Carlton Astronomy Campus
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Carlton Astronomy Campus clear sky chart from 2005-11-06 to 2026-04-02.

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	12% 6% 2 7% 5% 5% 5% 4 6% 7% 40%	130532
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	131449
Transparency	Day	26% 38% 8% 12% 12% 5%	134143
	Night	26% 37% 7% 12% 12% 6%	135915
Seeing	Day	15% 9% 22% 24% 21% 9%	16717
	Night	15% 8% 23% 29% 20% 4	26545

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	17% 9% 3 8% 4 3 3 3 4 6% 41%	5289
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 3 5% 52%	5310
	21st to end	Day	22% 10% 3 7% 4 4 4 3 4 4 36%	5842
February	1st to 10th	Day	16% 8% 3 7% 5% 4 5% 3 5% 5% 40%	2100
	11th to 20th	Day	18% 9% 3 6% 4 3 4 3 4 6% 39%	2048
	21st to end	Day	17% 8% 3 7% 4 4 5% 3 4 7% 37%	1730
March	1st to 10th	Day	19% 9% 3 8% 5% 4 5% 3 5% 6% 33%	5036
	11th to 20th	Day	14% 6% 2 5% 4 5% 4 3 5% 6% 45%	5090
	21st to end	Day	14% 8% 3 8% 6% 5% 4 4 4 5% 41%	5718
April	1st to 10th	Day	10% 7% 3 6% 5% 5% 5% 5% 5% 7% 42%	2412
	11th to 20th	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 41%	2374
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 6% 8% 43%	2400
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 6% 4 6% 8% 32%	5405
	11th to 20th	Day	11% 5% 2 6% 4 5% 6% 4 6% 9% 40%	5404
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	5932
June	1st to 10th	Day	9% 5% 3 8% 7% 6% 7% 6% 9% 10% 32%	2589
	11th to 20th	Day	6% 6% 2 7% 7% 7% 7% 6% 8% 10% 34%	2600
	21st to end	Day	7% 6% 2 8% 8% 7% 7% 6% 9% 10% 30%	2600
July	1st to 10th	Day	6% 5% 2 9% 7% 7% 9% 6% 10% 11% 28%	5900
	11th to 20th	Day	7% 4 2 7% 6% 7% 7% 6% 9% 11% 32%	5900
	21st to end	Day	9% 7% 3 8% 5% 7% 7% 5% 7% 11% 31%	6269
August	1st to 10th	Day	3 3 2 5% 4 4 5% 5% 6% 7% 57%	3905
	11th to 20th	Day	7% 4 2 5% 4 5% 6% 5% 9% 10% 44%	3849
	21st to end	Day	10% 6% 3 8% 6% 6% 6% 4 6% 7% 37%	4312
September	1st to 10th	Day	10% 5% 3 8% 6% 5% 6% 4 8% 8% 36%	2400
	11th to 20th	Day	12% 6% 2 7% 4 6% 5% 5% 6% 8% 40%	2400
	21st to end	Day	10% 6% 1 6% 4 4 3 3 5% 7% 51%	2180
October	1st to 10th	Day	7% 4 2 5% 5% 4 4 4 5% 7% 55%	2000
	11th to 20th	Day	9% 5% 1 5% 3 3 4 2 5% 5% 59%	2000
	21st to end	Day	11% 4 2 5% 4 3 3 2 4 5% 57%	2200
November	1st to 10th	Day	10% 6% 2 4 4 3 3 4 5% 6% 53%	2058
	11th to 20th	Day	9% 7% 2 5% 4 4 3 3 5% 5% 53%	2100
	21st to end	Day	14% 6% 2 6% 4 4 4 4 5% 6% 46%	1941
December	1st to 10th	Day	17% 9% 2 6% 4 4 3 3 5% 5% 41%	3020
	11th to 20th	Day	12% 7% 3 7% 4 4 5% 3 5% 6% 43%	3632
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 6% 46%	4587
January	1st to 10th	Night	21% 8% 2 5% 3 3 3 3 4 5% 43%	7634
	11th to 20th	Night	15% 7% 3 5% 3 3 3 2 4 5% 49%	7680
	21st to end	Night	25% 9% 3 6% 4 4 4 3 4 5% 35%	8366
February	1st to 10th	Night	15% 9% 3 6% 4 4 4 3 4 6% 43%	2520
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 5% 6% 40%	2458
	21st to end	Night	20% 8% 3 5% 4 3 4 4 5% 5% 40%	2076
March	1st to 10th	Night	19% 11% 3 6% 3 4 4 3 5% 6% 37%	5079
	11th to 20th	Night	14% 8% 2 6% 4 3 4 3 4 5% 45%	5090
	21st to end	Night	18% 8% 2 6% 4 4 4 3 4 5% 43%	5451
April	1st to 10th	Night	13% 7% 2 4 4 4 3 3 6% 8% 44%	2010
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 5% 7% 46%	1975
	21st to end	Night	10% 5% 2 5% 4 3 3 4 6% 7% 52%	2000
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 4 6% 8% 39%	4490
	11th to 20th	Night	15% 7% 2 5% 4 3 4 4 6% 8% 42%	4489
	21st to end	Night	10% 6% 2 6% 5% 5% 6% 4 7% 10% 39%	4922
June	1st to 10th	Night	7% 5% 2 7% 5% 4 4 5% 7% 11% 43%	1805
	11th to 20th	Night	3 4 1 5% 4 5% 5% 6% 9% 11% 47%	1800
	21st to end	Night	6% 5% 1 6% 6% 6% 6% 6% 9% 12% 38%	1800
July	1st to 10th	Night	6% 5% 3 8% 5% 6% 7% 6% 9% 11% 35%	4078
	11th to 20th	Night	5% 4 2 5% 4 6% 6% 6% 10% 14% 39%	4077
	21st to end	Night	6% 6% 3 7% 5% 5% 7% 6% 7% 12% 35%	4579
August	1st to 10th	Night	2 2 1 3 3 3 3 4 6% 11% 62%	3244
	11th to 20th	Night	3 3 1 5% 4 5% 6% 5% 8% 14% 46%	3200
	21st to end	Night	7% 5% 2 7% 5% 6% 5% 5% 6% 8% 42%	3586
September	1st to 10th	Night	8% 6% 2 6% 5% 5% 6% 4 7% 9% 43%	2000
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 4 7% 10% 45%	2000
	21st to end	Night	8% 5% 2 5% 4 4 4 3 6% 8% 52%	2193
October	1st to 10th	Night	7% 4 1 4 3 2 3 4 5% 6% 60%	2400
	11th to 20th	Night	8% 5% 2 4 3 3 3 4 4 5% 60%	2400
	21st to end	Night	11% 6% 2 4 4 3 2 3 4 5% 57%	2640
November	1st to 10th	Night	12% 4 1 4 4 3 3 2 4 6% 55%	2460
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 3 5% 56%	2520
	21st to end	Night	14% 7% 2 5% 4 3 4 3 5% 6% 48%	2669
December	1st to 10th	Night	21% 7% 2 6% 4 4 3 3 3 5% 43%	4380
	11th to 20th	Night	12% 10% 3 7% 4 3 3 3 5% 5% 44%	4745
	21st to end	Night	20% 10% 2 5% 3 3 2 2 3 4 46%	6633

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	38% 21% 5% 16% 18% 2	5290
	11th to 20th	Day	27% 21% 4 14% 26% 7%	5310
	21st to end	Day	44% 22% 3 7% 12% 12%	5842
February	1st to 10th	Day	19% 59% 3 7% 8% 5%	2100
	11th to 20th	Day	22% 59% 3 8% 8% 2	2058
	21st to end	Day	22% 59% 3 3 8% 6%	1730
March	1st to 10th	Day	40% 29% 4 9% 12% 6%	5036
	11th to 20th	Day	29% 29% 4 11% 13% 15%	5090
	21st to end	Day	35% 29% 8% 11% 13% 6%	5720
April	1st to 10th	Day	17% 61% 2 7% 8% 4	2412
	11th to 20th	Day	19% 62% 3 6% 6% 4	2374
	21st to end	Day	15% 63% 5% 8% 6% 3	2400
May	1st to 10th	Day	36% 32% 8% 16% 6% 2	5440
	11th to 20th	Day	29% 31% 12% 14% 13% 1	5408
	21st to end	Day	24% 32% 11% 21% 11% 2	5945
June	1st to 10th	Day	17% 63% 5% 7% 7% 1	2589
	11th to 20th	Day	20% 64% 5% 6% 4 1	2600
	21st to end	Day	18% 63% 8% 7% 3	2600
July	1st to 10th	Day	31% 32% 17% 14% 5%	5836
	11th to 20th	Day	29% 32% 24% 13% 1	5831
	21st to end	Day	33% 30% 12% 20% 4 1	6089
August	1st to 10th	Day	10% 37% 6% 25% 21% 1	3893
	11th to 20th	Day	15% 39% 15% 23% 8% 1	3885
	21st to end	Day	28% 38% 9% 10% 14% 1	4311
September	1st to 10th	Day	17% 59% 7% 8% 6% 2	2400
	11th to 20th	Day	14% 58% 6% 11% 9% 2	2400
	21st to end	Day	14% 58% 6% 12% 7% 3	2180
October	1st to 10th	Day	13% 36% 4 8% 12% 26%	3260
	11th to 20th	Day	6% 35% 4 11% 17% 26%	3240
	21st to end	Day	13% 35% 2 8% 28% 14%	3538
November	1st to 10th	Day	19% 54% 4 9% 9% 5%	2058
	11th to 20th	Day	18% 54% 4 8% 11% 6%	2100
	21st to end	Day	18% 55% 4 9% 11% 4	1941
December	1st to 10th	Day	32% 35% 3 8% 17% 4	3020
	11th to 20th	Day	26% 32% 6% 10% 17% 9%	3630
	21st to end	Day	30% 27% 4 15% 21% 3	4587
January	1st to 10th	Night	37% 21% 7% 15% 16% 5%	7635
	11th to 20th	Night	31% 21% 3 11% 23% 11%	7680
	21st to end	Night	44% 22% 3 10% 13% 7%	8366
February	1st to 10th	Night	17% 59% 3 7% 10% 4	2520
	11th to 20th	Night	21% 58% 2 8% 8% 3	2459
	21st to end	Night	20% 59% 3 6% 6% 6%	2076
March	1st to 10th	Night	38% 30% 6% 6% 14% 6%	5079
	11th to 20th	Night	28% 30% 4 11% 12% 15%	5090
	21st to end	Night	34% 29% 6% 14% 13% 5%	5451
April	1st to 10th	Night	17% 62% 3 6% 8% 4	2010
	11th to 20th	Night	16% 63% 4 7% 7% 3	1975
	21st to end	Night	14% 62% 5% 8% 8% 3	2000
May	1st to 10th	Night	33% 32% 10% 14% 9% 2	4520
	11th to 20th	Night	30% 30% 12% 17% 10%	4490
	21st to end	Night	28% 32% 12% 20% 7% 1	4931
June	1st to 10th	Night	12% 64% 6% 10% 8% 1	1805
	11th to 20th	Night	13% 65% 9% 8% 4 1	1800
	21st to end	Night	16% 63% 9% 9% 3	1800
July	1st to 10th	Night	29% 34% 20% 13% 4	4035
	11th to 20th	Night	25% 34% 30% 9% 1	4026
	21st to end	Night	31% 32% 12% 20% 4	4447
August	1st to 10th	Night	8% 37% 9% 23% 22% 1	3232
	11th to 20th	Night	15% 40% 17% 24% 4 1	3230
	21st to end	Night	26% 39% 8% 12% 12% 3	3586
September	1st to 10th	Night	14% 60% 8% 9% 7% 3	2000
	11th to 20th	Night	12% 59% 8% 12% 7% 3	2000
	21st to end	Night	15% 58% 6% 12% 7% 3	2193
October	1st to 10th	Night	13% 36% 5% 8% 14% 24%	3920
	11th to 20th	Night	7% 35% 2 14% 17% 25%	3899
	21st to end	Night	12% 35% 3 6% 28% 15%	4252
November	1st to 10th	Night	20% 54% 5% 7% 10% 5%	2460
	11th to 20th	Night	18% 54% 3 8% 11% 7%	2520
	21st to end	Night	17% 54% 4 9% 12% 5%	2669
December	1st to 10th	Night	33% 34% 2 10% 18% 2	4380
	11th to 20th	Night	28% 32% 5% 11% 16% 9%	4746
	21st to end	Night	33% 26% 4 14% 18% 5%	6633

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	24% 23% 34% 15% 4 1	330
	11th to 20th	Day	23% 18% 38% 16% 4	330
	21st to end	Day	29% 23% 35% 12% 2	363
February	1st to 10th	Day	24% 20% 36% 17% 3	330
	11th to 20th	Day	37% 16% 31% 12% 4 1	330
	21st to end	Day	24% 29% 39% 6% 2	273
March	1st to 10th	Day	25% 19% 38% 15% 3	330
	11th to 20th	Day	27% 21% 36% 13% 2	330
	21st to end	Day	18% 20% 31% 25% 5%	421
April	1st to 10th	Day	19% 18% 35% 21% 7% 1	504
	11th to 20th	Day	24% 15% 31% 19% 9% 2	492
	21st to end	Day	12% 11% 32% 29% 12% 2	500
May	1st to 10th	Day	17% 10% 30% 29% 12% 3	503
	11th to 20th	Day	14% 6% 27% 29% 17% 8%	527
	21st to end	Day	8% 6% 21% 32% 22% 9%	573
June	1st to 10th	Day	9% 4 15% 34% 28% 9%	719
	11th to 20th	Day	8% 4 16% 34% 29% 9%	730
	21st to end	Day	9% 3 11% 29% 29% 18%	730
July	1st to 10th	Day	6% 3 6% 28% 39% 17%	721
	11th to 20th	Day	6% 2 4 26% 41% 22%	718
	21st to end	Day	9% 3 7% 24% 41% 17%	729
August	1st to 10th	Day	5% 2 5% 20% 44% 24%	526
	11th to 20th	Day	9% 1 4 15% 43% 29%	519
	21st to end	Day	13% 4 7% 24% 29% 23%	583
September	1st to 10th	Day	12% 3 9% 26% 33% 17%	530
	11th to 20th	Day	12% 3 11% 23% 35% 15%	530
	21st to end	Day	13% 4 12% 28% 32% 11%	510
October	1st to 10th	Day	8% 6% 23% 39% 18% 6%	330
	11th to 20th	Day	9% 7% 24% 35% 20% 5%	330
	21st to end	Day	19% 9% 27% 28% 13% 4	363
November	1st to 10th	Day	15% 6% 32% 31% 13% 2	330
	11th to 20th	Day	14% 12% 35% 27% 11% 1	330
	21st to end	Day	18% 16% 37% 20% 8% 2	330
December	1st to 10th	Day	23% 14% 36% 20% 6% 1	330
	11th to 20th	Day	20% 13% 38% 22% 7%	330
	21st to end	Day	26% 17% 32% 19% 5% 1	363
January	1st to 10th	Night	25% 14% 33% 22% 5%	1050
	11th to 20th	Night	22% 15% 31% 24% 8%	1050
	21st to end	Night	26% 13% 32% 24% 4	1145
February	1st to 10th	Night	20% 16% 37% 24% 3	840
	11th to 20th	Night	27% 16% 29% 23% 4	823
	21st to end	Night	25% 21% 34% 18% 3	692
March	1st to 10th	Night	21% 14% 34% 26% 6%	656
	11th to 20th	Night	18% 14% 33% 29% 6%	630
	21st to end	Night	16% 11% 39% 27% 7%	693
April	1st to 10th	Night	18% 11% 36% 26% 8% 1	603
	11th to 20th	Night	17% 9% 34% 28% 10% 1	593
	21st to end	Night	11% 9% 35% 31% 14% 1	600
May	1st to 10th	Night	12% 5% 30% 38% 14% 1	600
	11th to 20th	Night	12% 5% 25% 36% 20% 3	591
	21st to end	Night	8% 4 19% 39% 25% 5%	650
June	1st to 10th	Night	8% 3 11% 40% 35% 5%	600
	11th to 20th	Night	4 1 12% 41% 34% 8%	600
	21st to end	Night	6% 2 8% 31% 38% 15%	600
July	1st to 10th	Night	4 2 5% 26% 49% 13%	585
	11th to 20th	Night	5% 2 5% 25% 46% 18%	582
	21st to end	Night	7% 1 6% 25% 49% 12%	657
August	1st to 10th	Night	4 1 4 25% 49% 17%	597
	11th to 20th	Night	6% 1 4 24% 48% 17%	594
	21st to end	Night	10% 1 6% 27% 40% 16%	660
September	1st to 10th	Night	9% 3 6% 28% 43% 12%	600
	11th to 20th	Night	10% 3 7% 31% 43% 8%	600
	21st to end	Night	9% 2 14% 32% 37% 5%	778
October	1st to 10th	Night	8% 3 19% 36% 31% 4	800
	11th to 20th	Night	9% 6% 23% 37% 22% 3	800
	21st to end	Night	15% 8% 21% 32% 22% 2	880
November	1st to 10th	Night	14% 6% 22% 33% 22% 2	820
	11th to 20th	Night	16% 9% 28% 33% 13% 1	840
	21st to end	Night	18% 9% 31% 28% 13% 1	840
December	1st to 10th	Night	21% 11% 29% 29% 9%	840
	11th to 20th	Night	21% 11% 31% 29% 8%	901
	21st to end	Night	27% 13% 27% 25% 7%	1155

Page generated at 2026-03-30 01:18:39. Version ac07

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