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 2025-04-11.

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	15% 11% 4 9% 5% 5% 4 4 5% 7% 30%	113950
	Night	14% 12% 4 10% 5% 5% 5% 4 5% 6% 30%	114212
Transparency	Day	51% 7% 13% 18% 10% 1	117590
	Night	51% 7% 12% 18% 11% 1	118736
Seeing	Day	18% 12% 28% 24% 13% 5%	15488
	Night	20% 10% 29% 29% 11% 1	24760

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	12% 18% 6% 25% 7% 5% 4 3 4 3 13%	3643
	11th to 20th	Day	11% 21% 10% 23% 7% 7% 4 3 3 4 8%	4026
	21st to end	Day	19% 19% 7% 14% 7% 6% 4 3 4 4 14%	4466
February	1st to 10th	Day	16% 18% 5% 13% 8% 6% 4 4 4 5% 17%	1800
	11th to 20th	Day	14% 14% 8% 14% 8% 6% 4 3 4 6% 18%	1753
	21st to end	Day	16% 16% 6% 12% 6% 5% 5% 3 4 4 21%	1650
March	1st to 10th	Day	17% 13% 7% 13% 7% 3 3 3 3 5% 26%	4799
	11th to 20th	Day	12% 13% 5% 10% 5% 4 4 3 5% 8% 30%	4980
	21st to end	Day	19% 13% 6% 10% 5% 4 4 3 4 4 28%	5465
April	1st to 10th	Day	16% 14% 4 9% 5% 4 4 3 5% 7% 28%	2271
	11th to 20th	Day	18% 11% 2 6% 4 4 4 3 5% 8% 33%	2428
	21st to end	Day	20% 10% 3 8% 5% 4 3 3 5% 6% 33%	2470
May	1st to 10th	Day	18% 9% 3 7% 4 4 5% 4 6% 7% 33%	4108
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 7% 9% 31%	4081
	21st to end	Day	12% 5% 2 5% 4 4 4 5% 5% 7% 46%	4833
June	1st to 10th	Day	14% 6% 2 7% 6% 5% 5% 5% 6% 8% 38%	2660
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	2832
	21st to end	Day	13% 6% 2 5% 5% 4 5% 4 6% 9% 40%	2850
July	1st to 10th	Day	11% 5% 2 6% 5% 4 5% 5% 7% 9% 40%	4760
	11th to 20th	Day	11% 5% 2 5% 4 5% 4 4 7% 10% 41%	4205
	21st to end	Day	8% 5% 2 7% 5% 6% 5% 5% 6% 9% 41%	4466
August	1st to 10th	Day	15% 6% 2 5% 4 5% 6% 5% 6% 10% 37%	4104
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 6% 7% 10% 33%	4044
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 5% 6% 9% 41%	4475
September	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 6% 8% 40%	2225
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 46%	2090
	21st to end	Day	18% 7% 2 6% 4 5% 4 4 7% 8% 35%	2090
October	1st to 10th	Day	17% 7% 2 7% 5% 4 4 3 7% 8% 36%	1941
	11th to 20th	Day	20% 10% 3 8% 5% 5% 5% 4 5% 6% 28%	1900
	21st to end	Day	25% 12% 4 9% 6% 5% 4 3 6% 6% 22%	2052
November	1st to 10th	Day	15% 12% 3 8% 5% 6% 4 4 6% 8% 31%	1762
	11th to 20th	Day	16% 15% 4 9% 5% 6% 5% 3 6% 7% 25%	1642
	21st to end	Day	19% 19% 6% 10% 5% 5% 5% 5% 4 6% 15%	1600
December	1st to 10th	Day	18% 21% 7% 12% 6% 6% 4 4 5% 6% 12%	2600
	11th to 20th	Day	21% 20% 6% 11% 6% 4 4 3 5% 4 16%	2986
	21st to end	Day	26% 21% 7% 13% 5% 4 3 3 4 5% 8%	3893
January	1st to 10th	Night	13% 15% 7% 23% 7% 5% 4 3 3 4 15%	6266
	11th to 20th	Night	12% 17% 8% 19% 9% 7% 6% 4 4 4 9%	5859
	21st to end	Night	18% 16% 7% 14% 5% 4 4 3 5% 5% 18%	6490
February	1st to 10th	Night	16% 16% 5% 11% 9% 6% 5% 4 6% 5% 19%	2600
	11th to 20th	Night	15% 15% 5% 9% 6% 6% 5% 5% 6% 7% 21%	2530
	21st to end	Night	16% 13% 5% 12% 8% 5% 5% 4 5% 5% 22%	1980
March	1st to 10th	Night	14% 11% 4 9% 6% 5% 5% 4 5% 6% 32%	5027
	11th to 20th	Night	10% 9% 4 9% 5% 5% 5% 4 7% 8% 35%	4979
	21st to end	Night	18% 11% 4 9% 5% 4 4 3 4 4 35%	5467
April	1st to 10th	Night	18% 11% 4 7% 5% 4 4 3 6% 6% 31%	2127
	11th to 20th	Night	17% 10% 2 6% 5% 4 4 3 5% 6% 36%	1724
	21st to end	Night	15% 9% 3 7% 5% 4 6% 4 6% 7% 33%	1710
May	1st to 10th	Night	19% 9% 2 6% 4 3 4 3 6% 7% 38%	2833
	11th to 20th	Night	10% 9% 3 8% 5% 5% 5% 5% 6% 7% 37%	2812
	21st to end	Night	11% 5% 2 4 3 3 4 3 6% 7% 51%	2738
June	1st to 10th	Night	13% 6% 2 5% 4 4 4 4 6% 9% 43%	1520
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 5% 8% 10% 41%	1343
	21st to end	Night	10% 5% 2 6% 4 4 6% 3 5% 9% 46%	1330
July	1st to 10th	Night	6% 5% 1 5% 4 5% 3 4 6% 11% 50%	2210
	11th to 20th	Night	9% 5% 1 5% 3 3 3 3 5% 9% 52%	2736
	21st to end	Night	6% 5% 2 5% 4 4 6% 4 7% 9% 48%	3087
August	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 6% 8% 45%	2825
	11th to 20th	Night	11% 8% 3 5% 4 4 4 3 5% 7% 47%	2785
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	3147
September	1st to 10th	Night	9% 7% 3 6% 5% 4 4 2 6% 7% 47%	1952
	11th to 20th	Night	10% 5% 1 4 4 4 4 3 5% 6% 53%	2090
	21st to end	Night	17% 7% 2 6% 4 4 3 3 5% 7% 40%	2090
October	1st to 10th	Night	14% 8% 3 6% 5% 4 4 3 5% 7% 42%	2238
	11th to 20th	Night	19% 10% 3 8% 4 4 3 3 5% 7% 34%	2280
	21st to end	Night	23% 10% 3 8% 5% 4 5% 3 5% 6% 27%	2541
November	1st to 10th	Night	15% 10% 3 7% 4 4 5% 3 6% 7% 37%	2536
	11th to 20th	Night	17% 14% 3 8% 5% 5% 5% 4 6% 7% 25%	2755
	21st to end	Night	21% 14% 5% 9% 6% 5% 5% 4 5% 6% 20%	2800
December	1st to 10th	Night	13% 18% 7% 15% 7% 6% 6% 4 6% 5% 13%	4580
	11th to 20th	Night	15% 22% 6% 12% 5% 4 4 3 4 5% 19%	5208
	21st to end	Night	23% 20% 7% 11% 6% 4 4 3 4 5% 13%	7017

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	75% 6% 8% 8% 3	3644
	11th to 20th	Day	80% 6% 7% 4 3	4060
	21st to end	Day	73% 6% 9% 6% 6%	4466
February	1st to 10th	Day	68% 6% 9% 12% 4 1	1800
	11th to 20th	Day	67% 5% 9% 13% 6%	1762
	21st to end	Day	65% 5% 10% 13% 6% 1	1650
March	1st to 10th	Day	61% 4 13% 16% 5%	4802
	11th to 20th	Day	51% 5% 17% 18% 7% 2	4980
	21st to end	Day	58% 5% 11% 16% 8% 1	5465
April	1st to 10th	Day	55% 5% 11% 19% 9% 1	2271
	11th to 20th	Day	49% 5% 13% 19% 13% 1	2428
	21st to end	Day	52% 5% 12% 18% 12% 2	2470
May	1st to 10th	Day	47% 5% 13% 20% 11% 3	4120
	11th to 20th	Day	46% 11% 14% 19% 8% 1	4085
	21st to end	Day	36% 7% 12% 20% 19% 5%	4833
June	1st to 10th	Day	42% 7% 13% 24% 13% 2	2660
	11th to 20th	Day	40% 7% 12% 22% 17% 2	2832
	21st to end	Day	39% 8% 12% 24% 16% 1	2850
July	1st to 10th	Day	38% 7% 14% 25% 14% 2	4686
	11th to 20th	Day	34% 8% 14% 31% 13% 1	4132
	21st to end	Day	37% 9% 13% 21% 19% 1	4324
August	1st to 10th	Day	40% 8% 14% 22% 14% 1	4091
	11th to 20th	Day	40% 10% 16% 24% 9% 1	4083
	21st to end	Day	36% 9% 14% 28% 12% 1	4475
September	1st to 10th	Day	39% 8% 14% 24% 13% 1	2225
	11th to 20th	Day	35% 6% 13% 29% 16%	2090
	21st to end	Day	45% 7% 13% 24% 10% 1	2090
October	1st to 10th	Day	36% 9% 16% 21% 17% 1	3227
	11th to 20th	Day	41% 7% 13% 20% 18%	3140
	21st to end	Day	50% 7% 17% 20% 6%	3366
November	1st to 10th	Day	51% 8% 11% 20% 8% 2	1762
	11th to 20th	Day	56% 7% 14% 13% 9% 1	1642
	21st to end	Day	68% 7% 11% 12% 3	1600
December	1st to 10th	Day	71% 8% 12% 7% 2	2600
	11th to 20th	Day	70% 6% 11% 8% 5%	2986
	21st to end	Day	78% 6% 8% 7% 1	3893
January	1st to 10th	Night	70% 7% 9% 10% 3	6266
	11th to 20th	Night	75% 8% 8% 7% 3	5900
	21st to end	Night	67% 6% 9% 11% 7%	6490
February	1st to 10th	Night	65% 6% 10% 13% 5%	2600
	11th to 20th	Night	60% 7% 12% 14% 7%	2532
	21st to end	Night	62% 5% 10% 16% 6%	1980
March	1st to 10th	Night	51% 5% 14% 19% 10% 1	5037
	11th to 20th	Night	43% 6% 19% 20% 11% 1	4980
	21st to end	Night	52% 5% 11% 18% 14% 1	5467
April	1st to 10th	Night	52% 6% 10% 18% 13% 1	2127
	11th to 20th	Night	48% 4 10% 19% 16% 2	1724
	21st to end	Night	47% 7% 10% 18% 15% 3	1710
May	1st to 10th	Night	44% 6% 11% 18% 16% 4	2840
	11th to 20th	Night	43% 8% 13% 24% 10% 2	2813
	21st to end	Night	30% 8% 13% 22% 22% 4	2739
June	1st to 10th	Night	36% 7% 11% 27% 16% 3	1520
	11th to 20th	Night	34% 8% 14% 24% 18% 2	1343
	21st to end	Night	34% 7% 14% 24% 19% 1	1330
July	1st to 10th	Night	28% 6% 13% 28% 18% 7%	2175
	11th to 20th	Night	28% 7% 14% 30% 19% 2	2686
	21st to end	Night	30% 10% 16% 22% 19% 3	2988
August	1st to 10th	Night	36% 7% 13% 26% 17% 1	2814
	11th to 20th	Night	36% 7% 14% 30% 12% 1	2812
	21st to end	Night	32% 8% 12% 30% 17% 1	3147
September	1st to 10th	Night	35% 7% 13% 25% 17% 2	1952
	11th to 20th	Night	30% 6% 13% 31% 19% 2	2090
	21st to end	Night	43% 7% 12% 24% 14%	2090
October	1st to 10th	Night	31% 6% 10% 27% 24% 1	3732
	11th to 20th	Night	43% 5% 15% 22% 16% 1	3779
	21st to end	Night	43% 7% 15% 24% 11%	4177
November	1st to 10th	Night	44% 7% 13% 23% 12% 1	2536
	11th to 20th	Night	55% 8% 12% 17% 7% 1	2755
	21st to end	Night	63% 8% 15% 11% 3	2800
December	1st to 10th	Night	68% 9% 12% 8% 3	4580
	11th to 20th	Night	66% 6% 9% 11% 7% 1	5208
	21st to end	Night	72% 6% 10% 8% 3	7017

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	20% 23% 31% 18% 6% 1	265
	11th to 20th	Day	16% 24% 43% 13% 3 1	300
	21st to end	Day	28% 15% 34% 18% 4	330
February	1st to 10th	Day	25% 22% 36% 13% 3	300
	11th to 20th	Day	26% 23% 34% 15% 1	300
	21st to end	Day	27% 24% 36% 11% 1	249
March	1st to 10th	Day	24% 15% 38% 17% 5% 1	440
	11th to 20th	Day	23% 16% 33% 22% 4 2	500
	21st to end	Day	29% 14% 31% 20% 5% 1	550
April	1st to 10th	Day	22% 18% 32% 21% 7% 1	500
	11th to 20th	Day	26% 15% 27% 19% 9% 4	452
	21st to end	Day	24% 14% 29% 22% 9% 2	460
May	1st to 10th	Day	21% 12% 27% 26% 11% 3	463
	11th to 20th	Day	17% 9% 26% 23% 18% 6%	487
	21st to end	Day	13% 7% 25% 28% 19% 8%	529
June	1st to 10th	Day	14% 5% 32% 31% 14% 4	490
	11th to 20th	Day	11% 9% 28% 33% 15% 5%	662
	21st to end	Day	14% 9% 28% 31% 15% 4	680
July	1st to 10th	Day	10% 7% 21% 29% 24% 9%	671
	11th to 20th	Day	14% 6% 25% 27% 19% 9%	533
	21st to end	Day	9% 5% 20% 33% 24% 10%	532
August	1st to 10th	Day	11% 4 19% 28% 24% 14%	486
	11th to 20th	Day	14% 4 18% 26% 24% 14%	479
	21st to end	Day	10% 6% 22% 27% 24% 12%	539
September	1st to 10th	Day	11% 6% 24% 27% 20% 12%	490
	11th to 20th	Day	9% 3 21% 23% 29% 14%	490
	21st to end	Day	17% 12% 23% 26% 14% 8%	490
October	1st to 10th	Day	19% 10% 25% 24% 16% 6%	490
	11th to 20th	Day	21% 14% 34% 22% 8% 2	490
	21st to end	Day	27% 15% 33% 20% 5% 1	501
November	1st to 10th	Day	22% 18% 33% 17% 9% 2	300
	11th to 20th	Day	28% 16% 32% 15% 6% 2	220
	21st to end	Day	27% 19% 35% 16% 4	200
December	1st to 10th	Day	26% 21% 31% 17% 6% 1	200
	11th to 20th	Day	28% 21% 31% 18% 4	200
	21st to end	Day	31% 20% 32% 14% 3 1	220
January	1st to 10th	Night	23% 18% 38% 16% 5% 1	901
	11th to 20th	Night	29% 16% 32% 20% 4	800
	21st to end	Night	22% 13% 35% 24% 6%	880
February	1st to 10th	Night	24% 15% 36% 22% 2	800
	11th to 20th	Night	24% 15% 32% 25% 5%	783
	21st to end	Night	22% 16% 37% 22% 3	660
March	1st to 10th	Night	20% 12% 37% 25% 6%	800
	11th to 20th	Night	19% 14% 36% 24% 7% 1	800
	21st to end	Night	23% 13% 29% 26% 8% 1	880
April	1st to 10th	Night	23% 15% 31% 27% 5%	800
	11th to 20th	Night	22% 11% 33% 24% 9% 1	596
	21st to end	Night	18% 9% 34% 30% 8% 1	570
May	1st to 10th	Night	21% 6% 28% 31% 13% 1	570
	11th to 20th	Night	15% 4 27% 39% 12% 2	561
	21st to end	Night	13% 3 28% 39% 16% 1	411
June	1st to 10th	Night	15% 4 29% 40% 12% 1	380
	11th to 20th	Night	13% 3 26% 42% 15%	380
	21st to end	Night	11% 5% 23% 45% 16% 1	380
July	1st to 10th	Night	6% 2 20% 48% 22% 1	370
	11th to 20th	Night	10% 3 19% 41% 26% 2	542
	21st to end	Night	10% 2 16% 46% 24% 2	624
August	1st to 10th	Night	9% 2 14% 43% 29% 3	567
	11th to 20th	Night	9% 3 12% 40% 32% 4	564
	21st to end	Night	10% 3 19% 38% 27% 3	637
September	1st to 10th	Night	11% 5% 21% 35% 23% 5%	760
	11th to 20th	Night	11% 4 16% 36% 29% 4	760
	21st to end	Night	19% 6% 21% 33% 17% 4	760
October	1st to 10th	Night	17% 7% 26% 31% 18% 2	760
	11th to 20th	Night	22% 12% 27% 27% 11% 1	760
	21st to end	Night	28% 10% 32% 26% 5%	836
November	1st to 10th	Night	19% 13% 28% 29% 10% 1	780
	11th to 20th	Night	24% 12% 37% 21% 6% 1	800
	21st to end	Night	28% 12% 30% 24% 6%	800
December	1st to 10th	Night	28% 16% 33% 19% 5%	800
	11th to 20th	Night	24% 17% 38% 18% 3	800
	21st to end	Night	28% 12% 36% 21% 3	888

Page generated at 2025-04-08 09:17:30. Version ac07

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