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 2024-09-14.

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 8% 5% 5% 4 4 5% 7% 31%	100728
	Night	16% 12% 4 8% 5% 4 4 3 5% 6% 32%	97030
Transparency	Day	52% 4 11% 21% 11% 1	100539
	Night	53% 3 10% 21% 11% 1	96890
Seeing	Day	20% 13% 28% 22% 12% 5%	14729
	Night	21% 10% 27% 28% 12% 1	24410

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	19% 22% 8% 14% 5% 4 3 3 4 5% 14%	2836
	11th to 20th	Day	15% 24% 7% 12% 5% 5% 6% 3 4 5% 13%	2840
	21st to end	Day	27% 20% 11% 12% 7% 4 3 2 3 2 9%	3124
February	1st to 10th	Day	18% 18% 6% 10% 6% 4 3 2 4 5% 22%	1710
	11th to 20th	Day	15% 13% 7% 15% 8% 5% 5% 3 4 4 21%	1751
	21st to end	Day	18% 14% 4 11% 6% 5% 3 4 5% 6% 25%	1570
March	1st to 10th	Day	22% 14% 7% 8% 4 4 3 3 4 6% 25%	3312
	11th to 20th	Day	13% 15% 5% 11% 5% 4 4 3 4 5% 33%	3480
	21st to end	Day	21% 12% 5% 9% 5% 4 3 3 4 5% 28%	3815
April	1st to 10th	Day	15% 14% 3 9% 6% 4 4 3 6% 6% 31%	2103
	11th to 20th	Day	18% 11% 3 7% 4 4 4 3 5% 6% 33%	2442
	21st to end	Day	20% 11% 3 7% 5% 4 4 3 5% 5% 33%	2470
May	1st to 10th	Day	19% 11% 3 7% 5% 5% 5% 4 5% 6% 30%	4082
	11th to 20th	Day	19% 8% 2 7% 5% 4 4 4 5% 8% 34%	4081
	21st to end	Day	11% 7% 2 7% 4 5% 4 5% 7% 8% 39%	4476
June	1st to 10th	Day	14% 7% 2 7% 4 5% 5% 5% 6% 8% 37%	2566
	11th to 20th	Day	12% 6% 3 7% 5% 5% 5% 4 6% 9% 38%	2670
	21st to end	Day	12% 5% 2 7% 4 5% 5% 5% 7% 9% 39%	2706
July	1st to 10th	Day	10% 6% 3 8% 5% 5% 6% 5% 7% 9% 37%	4290
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 4 7% 10% 43%	4120
	21st to end	Day	10% 7% 2 7% 5% 6% 5% 5% 6% 8% 39%	4466
August	1st to 10th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 8% 36%	4104
	11th to 20th	Day	11% 7% 2 6% 5% 4 6% 4 8% 10% 36%	4044
	21st to end	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 10% 40%	4486
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 6% 7% 43%	2138
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 3 5% 9% 42%	2014
	21st to end	Day	17% 8% 3 6% 4 4 4 3 5% 6% 40%	1980
October	1st to 10th	Day	15% 6% 3 7% 4 4 4 5% 7% 7% 38%	1883
	11th to 20th	Day	18% 10% 3 8% 4 4 4 3 5% 5% 36%	1800
	21st to end	Day	23% 12% 3 7% 4 4 5% 4 4 5% 29%	1949
November	1st to 10th	Day	12% 10% 3 9% 4 4 4 3 6% 7% 39%	1672
	11th to 20th	Day	18% 15% 4 7% 5% 4 3 3 4 6% 31%	1710
	21st to end	Day	18% 16% 5% 8% 6% 5% 5% 3 5% 6% 23%	1568
December	1st to 10th	Day	19% 19% 7% 11% 5% 4 4 3 4 5% 19%	1520
	11th to 20th	Day	18% 17% 7% 14% 5% 4 4 3 6% 6% 17%	1910
	21st to end	Day	23% 24% 8% 12% 5% 4 3 3 4 5% 10%	3040
January	1st to 10th	Night	16% 22% 8% 14% 6% 4 3 3 4 4 16%	4109
	11th to 20th	Night	14% 21% 9% 17% 6% 4 7% 4 4 3 11%	4120
	21st to end	Night	27% 19% 7% 13% 5% 3 3 2 3 3 15%	4532
February	1st to 10th	Night	21% 15% 4 12% 6% 5% 4 2 5% 5% 21%	2470
	11th to 20th	Night	16% 16% 5% 10% 6% 5% 5% 4 5% 4 23%	2314
	21st to end	Night	20% 14% 4 12% 6% 5% 5% 2 4 5% 24%	1884
March	1st to 10th	Night	21% 15% 4 7% 4 3 3 2 3 5% 33%	3526
	11th to 20th	Night	14% 11% 3 6% 5% 4 4 4 5% 6% 37%	3480
	21st to end	Night	18% 11% 3 8% 5% 3 4 4 5% 6% 32%	3817
April	1st to 10th	Night	15% 13% 3 7% 5% 5% 5% 4 5% 6% 32%	2078
	11th to 20th	Night	16% 10% 4 7% 5% 4 4 4 6% 7% 34%	1701
	21st to end	Night	16% 9% 2 8% 4 4 4 3 5% 7% 37%	1710
May	1st to 10th	Night	23% 10% 3 8% 5% 4 5% 3 5% 7% 28%	2813
	11th to 20th	Night	17% 7% 2 7% 5% 4 4 4 5% 8% 38%	2812
	21st to end	Night	10% 5% 2 5% 3 3 4 4 7% 9% 47%	3079
June	1st to 10th	Night	10% 7% 2 6% 5% 4 4 4 5% 6% 45%	1606
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 5% 9% 8% 42%	1498
	21st to end	Night	10% 5% 2 5% 5% 5% 6% 4 6% 9% 44%	1463
July	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 6% 9% 46%	2654
	11th to 20th	Night	9% 4 2 5% 4 4 5% 4 7% 9% 48%	2840
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 7% 9% 44%	3087
August	1st to 10th	Night	11% 8% 2 7% 5% 5% 4 4 7% 9% 39%	2825
	11th to 20th	Night	11% 8% 3 7% 4 5% 4 3 6% 7% 43%	2785
	21st to end	Night	7% 5% 2 6% 5% 5% 5% 5% 6% 8% 46%	3124
September	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 48%	2038
	11th to 20th	Night	11% 5% 1 5% 4 4 4 3 5% 6% 53%	2024
	21st to end	Night	14% 7% 3 6% 4 4 4 2 5% 6% 44%	1980
October	1st to 10th	Night	14% 6% 2 6% 4 4 4 3 6% 8% 43%	2075
	11th to 20th	Night	19% 8% 3 8% 5% 4 4 3 5% 6% 36%	2160
	21st to end	Night	23% 10% 3 6% 5% 4 4 4 5% 6% 30%	2402
November	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 5% 7% 40%	2406
	11th to 20th	Night	16% 14% 4 9% 6% 4 4 3 5% 6% 30%	2470
	21st to end	Night	21% 14% 4 8% 5% 5% 4 3 5% 6% 25%	2607
December	1st to 10th	Night	20% 19% 6% 9% 5% 4 3 3 4 5% 21%	2660
	11th to 20th	Night	17% 16% 6% 11% 6% 4 3 3 5% 5% 23%	3285
	21st to end	Night	24% 21% 7% 10% 5% 4 4 3 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	74% 2 8% 12% 3	2837
	11th to 20th	Day	74% 3 9% 10% 4	2840
	21st to end	Day	84% 2 6% 7% 2	3124
February	1st to 10th	Day	66% 2 10% 15% 6%	1710
	11th to 20th	Day	67% 3 7% 15% 7% 1	1760
	21st to end	Day	61% 3 12% 16% 7% 2	1570
March	1st to 10th	Day	63% 2 7% 22% 5%	3312
	11th to 20th	Day	55% 3 10% 23% 8% 1	3480
	21st to end	Day	59% 3 9% 18% 11% 1	3815
April	1st to 10th	Day	54% 4 10% 19% 12% 1	2103
	11th to 20th	Day	53% 3 11% 18% 13% 2	2442
	21st to end	Day	53% 3 10% 21% 11% 2	2470
May	1st to 10th	Day	56% 3 9% 22% 8% 1	4120
	11th to 20th	Day	50% 3 12% 25% 9% 1	4085
	21st to end	Day	42% 4 12% 23% 14% 5%	4489
June	1st to 10th	Day	44% 4 11% 26% 14% 1	2566
	11th to 20th	Day	43% 5% 13% 24% 15% 1	2670
	21st to end	Day	39% 5% 13% 25% 16% 2	2706
July	1st to 10th	Day	42% 5% 11% 23% 17% 2	4225
	11th to 20th	Day	35% 5% 12% 27% 20% 1	4051
	21st to end	Day	41% 6% 12% 18% 20% 2	4324
August	1st to 10th	Day	45% 6% 13% 24% 12%	4091
	11th to 20th	Day	40% 6% 13% 30% 11% 1	4083
	21st to end	Day	38% 6% 14% 29% 11% 2	4486
September	1st to 10th	Day	39% 5% 13% 23% 17% 4	2138
	11th to 20th	Day	39% 4 12% 27% 16% 2	2014
	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	73% 2 7% 14% 4	4109
	11th to 20th	Night	80% 2 6% 10% 2	4120
	21st to end	Night	77% 1 7% 12% 3	4532
February	1st to 10th	Night	66% 3 9% 15% 6%	2470
	11th to 20th	Night	63% 3 9% 17% 7%	2316
	21st to end	Night	64% 2 9% 14% 8% 2	1884
March	1st to 10th	Night	57% 2 9% 23% 9%	3526
	11th to 20th	Night	48% 3 13% 25% 10% 2	3480
	21st to end	Night	53% 3 7% 26% 10% 1	3817
April	1st to 10th	Night	51% 3 11% 18% 14% 2	2078
	11th to 20th	Night	49% 4 11% 18% 15% 3	1701
	21st to end	Night	47% 3 11% 22% 15% 3	1710
May	1st to 10th	Night	57% 3 8% 21% 9% 2	2840
	11th to 20th	Night	46% 3 11% 25% 13% 2	2813
	21st to end	Night	33% 4 13% 30% 16% 4	3087
June	1st to 10th	Night	39% 6% 11% 26% 16% 3	1606
	11th to 20th	Night	36% 5% 16% 23% 18% 2	1498
	21st to end	Night	36% 5% 14% 27% 16% 3	1463
July	1st to 10th	Night	35% 5% 12% 26% 18% 5%	2610
	11th to 20th	Night	32% 5% 13% 30% 18% 2	2786
	21st to end	Night	35% 6% 14% 19% 24% 2	2988
August	1st to 10th	Night	41% 6% 11% 25% 15% 1	2814
	11th to 20th	Night	40% 5% 11% 29% 14% 1	2812
	21st to end	Night	34% 6% 14% 29% 15% 2	3124
September	1st to 10th	Night	35% 5% 11% 27% 17% 4	2038
	11th to 20th	Night	32% 4 11% 29% 24% 2	2024
	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% 11% 2	297
February	1st to 10th	Day	31% 22% 33% 11% 3	270
	11th to 20th	Day	28% 27% 34% 10% 1	270
	21st to end	Day	30% 31% 31% 8%	225
March	1st to 10th	Day	28% 21% 32% 15% 5%	384
	11th to 20th	Day	28% 14% 34% 18% 5% 1	460
	21st to end	Day	32% 16% 29% 19% 4 1	506
April	1st to 10th	Day	24% 18% 33% 16% 8% 1	460
	11th to 20th	Day	27% 17% 26% 18% 8% 4	452
	21st to end	Day	26% 15% 34% 15% 9% 2	460
May	1st to 10th	Day	26% 13% 29% 21% 8% 3	463
	11th to 20th	Day	21% 9% 31% 23% 10% 5%	487
	21st to end	Day	14% 7% 26% 28% 19% 6%	529
June	1st to 10th	Day	16% 8% 28% 29% 15% 3	490
	11th to 20th	Day	15% 10% 22% 31% 18% 3	500
	21st to end	Day	12% 10% 29% 31% 14% 4	680
July	1st to 10th	Day	12% 4 22% 28% 21% 13%	590
	11th to 20th	Day	11% 6% 24% 28% 20% 11%	484
	21st to end	Day	10% 5% 22% 33% 22% 9%	532
August	1st to 10th	Day	11% 6% 18% 30% 23% 11%	486
	11th to 20th	Day	14% 5% 22% 27% 21% 11%	479
	21st to end	Day	10% 5% 18% 29% 24% 14%	539
September	1st to 10th	Day	15% 5% 21% 27% 20% 13%	490
	11th to 20th	Day	12% 5% 16% 24% 29% 14%	463
	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	836
February	1st to 10th	Night	28% 17% 33% 19% 3	760
	11th to 20th	Night	25% 19% 33% 20% 3	743
	21st to end	Night	29% 15% 35% 19% 2	628
March	1st to 10th	Night	25% 15% 32% 24% 4	760
	11th to 20th	Night	20% 15% 33% 24% 6% 1	760
	21st to end	Night	26% 8% 31% 26% 7% 1	836
April	1st to 10th	Night	22% 14% 33% 26% 4 1	760
	11th to 20th	Night	22% 10% 34% 24% 9% 1	577
	21st to end	Night	19% 7% 35% 31% 6% 1	570
May	1st to 10th	Night	21% 9% 26% 33% 10% 2	570
	11th to 20th	Night	19% 5% 26% 33% 15% 2	561
	21st to end	Night	12% 2 21% 39% 23% 2	617
June	1st to 10th	Night	12% 4 24% 42% 16% 1	466
	11th to 20th	Night	11% 4 22% 46% 17% 1	380
	21st to end	Night	12% 3 23% 40% 21% 1	513
July	1st to 10th	Night	10% 3 13% 40% 31% 3	555
	11th to 20th	Night	9% 4 16% 42% 25% 4	552
	21st to end	Night	9% 2 16% 41% 30% 2	624
August	1st to 10th	Night	10% 2 12% 45% 27% 3	567
	11th to 20th	Night	11% 4 15% 39% 28% 4	564
	21st to end	Night	10% 3 15% 35% 33% 4	627
September	1st to 10th	Night	13% 6% 14% 35% 28% 4	760
	11th to 20th	Night	12% 3 13% 36% 32% 5%	736
	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

Page generated at 2024-09-11 10:00:03. Version ac07

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