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 2025-02-21.

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% 30%	108929
	Night	15% 12% 4 10% 5% 4 4 3 5% 6% 31%	109574
Transparency	Day	53% 4 11% 20% 11% 1	112606
	Night	53% 3 10% 21% 11% 1	114037
Seeing	Day	20% 13% 28% 22% 12% 5%	15224
	Night	21% 10% 27% 28% 12% 1	25047

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	15% 20% 8% 22% 5% 4 3 3 4 4 13%	4056
	11th to 20th	Day	13% 22% 7% 16% 6% 6% 5% 2 3 4 14%	4060
	21st to end	Day	22% 19% 10% 12% 7% 5% 4 2 3 3 12%	4466
February	1st to 10th	Day	18% 18% 5% 11% 6% 4 4 2 4 5% 22%	1800
	11th to 20th	Day	15% 13% 7% 15% 8% 4 5% 3 4 4 20%	1846
	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	11% 7% 2 5% 5% 4 5% 3 5% 9% 44%	2090
	21st to end	Day	19% 9% 3 6% 4 4 4 3 5% 6% 39%	2090
October	1st to 10th	Day	14% 6% 3 7% 4 4 4 5% 7% 7% 40%	1987
	11th to 20th	Day	17% 10% 3 8% 4 4 4 3 5% 5% 37%	1900
	21st to end	Day	22% 11% 3 7% 4 4 4 4 4 6% 30%	2057
November	1st to 10th	Day	12% 10% 3 8% 4 4 4 3 6% 7% 39%	1762
	11th to 20th	Day	18% 15% 4 7% 5% 4 4 3 4 6% 30%	1800
	21st to end	Day	18% 17% 6% 8% 6% 5% 5% 3 5% 6% 22%	1650
December	1st to 10th	Day	19% 14% 6% 15% 5% 4 4 3 5% 5% 20%	2600
	11th to 20th	Day	21% 20% 8% 12% 4 3 3 2 5% 5% 17%	2990
	21st to end	Day	23% 23% 7% 13% 5% 5% 4 3 3 4 11%	4354
January	1st to 10th	Night	14% 19% 8% 22% 7% 4 3 2 3 3 14%	5889
	11th to 20th	Night	11% 21% 10% 17% 9% 6% 6% 4 3 3 10%	5900
	21st to end	Night	21% 18% 7% 13% 5% 3 4 2 3 4 19%	6490
February	1st to 10th	Night	21% 15% 4 12% 6% 5% 4 2 5% 4 21%	2600
	11th to 20th	Night	17% 16% 6% 10% 6% 5% 5% 4 5% 4 23%	2439
	21st to end	Night	20% 14% 4 12% 6% 5% 5% 2 4 5% 24%	1885
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	10% 5% 1 5% 3 3 4 3 5% 6% 54%	2090
	21st to end	Night	16% 8% 3 6% 4 4 4 2 5% 6% 42%	2090
October	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 5% 7% 44%	2191
	11th to 20th	Night	18% 8% 3 7% 5% 4 4 3 5% 6% 37%	2280
	21st to end	Night	22% 10% 3 6% 5% 4 4 4 6% 6% 31%	2536
November	1st to 10th	Night	14% 9% 2 6% 4 4 4 4 5% 7% 40%	2536
	11th to 20th	Night	16% 14% 4 8% 6% 4 4 3 5% 5% 30%	2600
	21st to end	Night	21% 15% 4 8% 5% 5% 4 3 5% 6% 25%	2745
December	1st to 10th	Night	16% 16% 7% 12% 6% 5% 4 3 5% 5% 21%	4580
	11th to 20th	Night	19% 18% 8% 13% 5% 4 3 3 4 5% 19%	5205
	21st to end	Night	22% 20% 6% 12% 6% 4 4 3 4 6% 13%	6582

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	76% 3 8% 9% 3	4057
	11th to 20th	Day	76% 2 8% 7% 6%	4060
	21st to end	Day	79% 2 6% 10% 3	4466
February	1st to 10th	Day	66% 3 10% 15% 6%	1800
	11th to 20th	Day	67% 3 7% 15% 6% 1	1855
	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% 26% 18% 2	2090
	21st to end	Day	48% 3 9% 23% 17% 1	2090
October	1st to 10th	Day	32% 5% 13% 30% 20% 1	3299
	11th to 20th	Day	44% 4 10% 22% 17% 3	3140
	21st to end	Day	43% 3 11% 25% 17% 1	3371
November	1st to 10th	Day	45% 4 11% 25% 15% 2	1762
	11th to 20th	Day	57% 2 9% 19% 10% 2	1800
	21st to end	Day	64% 4 9% 15% 7%	1650
December	1st to 10th	Day	66% 4 14% 14% 3	2600
	11th to 20th	Day	71% 3 9% 14% 4	2986
	21st to end	Day	80% 2 7% 8% 3	4354
January	1st to 10th	Night	77% 2 6% 12% 4	5889
	11th to 20th	Night	81% 2 6% 8% 3	5900
	21st to end	Night	71% 2 9% 12% 6%	6490
February	1st to 10th	Night	67% 3 9% 15% 6%	2600
	11th to 20th	Night	64% 3 9% 17% 7%	2441
	21st to end	Night	64% 2 9% 14% 8% 2	1885
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	31% 4 10% 29% 24% 2	2090
	21st to end	Night	43% 3 11% 22% 19% 1	2090
October	1st to 10th	Night	31% 4 8% 31% 25% 1	3659
	11th to 20th	Night	43% 5% 9% 22% 19% 3	3779
	21st to end	Night	38% 3 10% 29% 17% 2	4172
November	1st to 10th	Night	43% 4 11% 25% 15% 2	2536
	11th to 20th	Night	56% 2 9% 19% 11% 2	2600
	21st to end	Night	61% 3 12% 18% 6% 1	2745
December	1st to 10th	Night	65% 4 11% 16% 4	4580
	11th to 20th	Night	69% 3 9% 16% 3	5208
	21st to end	Night	75% 3 9% 11% 2	6582

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% 37% 10% 5% 1	300
	11th to 20th	Day	29% 28% 32% 9% 2 1	300
	21st to end	Day	28% 26% 33% 10% 2	330
February	1st to 10th	Day	31% 23% 33% 10% 3	300
	11th to 20th	Day	28% 26% 34% 10% 1	300
	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	11% 4 16% 24% 31% 14%	490
	21st to end	Day	23% 10% 22% 22% 13% 9%	490
October	1st to 10th	Day	13% 9% 27% 27% 19% 7%	490
	11th to 20th	Day	19% 16% 30% 23% 8% 3	490
	21st to end	Day	27% 15% 34% 18% 5%	506
November	1st to 10th	Day	16% 20% 38% 15% 10% 1	300
	11th to 20th	Day	30% 20% 29% 14% 6% 1	300
	21st to end	Day	28% 21% 37% 12% 1	225
December	1st to 10th	Day	23% 23% 35% 14% 4 2	200
	11th to 20th	Day	26% 25% 36% 13% 2	200
	21st to end	Day	29% 20% 37% 12% 1	307
January	1st to 10th	Night	26% 20% 34% 17% 4	800
	11th to 20th	Night	30% 18% 34% 16% 4	800
	21st to end	Night	28% 15% 34% 21% 3	880
February	1st to 10th	Night	28% 18% 33% 19% 2	800
	11th to 20th	Night	26% 19% 32% 20% 3	783
	21st to end	Night	28% 15% 35% 19% 2	629
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	11% 3 13% 36% 32% 5%	760
	21st to end	Night	18% 7% 22% 30% 19% 3	760
October	1st to 10th	Night	16% 6% 25% 33% 17% 3	760
	11th to 20th	Night	22% 12% 30% 26% 10% 1	760
	21st to end	Night	26% 12% 30% 26% 7%	836
November	1st to 10th	Night	19% 11% 31% 26% 11% 1	780
	11th to 20th	Night	26% 15% 32% 20% 6% 1	800
	21st to end	Night	29% 14% 30% 20% 6%	800
December	1st to 10th	Night	31% 15% 32% 19% 4	800
	11th to 20th	Night	28% 15% 33% 20% 4	800
	21st to end	Night	30% 15% 33% 19% 3	880

Page generated at 2025-02-18 03:26:27. Version ac07

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