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 2025-01-07.

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%	105192
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	103788
Transparency	Day	43% 5% 10% 22% 17% 4	108959
	Night	41% 5% 10% 22% 18% 5%	108159
Seeing	Day	17% 10% 22% 22% 18% 10%	14945
	Night	17% 8% 23% 28% 20% 4	24824

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% 14% 4 9% 5% 5% 5% 4 5% 5% 29%	2890
	11th to 20th	Day	22% 9% 4 11% 4 4 3 2 4 4 33%	2840
	21st to end	Day	23% 12% 6% 5% 3 2 4 3 4 5% 32%	3384
February	1st to 10th	Day	20% 12% 4 8% 3 3 3 2 4 5% 35%	1900
	11th to 20th	Day	21% 10% 3 9% 6% 4 3 3 4 4 34%	1848
	21st to end	Day	21% 10% 3 8% 4 4 3 3 4 6% 34%	1570
March	1st to 10th	Day	22% 12% 3 7% 4 4 4 3 5% 6% 30%	3120
	11th to 20th	Day	15% 8% 2 6% 4 4 3 3 5% 5% 45%	3384
	21st to end	Day	14% 7% 3 7% 5% 5% 4 3 6% 7% 39%	3927
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 6% 7% 44%	2280
	11th to 20th	Day	14% 8% 3 6% 4 4 4 3 5% 6% 44%	2254
	21st to end	Day	13% 6% 3 7% 5% 4 4 4 6% 9% 40%	2342
May	1st to 10th	Day	17% 7% 2 6% 4 4 5% 4 7% 8% 36%	4082
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 4 6% 7% 34%	4081
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	4476
June	1st to 10th	Day	10% 7% 2 6% 6% 4 5% 4 6% 10% 38%	2470
	11th to 20th	Day	5% 5% 2 5% 5% 4 6% 6% 8% 10% 43%	2470
	21st to end	Day	6% 5% 3 7% 5% 6% 6% 5% 8% 12% 39%	2579
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 6% 6% 8% 11% 35%	4244
	11th to 20th	Day	10% 6% 3 7% 5% 6% 7% 5% 8% 10% 34%	4120
	21st to end	Day	17% 11% 4 9% 6% 6% 7% 6% 7% 7% 21%	4466
August	1st to 10th	Day	5% 4 2 5% 4 5% 6% 5% 7% 11% 46%	3955
	11th to 20th	Day	7% 4 2 7% 6% 7% 6% 5% 8% 10% 39%	3729
	21st to end	Day	8% 4 2 6% 5% 5% 5% 4 6% 8% 49%	4180
September	1st to 10th	Day	12% 6% 2 7% 5% 5% 5% 4 8% 8% 39%	2214
	11th to 20th	Day	14% 6% 3 6% 4 5% 4 4 6% 7% 41%	2090
	21st to end	Day	13% 4 2 6% 4 4 5% 4 4 8% 45%	2090
October	1st to 10th	Day	11% 5% 2 4 3 4 3 3 4 5% 56%	2090
	11th to 20th	Day	12% 5% 1 5% 3 3 3 3 4 6% 54%	2076
	21st to end	Day	15% 6% 2 5% 3 3 3 4 4 6% 48%	1910
November	1st to 10th	Day	11% 5% 2 4 4 3 4 3 4 6% 53%	1762
	11th to 20th	Day	14% 7% 2 5% 4 3 4 4 5% 6% 47%	1800
	21st to end	Day	16% 8% 2 5% 4 4 4 4 5% 7% 42%	1800
December	1st to 10th	Day	17% 8% 2 7% 4 4 4 3 5% 4 43%	2930
	11th to 20th	Day	19% 14% 5% 9% 5% 4 4 3 3 4 30%	3373
	21st to end	Day	19% 12% 6% 7% 5% 4 4 3 6% 7% 29%	4466
January	1st to 10th	Night	18% 14% 3 7% 3 3 3 3 4 5% 36%	4188
	11th to 20th	Night	20% 12% 5% 7% 5% 4 3 3 3 4 35%	4120
	21st to end	Night	22% 14% 4 6% 4 3 3 3 3 4 35%	4267
February	1st to 10th	Night	19% 10% 3 8% 4 3 3 3 5% 6% 35%	2280
	11th to 20th	Night	22% 10% 3 8% 5% 4 3 2 4 4 35%	2218
	21st to end	Night	23% 11% 2 7% 4 4 3 3 5% 5% 32%	1884
March	1st to 10th	Night	23% 13% 3 7% 4 4 3 3 5% 5% 30%	3733
	11th to 20th	Night	13% 8% 2 5% 3 4 4 3 5% 6% 47%	3576
	21st to end	Night	15% 8% 3 8% 5% 6% 4 4 4 5% 39%	3732
April	1st to 10th	Night	13% 6% 2 5% 3 5% 3 3 5% 8% 47%	1900
	11th to 20th	Night	16% 7% 2 3 3 3 3 4 5% 7% 48%	1875
	21st to end	Night	13% 6% 2 5% 5% 4 4 4 6% 7% 45%	1835
May	1st to 10th	Night	15% 6% 2 5% 5% 4 4 4 6% 8% 41%	2813
	11th to 20th	Night	16% 8% 3 7% 4 4 5% 4 5% 6% 39%	2812
	21st to end	Night	7% 6% 2 6% 4 5% 6% 4 7% 11% 43%	3079
June	1st to 10th	Night	9% 6% 3 7% 4 4 5% 5% 7% 10% 41%	1710
	11th to 20th	Night	4 2 1 4 3 4 6% 4 7% 12% 54%	1710
	21st to end	Night	6% 4 2 5% 5% 4 5% 4 7% 11% 48%	1580
July	1st to 10th	Night	9% 5% 2 7% 5% 5% 6% 6% 7% 11% 37%	2702
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 4 7% 11% 42%	2840
	21st to end	Night	11% 12% 4 10% 7% 5% 6% 5% 7% 8% 25%	3087
August	1st to 10th	Night	4 3 1 5% 4 5% 4 4 6% 11% 54%	2970
	11th to 20th	Night	6% 4 2 5% 4 4 5% 4 6% 12% 48%	3100
	21st to end	Night	5% 4 1 4 4 4 4 4 6% 9% 54%	3476
September	1st to 10th	Night	9% 6% 2 6% 4 4 5% 3 6% 8% 46%	1947
	11th to 20th	Night	10% 6% 2 6% 4 5% 4 4 6% 7% 45%	2090
	21st to end	Night	10% 5% 1 6% 3 3 3 3 6% 7% 53%	2090
October	1st to 10th	Night	11% 4 2 4 3 3 3 2 4 5% 59%	2090
	11th to 20th	Night	15% 5% 2 5% 3 4 4 3 4 6% 51%	2102
	21st to end	Night	14% 7% 2 4 4 2 3 3 4 6% 50%	2684
November	1st to 10th	Night	13% 6% 2 6% 4 2 3 3 4 6% 51%	2536
	11th to 20th	Night	12% 8% 2 5% 3 3 3 3 6% 7% 48%	2600
	21st to end	Night	16% 9% 3 5% 3 3 3 4 5% 5% 43%	2600
December	1st to 10th	Night	19% 10% 3 7% 4 3 3 2 4 5% 40%	4250
	11th to 20th	Night	23% 11% 3 6% 3 3 3 2 4 5% 37%	4822
	21st to end	Night	18% 12% 5% 8% 4 5% 4 3 4 6% 31%	6490

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 5% 18% 15% 2	2891
	11th to 20th	Day	56% 3 6% 15% 19% 1	2840
	21st to end	Day	56% 2 7% 21% 13% 1	3384
February	1st to 10th	Day	54% 2 7% 18% 17% 2	1900
	11th to 20th	Day	55% 3 6% 18% 15% 4	1858
	21st to end	Day	52% 3 7% 16% 16% 5%	1570
March	1st to 10th	Day	54% 5% 7% 14% 13% 8%	3120
	11th to 20th	Day	41% 4 7% 23% 18% 6%	3384
	21st to end	Day	45% 4 11% 24% 15% 2	3927
April	1st to 10th	Day	40% 4 10% 23% 19% 4	2280
	11th to 20th	Day	41% 4 6% 20% 24% 5%	2254
	21st to end	Day	42% 4 8% 27% 16% 2	2342
May	1st to 10th	Day	45% 6% 11% 23% 13% 2	4120
	11th to 20th	Day	49% 5% 11% 23% 11%	4085
	21st to end	Day	41% 6% 11% 22% 19% 1	4489
June	1st to 10th	Day	41% 7% 13% 23% 15% 1	2470
	11th to 20th	Day	32% 6% 18% 24% 18% 2	2470
	21st to end	Day	37% 8% 18% 18% 15% 4	2579
July	1st to 10th	Day	39% 9% 21% 22% 7% 2	4179
	11th to 20th	Day	41% 8% 19% 24% 7% 2	4051
	21st to end	Day	59% 8% 11% 15% 6% 1	4324
August	1st to 10th	Day	29% 7% 15% 33% 15% 1	3942
	11th to 20th	Day	36% 7% 16% 25% 14% 2	3765
	21st to end	Day	31% 7% 11% 29% 20% 2	4179
September	1st to 10th	Day	41% 6% 13% 19% 16% 5%	2214
	11th to 20th	Day	41% 7% 11% 20% 20% 2	2090
	21st to end	Day	37% 5% 11% 22% 20% 6%	2090
October	1st to 10th	Day	24% 3 6% 17% 37% 12%	3480
	11th to 20th	Day	23% 3 5% 23% 31% 16%	3432
	21st to end	Day	28% 3 8% 24% 27% 10%	3122
November	1st to 10th	Day	33% 4 8% 21% 28% 7%	1762
	11th to 20th	Day	38% 3 6% 23% 24% 7%	1800
	21st to end	Day	42% 3 7% 22% 22% 4	1800
December	1st to 10th	Day	45% 3 4 23% 22% 2	2930
	11th to 20th	Day	59% 3 6% 18% 12% 2	3370
	21st to end	Day	54% 4 9% 21% 9% 2	4466
January	1st to 10th	Night	50% 3 7% 21% 17% 2	4188
	11th to 20th	Night	54% 3 6% 16% 16% 6%	4120
	21st to end	Night	55% 3 6% 18% 17% 2	4267
February	1st to 10th	Night	51% 3 9% 18% 17% 2	2280
	11th to 20th	Night	54% 2 6% 17% 15% 5%	2219
	21st to end	Night	54% 3 6% 17% 15% 5%	1884
March	1st to 10th	Night	56% 5% 8% 11% 15% 6%	3733
	11th to 20th	Night	38% 5% 7% 23% 17% 10%	3576
	21st to end	Night	48% 4 8% 22% 17% 1	3732
April	1st to 10th	Night	37% 4 11% 23% 21% 3	1900
	11th to 20th	Night	36% 4 8% 21% 23% 8%	1875
	21st to end	Night	38% 4 10% 28% 18% 2	1835
May	1st to 10th	Night	41% 7% 13% 24% 15% 1	2840
	11th to 20th	Night	45% 6% 13% 22% 14% 1	2813
	21st to end	Night	34% 8% 15% 26% 17% 1	3087
June	1st to 10th	Night	37% 7% 14% 25% 15% 2	1710
	11th to 20th	Night	23% 6% 19% 34% 16% 3	1710
	21st to end	Night	30% 7% 18% 26% 15% 4	1580
July	1st to 10th	Night	38% 9% 21% 22% 8% 1	2657
	11th to 20th	Night	34% 8% 20% 28% 7% 3	2786
	21st to end	Night	55% 9% 12% 16% 7% 1	2988
August	1st to 10th	Night	23% 6% 13% 38% 18% 1	2959
	11th to 20th	Night	28% 6% 23% 27% 14% 1	3130
	21st to end	Night	25% 7% 14% 30% 23% 2	3476
September	1st to 10th	Night	36% 5% 16% 22% 16% 6%	1947
	11th to 20th	Night	37% 6% 11% 20% 23% 2	2090
	21st to end	Night	31% 6% 9% 24% 23% 7%	2090
October	1st to 10th	Night	23% 4 7% 20% 31% 16%	3480
	11th to 20th	Night	23% 2 4 21% 30% 20%	3485
	21st to end	Night	25% 3 5% 26% 30% 11%	4422
November	1st to 10th	Night	35% 4 8% 21% 25% 8%	2536
	11th to 20th	Night	36% 3 7% 21% 26% 8%	2600
	21st to end	Night	43% 3 6% 22% 21% 5%	2600
December	1st to 10th	Night	49% 3 5% 16% 25% 2	4250
	11th to 20th	Night	51% 3 5% 20% 20% 2	4824
	21st to end	Night	54% 4 8% 23% 11% 1	6490

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% 26% 31% 11% 7%	288
	11th to 20th	Day	31% 24% 27% 14% 3 1	270
	21st to end	Day	23% 22% 37% 14% 3	297
February	1st to 10th	Day	29% 20% 33% 15% 2 1	270
	11th to 20th	Day	35% 22% 30% 9% 3 1	270
	21st to end	Day	36% 23% 29% 8% 3	225
March	1st to 10th	Day	28% 18% 34% 16% 4	270
	11th to 20th	Day	27% 16% 32% 19% 6% 1	403
	21st to end	Day	26% 17% 31% 17% 9%	506
April	1st to 10th	Day	21% 15% 27% 23% 11% 2	460
	11th to 20th	Day	19% 14% 32% 21% 13% 1	452
	21st to end	Day	18% 14% 32% 20% 14% 2	460
May	1st to 10th	Day	15% 13% 27% 26% 13% 6%	463
	11th to 20th	Day	13% 8% 24% 26% 19% 10%	487
	21st to end	Day	12% 5% 21% 27% 20% 16%	529
June	1st to 10th	Day	13% 6% 18% 27% 20% 17%	490
	11th to 20th	Day	5% 5% 13% 27% 26% 24%	490
	21st to end	Day	7% 4 16% 30% 25% 18%	599
July	1st to 10th	Day	9% 2 14% 25% 27% 23%	557
	11th to 20th	Day	10% 4 10% 22% 27% 27%	484
	21st to end	Day	11% 2 10% 22% 28% 26%	532
August	1st to 10th	Day	12% 3 10% 24% 30% 21%	486
	11th to 20th	Day	10% 3 13% 25% 36% 13%	479
	21st to end	Day	14% 1 9% 22% 31% 24%	539
September	1st to 10th	Day	11% 4 11% 22% 34% 18%	490
	11th to 20th	Day	14% 3 12% 23% 28% 20%	490
	21st to end	Day	16% 3 15% 28% 28% 10%	490
October	1st to 10th	Day	12% 4 18% 30% 24% 10%	490
	11th to 20th	Day	14% 9% 28% 28% 17% 4	490
	21st to end	Day	21% 9% 28% 28% 12% 3	359
November	1st to 10th	Day	18% 8% 32% 29% 9% 3	300
	11th to 20th	Day	14% 17% 35% 25% 7% 1	300
	21st to end	Day	22% 15% 33% 16% 11% 3	300
December	1st to 10th	Day	27% 20% 29% 18% 6% 1	300
	11th to 20th	Day	25% 20% 27% 18% 10%	300
	21st to end	Day	24% 18% 37% 15% 5% 1	330
January	1st to 10th	Night	24% 15% 35% 22% 5%	785
	11th to 20th	Night	26% 18% 30% 20% 5%	760
	21st to end	Night	21% 15% 36% 25% 3	836
February	1st to 10th	Night	23% 14% 34% 25% 4	760
	11th to 20th	Night	28% 12% 35% 22% 3	743
	21st to end	Night	29% 16% 33% 21% 2	628
March	1st to 10th	Night	22% 14% 31% 28% 5%	760
	11th to 20th	Night	21% 13% 34% 23% 8% 1	760
	21st to end	Night	20% 12% 32% 26% 10% 1	789
April	1st to 10th	Night	15% 13% 31% 30% 11% 1	570
	11th to 20th	Night	19% 8% 30% 31% 10% 2	563
	21st to end	Night	15% 8% 29% 34% 13% 1	570
May	1st to 10th	Night	14% 5% 27% 32% 19% 3	570
	11th to 20th	Night	15% 4 21% 34% 22% 4	561
	21st to end	Night	9% 2 14% 40% 30% 5%	617
June	1st to 10th	Night	10% 3 16% 32% 34% 5%	570
	11th to 20th	Night	5% 1 11% 36% 39% 9%	570
	21st to end	Night	7% 1 11% 32% 41% 7%	570
July	1st to 10th	Night	9% 1 9% 26% 42% 13%	555
	11th to 20th	Night	9% 3 6% 27% 44% 13%	552
	21st to end	Night	9% 2 7% 27% 42% 12%	624
August	1st to 10th	Night	7% 1 7% 23% 50% 11%	567
	11th to 20th	Night	8% 1 11% 32% 41% 8%	564
	21st to end	Night	10% 1 7% 28% 44% 11%	627
September	1st to 10th	Night	11% 2 12% 27% 41% 7%	617
	11th to 20th	Night	12% 2 13% 30% 36% 7%	760
	21st to end	Night	12% 1 17% 31% 35% 4	760
October	1st to 10th	Night	13% 4 17% 31% 28% 7%	760
	11th to 20th	Night	17% 7% 21% 34% 20% 1	760
	21st to end	Night	18% 6% 24% 33% 16% 3	836
November	1st to 10th	Night	17% 7% 22% 33% 18% 3	780
	11th to 20th	Night	16% 10% 33% 28% 13% 1	800
	21st to end	Night	21% 11% 26% 29% 11% 1	800
December	1st to 10th	Night	26% 10% 29% 28% 6%	800
	11th to 20th	Night	24% 11% 29% 27% 8% 1	800
	21st to end	Night	25% 12% 30% 27% 5%	880

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