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 2026-06-16.

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	15% 11% 4 9% 5% 5% 4 4 5% 7% 31%	137453
	Night	15% 12% 4 10% 5% 4 4 3 5% 6% 31%	134477
Transparency	Day	52% 4 10% 21% 11% 1	141086
	Night	53% 4 10% 20% 12% 1	138917
Seeing	Day	20% 13% 28% 21% 12% 5%	16992
	Night	21% 11% 27% 28% 12% 1	26729

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	16% 20% 7% 20% 5% 4 3 3 4 5% 13%	5258
	11th to 20th	Day	12% 21% 8% 17% 7% 5% 5% 2 4 4 15%	5280
	21st to end	Day	19% 17% 8% 13% 8% 7% 4 3 4 4 13%	5780
February	1st to 10th	Day	17% 18% 6% 11% 6% 4 4 3 5% 6% 22%	1890
	11th to 20th	Day	15% 14% 7% 15% 8% 4 5% 3 4 4 20%	1941
	21st to end	Day	17% 14% 5% 12% 6% 4 4 4 5% 6% 25%	1730
March	1st to 10th	Day	17% 12% 7% 12% 5% 4 4 3 4 5% 27%	4862
	11th to 20th	Day	12% 12% 5% 11% 4 4 4 3 4 5% 36%	5090
	21st to end	Day	19% 13% 5% 10% 5% 4 4 3 4 6% 26%	5586
April	1st to 10th	Day	15% 13% 3 9% 6% 4 4 3 6% 6% 30%	2325
	11th to 20th	Day	17% 12% 4 7% 5% 5% 4 3 5% 7% 32%	2702
	21st to end	Day	19% 11% 3 7% 5% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	20% 10% 3 8% 5% 5% 4 4 5% 6% 29%	7616
	11th to 20th	Day	16% 8% 3 8% 5% 4 5% 4 6% 8% 33%	7626
	21st to end	Day	17% 8% 3 7% 4 5% 4 4 6% 6% 36%	8391
June	1st to 10th	Day	13% 7% 2 7% 4 4 5% 5% 6% 8% 38%	2836
	11th to 20th	Day	13% 6% 3 8% 5% 5% 5% 4 6% 8% 37%	2881
	21st to end	Day	11% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	2848
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 6% 9% 41%	6140
	11th to 20th	Day	10% 6% 2 7% 5% 6% 6% 5% 8% 10% 37%	5900
	21st to end	Day	11% 7% 2 7% 5% 5% 5% 4 7% 8% 39%	6424
August	1st to 10th	Day	12% 7% 3 7% 5% 5% 5% 4 6% 8% 37%	4234
	11th to 20th	Day	10% 7% 2 6% 4 5% 6% 4 8% 10% 37%	4174
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4628
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 4 6% 7% 45%	2250
	11th to 20th	Day	11% 6% 2 5% 5% 4 5% 3 5% 9% 45%	2200
	21st to end	Day	18% 9% 3 6% 4 4 4 3 5% 6% 39%	2200
October	1st to 10th	Day	14% 6% 3 6% 4 4 4 4 7% 7% 42%	2091
	11th to 20th	Day	17% 10% 3 7% 4 4 4 3 5% 5% 38%	2000
	21st to end	Day	21% 11% 3 7% 4 4 4 4 4 6% 31%	2165
November	1st to 10th	Day	12% 10% 3 8% 4 4 4 3 6% 7% 38%	1852
	11th to 20th	Day	18% 15% 4 7% 5% 4 4 3 4 6% 30%	1890
	21st to end	Day	18% 17% 6% 9% 6% 5% 4 3 5% 6% 22%	1732
December	1st to 10th	Day	19% 15% 6% 15% 5% 4 4 3 5% 5% 20%	2680
	11th to 20th	Day	21% 20% 8% 12% 4 3 3 3 5% 5% 18%	3070
	21st to end	Day	23% 23% 7% 13% 5% 5% 4 3 3 4 11%	4451
January	1st to 10th	Night	14% 20% 7% 22% 8% 4 3 3 3 3 12%	7635
	11th to 20th	Night	10% 19% 10% 17% 8% 6% 6% 4 3 3 12%	7680
	21st to end	Night	18% 17% 6% 12% 5% 5% 5% 3 4 5% 20%	8401
February	1st to 10th	Night	20% 14% 4 12% 6% 5% 4 2 5% 4 22%	2730
	11th to 20th	Night	16% 15% 6% 10% 6% 5% 5% 4 5% 5% 22%	2564
	21st to end	Night	19% 14% 5% 12% 6% 5% 5% 2 4 5% 25%	2076
March	1st to 10th	Night	21% 13% 4 9% 4 3 3 2 3 5% 32%	5196
	11th to 20th	Night	12% 10% 3 6% 4 4 4 4 6% 7% 40%	5090
	21st to end	Night	17% 12% 4 9% 5% 4 4 3 4 5% 33%	5588
April	1st to 10th	Night	17% 13% 3 7% 5% 4 4 4 5% 6% 31%	2296
	11th to 20th	Night	15% 10% 4 7% 5% 5% 5% 4 6% 7% 34%	1882
	21st to end	Night	16% 9% 3 8% 4 4 4 3 5% 7% 37%	1890
May	1st to 10th	Night	23% 10% 3 8% 5% 5% 5% 4 6% 6% 26%	5237
	11th to 20th	Night	16% 8% 2 7% 4 4 4 4 5% 8% 38%	5234
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 7% 39%	5761
June	1st to 10th	Night	10% 7% 3 6% 5% 4 4 4 6% 6% 45%	1775
	11th to 20th	Night	9% 6% 2 6% 5% 4 5% 5% 8% 9% 41%	1617
	21st to end	Night	9% 5% 2 5% 5% 5% 6% 4 6% 9% 44%	1540
July	1st to 10th	Night	8% 4 2 6% 4 5% 5% 4 6% 10% 44%	3794
	11th to 20th	Night	8% 5% 2 7% 4 5% 5% 4 7% 10% 42%	4057
	21st to end	Night	8% 5% 2 7% 4 4 5% 4 6% 8% 46%	4427
August	1st to 10th	Night	11% 7% 2 7% 5% 4 4 3 6% 9% 40%	2915
	11th to 20th	Night	11% 8% 3 7% 4 4 4 3 6% 7% 43%	2875
	21st to end	Night	6% 5% 2 6% 5% 5% 5% 5% 6% 8% 46%	3223
September	1st to 10th	Night	11% 6% 2 5% 4 4 4 4 5% 7% 49%	2146
	11th to 20th	Night	10% 5% 1 5% 3 3 4 3 5% 6% 56%	2200
	21st to end	Night	16% 7% 3 6% 4 4 4 2 5% 6% 42%	2200
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 45%	2307
	11th to 20th	Night	17% 8% 2 7% 5% 4 4 3 5% 6% 38%	2400
	21st to end	Night	22% 9% 3 6% 5% 4 4 4 6% 6% 32%	2670
November	1st to 10th	Night	14% 9% 2 6% 4 4 4 4 5% 7% 41%	2666
	11th to 20th	Night	16% 14% 4 8% 6% 4 4 3 5% 5% 31%	2730
	21st to end	Night	21% 15% 4 8% 5% 4 4 3 5% 6% 25%	2883
December	1st to 10th	Night	16% 16% 7% 12% 6% 5% 4 3 5% 5% 21%	4720
	11th to 20th	Night	18% 18% 8% 13% 5% 4 3 3 4 5% 19%	5345
	21st to end	Night	22% 20% 6% 12% 6% 4 4 3 4 6% 13%	6727

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	75% 3 10% 10% 3	5259
	11th to 20th	Day	75% 2 8% 8% 8%	5280
	21st to end	Day	77% 2 6% 12% 3	5780
February	1st to 10th	Day	65% 3 10% 15% 7%	1890
	11th to 20th	Day	67% 3 7% 15% 6% 1	1950
	21st to end	Day	61% 3 12% 16% 7% 2	1730
March	1st to 10th	Day	62% 2 7% 23% 6%	4862
	11th to 20th	Day	51% 3 11% 25% 8% 1	5090
	21st to end	Day	60% 3 7% 18% 11% 1	5586
April	1st to 10th	Day	55% 4 10% 19% 12% 1	2325
	11th to 20th	Day	53% 4 11% 18% 13% 2	2702
	21st to end	Day	53% 3 10% 22% 11% 2	2730
May	1st to 10th	Day	56% 4 9% 20% 9% 2	7651
	11th to 20th	Day	48% 5% 13% 23% 12% 1	7630
	21st to end	Day	47% 4 9% 20% 16% 4	8405
June	1st to 10th	Day	43% 5% 11% 26% 15% 1	2836
	11th to 20th	Day	44% 5% 13% 23% 14% 1	2881
	21st to end	Day	38% 6% 13% 25% 16% 2	2848
July	1st to 10th	Day	38% 5% 11% 29% 16% 1	6075
	11th to 20th	Day	39% 7% 13% 24% 16% 1	5831
	21st to end	Day	40% 6% 11% 21% 20% 2	6240
August	1st to 10th	Day	44% 6% 14% 24% 13%	4221
	11th to 20th	Day	39% 6% 13% 30% 11% 1	4213
	21st to end	Day	37% 6% 14% 30% 11% 2	4628
September	1st to 10th	Day	38% 5% 13% 24% 17% 4	2250
	11th to 20th	Day	37% 4 12% 27% 17% 2	2200
	21st to end	Day	47% 3 9% 22% 18% 1	2200
October	1st to 10th	Day	31% 5% 13% 31% 20% 1	3403
	11th to 20th	Day	44% 4 10% 22% 18% 3	3240
	21st to end	Day	43% 3 11% 25% 17% 1	3479
November	1st to 10th	Day	45% 4 11% 24% 14% 1	1852
	11th to 20th	Day	57% 2 9% 20% 10% 2	1890
	21st to end	Day	64% 3 9% 16% 7%	1732
December	1st to 10th	Day	67% 4 13% 13% 3	2680
	11th to 20th	Day	70% 3 9% 15% 4	3066
	21st to end	Day	80% 2 7% 8% 3	4451
January	1st to 10th	Night	78% 2 6% 11% 3	7636
	11th to 20th	Night	77% 2 6% 9% 4	7680
	21st to end	Night	67% 3 9% 14% 6%	8401
February	1st to 10th	Night	66% 3 9% 16% 6%	2730
	11th to 20th	Night	64% 3 9% 17% 7%	2566
	21st to end	Night	64% 2 9% 15% 8% 2	2076
March	1st to 10th	Night	57% 2 10% 21% 9%	5196
	11th to 20th	Night	44% 3 12% 29% 10% 2	5090
	21st to end	Night	53% 3 7% 22% 14% 1	5588
April	1st to 10th	Night	53% 3 11% 18% 14% 2	2296
	11th to 20th	Night	49% 4 11% 18% 15% 3	1882
	21st to end	Night	48% 3 10% 22% 15% 2	1890
May	1st to 10th	Night	58% 5% 8% 16% 10% 3	5264
	11th to 20th	Night	45% 4 10% 24% 15% 1	5235
	21st to end	Night	45% 4 11% 23% 15% 3	5771
June	1st to 10th	Night	38% 6% 10% 25% 17% 4	1775
	11th to 20th	Night	37% 5% 16% 23% 18% 2	1617
	21st to end	Night	36% 5% 14% 27% 16% 3	1540
July	1st to 10th	Night	34% 6% 14% 26% 17% 4	3752
	11th to 20th	Night	36% 7% 14% 28% 14% 1	4006
	21st to end	Night	35% 5% 12% 20% 26% 2	4297
August	1st to 10th	Night	40% 6% 12% 26% 16% 1	2904
	11th to 20th	Night	39% 5% 11% 29% 14% 1	2902
	21st to end	Night	34% 6% 14% 30% 15% 2	3223
September	1st to 10th	Night	34% 5% 11% 29% 17% 4	2146
	11th to 20th	Night	30% 4 11% 30% 25% 2	2200
	21st to end	Night	43% 3 11% 22% 19% 2	2200
October	1st to 10th	Night	31% 4 8% 31% 26% 1	3775
	11th to 20th	Night	42% 4 9% 22% 20% 2	3899
	21st to end	Night	38% 3 10% 29% 17% 2	4306
November	1st to 10th	Night	42% 4 11% 25% 16% 2	2666
	11th to 20th	Night	56% 2 9% 20% 11% 2	2730
	21st to end	Night	61% 3 12% 18% 6% 1	2883
December	1st to 10th	Night	65% 4 11% 16% 4	4720
	11th to 20th	Night	69% 3 9% 16% 3	5348
	21st to end	Night	74% 3 9% 11% 2	6727

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	27% 20% 38% 9% 4 1	330
	11th to 20th	Day	32% 28% 30% 8% 2 1	330
	21st to end	Day	29% 26% 33% 10% 2	363
February	1st to 10th	Day	29% 23% 34% 11% 3	330
	11th to 20th	Day	28% 28% 33% 9% 1	330
	21st to end	Day	27% 32% 33% 8%	273
March	1st to 10th	Day	27% 21% 34% 14% 4	456
	11th to 20th	Day	29% 15% 34% 16% 5% 1	540
	21st to end	Day	30% 19% 28% 19% 4 1	594
April	1st to 10th	Day	26% 19% 31% 16% 7% 1	540
	11th to 20th	Day	27% 19% 26% 17% 7% 4	532
	21st to end	Day	26% 14% 34% 15% 9% 1	540
May	1st to 10th	Day	27% 15% 29% 20% 7% 2	543
	11th to 20th	Day	21% 9% 33% 24% 9% 4	567
	21st to end	Day	17% 8% 26% 25% 17% 6%	617
June	1st to 10th	Day	16% 8% 29% 29% 14% 4	570
	11th to 20th	Day	16% 11% 23% 31% 17% 3	561
	21st to end	Day	11% 9% 28% 31% 16% 5%	730
July	1st to 10th	Day	11% 4 22% 29% 22% 12%	635
	11th to 20th	Day	11% 6% 24% 28% 21% 10%	524
	21st to end	Day	10% 5% 21% 32% 23% 9%	576
August	1st to 10th	Day	10% 6% 18% 31% 24% 11%	526
	11th to 20th	Day	13% 5% 22% 27% 22% 11%	519
	21st to end	Day	9% 6% 19% 30% 23% 14%	583
September	1st to 10th	Day	14% 5% 21% 26% 20% 14%	530
	11th to 20th	Day	10% 4 15% 25% 31% 16%	530
	21st to end	Day	22% 10% 22% 22% 14% 9%	530
October	1st to 10th	Day	13% 8% 25% 27% 20% 7%	530
	11th to 20th	Day	19% 15% 32% 24% 8% 3	530
	21st to end	Day	27% 16% 34% 18% 5%	548
November	1st to 10th	Day	18% 21% 37% 14% 10% 1	330
	11th to 20th	Day	31% 21% 28% 13% 5% 1	330
	21st to end	Day	30% 21% 36% 13% 1	247
December	1st to 10th	Day	23% 22% 36% 14% 4 1	220
	11th to 20th	Day	27% 25% 34% 12% 1	220
	21st to end	Day	32% 20% 35% 12% 1	338
January	1st to 10th	Night	26% 19% 33% 17% 4	840
	11th to 20th	Night	29% 19% 33% 15% 4	840
	21st to end	Night	27% 16% 34% 21% 3	924
February	1st to 10th	Night	27% 17% 34% 19% 2	840
	11th to 20th	Night	25% 20% 32% 20% 3	823
	21st to end	Night	28% 16% 35% 19% 2	692
March	1st to 10th	Night	25% 15% 33% 23% 4	840
	11th to 20th	Night	21% 16% 33% 23% 6% 1	840
	21st to end	Night	26% 10% 32% 25% 7% 1	924
April	1st to 10th	Night	24% 14% 33% 25% 4 1	840
	11th to 20th	Night	22% 11% 34% 25% 8% 1	638
	21st to end	Night	19% 8% 35% 31% 6% 1	630
May	1st to 10th	Night	22% 8% 27% 33% 9% 1	630
	11th to 20th	Night	19% 7% 26% 33% 14% 1	621
	21st to end	Night	13% 3 21% 40% 21% 2	683
June	1st to 10th	Night	12% 4 25% 42% 16% 1	515
	11th to 20th	Night	11% 4 24% 45% 16% 1	410
	21st to end	Night	11% 3 23% 40% 22% 1	540
July	1st to 10th	Night	9% 3 13% 41% 30% 3	585
	11th to 20th	Night	9% 4 16% 43% 24% 3	582
	21st to end	Night	9% 2 16% 40% 31% 2	657
August	1st to 10th	Night	10% 2 12% 46% 28% 3	597
	11th to 20th	Night	11% 4 15% 38% 29% 4	594
	21st to end	Night	10% 3 15% 35% 33% 4	660
September	1st to 10th	Night	13% 6% 14% 35% 28% 4	800
	11th to 20th	Night	11% 3 12% 35% 34% 6%	800
	21st to end	Night	18% 7% 22% 30% 20% 3	800
October	1st to 10th	Night	15% 6% 24% 33% 18% 3	800
	11th to 20th	Night	21% 12% 30% 27% 10% 1	800
	21st to end	Night	26% 11% 31% 26% 6%	880
November	1st to 10th	Night	19% 12% 32% 26% 10% 1	820
	11th to 20th	Night	26% 16% 31% 20% 6% 1	840
	21st to end	Night	30% 14% 30% 20% 6%	840
December	1st to 10th	Night	30% 15% 32% 19% 4	840
	11th to 20th	Night	27% 16% 34% 20% 4	840
	21st to end	Night	30% 16% 33% 19% 2	924

Page generated at 2026-06-13 02:31:39. Version ac07

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