Casey Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Casey Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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% 9% 3 8% 5% 4 4 4 5% 7% 38%	150137
	Night	12% 10% 4 9% 5% 4 4 4 5% 7% 37%	141671
Transparency	Day	43% 6% 12% 26% 12% 1	153649
	Night	46% 5% 12% 23% 13% 1	146235
Seeing	Day	17% 10% 29% 26% 14% 4	18696
	Night	17% 9% 31% 32% 11% 1	28450

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	9% 13% 7% 18% 7% 6% 4 3 4 5% 23%	4691
	11th to 20th	Day	6% 13% 7% 14% 6% 6% 7% 9% 7% 4 21%	4710
	21st to end	Day	7% 12% 6% 10% 5% 5% 6% 4 5% 7% 33%	5573
February	1st to 10th	Day	8% 13% 5% 11% 6% 5% 5% 5% 5% 6% 31%	1890
	11th to 20th	Day	9% 11% 4 9% 5% 6% 4 4 6% 7% 35%	1844
	21st to end	Day	12% 12% 5% 9% 6% 4 5% 4 4 7% 32%	1712
March	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 7% 48%	4692
	11th to 20th	Day	11% 8% 3 8% 4 5% 4 3 6% 8% 40%	5090
	21st to end	Day	22% 11% 3 10% 5% 4 4 3 5% 6% 28%	5817
April	1st to 10th	Day	16% 13% 4 7% 5% 4 4 3 5% 6% 33%	2520
	11th to 20th	Day	21% 11% 4 8% 5% 4 4 3 5% 6% 30%	2494
	21st to end	Day	19% 12% 3 7% 6% 5% 4 4 5% 6% 29%	2618
May	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 5% 7% 47%	8212
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 5% 7% 43%	8222
	21st to end	Day	15% 8% 2 6% 5% 4 4 4 5% 7% 39%	9047
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 3 6% 9% 42%	2940
	11th to 20th	Day	14% 6% 3 7% 6% 5% 4 4 6% 8% 36%	2940
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 9% 39%	2940
July	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 10% 39%	8280
	11th to 20th	Day	7% 4 1 4 3 3 4 3 6% 10% 56%	8280
	21st to end	Day	9% 6% 2 6% 4 4 4 4 6% 9% 46%	8990
August	1st to 10th	Day	11% 7% 2 6% 4 4 5% 4 6% 7% 44%	6208
	11th to 20th	Day	12% 6% 2 6% 4 5% 4 4 6% 9% 43%	5948
	21st to end	Day	10% 6% 2 6% 4 5% 5% 3 7% 8% 44%	6103
September	1st to 10th	Day	12% 7% 2 5% 4 3 4 3 5% 8% 47%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 9% 43%	2520
	21st to end	Day	14% 7% 2 5% 4 4 5% 4 6% 7% 43%	2204
October	1st to 10th	Day	15% 7% 3 5% 4 3 4 3 6% 6% 44%	2000
	11th to 20th	Day	16% 11% 2 7% 5% 4 4 3 5% 5% 36%	2000
	21st to end	Day	20% 11% 3 7% 5% 3 3 2 4 5% 36%	2085
November	1st to 10th	Day	13% 10% 3 9% 4 5% 4 3 6% 6% 37%	1839
	11th to 20th	Day	14% 14% 5% 9% 6% 4 4 3 5% 5% 29%	1680
	21st to end	Day	14% 13% 4 8% 6% 4 3 3 5% 5% 35%	1680
December	1st to 10th	Day	7% 11% 5% 16% 13% 7% 6% 4 5% 5% 22%	2690
	11th to 20th	Day	12% 16% 6% 11% 7% 4 4 3 3 5% 28%	3088
	21st to end	Day	20% 21% 8% 16% 6% 4 3 2 3 3 14%	4070
January	1st to 10th	Night	9% 14% 7% 14% 7% 4 3 3 4 6% 29%	8232
	11th to 20th	Night	7% 14% 6% 14% 7% 6% 8% 8% 6% 4 20%	8280
	21st to end	Night	7% 14% 5% 11% 6% 5% 5% 3 5% 6% 32%	8626
February	1st to 10th	Night	7% 12% 5% 11% 7% 6% 5% 4 5% 6% 33%	2730
	11th to 20th	Night	8% 10% 4 10% 5% 6% 5% 4 5% 6% 38%	2663
	21st to end	Night	12% 11% 3 9% 6% 5% 5% 3 5% 5% 35%	2093
March	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 5% 8% 45%	5411
	11th to 20th	Night	10% 8% 3 9% 5% 5% 5% 4 5% 9% 37%	5090
	21st to end	Night	24% 11% 4 9% 4 4 3 2 4 6% 28%	5354
April	1st to 10th	Night	19% 11% 3 6% 4 4 3 4 5% 6% 35%	2100
	11th to 20th	Night	21% 9% 3 7% 4 4 3 4 4 5% 35%	2075
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 8% 36%	1998
May	1st to 10th	Night	12% 6% 2 6% 4 4 5% 3 5% 7% 44%	4671
	11th to 20th	Night	15% 7% 2 5% 4 3 4 3 4 7% 46%	4670
	21st to end	Night	14% 8% 3 8% 5% 4 5% 4 5% 6% 38%	5139
June	1st to 10th	Night	12% 6% 1 5% 5% 4 4 5% 6% 8% 44%	1680
	11th to 20th	Night	12% 6% 3 7% 4 4 4 3 7% 7% 43%	1680
	21st to end	Night	9% 5% 2 5% 4 4 4 4 6% 8% 47%	1680
July	1st to 10th	Night	10% 6% 3 7% 4 4 5% 4 6% 9% 42%	4708
	11th to 20th	Night	6% 3 1 4 4 3 4 3 6% 10% 56%	4707
	21st to end	Night	7% 5% 2 6% 4 4 4 4 7% 9% 50%	5117
August	1st to 10th	Night	9% 6% 2 7% 4 4 4 3 6% 7% 48%	3958
	11th to 20th	Night	12% 6% 3 7% 4 4 4 3 5% 7% 45%	4118
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 7% 49%	5023
September	1st to 10th	Night	10% 6% 2 5% 4 4 3 4 5% 7% 52%	2100
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 8% 50%	2100
	21st to end	Night	14% 5% 2 5% 4 3 4 3 6% 6% 49%	2348
October	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 50%	2400
	11th to 20th	Night	16% 10% 3 6% 4 4 3 3 5% 7% 40%	2400
	21st to end	Night	17% 11% 3 7% 4 4 4 4 5% 5% 37%	2754
November	1st to 10th	Night	14% 10% 4 6% 4 4 4 4 4 7% 40%	2677
	11th to 20th	Night	14% 12% 5% 9% 5% 4 4 3 5% 6% 33%	2940
	21st to end	Night	15% 12% 4 10% 4 4 4 3 4 6% 34%	2940
December	1st to 10th	Night	8% 10% 6% 13% 8% 7% 5% 4 5% 7% 29%	4720
	11th to 20th	Night	11% 18% 7% 14% 5% 4 5% 4 4 5% 22%	5339
	21st to end	Night	22% 17% 6% 14% 6% 4 3 2 3 5% 18%	7150

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	64% 4 10% 16% 6%	4692
	11th to 20th	Day	62% 4 7% 17% 10%	4710
	21st to end	Day	50% 5% 13% 23% 9%	5573
February	1st to 10th	Day	51% 6% 13% 19% 10% 1	1890
	11th to 20th	Day	47% 5% 12% 26% 9% 1	1853
	21st to end	Day	51% 5% 11% 23% 9% 1	1712
March	1st to 10th	Day	38% 3 15% 26% 14% 4	4692
	11th to 20th	Day	41% 4 14% 27% 11% 1	5090
	21st to end	Day	57% 5% 10% 17% 10% 1	5819
April	1st to 10th	Day	51% 5% 11% 19% 13% 1	2520
	11th to 20th	Day	54% 5% 10% 19% 11% 2	2494
	21st to end	Day	55% 6% 12% 18% 9% 1	2618
May	1st to 10th	Day	34% 6% 10% 31% 18% 1	8250
	11th to 20th	Day	39% 6% 9% 26% 18% 2	8226
	21st to end	Day	43% 6% 9% 27% 13% 2	9061
June	1st to 10th	Day	37% 7% 13% 29% 13% 2	2940
	11th to 20th	Day	44% 7% 13% 25% 11% 1	2940
	21st to end	Day	39% 8% 17% 26% 10% 2	2940
July	1st to 10th	Day	38% 7% 17% 31% 7%	8211
	11th to 20th	Day	23% 6% 14% 39% 17% 1	8205
	21st to end	Day	34% 6% 13% 28% 17% 1	8794
August	1st to 10th	Day	37% 6% 13% 31% 12% 1	6193
	11th to 20th	Day	37% 7% 16% 31% 10%	5987
	21st to end	Day	36% 6% 15% 31% 12% 1	6102
September	1st to 10th	Day	36% 6% 14% 29% 14%	2520
	11th to 20th	Day	38% 7% 17% 30% 8% 1	2520
	21st to end	Day	40% 6% 13% 30% 11%	2204
October	1st to 10th	Day	28% 4 8% 38% 21%	3260
	11th to 20th	Day	40% 5% 11% 25% 16% 2	3240
	21st to end	Day	43% 4 14% 30% 9%	3349
November	1st to 10th	Day	46% 6% 14% 20% 13% 1	1839
	11th to 20th	Day	56% 5% 11% 19% 8% 1	1680
	21st to end	Day	51% 5% 15% 21% 6% 1	1680
December	1st to 10th	Day	63% 5% 11% 16% 5%	2690
	11th to 20th	Day	59% 5% 10% 19% 7% 1	3085
	21st to end	Day	76% 3 7% 10% 4	4070
January	1st to 10th	Night	57% 4 13% 18% 7%	8233
	11th to 20th	Night	63% 5% 8% 13% 10%	8280
	21st to end	Night	52% 4 11% 20% 12%	8626
February	1st to 10th	Night	53% 4 12% 20% 9% 2	2730
	11th to 20th	Night	45% 6% 12% 27% 9% 1	2664
	21st to end	Night	49% 5% 13% 22% 10% 2	2093
March	1st to 10th	Night	37% 5% 15% 26% 13% 4	5411
	11th to 20th	Night	43% 6% 15% 23% 12% 1	5090
	21st to end	Night	59% 5% 9% 16% 12%	5354
April	1st to 10th	Night	49% 5% 9% 18% 17% 2	2100
	11th to 20th	Night	50% 5% 9% 19% 15% 2	2075
	21st to end	Night	48% 5% 8% 22% 15% 2	1998
May	1st to 10th	Night	37% 6% 9% 26% 21% 1	4695
	11th to 20th	Night	38% 4 9% 23% 21% 4	4671
	21st to end	Night	45% 7% 9% 24% 13% 2	5149
June	1st to 10th	Night	35% 8% 13% 26% 17% 1	1680
	11th to 20th	Night	37% 6% 14% 23% 19% 1	1680
	21st to end	Night	31% 8% 15% 32% 12% 2	1680
July	1st to 10th	Night	37% 7% 17% 29% 10% 1	4670
	11th to 20th	Night	23% 5% 13% 38% 20% 1	4662
	21st to end	Night	30% 7% 13% 30% 18% 2	5002
August	1st to 10th	Night	34% 6% 12% 30% 16% 2	3949
	11th to 20th	Night	37% 7% 15% 27% 12% 1	4145
	21st to end	Night	34% 7% 16% 30% 13% 1	5023
September	1st to 10th	Night	33% 5% 12% 33% 16% 1	2100
	11th to 20th	Night	33% 6% 16% 31% 14%	2100
	21st to end	Night	34% 6% 12% 31% 15% 1	2348
October	1st to 10th	Night	26% 3 6% 35% 26% 4	3920
	11th to 20th	Night	33% 5% 11% 28% 21% 1	3899
	21st to end	Night	47% 4 10% 24% 13%	4440
November	1st to 10th	Night	44% 4 12% 25% 13% 1	2677
	11th to 20th	Night	51% 5% 14% 17% 11% 1	2940
	21st to end	Night	51% 5% 14% 22% 7% 1	2940
December	1st to 10th	Night	54% 7% 16% 18% 4	4720
	11th to 20th	Night	65% 4 10% 15% 6%	5341
	21st to end	Night	70% 4 8% 13% 6%	7150

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	15% 15% 32% 24% 12% 3	330
	11th to 20th	Day	15% 12% 38% 27% 6% 2	330
	21st to end	Day	16% 12% 36% 25% 8% 2	363
February	1st to 10th	Day	17% 13% 40% 22% 7% 2	330
	11th to 20th	Day	13% 17% 38% 24% 7%	330
	21st to end	Day	19% 12% 42% 24% 4	273
March	1st to 10th	Day	19% 14% 36% 20% 10% 1	330
	11th to 20th	Day	22% 13% 33% 22% 8% 2	330
	21st to end	Day	23% 12% 32% 22% 8% 3	468
April	1st to 10th	Day	26% 15% 33% 19% 6% 1	540
	11th to 20th	Day	29% 14% 24% 23% 8% 2	532
	21st to end	Day	25% 14% 29% 21% 9% 2	638
May	1st to 10th	Day	19% 13% 29% 28% 8% 2	753
	11th to 20th	Day	21% 13% 26% 25% 12% 3	775
	21st to end	Day	19% 9% 24% 28% 15% 4	844
June	1st to 10th	Day	12% 8% 25% 34% 17% 5%	780
	11th to 20th	Day	16% 9% 35% 27% 11% 2	780
	21st to end	Day	14% 8% 31% 26% 15% 6%	780
July	1st to 10th	Day	10% 5% 19% 30% 26% 11%	771
	11th to 20th	Day	9% 6% 25% 30% 21% 9%	768
	21st to end	Day	8% 4 24% 36% 20% 8%	849
August	1st to 10th	Day	11% 5% 22% 31% 23% 8%	776
	11th to 20th	Day	12% 4 22% 30% 25% 6%	765
	21st to end	Day	13% 5% 25% 30% 21% 6%	638
September	1st to 10th	Day	12% 10% 26% 27% 19% 6%	570
	11th to 20th	Day	13% 7% 28% 31% 18% 3	570
	21st to end	Day	16% 11% 26% 26% 17% 4	447
October	1st to 10th	Day	16% 11% 30% 26% 12% 4	330
	11th to 20th	Day	22% 18% 32% 20% 7% 1	330
	21st to end	Day	25% 17% 33% 15% 7% 2	363
November	1st to 10th	Day	19% 13% 38% 20% 8% 1	330
	11th to 20th	Day	20% 18% 36% 19% 6%	330
	21st to end	Day	23% 15% 38% 18% 6%	330
December	1st to 10th	Day	15% 13% 40% 24% 7% 1	330
	11th to 20th	Day	20% 14% 39% 18% 8% 1	330
	21st to end	Day	26% 13% 38% 19% 4 1	363
January	1st to 10th	Night	14% 12% 34% 31% 9% 1	1050
	11th to 20th	Night	18% 10% 34% 30% 6% 1	1050
	21st to end	Night	19% 9% 32% 32% 7% 1	1155
February	1st to 10th	Night	14% 12% 38% 29% 7% 1	1050
	11th to 20th	Night	13% 12% 39% 28% 8%	1029
	21st to end	Night	18% 13% 33% 30% 6%	709
March	1st to 10th	Night	16% 11% 37% 29% 6% 1	810
	11th to 20th	Night	18% 12% 32% 32% 5% 1	630
	21st to end	Night	22% 9% 31% 26% 10% 1	693
April	1st to 10th	Night	23% 13% 34% 24% 5% 1	630
	11th to 20th	Night	25% 11% 28% 29% 7%	623
	21st to end	Night	23% 9% 32% 26% 9% 1	630
May	1st to 10th	Night	21% 10% 31% 31% 7%	630
	11th to 20th	Night	19% 8% 31% 31% 10% 1	621
	21st to end	Night	14% 7% 26% 40% 13% 1	683
June	1st to 10th	Night	13% 5% 28% 43% 12%	630
	11th to 20th	Night	15% 5% 31% 38% 10%	630
	21st to end	Night	10% 7% 23% 43% 17% 1	630
July	1st to 10th	Night	8% 3 22% 36% 28% 2	615
	11th to 20th	Night	8% 5% 22% 41% 23% 2	612
	21st to end	Night	6% 3 20% 45% 23% 2	690
August	1st to 10th	Night	8% 4 15% 46% 27% 1	627
	11th to 20th	Night	12% 2 19% 40% 24% 3	624
	21st to end	Night	13% 3 23% 37% 22% 2	693
September	1st to 10th	Night	12% 4 24% 39% 18% 4	630
	11th to 20th	Night	12% 4 27% 36% 18% 2	630
	21st to end	Night	15% 7% 29% 31% 16% 1	780
October	1st to 10th	Night	17% 7% 29% 32% 14% 1	800
	11th to 20th	Night	19% 10% 34% 29% 7% 1	800
	21st to end	Night	23% 11% 29% 30% 8%	880
November	1st to 10th	Night	19% 9% 34% 30% 7%	831
	11th to 20th	Night	19% 13% 37% 24% 7%	1050
	21st to end	Night	21% 14% 34% 24% 7%	1050
December	1st to 10th	Night	18% 10% 35% 28% 8% 1	1050
	11th to 20th	Night	22% 10% 35% 25% 7% 1	1050
	21st to end	Night	27% 10% 31% 26% 6%	1155

Page generated at 2026-09-26 03:18:40. Version ac07

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