Sandilands Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandilands Observatory 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	11% 10% 4 9% 6% 5% 5% 4 6% 7% 33%	135101
	Night	10% 11% 4 11% 6% 5% 5% 4 6% 6% 30%	135545
Transparency	Day	49% 6% 13% 22% 9% 1	134797
	Night	53% 5% 12% 21% 8% 1	135431
Seeing	Day	18% 16% 35% 21% 8% 2	18095
	Night	17% 12% 35% 26% 9% 1	28205

By Month

Percentage Cloud Cover NW (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	6% 13% 7% 18% 7% 7% 6% 5% 5% 5% 20%	4125
	11th to 20th	Day	4 12% 6% 15% 9% 9% 12% 8% 8% 5% 11%	4119
	21st to end	Day	4 16% 8% 14% 7% 7% 8% 6% 6% 7% 17%	5026
February	1st to 10th	Day	7% 13% 6% 11% 9% 7% 8% 5% 8% 6% 19%	1880
	11th to 20th	Day	6% 10% 5% 9% 8% 8% 7% 5% 6% 8% 26%	1844
	21st to end	Day	5% 11% 4 15% 9% 8% 7% 4 6% 6% 25%	1579
March	1st to 10th	Day	9% 13% 4 9% 8% 4 5% 4 5% 6% 33%	4821
	11th to 20th	Day	9% 10% 4 13% 8% 5% 5% 4 6% 8% 28%	5088
	21st to end	Day	12% 14% 6% 10% 6% 5% 4 3 4 6% 31%	5585
April	1st to 10th	Day	12% 12% 4 9% 5% 4 4 4 5% 6% 34%	2425
	11th to 20th	Day	14% 11% 4 9% 5% 5% 4 3 6% 7% 33%	2572
	21st to end	Day	17% 10% 4 7% 5% 5% 5% 4 6% 6% 31%	2600
May	1st to 10th	Day	13% 8% 3 6% 4 3 5% 3 5% 6% 45%	6217
	11th to 20th	Day	17% 9% 2 7% 5% 4 5% 4 6% 8% 33%	6443
	21st to end	Day	15% 9% 3 6% 4 4 4 3 5% 6% 43%	7428
June	1st to 10th	Day	15% 7% 3 6% 6% 5% 5% 4 6% 8% 34%	3000
	11th to 20th	Day	15% 7% 3 7% 5% 5% 6% 4 7% 9% 32%	3000
	21st to end	Day	14% 7% 2 7% 7% 6% 6% 5% 7% 9% 31%	3000
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	6800
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 5% 6% 9% 43%	6800
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 4 7% 9% 41%	7400
August	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 41%	4322
	11th to 20th	Day	11% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	4171
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 9% 40%	4675
September	1st to 10th	Day	13% 5% 2 6% 5% 4 4 4 5% 7% 44%	2456
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 4 6% 8% 35%	2243
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 6% 41%	2191
October	1st to 10th	Day	13% 9% 3 6% 4 4 4 4 5% 8% 41%	3260
	11th to 20th	Day	12% 12% 4 8% 4 3 4 2 5% 8% 37%	3240
	21st to end	Day	15% 13% 3 8% 5% 4 5% 3 6% 7% 29%	3182
November	1st to 10th	Day	14% 18% 5% 12% 5% 4 3 4 4 5% 25%	1647
	11th to 20th	Day	11% 19% 5% 14% 6% 4 3 3 4 5% 24%	1680
	21st to end	Day	12% 15% 7% 15% 10% 6% 4 3 4 5% 19%	1638
December	1st to 10th	Day	11% 16% 6% 17% 11% 8% 6% 4 6% 5% 11%	2360
	11th to 20th	Day	8% 16% 5% 15% 7% 4 6% 5% 7% 7% 20%	2709
	21st to end	Day	9% 15% 7% 19% 7% 6% 5% 4 6% 5% 17%	3575
January	1st to 10th	Night	5% 13% 6% 18% 9% 8% 7% 6% 7% 6% 15%	8799
	11th to 20th	Night	5% 15% 6% 13% 9% 10% 10% 9% 9% 3 10%	8809
	21st to end	Night	6% 15% 6% 12% 6% 5% 9% 5% 8% 7% 20%	9170
February	1st to 10th	Night	7% 14% 4 12% 9% 7% 7% 5% 7% 7% 23%	2739
	11th to 20th	Night	6% 10% 3 9% 6% 7% 7% 6% 8% 7% 31%	2663
	21st to end	Night	7% 10% 5% 14% 8% 6% 6% 5% 5% 7% 27%	2244
March	1st to 10th	Night	8% 13% 4 12% 7% 5% 5% 4 6% 6% 30%	5262
	11th to 20th	Night	6% 11% 5% 10% 8% 6% 4 4 6% 8% 33%	5090
	21st to end	Night	13% 14% 5% 11% 5% 5% 4 4 5% 5% 30%	5588
April	1st to 10th	Night	12% 10% 3 7% 4 4 5% 4 6% 7% 38%	1992
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 3 5% 6% 36%	1779
	21st to end	Night	13% 10% 3 8% 5% 4 5% 3 5% 7% 36%	1800
May	1st to 10th	Night	14% 8% 2 5% 3 3 3 2 5% 6% 47%	3763
	11th to 20th	Night	17% 8% 3 7% 4 4 5% 3 5% 7% 37%	3471
	21st to end	Night	13% 8% 2 5% 3 3 3 3 5% 6% 47%	3470
June	1st to 10th	Night	12% 7% 3 7% 4 4 4 4 7% 10% 41%	1400
	11th to 20th	Night	13% 6% 3 6% 5% 4 4 4 6% 7% 43%	1400
	21st to end	Night	13% 6% 2 5% 4 4 4 5% 6% 8% 43%	1400
July	1st to 10th	Night	8% 5% 2 5% 3 4 4 4 6% 8% 51%	3180
	11th to 20th	Night	7% 4 2 5% 4 4 4 4 7% 10% 49%	3180
	21st to end	Night	7% 4 2 5% 4 4 5% 4 7% 10% 47%	3466
August	1st to 10th	Night	5% 4 2 7% 4 5% 6% 5% 7% 9% 46%	2833
	11th to 20th	Night	11% 5% 1 5% 3 4 4 3 6% 9% 50%	2888
	21st to end	Night	11% 7% 2 6% 3 5% 4 4 6% 8% 45%	3234
September	1st to 10th	Night	11% 5% 2 5% 4 4 3 2 6% 7% 52%	1943
	11th to 20th	Night	15% 6% 4 6% 4 3 4 4 6% 8% 40%	2155
	21st to end	Night	18% 7% 2 5% 4 3 4 3 5% 6% 42%	2200
October	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 4 6% 44%	3908
	11th to 20th	Night	13% 11% 3 7% 4 4 5% 3 5% 7% 38%	3899
	21st to end	Night	13% 13% 4 9% 5% 4 5% 3 5% 6% 34%	4617
November	1st to 10th	Night	14% 17% 5% 9% 6% 5% 4 3 5% 5% 27%	2871
	11th to 20th	Night	12% 18% 5% 14% 7% 5% 4 3 3 5% 24%	2940
	21st to end	Night	12% 16% 6% 15% 10% 6% 5% 3 4 5% 18%	2979
December	1st to 10th	Night	9% 18% 7% 19% 11% 7% 5% 3 4 4 13%	5050
	11th to 20th	Night	8% 14% 6% 15% 9% 6% 6% 7% 6% 6% 18%	5718
	21st to end	Night	8% 13% 8% 21% 9% 7% 6% 4 4 4 17%	7645

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	66% 4 8% 15% 7%	4125
	11th to 20th	Day	69% 4 9% 14% 5%	4140
	21st to end	Day	66% 4 8% 17% 6%	5026
February	1st to 10th	Day	60% 7% 11% 13% 8% 1	1880
	11th to 20th	Day	53% 5% 12% 20% 9%	1853
	21st to end	Day	59% 6% 10% 15% 9% 1	1579
March	1st to 10th	Day	51% 5% 14% 16% 11% 3	4821
	11th to 20th	Day	52% 7% 13% 19% 9%	5090
	21st to end	Day	55% 5% 8% 21% 11%	5585
April	1st to 10th	Day	50% 5% 10% 22% 12% 1	2425
	11th to 20th	Day	51% 4 11% 21% 12% 1	2572
	21st to end	Day	52% 6% 12% 20% 9% 1	2600
May	1st to 10th	Day	39% 5% 11% 25% 19% 1	6217
	11th to 20th	Day	47% 6% 10% 23% 14% 1	6447
	21st to end	Day	42% 5% 7% 24% 18% 3	7428
June	1st to 10th	Day	46% 7% 14% 21% 11% 1	3000
	11th to 20th	Day	46% 8% 14% 22% 9%	3000
	21st to end	Day	48% 8% 16% 20% 7%	3000
July	1st to 10th	Day	36% 8% 21% 28% 6% 1	6726
	11th to 20th	Day	37% 7% 19% 31% 6%	6719
	21st to end	Day	39% 7% 18% 29% 6%	7193
August	1st to 10th	Day	37% 7% 16% 28% 11%	4305
	11th to 20th	Day	38% 8% 21% 28% 6%	4210
	21st to end	Day	40% 5% 21% 28% 5%	4674
September	1st to 10th	Day	38% 7% 17% 30% 8% 1	2456
	11th to 20th	Day	47% 8% 18% 23% 4	2243
	21st to end	Day	45% 5% 13% 31% 6%	2191
October	1st to 10th	Day	42% 5% 16% 30% 8%	3260
	11th to 20th	Day	46% 4 13% 23% 14%	3240
	21st to end	Day	54% 5% 15% 22% 4	3182
November	1st to 10th	Day	61% 6% 12% 15% 5% 1	1647
	11th to 20th	Day	64% 5% 14% 13% 4 1	1680
	21st to end	Day	68% 5% 14% 11% 2	1638
December	1st to 10th	Day	75% 7% 9% 8% 2	2360
	11th to 20th	Day	61% 4 12% 15% 7%	2710
	21st to end	Day	68% 4 7% 17% 3	3575
January	1st to 10th	Night	68% 4 10% 11% 7%	8800
	11th to 20th	Night	71% 4 7% 14% 4	8850
	21st to end	Night	61% 5% 10% 18% 7%	9170
February	1st to 10th	Night	57% 5% 13% 15% 9%	2739
	11th to 20th	Night	49% 5% 16% 21% 9%	2664
	21st to end	Night	55% 6% 12% 17% 9%	2244
March	1st to 10th	Night	53% 5% 14% 14% 10% 3	5262
	11th to 20th	Night	48% 5% 16% 21% 9% 1	5090
	21st to end	Night	55% 5% 9% 20% 11% 1	5588
April	1st to 10th	Night	44% 5% 11% 23% 15% 2	1992
	11th to 20th	Night	49% 3 9% 21% 16% 2	1779
	21st to end	Night	47% 5% 10% 25% 12% 1	1800
May	1st to 10th	Night	38% 5% 10% 27% 20%	3763
	11th to 20th	Night	45% 5% 10% 23% 15% 1	3471
	21st to end	Night	37% 6% 7% 27% 20% 4	3470
June	1st to 10th	Night	39% 7% 18% 22% 13% 1	1400
	11th to 20th	Night	40% 8% 13% 24% 13% 1	1400
	21st to end	Night	38% 8% 20% 26% 8% 1	1400
July	1st to 10th	Night	30% 7% 22% 33% 6% 3	3145
	11th to 20th	Night	29% 6% 21% 38% 6%	3138
	21st to end	Night	31% 7% 23% 31% 7%	3368
August	1st to 10th	Night	32% 6% 16% 35% 10%	2826
	11th to 20th	Night	31% 6% 25% 32% 6%	2915
	21st to end	Night	36% 5% 21% 30% 8%	3234
September	1st to 10th	Night	33% 5% 16% 33% 12%	1943
	11th to 20th	Night	42% 7% 18% 25% 9% 1	2155
	21st to end	Night	42% 5% 14% 29% 9%	2200
October	1st to 10th	Night	41% 5% 11% 31% 11%	3908
	11th to 20th	Night	46% 4 14% 27% 8%	3899
	21st to end	Night	51% 6% 13% 24% 6%	4617
November	1st to 10th	Night	59% 5% 13% 16% 6% 1	2871
	11th to 20th	Night	63% 6% 11% 14% 5%	2940
	21st to end	Night	69% 6% 12% 10% 4	2979
December	1st to 10th	Night	76% 4 9% 8% 2	5050
	11th to 20th	Night	64% 6% 11% 11% 9%	5716
	21st to end	Night	70% 4 7% 14% 4	7645

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	14% 18% 38% 20% 8% 2	330
	11th to 20th	Day	14% 12% 44% 25% 5%	330
	21st to end	Day	16% 16% 43% 18% 4 2	363
February	1st to 10th	Day	19% 18% 38% 20% 5% 1	330
	11th to 20th	Day	16% 17% 39% 20% 6% 2	330
	21st to end	Day	17% 22% 40% 16% 5% 1	273
March	1st to 10th	Day	18% 20% 38% 15% 7% 2	330
	11th to 20th	Day	19% 22% 37% 16% 5% 1	330
	21st to end	Day	17% 22% 37% 20% 4 1	363
April	1st to 10th	Day	19% 17% 37% 21% 6% 1	517
	11th to 20th	Day	23% 21% 32% 18% 6% 1	690
	21st to end	Day	22% 19% 33% 19% 6% 1	700
May	1st to 10th	Day	22% 20% 33% 19% 5% 1	703
	11th to 20th	Day	23% 18% 31% 19% 7% 2	725
	21st to end	Day	21% 14% 33% 25% 5% 2	789
June	1st to 10th	Day	20% 16% 32% 20% 9% 3	730
	11th to 20th	Day	20% 17% 37% 19% 5% 2	730
	21st to end	Day	17% 14% 32% 25% 9% 3	730
July	1st to 10th	Day	10% 13% 36% 22% 13% 5%	721
	11th to 20th	Day	13% 14% 35% 24% 12% 2	718
	21st to end	Day	10% 12% 35% 25% 14% 5%	794
August	1st to 10th	Day	11% 11% 33% 28% 15% 3	726
	11th to 20th	Day	14% 10% 30% 28% 13% 5%	715
	21st to end	Day	16% 12% 33% 25% 12% 1	803
September	1st to 10th	Day	16% 13% 34% 25% 10% 1	586
	11th to 20th	Day	20% 16% 40% 18% 6%	373
	21st to end	Day	24% 16% 37% 17% 4 2	330
October	1st to 10th	Day	26% 14% 35% 17% 6% 2	330
	11th to 20th	Day	26% 18% 34% 19% 3	330
	21st to end	Day	29% 17% 31% 19% 3 1	363
November	1st to 10th	Day	26% 17% 35% 17% 5% 1	330
	11th to 20th	Day	24% 21% 37% 14% 3	330
	21st to end	Day	26% 22% 34% 16% 2	330
December	1st to 10th	Day	18% 18% 36% 21% 7% 1	330
	11th to 20th	Day	19% 19% 46% 13% 2	330
	21st to end	Day	17% 14% 41% 17% 9% 2	363
January	1st to 10th	Night	12% 17% 36% 24% 10% 1	1050
	11th to 20th	Night	17% 13% 40% 23% 7% 1	1050
	21st to end	Night	15% 15% 39% 24% 6% 1	1155
February	1st to 10th	Night	15% 14% 34% 28% 8%	1050
	11th to 20th	Night	12% 12% 37% 27% 10% 2	1029
	21st to end	Night	11% 17% 39% 23% 9% 1	860
March	1st to 10th	Night	16% 13% 34% 27% 8% 1	738
	11th to 20th	Night	11% 14% 43% 23% 7% 1	630
	21st to end	Night	20% 12% 30% 25% 11% 2	693
April	1st to 10th	Night	17% 12% 34% 29% 8% 1	603
	11th to 20th	Night	22% 12% 31% 26% 9% 1	593
	21st to end	Night	18% 11% 36% 28% 7%	600
May	1st to 10th	Night	19% 11% 39% 24% 7%	600
	11th to 20th	Night	20% 10% 35% 25% 9% 1	591
	21st to end	Night	20% 11% 36% 25% 8%	650
June	1st to 10th	Night	16% 10% 32% 29% 13%	600
	11th to 20th	Night	16% 9% 33% 29% 12% 1	600
	21st to end	Night	15% 8% 26% 34% 15% 1	600
July	1st to 10th	Night	7% 5% 33% 35% 19% 1	585
	11th to 20th	Night	8% 7% 31% 36% 16% 1	582
	21st to end	Night	9% 4 28% 39% 17% 2	657
August	1st to 10th	Night	10% 5% 29% 39% 17% 1	597
	11th to 20th	Night	12% 7% 25% 32% 20% 4	594
	21st to end	Night	16% 7% 27% 34% 15% 1	660
September	1st to 10th	Night	14% 7% 34% 32% 12% 2	600
	11th to 20th	Night	17% 10% 33% 29% 11%	600
	21st to end	Night	21% 10% 33% 25% 9% 1	600
October	1st to 10th	Night	23% 13% 37% 21% 6%	788
	11th to 20th	Night	25% 10% 35% 22% 7% 1	800
	21st to end	Night	27% 12% 32% 22% 6% 1	1070
November	1st to 10th	Night	22% 16% 34% 19% 8% 1	1025
	11th to 20th	Night	22% 15% 34% 21% 6% 1	1050
	21st to end	Night	21% 19% 38% 16% 4 1	1050
December	1st to 10th	Night	14% 13% 39% 25% 7% 1	1050
	11th to 20th	Night	20% 17% 39% 19% 5% 1	1050
	21st to end	Night	19% 14% 36% 23% 8% 1	1155

Page generated at 2026-03-30 01:18:44. Version ac07

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