Hume Cronyn Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hume Cronyn Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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	14% 11% 4 9% 5% 5% 5% 4 5% 7% 31%	144345
	Night	14% 12% 4 10% 5% 5% 5% 4 5% 6% 31%	137424
Transparency	Day	50% 7% 12% 19% 11% 1	147942
	Night	51% 7% 11% 18% 12% 1	141916
Seeing	Day	18% 12% 28% 23% 13% 5%	17339
	Night	19% 10% 29% 29% 11% 1	26358

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% 7% 25% 7% 5% 4 3 3 3 11%	4729
	11th to 20th	Day	10% 23% 11% 23% 8% 7% 4 2 2 3 6%	5246
	21st to end	Day	15% 18% 7% 14% 8% 7% 5% 3 4 5% 13%	5780
February	1st to 10th	Day	15% 17% 5% 13% 8% 6% 4 4 4 5% 17%	1890
	11th to 20th	Day	14% 14% 8% 14% 8% 6% 4 3 4 6% 18%	1843
	21st to end	Day	16% 17% 6% 12% 6% 5% 5% 3 4 4 21%	1730
March	1st to 10th	Day	17% 13% 7% 13% 7% 3 3 3 3 5% 26%	4906
	11th to 20th	Day	12% 13% 5% 10% 5% 4 4 3 5% 8% 30%	5090
	21st to end	Day	19% 13% 6% 10% 5% 4 4 3 4 4 28%	5586
April	1st to 10th	Day	17% 15% 4 9% 5% 4 4 3 5% 7% 28%	2385
	11th to 20th	Day	18% 12% 3 7% 4 4 4 3 5% 8% 33%	2686
	21st to end	Day	19% 10% 3 7% 5% 4 3 3 5% 6% 34%	2730
May	1st to 10th	Day	18% 10% 3 8% 4 4 4 3 5% 7% 33%	7642
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 6% 9% 34%	7626
	21st to end	Day	16% 7% 2 6% 4 4 4 4 5% 6% 43%	9056
June	1st to 10th	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	2940
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 39%	3130
	21st to end	Day	13% 6% 2 6% 5% 4 5% 4 6% 9% 41%	3150
July	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	8879
	11th to 20th	Day	8% 4 2 5% 5% 5% 5% 4 7% 10% 46%	7849
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 6% 9% 44%	8340
August	1st to 10th	Day	14% 6% 2 5% 4 4 5% 5% 6% 9% 38%	4364
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 6% 7% 10% 33%	4226
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 9% 41%	4616
September	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 6% 8% 41%	2342
	11th to 20th	Day	11% 5% 2 5% 4 4 4 4 5% 8% 48%	2200
	21st to end	Day	18% 6% 2 6% 4 5% 4 4 7% 8% 35%	2200
October	1st to 10th	Day	17% 7% 2 7% 5% 4 4 3 7% 8% 37%	2043
	11th to 20th	Day	20% 9% 3 8% 5% 5% 5% 4 5% 6% 29%	2000
	21st to end	Day	24% 12% 4 9% 5% 5% 4 3 6% 6% 23%	2160
November	1st to 10th	Day	15% 12% 3 8% 5% 5% 4 4 6% 8% 30%	1852
	11th to 20th	Day	16% 15% 4 9% 5% 6% 5% 3 6% 7% 25%	1724
	21st to end	Day	19% 19% 6% 10% 5% 5% 5% 5% 5% 6% 15%	1680
December	1st to 10th	Day	18% 21% 7% 12% 6% 5% 4 4 5% 6% 13%	2680
	11th to 20th	Day	21% 20% 6% 11% 6% 4 4 3 5% 4 16%	3066
	21st to end	Day	26% 21% 7% 13% 5% 4 3 3 4 5% 8%	3979
January	1st to 10th	Night	14% 16% 7% 23% 7% 5% 4 3 3 4 13%	8121
	11th to 20th	Night	10% 19% 8% 19% 9% 6% 6% 4 4 4 11%	7639
	21st to end	Night	14% 15% 6% 15% 7% 6% 6% 3 5% 5% 19%	8401
February	1st to 10th	Night	15% 16% 5% 11% 8% 6% 5% 4 5% 5% 20%	2730
	11th to 20th	Night	15% 14% 5% 9% 6% 6% 5% 5% 6% 7% 22%	2660
	21st to end	Night	15% 14% 5% 12% 8% 5% 5% 4 6% 5% 22%	2076
March	1st to 10th	Night	14% 11% 4 9% 6% 5% 5% 4 5% 6% 32%	5140
	11th to 20th	Night	10% 9% 4 9% 5% 5% 4 4 7% 8% 35%	5089
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 4 4 35%	5588
April	1st to 10th	Night	17% 11% 4 7% 5% 4 4 3 6% 6% 31%	2233
	11th to 20th	Night	17% 10% 2 6% 6% 4 4 3 5% 6% 36%	1900
	21st to end	Night	14% 9% 3 7% 6% 4 6% 4 5% 8% 34%	1890
May	1st to 10th	Night	19% 8% 3 7% 4 4 4 3 5% 6% 35%	5257
	11th to 20th	Night	13% 10% 3 8% 5% 4 4 4 5% 6% 37%	5234
	21st to end	Night	18% 7% 2 4 3 3 4 3 5% 6% 44%	5114
June	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 9% 45%	1680
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 8% 10% 41%	1484
	21st to end	Night	10% 5% 2 6% 4 4 6% 3 5% 9% 46%	1470
July	1st to 10th	Night	7% 6% 2 6% 4 4 4 4 6% 10% 47%	4110
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 6% 10% 50%	5084
	21st to end	Night	8% 5% 2 6% 4 4 5% 3 6% 8% 49%	5737
August	1st to 10th	Night	11% 6% 2 6% 4 4 4 4 6% 8% 45%	3005
	11th to 20th	Night	11% 8% 3 5% 3 4 4 3 5% 7% 47%	2911
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	3247
September	1st to 10th	Night	8% 7% 3 6% 4 4 4 3 6% 7% 48%	2055
	11th to 20th	Night	10% 5% 1 4 4 4 4 3 5% 6% 55%	2200
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 6% 41%	2200
October	1st to 10th	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 43%	2356
	11th to 20th	Night	19% 10% 3 8% 4 4 3 3 5% 7% 35%	2400
	21st to end	Night	22% 10% 3 8% 5% 4 5% 3 5% 6% 28%	2675
November	1st to 10th	Night	14% 10% 3 7% 4 4 5% 3 5% 7% 37%	2666
	11th to 20th	Night	16% 14% 3 8% 5% 5% 5% 4 6% 7% 25%	2893
	21st to end	Night	21% 15% 5% 9% 6% 4 5% 4 5% 6% 20%	2940
December	1st to 10th	Night	13% 18% 7% 15% 7% 6% 6% 4 6% 5% 13%	4720
	11th to 20th	Night	15% 22% 6% 12% 5% 4 5% 3 4 5% 19%	5348
	21st to end	Night	23% 20% 7% 11% 6% 4 4 3 4 5% 13%	7171

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% 8% 7% 2	4730
	11th to 20th	Day	82% 5% 6% 4 2	5280
	21st to end	Day	71% 7% 9% 7% 6%	5780
February	1st to 10th	Day	68% 6% 9% 12% 5%	1890
	11th to 20th	Day	67% 5% 9% 13% 6%	1852
	21st to end	Day	65% 5% 11% 13% 5% 1	1730
March	1st to 10th	Day	61% 4 13% 16% 5%	4909
	11th to 20th	Day	52% 5% 16% 18% 7% 2	5090
	21st to end	Day	58% 5% 11% 16% 8% 1	5586
April	1st to 10th	Day	56% 5% 10% 19% 9% 1	2385
	11th to 20th	Day	50% 5% 12% 19% 13% 1	2686
	21st to end	Day	51% 5% 12% 19% 12% 2	2730
May	1st to 10th	Day	48% 6% 12% 20% 10% 4	7651
	11th to 20th	Day	45% 9% 13% 20% 12% 1	7630
	21st to end	Day	42% 6% 9% 20% 20% 3	9057
June	1st to 10th	Day	41% 7% 12% 24% 14% 2	2940
	11th to 20th	Day	41% 7% 12% 22% 17% 2	3130
	21st to end	Day	38% 9% 13% 24% 16%	3150
July	1st to 10th	Day	38% 8% 13% 25% 14% 1	8806
	11th to 20th	Day	30% 7% 13% 32% 16% 1	7776
	21st to end	Day	33% 8% 13% 23% 21% 1	8156
August	1st to 10th	Day	40% 8% 14% 22% 14% 1	4351
	11th to 20th	Day	40% 10% 16% 25% 9% 1	4265
	21st to end	Day	36% 9% 14% 28% 12% 1	4616
September	1st to 10th	Day	39% 8% 14% 25% 13% 1	2342
	11th to 20th	Day	34% 6% 14% 31% 16%	2200
	21st to end	Day	45% 7% 13% 23% 11% 1	2200
October	1st to 10th	Day	36% 9% 15% 22% 17% 1	3329
	11th to 20th	Day	41% 7% 13% 21% 18%	3240
	21st to end	Day	49% 7% 17% 20% 7%	3474
November	1st to 10th	Day	51% 8% 11% 21% 8% 2	1852
	11th to 20th	Day	56% 7% 14% 14% 8% 1	1724
	21st to end	Day	68% 7% 11% 12% 3	1680
December	1st to 10th	Day	71% 8% 12% 8% 2	2680
	11th to 20th	Day	70% 6% 11% 8% 5%	3066
	21st to end	Day	78% 6% 8% 7% 1	3979
January	1st to 10th	Night	72% 8% 9% 8% 3	8121
	11th to 20th	Night	74% 7% 7% 7% 4	7680
	21st to end	Night	65% 6% 8% 13% 7%	8401
February	1st to 10th	Night	64% 6% 10% 14% 6%	2730
	11th to 20th	Night	59% 7% 13% 14% 7%	2662
	21st to end	Night	63% 5% 10% 15% 6%	2076
March	1st to 10th	Night	51% 5% 14% 19% 10% 1	5150
	11th to 20th	Night	44% 6% 19% 20% 10% 1	5090
	21st to end	Night	52% 5% 11% 18% 14% 1	5588
April	1st to 10th	Night	52% 6% 11% 18% 13% 1	2233
	11th to 20th	Night	48% 5% 10% 19% 16% 2	1900
	21st to end	Night	46% 7% 10% 18% 15% 3	1890
May	1st to 10th	Night	48% 6% 11% 14% 15% 6%	5264
	11th to 20th	Night	46% 7% 11% 23% 12% 2	5235
	21st to end	Night	40% 6% 10% 23% 19% 3	5115
June	1st to 10th	Night	35% 6% 11% 26% 18% 3	1680
	11th to 20th	Night	35% 8% 14% 25% 17% 2	1484
	21st to end	Night	34% 7% 14% 24% 20% 1	1470
July	1st to 10th	Night	31% 7% 15% 28% 16% 4	4075
	11th to 20th	Night	29% 6% 11% 29% 23% 2	5034
	21st to end	Night	31% 8% 12% 21% 25% 3	5606
August	1st to 10th	Night	36% 7% 13% 25% 17% 2	2994
	11th to 20th	Night	36% 7% 14% 30% 12% 1	2938
	21st to end	Night	31% 8% 12% 31% 16% 1	3247
September	1st to 10th	Night	35% 7% 13% 27% 16% 2	2055
	11th to 20th	Night	29% 6% 13% 31% 19% 2	2200
	21st to end	Night	43% 7% 12% 23% 14% 1	2200
October	1st to 10th	Night	30% 6% 10% 28% 24% 1	3850
	11th to 20th	Night	43% 5% 14% 22% 16% 1	3899
	21st to end	Night	43% 7% 15% 24% 11% 1	4311
November	1st to 10th	Night	44% 7% 13% 23% 12% 1	2666
	11th to 20th	Night	54% 8% 12% 17% 7% 1	2893
	21st to end	Night	63% 7% 14% 11% 3	2940
December	1st to 10th	Night	68% 9% 12% 8% 3	4720
	11th to 20th	Night	65% 6% 10% 11% 7% 1	5348
	21st to end	Night	72% 6% 10% 9% 3	7171

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% 24% 32% 17% 5% 1	292
	11th to 20th	Day	18% 25% 41% 12% 3 1	330
	21st to end	Day	28% 17% 33% 18% 4	363
February	1st to 10th	Day	24% 22% 36% 14% 4	330
	11th to 20th	Day	26% 23% 34% 16% 1	330
	21st to end	Day	26% 25% 37% 11% 1	273
March	1st to 10th	Day	26% 15% 37% 17% 5% 1	477
	11th to 20th	Day	24% 17% 32% 21% 4 1	540
	21st to end	Day	29% 15% 32% 19% 5% 1	594
April	1st to 10th	Day	22% 19% 31% 20% 7% 1	540
	11th to 20th	Day	25% 17% 27% 17% 9% 3	532
	21st to end	Day	23% 14% 31% 22% 8% 2	540
May	1st to 10th	Day	21% 14% 29% 24% 9% 3	543
	11th to 20th	Day	17% 11% 29% 22% 16% 5%	567
	21st to end	Day	15% 9% 26% 26% 16% 7%	617
June	1st to 10th	Day	13% 6% 33% 30% 14% 4	570
	11th to 20th	Day	12% 10% 30% 31% 13% 4	760
	21st to end	Day	13% 8% 28% 33% 14% 4	780
July	1st to 10th	Day	11% 7% 22% 29% 23% 8%	771
	11th to 20th	Day	14% 6% 26% 28% 18% 8%	619
	21st to end	Day	9% 5% 21% 33% 24% 9%	620
August	1st to 10th	Day	11% 4 19% 30% 24% 13%	566
	11th to 20th	Day	14% 4 19% 26% 22% 14%	535
	21st to end	Day	9% 6% 23% 27% 24% 11%	583
September	1st to 10th	Day	11% 6% 24% 26% 20% 12%	530
	11th to 20th	Day	8% 3 20% 23% 31% 15%	530
	21st to end	Day	16% 12% 22% 27% 15% 8%	530
October	1st to 10th	Day	18% 9% 25% 24% 18% 6%	530
	11th to 20th	Day	20% 14% 35% 22% 7% 2	530
	21st to end	Day	26% 16% 33% 19% 5% 1	543
November	1st to 10th	Day	23% 18% 33% 15% 8% 2	330
	11th to 20th	Day	31% 17% 30% 15% 6% 2	242
	21st to end	Day	27% 20% 34% 15% 4	220
December	1st to 10th	Day	26% 20% 30% 18% 5%	220
	11th to 20th	Day	28% 22% 30% 17% 4	220
	21st to end	Day	31% 21% 31% 14% 2 1	242
January	1st to 10th	Night	23% 18% 38% 16% 4 1	946
	11th to 20th	Night	28% 17% 32% 20% 3	840
	21st to end	Night	21% 14% 35% 24% 6%	924
February	1st to 10th	Night	24% 15% 36% 23% 3	840
	11th to 20th	Night	24% 15% 32% 25% 4	823
	21st to end	Night	23% 17% 36% 22% 3	692
March	1st to 10th	Night	20% 13% 36% 25% 6%	840
	11th to 20th	Night	21% 14% 35% 23% 6% 1	840
	21st to end	Night	23% 13% 30% 26% 8% 1	924
April	1st to 10th	Night	23% 15% 31% 26% 5%	840
	11th to 20th	Night	21% 12% 33% 24% 8% 1	656
	21st to end	Night	18% 10% 34% 31% 7%	630
May	1st to 10th	Night	21% 7% 29% 30% 12% 1	630
	11th to 20th	Night	15% 5% 29% 37% 12% 2	621
	21st to end	Night	14% 4 28% 38% 14% 1	455
June	1st to 10th	Night	15% 4 29% 40% 12% 1	420
	11th to 20th	Night	14% 4 27% 42% 14%	420
	21st to end	Night	10% 5% 24% 45% 15%	420
July	1st to 10th	Night	6% 2 20% 49% 21% 1	410
	11th to 20th	Night	10% 3 19% 40% 26% 2	601
	21st to end	Night	10% 3 16% 46% 24% 2	690
August	1st to 10th	Night	9% 2 14% 43% 30% 3	627
	11th to 20th	Night	9% 3 13% 40% 31% 4	606
	21st to end	Night	9% 3 19% 39% 27% 3	671
September	1st to 10th	Night	10% 5% 22% 34% 24% 5%	800
	11th to 20th	Night	10% 4 16% 37% 30% 4	800
	21st to end	Night	19% 6% 20% 33% 18% 4	800
October	1st to 10th	Night	16% 6% 26% 32% 18% 2	800
	11th to 20th	Night	22% 12% 28% 28% 10% 1	800
	21st to end	Night	27% 10% 32% 26% 5%	880
November	1st to 10th	Night	19% 14% 28% 29% 10% 1	820
	11th to 20th	Night	23% 13% 37% 20% 6% 1	840
	21st to end	Night	28% 13% 30% 23% 6%	840
December	1st to 10th	Night	28% 15% 33% 19% 5%	840
	11th to 20th	Night	24% 18% 38% 18% 3	840
	21st to end	Night	28% 13% 36% 20% 3	932

Page generated at 2026-08-12 03:37:19. Version ac07

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