Cat Creek Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cat Creek Observing Site clear sky chart from 2005-11-06 to 2026-05-02.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 38%	132111
	Night	14% 8% 3 7% 4 4 4 3 5% 7% 41%	130369
Transparency	Day	43% 5% 11% 21% 16% 3	135836
	Night	41% 4 10% 21% 18% 4	134715
Seeing	Day	17% 11% 23% 22% 17% 10%	16666
	Night	17% 8% 24% 28% 19% 3	26547

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% 13% 5% 10% 4 4 4 3 4 4 32%	5258
	11th to 20th	Day	15% 7% 5% 11% 3 3 2 2 3 4 45%	5280
	21st to end	Day	21% 9% 5% 7% 5% 3 4 3 4 6% 33%	6274
February	1st to 10th	Day	19% 12% 4 8% 4 3 4 3 4 5% 35%	2100
	11th to 20th	Day	21% 11% 3 9% 6% 3 3 3 4 5% 33%	2048
	21st to end	Day	20% 9% 2 8% 4 4 3 3 4 6% 36%	1730
March	1st to 10th	Day	17% 11% 4 7% 5% 4 4 3 5% 6% 33%	4580
	11th to 20th	Day	13% 8% 3 6% 4 4 3 3 5% 5% 46%	4949
	21st to end	Day	13% 7% 3 7% 4 4 4 3 5% 7% 42%	5755
April	1st to 10th	Day	13% 7% 2 5% 4 4 4 3 6% 7% 43%	2520
	11th to 20th	Day	12% 8% 3 6% 4 4 4 3 5% 7% 46%	2494
	21st to end	Day	13% 7% 3 7% 5% 4 5% 3 6% 8% 39%	2589
May	1st to 10th	Day	20% 8% 2 6% 5% 4 6% 4 7% 8% 29%	5875
	11th to 20th	Day	17% 10% 4 9% 5% 6% 5% 4 6% 7% 28%	5861
	21st to end	Day	16% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	6433
June	1st to 10th	Day	9% 7% 2 6% 6% 4 5% 4 6% 10% 39%	2600
	11th to 20th	Day	6% 5% 3 5% 5% 5% 6% 6% 8% 10% 42%	2600
	21st to end	Day	6% 5% 2 7% 5% 6% 6% 5% 8% 12% 39%	2715
July	1st to 10th	Day	7% 5% 2 7% 5% 6% 6% 6% 8% 11% 38%	6080
	11th to 20th	Day	8% 6% 2 7% 5% 6% 7% 5% 9% 12% 32%	5900
	21st to end	Day	12% 8% 3 8% 6% 6% 7% 6% 8% 10% 24%	6424
August	1st to 10th	Day	5% 4 2 6% 4 5% 6% 5% 7% 11% 45%	4080
	11th to 20th	Day	7% 4 2 7% 6% 7% 6% 5% 8% 10% 39%	3849
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 4 7% 8% 39%	2330
	11th to 20th	Day	13% 6% 3 6% 4 5% 4 4 6% 7% 42%	2200
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 5% 8% 45%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 4 3 3 4 5% 56%	2200
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 4 6% 55%	2185
	21st to end	Day	15% 6% 2 5% 3 3 3 4 4 6% 48%	2010
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 4 6% 53%	1852
	11th to 20th	Day	14% 7% 2 5% 3 3 4 4 5% 6% 48%	1890
	21st to end	Day	16% 8% 2 5% 4 4 4 4 5% 7% 42%	1890
December	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 4 4 42%	3020
	11th to 20th	Day	18% 14% 4 9% 5% 4 4 3 3 4 32%	3463
	21st to end	Day	19% 12% 6% 7% 5% 4 4 3 6% 7% 29%	4565
January	1st to 10th	Night	17% 11% 3 8% 3 3 3 3 4 5% 40%	7635
	11th to 20th	Night	16% 10% 4 7% 4 3 2 2 3 4 44%	7680
	21st to end	Night	19% 11% 4 7% 5% 3 3 2 3 5% 37%	7888
February	1st to 10th	Night	19% 10% 3 8% 4 3 3 3 5% 6% 35%	2520
	11th to 20th	Night	22% 10% 3 8% 4 4 3 2 4 4 34%	2458
	21st to end	Night	21% 11% 2 7% 4 4 3 3 6% 5% 34%	2076
March	1st to 10th	Night	20% 13% 3 8% 4 4 4 3 5% 6% 31%	5493
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 47%	5231
	21st to end	Night	15% 7% 2 8% 5% 5% 4 3 4 6% 41%	5459
April	1st to 10th	Night	13% 6% 2 5% 3 5% 3 3 5% 9% 46%	2100
	11th to 20th	Night	15% 6% 2 3 3 3 3 4 5% 7% 50%	2075
	21st to end	Night	13% 7% 2 5% 5% 4 4 3 6% 7% 44%	2028
May	1st to 10th	Night	16% 9% 3 8% 6% 4 5% 4 6% 8% 32%	4042
	11th to 20th	Night	18% 10% 3 9% 5% 5% 5% 4 5% 6% 29%	4032
	21st to end	Night	14% 8% 3 7% 4 4 5% 4 6% 9% 37%	4421
June	1st to 10th	Night	9% 6% 3 7% 4 4 5% 5% 7% 10% 41%	1800
	11th to 20th	Night	5% 3 1 4 3 4 6% 4 7% 11% 52%	1800
	21st to end	Night	5% 4 2 5% 5% 4 5% 4 6% 11% 49%	1663
July	1st to 10th	Night	9% 5% 2 6% 4 4 6% 6% 7% 10% 40%	3855
	11th to 20th	Night	8% 5% 2 6% 5% 5% 6% 5% 9% 13% 37%	4057
	21st to end	Night	8% 9% 3 9% 6% 5% 6% 5% 8% 12% 29%	4427
August	1st to 10th	Night	4 3 1 5% 4 5% 4 4 6% 11% 54%	3064
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 11% 49%	3200
	21st to end	Night	5% 4 2 4 4 4 4 4 6% 9% 53%	3586
September	1st to 10th	Night	9% 6% 2 7% 4 4 5% 3 6% 8% 46%	2050
	11th to 20th	Night	10% 6% 2 6% 4 5% 4 4 6% 7% 46%	2200
	21st to end	Night	11% 5% 2 6% 3 4 3 3 6% 7% 51%	2200
October	1st to 10th	Night	11% 4 2 4 3 3 3 3 4 5% 58%	2200
	11th to 20th	Night	14% 5% 2 5% 3 4 3 3 4 5% 53%	2213
	21st to end	Night	14% 7% 2 4 4 2 3 3 4 6% 50%	2825
November	1st to 10th	Night	13% 6% 2 6% 4 2 3 3 4 6% 51%	2666
	11th to 20th	Night	12% 7% 2 5% 3 3 3 3 5% 7% 49%	2730
	21st to end	Night	16% 9% 3 5% 3 3 3 4 5% 5% 43%	2730
December	1st to 10th	Night	19% 10% 3 7% 4 3 3 3 4 5% 39%	4380
	11th to 20th	Night	22% 11% 3 6% 3 3 3 2 4 5% 38%	4952
	21st to end	Night	18% 12% 5% 8% 4 5% 4 3 4 6% 31%	6633

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	56% 4 8% 20% 11% 1	5259
	11th to 20th	Day	44% 2 4 18% 30% 2	5280
	21st to end	Day	54% 2 7% 21% 12% 4	6274
February	1st to 10th	Day	54% 2 7% 18% 17% 2	2100
	11th to 20th	Day	55% 3 6% 18% 15% 4	2058
	21st to end	Day	50% 3 7% 17% 18% 6%	1730
March	1st to 10th	Day	51% 5% 7% 13% 17% 8%	4580
	11th to 20th	Day	40% 4 7% 20% 20% 10%	4949
	21st to end	Day	43% 3 10% 27% 16% 1	5757
April	1st to 10th	Day	39% 5% 11% 25% 18% 3	2520
	11th to 20th	Day	39% 4 6% 22% 24% 5%	2494
	21st to end	Day	43% 5% 9% 26% 16% 2	2589
May	1st to 10th	Day	50% 6% 13% 20% 9% 1	5910
	11th to 20th	Day	54% 6% 13% 18% 8%	5865
	21st to end	Day	47% 6% 9% 21% 15% 2	6447
June	1st to 10th	Day	40% 7% 13% 23% 16% 1	2600
	11th to 20th	Day	34% 6% 18% 23% 17% 2	2600
	21st to end	Day	37% 8% 18% 19% 15% 3	2715
July	1st to 10th	Day	37% 9% 19% 25% 10% 1	6015
	11th to 20th	Day	39% 10% 23% 22% 5% 1	5831
	21st to end	Day	49% 10% 18% 17% 4	6240
August	1st to 10th	Day	30% 7% 15% 33% 15% 1	4067
	11th to 20th	Day	36% 7% 16% 26% 14% 2	3885
	21st to end	Day	32% 7% 11% 29% 20% 2	4311
September	1st to 10th	Day	41% 6% 13% 20% 16% 5%	2330
	11th to 20th	Day	40% 7% 11% 21% 20% 2	2200
	21st to end	Day	37% 5% 11% 21% 20% 5%	2200
October	1st to 10th	Day	24% 3 6% 18% 37% 12%	3590
	11th to 20th	Day	23% 3 5% 23% 30% 17%	3541
	21st to end	Day	29% 3 8% 24% 27% 10%	3222
November	1st to 10th	Day	34% 4 8% 20% 28% 6%	1852
	11th to 20th	Day	37% 3 7% 23% 23% 7%	1890
	21st to end	Day	42% 3 7% 22% 21% 4	1890
December	1st to 10th	Day	46% 3 4 23% 21% 2	3020
	11th to 20th	Day	58% 3 7% 18% 13% 3	3460
	21st to end	Day	54% 4 9% 21% 9% 2	4565
January	1st to 10th	Night	48% 3 14% 20% 15% 1	7636
	11th to 20th	Night	45% 2 5% 17% 26% 4	7680
	21st to end	Night	52% 2 7% 17% 16% 6%	7888
February	1st to 10th	Night	51% 3 10% 19% 16% 2	2520
	11th to 20th	Night	55% 2 6% 17% 14% 5%	2459
	21st to end	Night	52% 3 6% 18% 16% 5%	2076
March	1st to 10th	Night	54% 4 7% 14% 18% 4	5493
	11th to 20th	Night	37% 4 6% 20% 20% 13%	5231
	21st to end	Night	45% 3 8% 24% 17% 2	5459
April	1st to 10th	Night	36% 4 13% 23% 21% 3	2100
	11th to 20th	Night	34% 4 9% 23% 23% 8%	2075
	21st to end	Night	39% 4 11% 27% 18% 2	2028
May	1st to 10th	Night	48% 7% 14% 19% 11% 1	4069
	11th to 20th	Night	54% 6% 13% 17% 10%	4033
	21st to end	Night	43% 7% 13% 24% 13% 1	4429
June	1st to 10th	Night	37% 7% 14% 25% 15% 2	1800
	11th to 20th	Night	25% 6% 18% 32% 16% 3	1800
	21st to end	Night	30% 7% 17% 26% 16% 4	1663
July	1st to 10th	Night	36% 9% 20% 22% 11% 2	3812
	11th to 20th	Night	34% 10% 24% 24% 5% 2	4006
	21st to end	Night	46% 10% 22% 17% 5% 1	4297
August	1st to 10th	Night	24% 6% 13% 37% 18% 1	3053
	11th to 20th	Night	28% 6% 23% 27% 14% 1	3230
	21st to end	Night	25% 7% 14% 30% 23% 2	3586
September	1st to 10th	Night	36% 5% 16% 21% 16% 6%	2050
	11th to 20th	Night	36% 6% 11% 22% 23% 2	2200
	21st to end	Night	32% 6% 10% 24% 23% 6%	2200
October	1st to 10th	Night	23% 4 7% 20% 31% 15%	3590
	11th to 20th	Night	23% 2 4 21% 31% 19%	3596
	21st to end	Night	25% 3 5% 25% 30% 11%	4563
November	1st to 10th	Night	35% 3 8% 21% 25% 8%	2666
	11th to 20th	Night	36% 3 7% 22% 25% 8%	2730
	21st to end	Night	44% 3 6% 22% 21% 5%	2730
December	1st to 10th	Night	49% 3 5% 16% 24% 2	4380
	11th to 20th	Night	50% 3 5% 19% 21% 2	4954
	21st to end	Night	54% 4 8% 23% 11% 1	6633

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	25% 25% 32% 12% 6%	330
	11th to 20th	Day	29% 25% 29% 13% 3 1	330
	21st to end	Day	24% 23% 37% 13% 3	363
February	1st to 10th	Day	26% 22% 36% 13% 2 1	330
	11th to 20th	Day	35% 24% 29% 9% 2 1	330
	21st to end	Day	33% 23% 30% 10% 4	273
March	1st to 10th	Day	27% 21% 34% 14% 4	330
	11th to 20th	Day	27% 18% 32% 17% 5% 1	477
	21st to end	Day	24% 17% 33% 18% 8%	594
April	1st to 10th	Day	20% 16% 29% 23% 10% 2	540
	11th to 20th	Day	17% 17% 32% 21% 12% 1	532
	21st to end	Day	18% 14% 31% 21% 14% 2	540
May	1st to 10th	Day	18% 14% 27% 25% 12% 5%	507
	11th to 20th	Day	13% 10% 25% 25% 18% 9%	527
	21st to end	Day	13% 6% 22% 26% 18% 15%	573
June	1st to 10th	Day	13% 5% 19% 27% 20% 16%	530
	11th to 20th	Day	6% 6% 13% 27% 25% 23%	530
	21st to end	Day	7% 4 15% 29% 25% 19%	645
July	1st to 10th	Day	9% 2 14% 27% 27% 22%	601
	11th to 20th	Day	10% 4 10% 22% 27% 27%	524
	21st to end	Day	11% 2 10% 22% 30% 26%	576
August	1st to 10th	Day	12% 4 10% 23% 31% 20%	526
	11th to 20th	Day	10% 3 12% 24% 36% 15%	519
	21st to end	Day	13% 1 10% 22% 30% 22%	583
September	1st to 10th	Day	12% 4 12% 22% 33% 17%	530
	11th to 20th	Day	13% 3 12% 25% 29% 19%	530
	21st to end	Day	15% 3 16% 29% 27% 9%	530
October	1st to 10th	Day	12% 4 19% 31% 24% 10%	530
	11th to 20th	Day	13% 9% 28% 29% 17% 3	530
	21st to end	Day	22% 10% 29% 26% 11% 3	393
November	1st to 10th	Day	18% 9% 31% 29% 9% 3	330
	11th to 20th	Day	14% 18% 37% 23% 8% 1	330
	21st to end	Day	23% 15% 34% 15% 10% 3	330
December	1st to 10th	Day	26% 20% 31% 17% 5% 1	330
	11th to 20th	Day	24% 20% 29% 17% 9%	330
	21st to end	Day	24% 21% 36% 14% 5% 1	363
January	1st to 10th	Night	24% 15% 35% 22% 4	840
	11th to 20th	Night	25% 18% 32% 20% 4	840
	21st to end	Night	21% 16% 37% 25% 2	924
February	1st to 10th	Night	23% 14% 34% 25% 4	840
	11th to 20th	Night	29% 15% 33% 21% 3	823
	21st to end	Night	28% 17% 32% 21% 2	692
March	1st to 10th	Night	22% 15% 31% 28% 5%	840
	11th to 20th	Night	21% 13% 35% 23% 7% 1	840
	21st to end	Night	19% 13% 34% 25% 9%	872
April	1st to 10th	Night	15% 13% 31% 30% 10%	630
	11th to 20th	Night	17% 8% 31% 33% 9% 2	623
	21st to end	Night	15% 9% 29% 33% 13% 1	630
May	1st to 10th	Night	14% 6% 28% 31% 18% 3	603
	11th to 20th	Night	15% 4 22% 33% 21% 4	591
	21st to end	Night	10% 3 15% 39% 28% 5%	650
June	1st to 10th	Night	10% 3 17% 32% 33% 5%	600
	11th to 20th	Night	6% 1 12% 36% 37% 9%	600
	21st to end	Night	7% 1 10% 31% 43% 8%	600
July	1st to 10th	Night	9% 1 9% 25% 43% 12%	585
	11th to 20th	Night	9% 2 6% 26% 45% 13%	582
	21st to end	Night	9% 2 7% 26% 43% 13%	657
August	1st to 10th	Night	8% 1 8% 24% 50% 11%	597
	11th to 20th	Night	8% 1 11% 31% 42% 8%	594
	21st to end	Night	10% 1 8% 29% 42% 10%	660
September	1st to 10th	Night	10% 2 13% 28% 40% 6%	650
	11th to 20th	Night	12% 2 13% 30% 37% 7%	800
	21st to end	Night	12% 2 17% 31% 35% 4	800
October	1st to 10th	Night	13% 4 18% 31% 28% 7%	800
	11th to 20th	Night	16% 7% 21% 35% 20% 1	800
	21st to end	Night	18% 6% 26% 32% 15% 3	880
November	1st to 10th	Night	17% 7% 22% 34% 18% 3	820
	11th to 20th	Night	16% 10% 32% 29% 12% 1	840
	21st to end	Night	22% 12% 26% 28% 11% 1	840
December	1st to 10th	Night	27% 11% 29% 28% 6%	840
	11th to 20th	Night	24% 11% 30% 27% 8%	840
	21st to end	Night	26% 13% 30% 27% 5%	924

Page generated at 2026-04-29 01:21:46. Version ac07

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