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 2024-11-02.

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	13% 9% 3 8% 5% 4 4 4 5% 7% 37%	101598
	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 38%	97491
Transparency	Day	45% 6% 13% 24% 11% 1	101391
	Night	46% 6% 12% 23% 12% 1	97369
Seeing	Day	16% 10% 29% 27% 14% 4	16009
	Night	17% 8% 30% 32% 11% 1	25858

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	11% 18% 8% 15% 5% 4 4 2 5% 5% 23%	2527
	11th to 20th	Day	7% 12% 5% 15% 7% 9% 9% 8% 6% 3 18%	2530
	21st to end	Day	13% 19% 7% 13% 5% 4 3 3 4 6% 22%	3007
February	1st to 10th	Day	8% 12% 5% 10% 6% 5% 5% 4 5% 6% 32%	1710
	11th to 20th	Day	10% 11% 4 8% 5% 6% 4 4 6% 8% 34%	1664
	21st to end	Day	12% 12% 5% 9% 5% 5% 6% 4 4 7% 31%	1553
March	1st to 10th	Day	14% 9% 3 6% 4 3 3 3 3 5% 47%	3202
	11th to 20th	Day	10% 9% 3 10% 5% 5% 4 3 5% 7% 40%	3480
	21st to end	Day	22% 13% 4 11% 5% 4 4 3 4 5% 26%	3973
April	1st to 10th	Day	16% 13% 4 8% 5% 4 3 3 5% 6% 33%	2280
	11th to 20th	Day	22% 10% 4 8% 5% 4 4 3 4 6% 31%	2254
	21st to end	Day	20% 12% 4 7% 6% 5% 4 4 5% 6% 29%	2368
May	1st to 10th	Day	17% 6% 2 6% 5% 4 4 3 5% 7% 40%	4399
	11th to 20th	Day	14% 7% 2 6% 5% 5% 4 4 6% 7% 40%	4399
	21st to end	Day	17% 8% 3 7% 5% 4 4 3 5% 6% 37%	4824
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	12% 10% 3 8% 4 5% 4 4 6% 6% 38%	1670
	11th to 20th	Day	15% 14% 6% 9% 6% 4 4 3 5% 5% 27%	1520
	21st to end	Day	14% 12% 4 7% 5% 4 3 3 5% 5% 37%	1520
December	1st to 10th	Day	11% 13% 5% 13% 9% 6% 6% 4 5% 6% 24%	1520
	11th to 20th	Day	11% 15% 6% 11% 8% 5% 5% 4 3 5% 27%	1918
	21st to end	Day	21% 19% 7% 15% 6% 3 3 2 3 3 18%	2783
January	1st to 10th	Night	9% 17% 9% 14% 5% 4 4 3 4 4 29%	4428
	11th to 20th	Night	8% 12% 6% 14% 7% 7% 7% 8% 6% 4 20%	4440
	21st to end	Night	13% 20% 8% 12% 5% 5% 3 3 4 4 23%	4659
February	1st to 10th	Night	6% 12% 5% 12% 7% 6% 5% 4 5% 5% 34%	2470
	11th to 20th	Night	8% 10% 3 9% 5% 6% 5% 4 6% 6% 38%	2403
	21st to end	Night	13% 11% 3 9% 6% 4 5% 3 5% 5% 35%	1900
March	1st to 10th	Night	9% 8% 3 6% 4 4 4 4 5% 8% 45%	3680
	11th to 20th	Night	8% 8% 4 10% 6% 5% 5% 4 5% 7% 39%	3480
	21st to end	Night	25% 11% 4 11% 5% 4 3 2 3 5% 28%	3658
April	1st to 10th	Night	19% 11% 3 6% 4 4 3 4 5% 6% 35%	1900
	11th to 20th	Night	22% 9% 3 7% 4 4 3 4 5% 5% 35%	1875
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 8% 36%	1808
May	1st to 10th	Night	17% 7% 3 7% 5% 4 5% 4 5% 7% 37%	2506
	11th to 20th	Night	14% 6% 2 5% 3 2 3 3 4 7% 50%	2506
	21st to end	Night	14% 7% 3 8% 4 4 4 3 5% 7% 41%	2743
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	13% 10% 4 6% 4 4 4 4 4 7% 41%	2440
	11th to 20th	Night	14% 12% 5% 9% 6% 4 4 4 5% 6% 32%	2660
	21st to end	Night	15% 11% 4 8% 4 4 4 3 5% 6% 36%	2660
December	1st to 10th	Night	12% 13% 5% 11% 7% 4 4 3 4 6% 30%	2660
	11th to 20th	Night	12% 16% 5% 13% 5% 5% 4 3 4 5% 26%	3279
	21st to end	Night	25% 16% 5% 11% 5% 4 3 2 3 5% 21%	4884

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 12% 16% 4	2528
	11th to 20th	Day	65% 7% 8% 14% 6%	2530
	21st to end	Day	62% 6% 14% 15% 4	3007
February	1st to 10th	Day	50% 6% 13% 19% 10% 1	1710
	11th to 20th	Day	47% 5% 13% 24% 9% 1	1673
	21st to end	Day	52% 5% 10% 21% 10% 2	1553
March	1st to 10th	Day	41% 4 13% 25% 16% 2	3202
	11th to 20th	Day	43% 5% 11% 30% 11% 1	3480
	21st to end	Day	59% 6% 9% 19% 6% 1	3973
April	1st to 10th	Day	51% 6% 10% 20% 12% 1	2280
	11th to 20th	Day	54% 5% 10% 19% 11% 2	2254
	21st to end	Day	55% 6% 12% 18% 9% 1	2368
May	1st to 10th	Day	42% 7% 11% 27% 12% 1	4440
	11th to 20th	Day	39% 8% 13% 27% 13% 1	4403
	21st to end	Day	46% 7% 10% 22% 12% 2	4837
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	40% 6% 12% 30% 12%	1900
	11th to 20th	Day	48% 6% 12% 22% 10% 1	1900
	21st to end	Day	50% 4 14% 23% 8%	1981
November	1st to 10th	Day	45% 6% 14% 20% 14% 2	1670
	11th to 20th	Day	57% 6% 12% 17% 7% 1	1520
	21st to end	Day	48% 5% 16% 22% 7% 1	1520
December	1st to 10th	Day	60% 6% 13% 16% 5%	1520
	11th to 20th	Day	60% 6% 14% 14% 6% 1	1915
	21st to end	Day	72% 4 7% 13% 4	2783
January	1st to 10th	Night	59% 5% 11% 19% 6%	4428
	11th to 20th	Night	62% 6% 11% 14% 6%	4440
	21st to end	Night	64% 5% 9% 16% 5% 1	4659
February	1st to 10th	Night	52% 4 12% 21% 9% 2	2470
	11th to 20th	Night	45% 6% 13% 27% 8% 1	2404
	21st to end	Night	49% 5% 13% 22% 10% 2	1900
March	1st to 10th	Night	36% 6% 14% 29% 13% 2	3680
	11th to 20th	Night	42% 7% 10% 26% 13% 1	3480
	21st to end	Night	61% 4 7% 19% 8% 1	3658
April	1st to 10th	Night	49% 6% 9% 18% 17% 2	1900
	11th to 20th	Night	50% 6% 8% 19% 16% 2	1875
	21st to end	Night	47% 6% 9% 22% 15% 2	1808
May	1st to 10th	Night	44% 8% 13% 16% 18% 1	2530
	11th to 20th	Night	34% 4 8% 32% 19% 4	2507
	21st to end	Night	40% 7% 12% 24% 17% 1	2751
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	36% 5% 9% 33% 16% 2	2280
	11th to 20th	Night	43% 6% 13% 23% 14% 1	2280
	21st to end	Night	49% 5% 12% 22% 11% 1	2616
November	1st to 10th	Night	43% 4 13% 25% 14% 1	2440
	11th to 20th	Night	52% 6% 14% 16% 11% 1	2660
	21st to end	Night	49% 5% 15% 23% 7% 1	2660
December	1st to 10th	Night	55% 5% 14% 20% 5%	2660
	11th to 20th	Night	59% 5% 12% 17% 5%	3281
	21st to end	Night	67% 4 10% 14% 4	4884

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	16% 14% 29% 24% 13% 3	270
	11th to 20th	Day	16% 10% 37% 27% 7% 2	270
	21st to end	Day	18% 8% 36% 28% 9% 2	297
February	1st to 10th	Day	14% 13% 40% 23% 7% 2	270
	11th to 20th	Day	14% 16% 37% 24% 8%	270
	21st to end	Day	18% 10% 40% 27% 4	225
March	1st to 10th	Day	19% 14% 34% 20% 12% 1	270
	11th to 20th	Day	19% 12% 32% 24% 10% 2	270
	21st to end	Day	23% 10% 32% 23% 9% 3	392
April	1st to 10th	Day	26% 13% 32% 21% 7% 1	460
	11th to 20th	Day	29% 12% 25% 24% 9% 2	452
	21st to end	Day	26% 14% 27% 22% 9% 2	548
May	1st to 10th	Day	20% 13% 30% 27% 8% 2	653
	11th to 20th	Day	20% 12% 27% 25% 13% 4	675
	21st to end	Day	19% 9% 24% 28% 15% 4	734
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	26% 17% 34% 15% 6% 2	330
November	1st to 10th	Day	18% 11% 35% 24% 10% 1	273
	11th to 20th	Day	21% 16% 37% 21% 6%	270
	21st to end	Day	21% 15% 38% 18% 7%	270
December	1st to 10th	Day	16% 11% 38% 25% 8% 1	270
	11th to 20th	Day	19% 13% 40% 18% 9% 1	270
	21st to end	Day	26% 11% 38% 21% 3 1	297
January	1st to 10th	Night	13% 11% 35% 31% 9% 1	950
	11th to 20th	Night	18% 9% 34% 30% 7% 1	950
	21st to end	Night	20% 8% 31% 32% 8% 1	1045
February	1st to 10th	Night	13% 10% 38% 30% 8% 1	950
	11th to 20th	Night	13% 11% 39% 29% 8% 1	929
	21st to end	Night	19% 12% 32% 30% 7%	644
March	1st to 10th	Night	15% 10% 37% 30% 6% 1	733
	11th to 20th	Night	17% 11% 32% 34% 6% 1	570
	21st to end	Night	23% 8% 29% 27% 11% 2	627
April	1st to 10th	Night	22% 13% 34% 25% 5% 1	570
	11th to 20th	Night	26% 10% 29% 29% 7%	563
	21st to end	Night	23% 9% 32% 26% 9% 1	570
May	1st to 10th	Night	22% 10% 30% 31% 6%	570
	11th to 20th	Night	19% 7% 30% 31% 11% 1	561
	21st to end	Night	14% 7% 25% 40% 13% 1	617
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% 9% 33% 32% 7%	757
	11th to 20th	Night	19% 12% 38% 24% 7%	950
	21st to end	Night	21% 13% 34% 25% 7%	950
December	1st to 10th	Night	19% 10% 35% 28% 8% 1	950
	11th to 20th	Night	22% 9% 36% 25% 8% 1	950
	21st to end	Night	28% 10% 31% 25% 6%	1045

Page generated at 2024-10-30 12:19:20. Version ac07

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