% 5% 11% 17% 5%	3369
November	1st to 10th	Day	61% 6% 10% 15% 7%	1708
	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	80% 7% 7% 4 2	4736
	11th to 20th	Night	86% 4 5% 3 2	4155
	21st to end	Night	79% 6% 9% 4 2 1	4532
February	1st to 10th	Night	75% 5% 7% 10% 2	2470
	11th to 20th	Night	68% 5% 10% 11% 5%	2405
	21st to end	Night	65% 6% 11% 11% 7%	1938
March	1st to 10th	Night	53% 7% 12% 19% 7% 1	3481
	11th to 20th	Night	54% 7% 12% 17% 9% 1	3480
	21st to end	Night	52% 7% 11% 18% 11% 1	3817
April	1st to 10th	Night	58% 6% 8% 16% 12% 1	1985
	11th to 20th	Night	50% 5% 8% 20% 16% 1	1733
	21st to end	Night	54% 6% 9% 14% 15% 2	1710
May	1st to 10th	Night	43% 6% 11% 20% 15% 5%	2840
	11th to 20th	Night	42% 9% 12% 23% 12% 2	2628
	21st to end	Night	34% 7% 12% 23% 20% 3	2739
June	1st to 10th	Night	42% 5% 12% 23% 16% 2	1473
	11th to 20th	Night	37% 8% 13% 24% 17% 1	1330
	21st to end	Night	40% 6% 12% 26% 14% 1	1330
July	1st to 10th	Night	34% 6% 14% 23% 19% 4	2175
	11th to 20th	Night	35% 5% 11% 33% 15% 1	2398
	21st to end	Night	36% 8% 12% 27% 16% 1	2988
August	1st to 10th	Night	39% 5% 11% 28% 16% 1	2814
	11th to 20th	Night	44% 5% 15% 23% 12% 1	2812
	21st to end	Night	38% 7% 11% 31% 13% 1	3172
September	1st to 10th	Night	36% 7% 13% 28% 14% 2	1900
	11th to 20th	Night	35% 5% 13% 28% 18% 1	2087
	21st to end	Night	46% 7% 12% 24% 10% 1	2090
October	1st to 10th	Night	41% 6% 9% 24% 18% 1	3790
	11th to 20th	Night	51% 4 12% 18% 14%	3779
	21st to end	Night	53% 7% 15% 20% 6%	4177
November	1st to 10th	Night	59% 6% 11% 17% 7% 1	2587
	11th to 20th	Night	64% 6% 12% 12% 4 2	2667
	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% 29% 36% 11% 4	270
	21st to end	Day	25% 23% 34% 16% 3	297
February	1st to 10th	Day	26% 29% 33% 9% 3	270
	11th to 20th	Day	24% 27% 39% 9% 1	270
	21st to end	Day	31% 32% 29% 8%	225
March	1st to 10th	Day	25% 20% 36% 14% 5%	418
	11th to 20th	Day	24% 18% 37% 16% 3 1	460
	21st to end	Day	27% 18% 34% 17% 5%	506
April	1st to 10th	Day	23% 22% 33% 16% 5% 1	460
	11th to 20th	Day	31% 21% 24% 13% 8% 4	452
	21st to end	Day	29% 15% 30% 19% 5% 2	460
May	1st to 10th	Day	23% 14% 31% 23% 8% 2	463
	11th to 20th	Day	19% 13% 28% 23% 13% 4	487
	21st to end	Day	20% 11% 26% 27% 12% 4	529
June	1st to 10th	Day	18% 15% 27% 30% 9% 2	532
	11th to 20th	Day	15% 12% 32% 29% 9% 3	680
	21st to end	Day	19% 11% 35% 24% 9% 2	680
July	1st to 10th	Day	12% 10% 29% 29% 16% 4	671
	11th to 20th	Day	14% 11% 28% 27% 13% 7%	577
	21st to end	Day	9% 7% 33% 31% 14% 5%	532
August	1st to 10th	Day	13% 11% 23% 31% 15% 8%	486
	11th to 20th	Day	16% 7% 21% 27% 20% 9%	479
	21st to end	Day	15% 7% 29% 26% 17% 6%	539
September	1st to 10th	Day	17% 13% 24% 24% 14% 7%	490
	11th to 20th	Day	12% 9% 21% 26% 22% 11%	490
	21st to end	Day	17% 16% 27% 22% 14% 4	490
October	1st to 10th	Day	25% 13% 27% 19% 12% 3	490
	11th to 20th	Day	26% 20% 31% 17% 4 1	490
	21st to end	Day	36% 19% 26% 15% 4	504
November	1st to 10th	Day	30% 22% 27% 15% 5%	273
	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	26% 21% 32% 17% 2 1	782
	21st to end	Night	18% 16% 40% 20% 5%	836
February	1st to 10th	Night	21% 25% 35% 16% 2	760
	11th to 20th	Night	21% 20% 36% 18% 3	743
	21st to end	Night	22% 24% 35% 16% 3	628
March	1st to 10th	Night	19% 19% 39% 19% 4	760
	11th to 20th	Night	19% 22% 37% 17% 5% 1	760
	21st to end	Night	21% 15% 31% 25% 7% 1	836
April	1st to 10th	Night	25% 17% 33% 23% 3	760
	11th to 20th	Night	22% 18% 29% 22% 8% 2	609
	21st to end	Night	25% 13% 28% 25% 9%	570
May	1st to 10th	Night	19% 9% 32% 30% 10% 1	570
	11th to 20th	Night	17% 8% 30% 33% 11%	454
	21st to end	Night	13% 5% 31% 37% 14%	411
June	1st to 10th	Night	17% 7% 33% 33% 10%	380
	11th to 20th	Night	12% 8% 31% 37% 12%	380
	21st to end	Night	15% 7% 31% 33% 14% 1	380
July	1st to 10th	Night	9% 5% 20% 48% 18% 1	370
	11th to 20th	Night	14% 6% 25% 36% 19% 1	410
	21st to end	Night	11% 5% 22% 42% 19% 1	624
August	1st to 10th	Night	12% 3 23% 37% 24% 1	567
	11th to 20th	Night	12% 3 20% 37% 25% 3	564
	21st to end	Night	15% 5% 27% 27% 24% 2	649
September	1st to 10th	Night	14% 8% 25% 31% 18% 4	760
	11th to 20th	Night	15% 5% 24% 34% 18% 5%	760
	21st to end	Night	19% 10% 24% 30% 14% 3	760
October	1st to 10th	Night	23% 11% 29% 25% 12% 1	760
	11th to 20th	Night	28% 14% 27% 22% 7% 1	760
	21st to end	Night	30% 16% 31% 18% 3	836
November	1st to 10th	Night	28% 16% 30% 20% 6%	780
	11th to 20th	Night	25% 23% 30% 17% 4 1	763
	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 2024-11-08 06:44:47. Version ac07

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