Umatilla
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Umatilla clear sky chart from 2005-11-06 to 2025-12-18.

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	7% 7% 3 6% 4 4 4 3 5% 6% 51%	136214
	Night	9% 8% 3 7% 4 4 4 4 6% 7% 44%	132109
Transparency	Day	33% 6% 11% 22% 19% 9%	130932
	Night	39% 7% 12% 21% 14% 6%	128393
Seeing	Day	10% 9% 20% 26% 22% 12%	15775
	Night	13% 7% 22% 35% 21% 3	24887

By Month

Percentage Cloud Cover SW (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% 16% 7% 15% 6% 5% 5% 4 5% 6% 13%	3639
	11th to 20th	Day	15% 13% 4 7% 5% 5% 5% 4 7% 7% 29%	3640
	21st to end	Day	16% 13% 5% 11% 6% 4 5% 4 6% 6% 24%	4004
February	1st to 10th	Day	13% 15% 6% 11% 6% 5% 4 4 6% 5% 24%	1755
	11th to 20th	Day	12% 14% 4 9% 5% 5% 5% 3 5% 6% 31%	1825
	21st to end	Day	10% 10% 4 8% 7% 6% 5% 6% 7% 8% 30%	1650
March	1st to 10th	Day	7% 9% 4 9% 5% 5% 6% 5% 8% 10% 34%	4518
	11th to 20th	Day	7% 10% 3 7% 4 4 4 4 5% 7% 45%	4766
	21st to end	Day	8% 9% 4 11% 7% 6% 6% 4 6% 8% 30%	5921
April	1st to 10th	Day	7% 8% 3 6% 6% 5% 5% 5% 7% 8% 39%	2400
	11th to 20th	Day	7% 7% 3 5% 4 4 4 4 6% 8% 48%	2369
	21st to end	Day	6% 5% 2 5% 4 4 5% 4 6% 8% 51%	2545
May	1st to 10th	Day	8% 4 2 4 3 3 3 3 6% 8% 57%	6313
	11th to 20th	Day	5% 5% 2 5% 4 4 3 3 5% 6% 57%	6277
	21st to end	Day	5% 5% 2 5% 4 4 4 3 5% 8% 57%	6902
June	1st to 10th	Day	9% 6% 2 5% 4 3 4 4 6% 7% 51%	2800
	11th to 20th	Day	7% 4 2 4 3 4 4 4 5% 7% 57%	2800
	21st to end	Day	3 3 1 3 2 2 3 2 5% 6% 68%	2800
July	1st to 10th	Day	2 2 1 2 2 2 2 2 3 3 80%	6319
	11th to 20th	Day	1 1 1 2 1 1 1 2 2 3 84%	6320
	21st to end	Day	1 1 1 2 2 2 2 2 4 5% 79%	6880
August	1st to 10th	Day	3 3 2 4 2 3 2 2 4 6% 68%	6210
	11th to 20th	Day	2 2 1 2 2 2 2 2 4 5% 75%	5787
	21st to end	Day	4 4 2 4 3 3 3 3 5% 5% 64%	5984
September	1st to 10th	Day	2 2 1 2 2 2 3 2 5% 7% 72%	2400
	11th to 20th	Day	3 4 2 5% 3 4 4 3 5% 6% 60%	2357
	21st to end	Day	6% 5% 2 5% 4 3 4 3 4 7% 58%	2200
October	1st to 10th	Day	6% 4 1 4 3 3 3 2 5% 6% 63%	3316
	11th to 20th	Day	4 5% 2 6% 5% 4 6% 4 8% 10% 46%	3242
	21st to end	Day	10% 8% 3 7% 5% 5% 5% 4 6% 8% 38%	3200
November	1st to 10th	Day	13% 10% 4 9% 7% 5% 6% 5% 5% 7% 31%	1854
	11th to 20th	Day	15% 14% 5% 10% 6% 6% 6% 4 5% 7% 24%	1753
	21st to end	Day	11% 14% 6% 13% 6% 6% 6% 5% 6% 6% 22%	1680
December	1st to 10th	Day	11% 9% 5% 14% 9% 7% 7% 4 7% 5% 22%	2700
	11th to 20th	Day	17% 17% 7% 14% 8% 6% 5% 3 4 4 14%	3084
	21st to end	Day	16% 14% 5% 14% 8% 7% 6% 4 4 5% 16%	4004
January	1st to 10th	Night	19% 16% 7% 14% 7% 5% 5% 4 5% 5% 13%	6306
	11th to 20th	Night	13% 12% 3 8% 4 4 4 4 7% 10% 31%	6320
	21st to end	Night	14% 12% 5% 12% 6% 4 4 3 6% 9% 25%	6952
February	1st to 10th	Night	15% 13% 4 8% 6% 5% 5% 4 5% 7% 29%	2640
	11th to 20th	Night	11% 11% 4 9% 6% 4 4 4 6% 7% 34%	2461
	21st to end	Night	11% 10% 3 8% 6% 6% 5% 4 5% 6% 35%	1980
March	1st to 10th	Night	7% 8% 3 7% 5% 5% 5% 5% 7% 9% 38%	5376
	11th to 20th	Night	9% 9% 3 7% 4 4 4 4 5% 6% 44%	5230
	21st to end	Night	7% 9% 4 10% 7% 7% 7% 5% 9% 7% 28%	5053
April	1st to 10th	Night	10% 11% 3 7% 6% 5% 5% 5% 5% 8% 37%	2000
	11th to 20th	Night	8% 8% 3 7% 5% 4 4 3 5% 7% 48%	1979
	21st to end	Night	7% 6% 2 6% 4 5% 5% 4 6% 8% 47%	1854
May	1st to 10th	Night	6% 4 2 5% 4 4 5% 3 4 6% 57%	3645
	11th to 20th	Night	6% 6% 2 5% 3 4 4 3 5% 7% 54%	3616
	21st to end	Night	5% 5% 1 4 4 3 4 4 6% 8% 55%	3972
June	1st to 10th	Night	9% 4 2 4 5% 4 4 4 7% 8% 49%	1600
	11th to 20th	Night	7% 7% 2 5% 4 4 4 3 4 6% 54%	1600
	21st to end	Night	4 3 1 4 3 4 4 3 5% 5% 64%	1600
July	1st to 10th	Night	2 2 1 3 1 2 3 2 4 5% 76%	3640
	11th to 20th	Night	1 1 1 1 1 2 2 3 4 84%	3640
	21st to end	Night	3 2 1 2 1 1 1 2 3 6% 78%	3970
August	1st to 10th	Night	3 2 1 4 3 3 4 3 5% 7% 64%	3582
	11th to 20th	Night	2 2 1 3 2 3 3 3 4 5% 73%	4045
	21st to end	Night	3 3 2 5% 2 3 3 3 5% 6% 65%	5015
September	1st to 10th	Night	3 2 1 3 2 2 3 2 4 5% 72%	2000
	11th to 20th	Night	3 4 2 4 3 4 4 3 5% 7% 61%	2041
	21st to end	Night	5% 4 2 5% 3 3 4 3 5% 7% 60%	2200
October	1st to 10th	Night	5% 3 1 4 3 3 3 2 4 5% 67%	3882
	11th to 20th	Night	6% 5% 1 5% 4 5% 5% 4 7% 8% 51%	3913
	21st to end	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 42%	4600
November	1st to 10th	Night	14% 9% 3 9% 5% 5% 5% 4 5% 7% 35%	2665
	11th to 20th	Night	16% 11% 4 8% 6% 5% 4 4 6% 7% 29%	2866
	21st to end	Night	14% 11% 3 8% 5% 6% 6% 4 6% 7% 29%	2940
December	1st to 10th	Night	9% 9% 5% 10% 6% 5% 6% 4 8% 10% 29%	4700
	11th to 20th	Night	20% 18% 5% 13% 6% 6% 6% 5% 5% 6% 11%	5274
	21st to end	Night	17% 14% 5% 9% 6% 5% 5% 4 5% 6% 21%	6952

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	70% 9% 11% 7% 2	3639
	11th to 20th	Day	54% 10% 17% 16% 3	3640
	21st to end	Day	58% 10% 10% 16% 6%	4004
February	1st to 10th	Day	60% 9% 16% 11% 4	1755
	11th to 20th	Day	53% 9% 16% 16% 7%	1834
	21st to end	Day	49% 9% 16% 16% 8% 1	1650
March	1st to 10th	Day	42% 6% 13% 24% 15%	4518
	11th to 20th	Day	38% 5% 13% 31% 12%	4766
	21st to end	Day	48% 9% 17% 19% 7%	5921
April	1st to 10th	Day	39% 8% 16% 23% 13% 1	2400
	11th to 20th	Day	33% 6% 16% 25% 17% 4	2372
	21st to end	Day	29% 7% 14% 28% 18% 2	2545
May	1st to 10th	Day	24% 4 8% 28% 30% 6%	6313
	11th to 20th	Day	27% 5% 10% 28% 24% 7%	6280
	21st to end	Day	25% 6% 16% 30% 21% 3	6904
June	1st to 10th	Day	32% 6% 11% 26% 20% 5%	2800
	11th to 20th	Day	25% 5% 11% 27% 25% 7%	2800
	21st to end	Day	17% 4 9% 31% 28% 12%	2800
July	1st to 10th	Day	11% 2 4 15% 31% 38%	6251
	11th to 20th	Day	8% 2 3 10% 32% 45%	6245
	21st to end	Day	9% 2 6% 24% 35% 24%	6681
August	1st to 10th	Day	14% 2 6% 15% 34% 28%	4524
	11th to 20th	Day	12% 3 6% 35% 31% 14%	4189
	21st to end	Day	18% 3 8% 28% 32% 12%	4311
September	1st to 10th	Day	12% 4 8% 28% 31% 16%	2400
	11th to 20th	Day	23% 6% 11% 26% 25% 9%	2357
	21st to end	Day	27% 5% 13% 33% 18% 5%	2200
October	1st to 10th	Day	23% 3 11% 36% 23% 4	3316
	11th to 20th	Day	30% 7% 21% 27% 14% 1	3242
	21st to end	Day	42% 8% 13% 27% 10% 1	3200
November	1st to 10th	Day	50% 9% 16% 19% 6% 1	1854
	11th to 20th	Day	60% 9% 13% 14% 3	1753
	21st to end	Day	58% 10% 17% 11% 4	1680
December	1st to 10th	Day	60% 10% 14% 13% 2	2700
	11th to 20th	Day	74% 7% 10% 8% 1	3084
	21st to end	Day	69% 8% 11% 11% 1	4004
January	1st to 10th	Night	73% 9% 10% 7% 1	6306
	11th to 20th	Night	47% 10% 19% 19% 4	6320
	21st to end	Night	54% 10% 12% 16% 8%	6952
February	1st to 10th	Night	55% 9% 18% 15% 3	2640
	11th to 20th	Night	49% 9% 15% 20% 6%	2463
	21st to end	Night	48% 8% 15% 19% 8% 1	1980
March	1st to 10th	Night	38% 7% 12% 26% 16% 1	5376
	11th to 20th	Night	39% 5% 12% 31% 12%	5230
	21st to end	Night	47% 11% 15% 22% 4	5053
April	1st to 10th	Night	43% 7% 16% 23% 12%	2000
	11th to 20th	Night	36% 5% 12% 27% 15% 4	1979
	21st to end	Night	32% 8% 14% 24% 17% 4	1854
May	1st to 10th	Night	27% 5% 9% 21% 33% 5%	3642
	11th to 20th	Night	28% 4 12% 32% 20% 3	3616
	21st to end	Night	24% 7% 19% 32% 15% 2	3972
June	1st to 10th	Night	31% 7% 12% 26% 18% 6%	1600
	11th to 20th	Night	30% 6% 10% 29% 21% 4	1600
	21st to end	Night	20% 5% 8% 27% 26% 13%	1600
July	1st to 10th	Night	12% 3 4 20% 25% 37%	3600
	11th to 20th	Night	6% 2 4 11% 32% 46%	3592
	21st to end	Night	10% 1 5% 24% 35% 26%	3857
August	1st to 10th	Night	16% 3 5% 13% 33% 30%	2601
	11th to 20th	Night	12% 4 6% 37% 26% 15%	2919
	21st to end	Night	15% 4 9% 25% 30% 17%	3608
September	1st to 10th	Night	15% 3 9% 25% 29% 19%	2000
	11th to 20th	Night	22% 5% 10% 25% 24% 14%	2041
	21st to end	Night	23% 6% 10% 28% 26% 7%	2200
October	1st to 10th	Night	21% 4 11% 34% 28% 2	3882
	11th to 20th	Night	29% 7% 17% 25% 20% 2	3913
	21st to end	Night	37% 6% 12% 30% 13% 1	4600
November	1st to 10th	Night	49% 7% 13% 24% 7%	2665
	11th to 20th	Night	54% 9% 14% 19% 4	2866
	21st to end	Night	51% 9% 19% 16% 5%	2940
December	1st to 10th	Night	46% 15% 18% 17% 3	4700
	11th to 20th	Night	73% 9% 11% 5% 1	5274
	21st to end	Night	62% 9% 15% 11% 3	6952

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	23% 18% 23% 21% 13% 4	200
	11th to 20th	Day	25% 13% 14% 26% 20% 4	200
	21st to end	Day	22% 6% 17% 29% 23% 3	220
February	1st to 10th	Day	22% 14% 21% 25% 13% 5%	355
	11th to 20th	Day	22% 13% 23% 22% 16% 4	430
	21st to end	Day	14% 14% 26% 25% 16% 4	414
March	1st to 10th	Day	16% 13% 27% 23% 16% 4	500
	11th to 20th	Day	14% 12% 22% 29% 16% 8%	500
	21st to end	Day	13% 13% 29% 28% 15% 3	550
April	1st to 10th	Day	13% 16% 25% 22% 15% 8%	500
	11th to 20th	Day	9% 15% 32% 22% 15% 7%	492
	21st to end	Day	6% 13% 27% 27% 18% 8%	500
May	1st to 10th	Day	10% 8% 24% 25% 25% 8%	503
	11th to 20th	Day	12% 9% 25% 22% 20% 13%	528
	21st to end	Day	9% 6% 22% 24% 24% 15%	573
June	1st to 10th	Day	10% 8% 19% 24% 23% 16%	530
	11th to 20th	Day	8% 12% 28% 27% 18% 8%	530
	21st to end	Day	3 8% 18% 26% 28% 17%	530
July	1st to 10th	Day	2 3 15% 31% 38% 11%	526
	11th to 20th	Day	2 5% 15% 32% 30% 16%	524
	21st to end	Day	1 3 13% 24% 35% 23%	575
August	1st to 10th	Day	5% 4 13% 24% 32% 23%	527
	11th to 20th	Day	2 3 12% 26% 26% 31%	517
	21st to end	Day	7% 5% 15% 26% 28% 19%	583
September	1st to 10th	Day	3 4 12% 29% 32% 21%	530
	11th to 20th	Day	5% 6% 17% 27% 28% 18%	530
	21st to end	Day	8% 8% 16% 27% 28% 13%	530
October	1st to 10th	Day	11% 7% 19% 25% 25% 13%	530
	11th to 20th	Day	10% 12% 17% 26% 23% 12%	523
	21st to end	Day	15% 11% 21% 34% 12% 7%	363
November	1st to 10th	Day	17% 12% 23% 28% 15% 4	330
	11th to 20th	Day	21% 12% 26% 21% 15% 5%	258
	21st to end	Day	12% 7% 25% 31% 19% 6%	220
December	1st to 10th	Day	15% 10% 24% 25% 13% 13%	220
	11th to 20th	Day	22% 11% 24% 30% 10% 2	214
	21st to end	Day	24% 8% 22% 29% 14% 3	220
January	1st to 10th	Night	26% 10% 22% 26% 14% 2	800
	11th to 20th	Night	20% 7% 19% 35% 18% 1	800
	21st to end	Night	17% 6% 18% 36% 21% 2	880
February	1st to 10th	Night	19% 9% 22% 33% 14% 2	800
	11th to 20th	Night	15% 10% 21% 34% 18% 2	784
	21st to end	Night	16% 13% 23% 31% 15% 1	660
March	1st to 10th	Night	14% 8% 24% 34% 17% 2	800
	11th to 20th	Night	11% 9% 24% 37% 19% 2	800
	21st to end	Night	13% 10% 31% 32% 13% 1	880
April	1st to 10th	Night	15% 8% 26% 32% 18% 2	800
	11th to 20th	Night	10% 11% 29% 28% 18% 3	791
	21st to end	Night	7% 9% 29% 34% 18% 2	654
May	1st to 10th	Night	7% 6% 28% 35% 20% 3	405
	11th to 20th	Night	10% 7% 21% 34% 24% 4	394
	21st to end	Night	6% 4 23% 42% 21% 3	433
June	1st to 10th	Night	9% 6% 28% 35% 20% 2	400
	11th to 20th	Night	10% 8% 36% 31% 14% 1	400
	21st to end	Night	5% 6% 24% 38% 26% 1	400
July	1st to 10th	Night	3 3 19% 46% 27% 1	390
	11th to 20th	Night	3 2 24% 46% 23% 1	388
	21st to end	Night	1 1 20% 46% 30% 2	438
August	1st to 10th	Night	3 2 16% 46% 29% 4	398
	11th to 20th	Night	4 2 12% 38% 35% 9%	584
	21st to end	Night	3 3 17% 40% 32% 6%	880
September	1st to 10th	Night	4 2 17% 38% 35% 5%	800
	11th to 20th	Night	5% 3 15% 36% 35% 7%	800
	21st to end	Night	6% 4 19% 31% 33% 7%	800
October	1st to 10th	Night	11% 4 18% 38% 27% 4	800
	11th to 20th	Night	12% 7% 19% 32% 24% 6%	800
	21st to end	Night	12% 6% 24% 35% 18% 5%	880
November	1st to 10th	Night	18% 7% 22% 35% 17% 1	820
	11th to 20th	Night	20% 9% 23% 31% 14% 3	840
	21st to end	Night	17% 7% 19% 35% 19% 3	840
December	1st to 10th	Night	18% 7% 18% 35% 18% 4	840
	11th to 20th	Night	22% 8% 20% 35% 14% 2	828
	21st to end	Night	25% 8% 23% 30% 13% 1	880

Page generated at 2025-12-14 22:13:50. Version ac07

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