CAA Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the CAA Observatory clear sky chart from 2005-11-06 to 2024-01-31.

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	16% 10% 4 7% 6% 3 5% 5% 5% 7% 31%	75001
	Night	16% 11% 4 7% 6% 3 4 4 5% 7% 33%	75081
Transparency	Day	53% 4 11% 20% 11% 1	74722
	Night	52% 4 11% 20% 12% 2	74880
Seeing	Day	20% 13% 28% 22% 13% 5%	13838
	Night	21% 10% 27% 28% 13% 1	23662

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	18% 16% 8% 11% 8% 4 4 5% 4 5% 17%	1706
	11th to 20th	Day	20% 18% 8% 13% 7% 3 4 3 4 4 15%	1710
	21st to end	Day	18% 19% 9% 13% 8% 3 4 4 5% 3 14%	1872
February	1st to 10th	Day	19% 17% 7% 10% 7% 3 4 3 4 4 21%	1620
	11th to 20th	Day	16% 12% 8% 13% 9% 4 5% 5% 5% 4 19%	1657
	21st to end	Day	18% 14% 4 11% 7% 3 4 4 4 6% 25%	1480
March	1st to 10th	Day	19% 13% 6% 8% 6% 3 4 4 4 6% 27%	1908
	11th to 20th	Day	16% 13% 5% 10% 6% 3 5% 3 4 5% 29%	1980
	21st to end	Day	22% 12% 4 8% 6% 3 3 4 4 6% 26%	2178
April	1st to 10th	Day	15% 12% 4 7% 7% 3 4 4 6% 6% 32%	1991
	11th to 20th	Day	18% 11% 3 6% 6% 3 5% 4 5% 7% 33%	2312
	21st to end	Day	21% 11% 4 6% 5% 2 4 4 5% 5% 33%	2340
May	1st to 10th	Day	18% 10% 3 7% 6% 4 6% 4 5% 7% 31%	2340
	11th to 20th	Day	19% 8% 3 7% 6% 3 5% 5% 6% 7% 31%	2301
	21st to end	Day	10% 6% 3 6% 5% 4 5% 6% 8% 9% 38%	2531
June	1st to 10th	Day	14% 6% 3 6% 5% 4 4 6% 5% 9% 38%	2431
	11th to 20th	Day	13% 6% 3 6% 6% 4 5% 5% 6% 9% 37%	2529
	21st to end	Day	12% 4 2 6% 5% 4 6% 5% 7% 9% 40%	2564
July	1st to 10th	Day	12% 5% 3 6% 5% 3 5% 6% 6% 8% 42%	2440
	11th to 20th	Day	11% 5% 2 5% 5% 4 5% 5% 7% 10% 41%	2340
	21st to end	Day	10% 6% 2 6% 6% 4 7% 6% 7% 9% 39%	2547
August	1st to 10th	Day	10% 5% 2 6% 6% 4 7% 6% 6% 8% 39%	2327
	11th to 20th	Day	12% 7% 3 5% 6% 4 6% 5% 7% 10% 35%	2303
	21st to end	Day	11% 5% 3 5% 6% 4 5% 6% 6% 9% 40%	2556
September	1st to 10th	Day	15% 6% 2 5% 5% 3 4 5% 6% 7% 42%	2026
	11th to 20th	Day	12% 6% 2 5% 6% 3 5% 5% 5% 9% 42%	1980
	21st to end	Day	17% 8% 3 5% 5% 3 5% 3 5% 6% 40%	1980
October	1st to 10th	Day	15% 6% 3 6% 5% 3 4 5% 7% 7% 38%	1883
	11th to 20th	Day	18% 10% 3 7% 5% 3 4 4 5% 5% 36%	1800
	21st to end	Day	23% 11% 4 6% 5% 4 4 5% 4 5% 29%	1949
November	1st to 10th	Day	12% 10% 3 7% 6% 3 4 3 5% 7% 39%	1672
	11th to 20th	Day	18% 14% 5% 6% 5% 3 4 4 4 6% 31%	1710
	21st to end	Day	18% 15% 6% 8% 6% 4 5% 4 4 6% 23%	1568
December	1st to 10th	Day	19% 18% 8% 10% 6% 3 4 3 3 5% 19%	1520
	11th to 20th	Day	18% 16% 7% 13% 5% 3 4 4 5% 6% 17%	1910
	21st to end	Day	24% 23% 8% 10% 6% 3 4 4 3 5% 10%	3040
January	1st to 10th	Night	18% 17% 6% 10% 7% 4 4 4 4 5% 21%	2459
	11th to 20th	Night	19% 18% 8% 11% 9% 4 4 3 4 4 17%	2470
	21st to end	Night	19% 17% 7% 11% 7% 3 4 3 4 4 20%	2711
February	1st to 10th	Night	22% 14% 5% 11% 7% 4 5% 3 5% 5% 21%	2340
	11th to 20th	Night	17% 14% 7% 9% 6% 4 5% 5% 5% 5% 22%	2188
	21st to end	Night	21% 13% 5% 10% 7% 3 6% 3 3 5% 25%	1776
March	1st to 10th	Night	17% 11% 5% 9% 6% 3 4 3 3 5% 33%	2051
	11th to 20th	Night	16% 9% 5% 6% 5% 3 4 4 5% 8% 34%	1980
	21st to end	Night	20% 11% 5% 7% 5% 3 4 5% 4 6% 32%	2178
April	1st to 10th	Night	15% 11% 4 6% 5% 3 5% 6% 5% 6% 33%	1969
	11th to 20th	Night	16% 9% 4 6% 6% 3 4 5% 6% 7% 35%	1611
	21st to end	Night	17% 8% 2 7% 5% 3 4 4 5% 7% 37%	1620
May	1st to 10th	Night	18% 9% 4 7% 7% 4 5% 5% 5% 7% 29%	1620
	11th to 20th	Night	17% 7% 3 6% 5% 3 4 5% 5% 8% 38%	1592
	21st to end	Night	9% 5% 2 4 5% 3 5% 6% 7% 10% 44%	1744
June	1st to 10th	Night	9% 7% 3 5% 6% 3 5% 5% 5% 7% 45%	1522
	11th to 20th	Night	10% 5% 2 5% 6% 3 5% 6% 8% 9% 41%	1420
	21st to end	Night	10% 4 2 4 4 4 6% 5% 6% 10% 45%	1385
July	1st to 10th	Night	9% 4 2 5% 4 4 5% 6% 6% 7% 47%	1512
	11th to 20th	Night	8% 3 2 4 5% 3 5% 5% 7% 10% 47%	1620
	21st to end	Night	8% 4 2 6% 5% 3 5% 5% 7% 8% 46%	1771
August	1st to 10th	Night	7% 4 2 5% 5% 4 4 5% 6% 11% 46%	1611
	11th to 20th	Night	9% 5% 2 5% 5% 4 5% 4 6% 9% 46%	1598
	21st to end	Night	9% 5% 2 4 5% 3 4 5% 6% 8% 49%	1782
September	1st to 10th	Night	12% 6% 2 4 4 2 5% 5% 5% 7% 47%	1930
	11th to 20th	Night	11% 4 2 4 4 2 4 4 5% 7% 54%	1980
	21st to end	Night	14% 7% 3 6% 5% 3 4 3 5% 6% 44%	1980
October	1st to 10th	Night	14% 6% 2 5% 5% 3 4 5% 5% 8% 43%	2075
	11th to 20th	Night	19% 8% 3 7% 6% 3 4 4 5% 6% 36%	2160
	21st to end	Night	23% 10% 3 5% 6% 3 4 4 5% 6% 30%	2402
November	1st to 10th	Night	15% 8% 3 6% 5% 3 4 4 5% 7% 40%	2406
	11th to 20th	Night	16% 13% 5% 7% 7% 4 4 4 5% 6% 31%	2470
	21st to end	Night	21% 13% 4 7% 6% 3 5% 4 5% 6% 25%	2607
December	1st to 10th	Night	20% 18% 8% 8% 6% 3 3 4 4 5% 22%	2660
	11th to 20th	Night	17% 15% 7% 10% 7% 3 4 4 5% 5% 23%	3285
	21st to end	Night	24% 20% 8% 8% 6% 3 4 4 4 5% 12%	4596

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	70% 3 9% 13% 4	1707
	11th to 20th	Day	74% 3 11% 9% 3	1710
	21st to end	Day	75% 3 8% 11% 3	1872
February	1st to 10th	Day	68% 2 10% 14% 6%	1620
	11th to 20th	Day	68% 3 8% 14% 6% 1	1666
	21st to end	Day	62% 3 12% 15% 7% 2	1480
March	1st to 10th	Day	60% 3 9% 22% 6%	1908
	11th to 20th	Day	59% 3 9% 18% 10% 1	1980
	21st to end	Day	60% 3 8% 19% 9% 1	2178
April	1st to 10th	Day	53% 4 10% 20% 12% 1	1991
	11th to 20th	Day	53% 3 11% 17% 13% 2	2312
	21st to end	Day	54% 3 9% 21% 11% 2	2340
May	1st to 10th	Day	54% 4 11% 19% 9% 2	2340
	11th to 20th	Day	51% 5% 11% 17% 14% 2	2305
	21st to end	Day	40% 6% 14% 26% 13% 1	2531
June	1st to 10th	Day	44% 4 12% 25% 14% 1	2431
	11th to 20th	Day	43% 5% 13% 23% 14% 1	2529
	21st to end	Day	39% 5% 13% 25% 16% 2	2564
July	1st to 10th	Day	41% 4 10% 25% 17% 2	2375
	11th to 20th	Day	38% 4 13% 27% 16% 2	2271
	21st to end	Day	40% 6% 12% 25% 15% 2	2405
August	1st to 10th	Day	41% 5% 12% 27% 14% 1	2314
	11th to 20th	Day	43% 4 15% 24% 11% 2	2303
	21st to end	Day	40% 5% 13% 23% 16% 4	2556
September	1st to 10th	Day	40% 5% 14% 22% 16% 4	2026
	11th to 20th	Day	40% 4 12% 26% 16% 2	1980
	21st to end	Day	46% 3 9% 24% 18% 1	1980
October	1st to 10th	Day	43% 4 10% 25% 16% 1	1883
	11th to 20th	Day	50% 3 11% 23% 12% 1	1800
	21st to end	Day	58% 3 11% 18% 9% 1	1949
November	1st to 10th	Day	45% 4 11% 24% 15% 2	1672
	11th to 20th	Day	56% 2 9% 20% 9% 2	1710
	21st to end	Day	63% 4 10% 16% 8%	1568
December	1st to 10th	Day	69% 2 11% 14% 4	1520
	11th to 20th	Day	69% 2 8% 17% 4	1906
	21st to end	Day	79% 2 9% 7% 2	3040
January	1st to 10th	Night	68% 3 8% 15% 7%	2459
	11th to 20th	Night	72% 2 10% 12% 3	2470
	21st to end	Night	68% 2 9% 15% 5%	2711
February	1st to 10th	Night	68% 3 9% 14% 5%	2340
	11th to 20th	Night	64% 3 9% 16% 8% 1	2190
	21st to end	Night	65% 2 8% 14% 8% 2	1776
March	1st to 10th	Night	56% 3 8% 23% 10% 1	2051
	11th to 20th	Night	49% 4 11% 22% 11% 3	1980
	21st to end	Night	55% 4 7% 22% 11% 2	2178
April	1st to 10th	Night	50% 3 12% 18% 15% 2	1969
	11th to 20th	Night	48% 4 11% 18% 16% 3	1611
	21st to end	Night	48% 3 11% 21% 15% 3	1620
May	1st to 10th	Night	54% 5% 10% 17% 12% 3	1620
	11th to 20th	Night	45% 5% 12% 19% 16% 3	1593
	21st to end	Night	35% 6% 16% 27% 15% 2	1745
June	1st to 10th	Night	38% 6% 11% 26% 16% 3	1522
	11th to 20th	Night	36% 5% 16% 22% 18% 2	1420
	21st to end	Night	35% 5% 14% 27% 16% 3	1385
July	1st to 10th	Night	36% 5% 11% 24% 19% 4	1468
	11th to 20th	Night	31% 5% 14% 28% 19% 3	1566
	21st to end	Night	35% 5% 14% 25% 18% 3	1672
August	1st to 10th	Night	32% 5% 13% 30% 20% 1	1600
	11th to 20th	Night	35% 5% 16% 29% 13% 2	1598
	21st to end	Night	32% 5% 14% 27% 18% 4	1782
September	1st to 10th	Night	36% 5% 12% 27% 15% 5%	1930
	11th to 20th	Night	32% 4 11% 29% 24% 2	1980
	21st to end	Night	41% 3 12% 23% 19% 2	1980
October	1st to 10th	Night	39% 4 12% 23% 20% 1	2075
	11th to 20th	Night	49% 3 9% 24% 14% 1	2160
	21st to end	Night	55% 3 10% 21% 10% 2	2402
November	1st to 10th	Night	43% 3 11% 25% 15% 3	2406
	11th to 20th	Night	56% 2 9% 20% 11% 2	2470
	21st to end	Night	60% 3 12% 18% 6% 1	2607
December	1st to 10th	Night	66% 2 10% 14% 6%	2660
	11th to 20th	Night	64% 3 11% 19% 3	3288
	21st to end	Night	76% 3 10% 10% 2	4596

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	28% 20% 35% 11% 5% 1	270
	11th to 20th	Day	29% 26% 32% 10% 2 1	270
	21st to end	Day	29% 25% 33% 10% 2	294
February	1st to 10th	Day	34% 22% 33% 10% 2	240
	11th to 20th	Day	28% 28% 34% 10%	240
	21st to end	Day	28% 32% 32% 7% 1	198
March	1st to 10th	Day	27% 22% 32% 15% 5%	348
	11th to 20th	Day	27% 14% 35% 18% 6% 1	420
	21st to end	Day	32% 15% 28% 20% 4 1	462
April	1st to 10th	Day	23% 17% 33% 17% 8% 1	420
	11th to 20th	Day	27% 15% 26% 18% 8% 5%	412
	21st to end	Day	27% 14% 33% 15% 9% 2	420
May	1st to 10th	Day	27% 13% 29% 21% 8% 3	423
	11th to 20th	Day	21% 9% 32% 23% 11% 4	447
	21st to end	Day	15% 6% 25% 28% 19% 7%	485
June	1st to 10th	Day	15% 8% 28% 30% 16% 4	450
	11th to 20th	Day	16% 11% 21% 30% 19% 3	459
	21st to end	Day	11% 9% 29% 32% 15% 4	630
July	1st to 10th	Day	11% 4 21% 29% 22% 14%	545
	11th to 20th	Day	12% 5% 23% 28% 21% 11%	444
	21st to end	Day	10% 5% 22% 32% 21% 9%	488
August	1st to 10th	Day	11% 4 17% 31% 24% 12%	446
	11th to 20th	Day	14% 5% 20% 26% 22% 12%	439
	21st to end	Day	10% 6% 18% 29% 23% 14%	495
September	1st to 10th	Day	15% 5% 20% 27% 19% 14%	450
	11th to 20th	Day	12% 5% 17% 23% 29% 14%	450
	21st to end	Day	20% 10% 23% 23% 14% 10%	450
October	1st to 10th	Day	14% 8% 26% 26% 19% 7%	450
	11th to 20th	Day	20% 16% 32% 24% 7% 2	450
	21st to end	Day	30% 16% 33% 17% 5%	464
November	1st to 10th	Day	16% 20% 36% 16% 11% 1	270
	11th to 20th	Day	29% 19% 30% 15% 6% 1	270
	21st to end	Day	26% 21% 39% 13% 1	203
December	1st to 10th	Day	23% 23% 35% 14% 4 2	180
	11th to 20th	Day	25% 24% 37% 13% 1	180
	21st to end	Day	29% 20% 37% 12% 1	276
January	1st to 10th	Night	27% 19% 33% 16% 4	760
	11th to 20th	Night	31% 17% 33% 16% 4	760
	21st to end	Night	27% 14% 34% 22% 3	835
February	1st to 10th	Night	29% 16% 34% 18% 2	720
	11th to 20th	Night	27% 18% 33% 20% 3	703
	21st to end	Night	29% 15% 35% 19% 2	592
March	1st to 10th	Night	24% 15% 33% 24% 4	720
	11th to 20th	Night	20% 14% 34% 24% 7% 1	720
	21st to end	Night	26% 8% 31% 26% 8% 1	792
April	1st to 10th	Night	21% 13% 33% 27% 5% 1	720
	11th to 20th	Night	22% 10% 34% 24% 9% 1	547
	21st to end	Night	20% 7% 35% 31% 6% 1	540
May	1st to 10th	Night	21% 8% 26% 33% 10% 1	540
	11th to 20th	Night	20% 5% 26% 33% 15% 1	531
	21st to end	Night	12% 2 21% 39% 23% 3	584
June	1st to 10th	Night	12% 4 24% 43% 17% 1	442
	11th to 20th	Night	11% 4 22% 45% 17% 1	360
	21st to end	Night	12% 2 22% 41% 22% 1	485
July	1st to 10th	Night	10% 2 12% 40% 32% 3	525
	11th to 20th	Night	9% 4 16% 42% 25% 4	522
	21st to end	Night	9% 2 16% 41% 29% 2	591
August	1st to 10th	Night	9% 1 12% 46% 28% 3	537
	11th to 20th	Night	10% 3 14% 40% 29% 4	534
	21st to end	Night	11% 3 15% 34% 33% 5%	594
September	1st to 10th	Night	13% 6% 13% 35% 28% 4	720
	11th to 20th	Night	12% 3 13% 35% 32% 5%	720
	21st to end	Night	17% 7% 22% 30% 20% 3	720
October	1st to 10th	Night	16% 6% 25% 33% 17% 4	720
	11th to 20th	Night	22% 12% 30% 26% 9% 1	720
	21st to end	Night	27% 12% 30% 25% 6%	792
November	1st to 10th	Night	20% 11% 31% 26% 11% 1	740
	11th to 20th	Night	26% 15% 32% 20% 6% 1	760
	21st to end	Night	28% 14% 30% 21% 7%	760
December	1st to 10th	Night	31% 15% 31% 19% 4	760
	11th to 20th	Night	27% 15% 34% 21% 4	760
	21st to end	Night	30% 16% 33% 19% 2	836

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