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 2025-05-26.

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	13% 9% 3 8% 5% 4 4 4 5% 7% 37%	114276
	Night	12% 10% 4 9% 5% 4 4 4 5% 6% 37%	115217
Transparency	Day	45% 6% 12% 24% 11% 1	117835
	Night	46% 5% 12% 23% 13% 1	119800
Seeing	Day	17% 10% 29% 26% 14% 4	16715
	Night	17% 9% 31% 32% 11% 1	26701

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	8% 14% 7% 18% 7% 6% 4 3 4 5% 26%	3617
	11th to 20th	Day	7% 12% 7% 13% 5% 7% 8% 8% 8% 4 20%	3620
	21st to end	Day	9% 16% 7% 12% 5% 4 4 3 4 6% 31%	4304
February	1st to 10th	Day	9% 13% 5% 11% 6% 5% 5% 5% 5% 6% 32%	1800
	11th to 20th	Day	10% 11% 4 9% 5% 6% 4 4 6% 8% 34%	1754
	21st to end	Day	12% 12% 5% 9% 6% 5% 6% 4 4 7% 32%	1633
March	1st to 10th	Day	14% 7% 2 5% 3 3 3 3 4 7% 48%	4590
	11th to 20th	Day	11% 8% 2 8% 4 5% 5% 3 6% 8% 41%	4980
	21st to end	Day	22% 11% 3 10% 5% 4 4 3 5% 6% 27%	5691
April	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 5% 6% 33%	2400
	11th to 20th	Day	21% 11% 4 8% 5% 4 4 3 5% 6% 31%	2374
	21st to end	Day	20% 12% 4 7% 6% 5% 4 4 5% 6% 29%	2493
May	1st to 10th	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 41%	4539
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 6% 7% 40%	4539
	21st to end	Day	17% 8% 2 7% 5% 4 4 3 5% 6% 38%	4895
June	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 6% 8% 43%	2660
	11th to 20th	Day	13% 6% 3 7% 6% 5% 4 4 6% 8% 37%	2660
	21st to end	Day	10% 6% 3 6% 5% 4 5% 5% 7% 9% 40%	2660
July	1st to 10th	Day	12% 5% 2 6% 4 6% 5% 5% 8% 10% 37%	4440
	11th to 20th	Day	6% 3 1 4 3 3 4 4 6% 10% 54%	4440
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 4 6% 9% 45%	4813
August	1st to 10th	Day	12% 7% 2 5% 4 4 5% 4 6% 8% 42%	4238
	11th to 20th	Day	12% 5% 2 6% 3 4 4 4 7% 8% 45%	4041
	21st to end	Day	11% 6% 3 7% 5% 5% 4 3 7% 8% 42%	4190
September	1st to 10th	Day	12% 7% 2 5% 3 3 4 3 4 8% 48%	2280
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 5% 8% 45%	2280
	21st to end	Day	15% 7% 2 5% 4 4 5% 4 6% 7% 43%	2014
October	1st to 10th	Day	15% 7% 3 5% 4 3 4 3 6% 6% 43%	1900
	11th to 20th	Day	16% 11% 2 7% 5% 4 4 4 5% 5% 36%	1900
	21st to end	Day	20% 11% 3 7% 5% 3 3 2 4 5% 36%	1981
November	1st to 10th	Day	13% 10% 3 8% 4 5% 4 3 6% 6% 37%	1750
	11th to 20th	Day	15% 14% 5% 9% 6% 4 4 3 5% 5% 28%	1600
	21st to end	Day	14% 13% 4 7% 6% 4 3 3 5% 5% 36%	1600
December	1st to 10th	Day	7% 10% 5% 16% 13% 7% 6% 4 5% 5% 23%	2610
	11th to 20th	Day	13% 17% 6% 11% 7% 4 4 3 3 5% 28%	3008
	21st to end	Day	20% 21% 8% 16% 6% 4 3 2 3 3 14%	3982
January	1st to 10th	Night	7% 13% 7% 14% 8% 4 3 2 4 6% 32%	6348
	11th to 20th	Night	8% 13% 6% 13% 7% 6% 8% 8% 6% 4 21%	6360
	21st to end	Night	9% 17% 7% 12% 6% 5% 3 3 4 4 30%	6659
February	1st to 10th	Night	7% 12% 5% 11% 7% 6% 5% 4 5% 5% 33%	2600
	11th to 20th	Night	8% 10% 3 10% 5% 6% 5% 4 5% 6% 38%	2533
	21st to end	Night	13% 11% 3 9% 6% 5% 5% 3 5% 5% 35%	1996
March	1st to 10th	Night	10% 7% 3 7% 4 4 4 4 5% 8% 45%	5292
	11th to 20th	Night	9% 8% 4 9% 6% 5% 5% 4 5% 9% 37%	4980
	21st to end	Night	25% 11% 4 9% 4 3 3 2 4 6% 28%	5238
April	1st to 10th	Night	18% 11% 3 6% 4 4 3 4 6% 6% 36%	2000
	11th to 20th	Night	22% 9% 3 7% 4 4 3 4 5% 5% 35%	1975
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 8% 36%	1903
May	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 4 5% 7% 38%	2586
	11th to 20th	Night	14% 6% 2 5% 3 2 4 2 4 7% 50%	2586
	21st to end	Night	14% 6% 3 8% 4 4 4 3 5% 7% 41%	2788
June	1st to 10th	Night	11% 6% 1 5% 5% 4 4 4 6% 8% 45%	1520
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 7% 7% 44%	1520
	21st to end	Night	9% 5% 2 5% 4 4 4 4 7% 8% 47%	1520
July	1st to 10th	Night	11% 4 2 5% 3 4 5% 4 6% 9% 46%	2530
	11th to 20th	Night	5% 3 1 5% 4 3 4 4 5% 8% 57%	2530
	21st to end	Night	5% 5% 2 5% 4 3 4 4 6% 8% 54%	2750
August	1st to 10th	Night	11% 6% 2 7% 4 5% 4 3 6% 7% 45%	2698
	11th to 20th	Night	12% 5% 2 6% 4 3 4 3 5% 7% 50%	2798
	21st to end	Night	12% 7% 2 7% 4 4 4 3 5% 6% 46%	3465
September	1st to 10th	Night	9% 5% 1 5% 4 3 3 4 5% 7% 52%	1900
	11th to 20th	Night	11% 5% 2 5% 4 3 4 3 5% 8% 51%	1900
	21st to end	Night	14% 5% 1 5% 3 3 3 3 6% 6% 50%	2145
October	1st to 10th	Night	14% 6% 2 5% 3 3 4 3 4 6% 50%	2280
	11th to 20th	Night	15% 9% 3 6% 4 4 3 3 5% 7% 41%	2280
	21st to end	Night	18% 11% 3 7% 4 4 4 4 5% 5% 36%	2616
November	1st to 10th	Night	14% 11% 4 6% 4 4 4 4 4 7% 40%	2546
	11th to 20th	Night	14% 12% 5% 9% 5% 4 4 4 5% 6% 32%	2800
	21st to end	Night	15% 12% 4 9% 5% 4 4 3 4 6% 35%	2800
December	1st to 10th	Night	8% 10% 6% 12% 8% 7% 5% 4 5% 7% 29%	4580
	11th to 20th	Night	12% 18% 7% 14% 6% 4 4 4 4 5% 22%	5199
	21st to end	Night	22% 17% 6% 14% 6% 4 3 2 3 5% 18%	6996

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	61% 4 10% 18% 6%	3618
	11th to 20th	Day	61% 5% 7% 17% 11%	3620
	21st to end	Day	54% 5% 15% 18% 7%	4304
February	1st to 10th	Day	51% 6% 13% 18% 10% 1	1800
	11th to 20th	Day	47% 5% 12% 25% 9% 1	1763
	21st to end	Day	52% 5% 11% 22% 9% 1	1633
March	1st to 10th	Day	38% 3 15% 26% 14% 4	4590
	11th to 20th	Day	41% 4 14% 28% 12% 1	4980
	21st to end	Day	57% 5% 9% 17% 11% 1	5693
April	1st to 10th	Day	51% 6% 10% 19% 12% 1	2400
	11th to 20th	Day	54% 5% 10% 19% 11% 2	2374
	21st to end	Day	55% 5% 11% 18% 9% 1	2493
May	1st to 10th	Day	41% 7% 11% 28% 13% 1	4580
	11th to 20th	Day	39% 7% 13% 27% 13% 1	4543
	21st to end	Day	46% 7% 10% 23% 12% 2	4908
June	1st to 10th	Day	35% 7% 14% 29% 13% 2	2660
	11th to 20th	Day	43% 7% 13% 25% 12% 1	2660
	21st to end	Day	37% 8% 17% 26% 10% 2	2660
July	1st to 10th	Day	39% 8% 18% 25% 10% 1	4371
	11th to 20th	Day	23% 7% 17% 35% 18% 1	4365
	21st to end	Day	34% 8% 15% 26% 17% 1	4662
August	1st to 10th	Day	38% 7% 13% 27% 14% 1	4223
	11th to 20th	Day	35% 7% 12% 34% 11%	4080
	21st to end	Day	39% 6% 14% 29% 12% 1	4189
September	1st to 10th	Day	35% 6% 14% 30% 15%	2280
	11th to 20th	Day	36% 7% 17% 31% 8% 1	2280
	21st to end	Day	39% 6% 13% 30% 11%	2014
October	1st to 10th	Day	28% 4 8% 38% 22%	3160
	11th to 20th	Day	40% 5% 11% 26% 16% 2	3140
	21st to end	Day	43% 4 14% 30% 9%	3245
November	1st to 10th	Day	46% 6% 14% 20% 13% 1	1750
	11th to 20th	Day	56% 6% 12% 18% 8% 1	1600
	21st to end	Day	50% 5% 16% 21% 7% 1	1600
December	1st to 10th	Day	63% 5% 11% 16% 5%	2610
	11th to 20th	Day	60% 5% 10% 18% 7% 1	3005
	21st to end	Day	76% 3 7% 10% 4	3982
January	1st to 10th	Night	55% 4 10% 21% 9%	6348
	11th to 20th	Night	62% 5% 9% 13% 11%	6360
	21st to end	Night	56% 4 11% 19% 8%	6659
February	1st to 10th	Night	53% 4 12% 20% 9% 2	2600
	11th to 20th	Night	45% 6% 12% 27% 9% 1	2534
	21st to end	Night	49% 5% 13% 21% 10% 2	1996
March	1st to 10th	Night	37% 5% 15% 25% 13% 5%	5292
	11th to 20th	Night	42% 6% 15% 23% 12% 1	4980
	21st to end	Night	59% 5% 9% 16% 12%	5238
April	1st to 10th	Night	49% 6% 9% 18% 17% 2	2000
	11th to 20th	Night	50% 6% 9% 19% 15% 2	1975
	21st to end	Night	47% 6% 9% 22% 15% 2	1903
May	1st to 10th	Night	44% 8% 12% 16% 19% 1	2610
	11th to 20th	Night	34% 4 8% 31% 19% 4	2587
	21st to end	Night	41% 7% 12% 24% 16% 1	2796
June	1st to 10th	Night	34% 8% 13% 27% 17% 2	1520
	11th to 20th	Night	36% 7% 15% 23% 19% 1	1520
	21st to end	Night	31% 8% 14% 32% 13% 3	1520
July	1st to 10th	Night	33% 7% 19% 26% 13% 2	2490
	11th to 20th	Night	23% 5% 14% 36% 20% 1	2482
	21st to end	Night	26% 7% 15% 29% 21% 3	2662
August	1st to 10th	Night	37% 7% 11% 27% 15% 2	2689
	11th to 20th	Night	34% 6% 13% 30% 15% 2	2825
	21st to end	Night	37% 6% 15% 26% 14% 2	3465
September	1st to 10th	Night	32% 6% 12% 33% 17% 1	1900
	11th to 20th	Night	31% 6% 15% 33% 15%	1900
	21st to end	Night	34% 6% 12% 32% 15% 1	2145
October	1st to 10th	Night	26% 3 6% 36% 26% 4	3800
	11th to 20th	Night	32% 5% 11% 29% 22% 1	3779
	21st to end	Night	48% 4 10% 24% 13%	4302
November	1st to 10th	Night	44% 4 13% 24% 14% 1	2546
	11th to 20th	Night	52% 6% 14% 17% 11% 1	2800
	21st to end	Night	51% 5% 14% 22% 7% 1	2800
December	1st to 10th	Night	53% 7% 17% 19% 4	4580
	11th to 20th	Night	65% 4 10% 15% 6%	5201
	21st to end	Night	70% 4 7% 13% 6%	6996

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% 14% 30% 25% 13% 3	300
	11th to 20th	Day	16% 12% 36% 27% 7% 2	300
	21st to end	Day	17% 11% 36% 26% 8% 2	330
February	1st to 10th	Day	16% 14% 40% 22% 6% 2	300
	11th to 20th	Day	13% 17% 37% 25% 8%	300
	21st to end	Day	19% 11% 41% 25% 4	249
March	1st to 10th	Day	19% 14% 35% 20% 11% 1	300
	11th to 20th	Day	20% 13% 32% 24% 9% 2	300
	21st to end	Day	24% 11% 31% 22% 9% 3	430
April	1st to 10th	Day	26% 14% 32% 20% 6% 1	500
	11th to 20th	Day	29% 14% 24% 23% 8% 1	492
	21st to end	Day	26% 14% 28% 21% 9% 2	593
May	1st to 10th	Day	19% 13% 30% 28% 9% 2	703
	11th to 20th	Day	21% 12% 26% 26% 13% 3	725
	21st to end	Day	19% 9% 24% 28% 15% 4	760
June	1st to 10th	Day	12% 7% 26% 33% 18% 5%	680
	11th to 20th	Day	14% 9% 35% 28% 11% 3	680
	21st to end	Day	13% 8% 30% 27% 15% 7%	680
July	1st to 10th	Day	10% 4 19% 29% 26% 11%	671
	11th to 20th	Day	9% 6% 25% 30% 21% 10%	668
	21st to end	Day	7% 4 24% 35% 21% 9%	739
August	1st to 10th	Day	11% 5% 22% 31% 23% 8%	676
	11th to 20th	Day	12% 4 20% 29% 27% 7%	665
	21st to end	Day	14% 5% 24% 29% 21% 6%	549
September	1st to 10th	Day	13% 8% 26% 28% 20% 6%	490
	11th to 20th	Day	12% 6% 27% 31% 20% 4	490
	21st to end	Day	16% 12% 28% 26% 14% 4	385
October	1st to 10th	Day	18% 10% 30% 26% 12% 4	300
	11th to 20th	Day	20% 17% 34% 21% 7% 1	300
	21st to end	Day	25% 18% 34% 15% 6% 2	330
November	1st to 10th	Day	20% 11% 37% 22% 9% 1	300
	11th to 20th	Day	20% 17% 38% 19% 6%	300
	21st to end	Day	22% 15% 38% 17% 7%	300
December	1st to 10th	Day	15% 12% 40% 25% 7% 1	300
	11th to 20th	Day	20% 14% 40% 17% 8% 1	300
	21st to end	Day	26% 12% 36% 21% 4 1	330
January	1st to 10th	Night	13% 12% 34% 31% 9% 1	1000
	11th to 20th	Night	19% 10% 34% 30% 6% 1	1000
	21st to end	Night	20% 9% 32% 31% 7% 1	1100
February	1st to 10th	Night	14% 11% 38% 29% 7% 1	1000
	11th to 20th	Night	12% 12% 39% 28% 8% 1	979
	21st to end	Night	18% 13% 33% 30% 6%	676
March	1st to 10th	Night	15% 10% 37% 30% 6% 1	771
	11th to 20th	Night	16% 12% 33% 33% 6% 1	600
	21st to end	Night	23% 9% 29% 27% 10% 2	660
April	1st to 10th	Night	23% 13% 34% 25% 5% 1	600
	11th to 20th	Night	25% 11% 29% 28% 7%	593
	21st to end	Night	24% 9% 32% 26% 9% 1	600
May	1st to 10th	Night	22% 10% 30% 31% 7%	600
	11th to 20th	Night	19% 8% 30% 32% 10% 1	591
	21st to end	Night	14% 7% 25% 40% 13% 1	634
June	1st to 10th	Night	12% 5% 28% 42% 12%	570
	11th to 20th	Night	15% 5% 30% 40% 11%	570
	21st to end	Night	9% 7% 23% 43% 17% 1	570
July	1st to 10th	Night	8% 3 22% 36% 28% 3	555
	11th to 20th	Night	8% 5% 21% 41% 23% 2	552
	21st to end	Night	6% 2 19% 45% 25% 3	624
August	1st to 10th	Night	8% 3 14% 47% 27% 1	567
	11th to 20th	Night	12% 2 19% 39% 25% 3	564
	21st to end	Night	14% 3 22% 36% 23% 2	627
September	1st to 10th	Night	12% 4 24% 39% 18% 4	570
	11th to 20th	Night	12% 4 28% 35% 19% 2	570
	21st to end	Night	15% 8% 30% 30% 16% 1	712
October	1st to 10th	Night	17% 7% 28% 32% 15% 1	760
	11th to 20th	Night	19% 10% 34% 30% 7% 1	760
	21st to end	Night	23% 11% 28% 30% 8%	836
November	1st to 10th	Night	19% 8% 34% 31% 7%	790
	11th to 20th	Night	20% 12% 37% 24% 7%	1000
	21st to end	Night	21% 13% 34% 25% 7%	1000
December	1st to 10th	Night	19% 10% 35% 28% 8% 1	1000
	11th to 20th	Night	22% 9% 36% 25% 8% 1	1000
	21st to end	Night	27% 10% 31% 26% 6%	1100

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