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 2024-11-11.

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%	105368
	Night	9% 8% 3 7% 5% 4 4 4 5% 6% 44%	102458
Transparency	Day	33% 6% 11% 22% 18% 10%	105114
	Night	39% 7% 12% 21% 14% 7%	102301
Seeing	Day	10% 9% 20% 26% 23% 12%	14439
	Night	13% 7% 22% 35% 21% 3	23503

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	16% 19% 7% 14% 6% 5% 4 4 5% 6% 15%	2539
	11th to 20th	Day	22% 18% 5% 8% 6% 5% 4 3 5% 5% 19%	2540
	21st to end	Day	22% 18% 7% 15% 8% 5% 5% 3 3 3 11%	2794
February	1st to 10th	Day	13% 14% 5% 11% 7% 5% 4 4 6% 6% 24%	1667
	11th to 20th	Day	12% 14% 4 9% 5% 5% 5% 3 5% 6% 31%	1731
	21st to end	Day	10% 10% 4 9% 7% 5% 5% 6% 7% 8% 30%	1570
March	1st to 10th	Day	8% 9% 3 7% 5% 5% 5% 5% 8% 10% 35%	3138
	11th to 20th	Day	6% 6% 2 5% 4 4 4 3 4 7% 54%	3321
	21st to end	Day	9% 7% 4 10% 7% 6% 6% 5% 7% 8% 32%	4130
April	1st to 10th	Day	8% 9% 3 6% 6% 5% 5% 5% 7% 8% 39%	2280
	11th to 20th	Day	7% 7% 3 5% 4 4 4 4 5% 8% 47%	2249
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 50%	2418
May	1st to 10th	Day	9% 5% 2 5% 3 3 4 3 6% 8% 52%	4413
	11th to 20th	Day	4 3 1 4 3 3 3 3 4 7% 65%	4377
	21st to end	Day	6% 6% 2 6% 4 4 5% 4 6% 8% 48%	4813
June	1st to 10th	Day	9% 6% 2 5% 4 3 4 4 6% 7% 50%	2660
	11th to 20th	Day	7% 4 2 4 3 4 4 4 5% 6% 57%	2660
	21st to end	Day	3 3 1 3 2 2 3 2 5% 6% 68%	2660
July	1st to 10th	Day	1 1 1 2 1 1 2 2 2 3 84%	4420
	11th to 20th	Day	1 1 3 1 1 2 2 3 4 82%	4420
	21st to end	Day	1 1 1 2 2 2 2 2 3 4 81%	4790
August	1st to 10th	Day	3 2 1 3 2 2 2 2 4 5% 73%	4400
	11th to 20th	Day	2 1 1 2 2 2 2 2 5% 6% 75%	4017
	21st to end	Day	3 4 1 4 2 3 3 2 4 5% 70%	4180
September	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 74%	2280
	11th to 20th	Day	3 4 2 6% 3 4 4 3 5% 6% 59%	2239
	21st to end	Day	6% 5% 2 5% 4 3 4 3 4 7% 57%	2090
October	1st to 10th	Day	6% 4 1 4 3 3 3 2 5% 6% 63%	3215
	11th to 20th	Day	4 5% 2 6% 5% 4 6% 4 8% 10% 46%	3143
	21st to end	Day	10% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	3101
November	1st to 10th	Day	13% 10% 3 9% 7% 5% 6% 5% 5% 7% 31%	1764
	11th to 20th	Day	14% 14% 5% 10% 5% 6% 6% 4 5% 6% 25%	1587
	21st to end	Day	11% 14% 5% 13% 6% 5% 6% 5% 7% 6% 22%	1520
December	1st to 10th	Day	12% 13% 5% 13% 9% 6% 6% 4 6% 5% 22%	1520
	11th to 20th	Day	15% 18% 8% 16% 9% 5% 5% 3 5% 4 12%	1928
	21st to end	Day	15% 15% 6% 15% 8% 7% 6% 4 4 5% 14%	2794
January	1st to 10th	Night	17% 16% 6% 12% 6% 5% 5% 4 5% 6% 16%	4406
	11th to 20th	Night	19% 16% 4 9% 5% 4 4 4 6% 8% 20%	4420
	21st to end	Night	18% 15% 6% 15% 7% 5% 5% 4 5% 5% 15%	4862
February	1st to 10th	Night	14% 13% 4 8% 6% 5% 5% 4 5% 7% 30%	2508
	11th to 20th	Night	11% 11% 4 8% 6% 4 4 4 6% 7% 33%	2335
	21st to end	Night	11% 10% 4 8% 6% 6% 5% 4 5% 6% 35%	1884
March	1st to 10th	Night	8% 9% 2 7% 5% 5% 5% 5% 7% 10% 38%	3736
	11th to 20th	Night	7% 6% 2 5% 4 4 4 4 6% 5% 54%	3655
	21st to end	Night	7% 8% 3 9% 7% 7% 7% 5% 9% 7% 29%	3522
April	1st to 10th	Night	10% 11% 3 7% 6% 4 4 5% 4 8% 37%	1900
	11th to 20th	Night	8% 8% 3 7% 5% 4 4 3 5% 7% 46%	1879
	21st to end	Night	7% 6% 2 7% 4 5% 5% 4 6% 8% 45%	1762
May	1st to 10th	Night	8% 5% 3 6% 4 4 5% 4 5% 6% 52%	2545
	11th to 20th	Night	5% 4 1 4 3 4 4 3 6% 7% 60%	2516
	21st to end	Night	4 5% 2 5% 5% 4 5% 4 6% 8% 51%	2762
June	1st to 10th	Night	9% 5% 2 5% 5% 4 4 4 7% 8% 47%	1520
	11th to 20th	Night	7% 8% 2 5% 4 4 4 3 4 6% 55%	1520
	21st to end	Night	4 3 1 4 3 3 4 3 5% 5% 64%	1520
July	1st to 10th	Night	1 1 1 2 1 1 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 1 1 2 2 2 3 4 82%	2540
	21st to end	Night	2 1 1 2 1 1 1 2 3 4 82%	2761
August	1st to 10th	Night	3 2 1 3 3 3 3 3 3 6% 70%	2529
	11th to 20th	Night	2 2 1 3 2 2 3 2 3 5% 75%	2805
	21st to end	Night	1 3 1 4 2 2 3 3 5% 5% 71%	3498
September	1st to 10th	Night	3 2 1 3 2 2 3 2 4 4 74%	1900
	11th to 20th	Night	4 4 2 4 3 4 4 3 5% 7% 60%	1939
	21st to end	Night	5% 4 2 4 3 3 4 3 5% 8% 59%	2090
October	1st to 10th	Night	5% 3 2 4 3 3 3 2 4 5% 67%	3763
	11th to 20th	Night	6% 5% 1 5% 4 5% 4 4 7% 8% 51%	3793
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 7% 9% 42%	4457
November	1st to 10th	Night	14% 9% 3 9% 5% 5% 4 4 5% 7% 36%	2535
	11th to 20th	Night	16% 11% 3 8% 6% 5% 4 5% 6% 7% 30%	2598
	21st to end	Night	13% 11% 3 8% 5% 6% 6% 4 6% 7% 30%	2660
December	1st to 10th	Night	14% 14% 5% 12% 7% 6% 4 4 5% 5% 24%	2660
	11th to 20th	Night	17% 18% 6% 13% 7% 6% 5% 4 5% 6% 13%	3276
	21st to end	Night	14% 15% 6% 10% 7% 6% 5% 5% 5% 6% 20%	4862

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	68% 9% 11% 10% 2	2539
	11th to 20th	Day	66% 8% 14% 10% 1	2540
	21st to end	Day	76% 7% 9% 7% 1	2794
February	1st to 10th	Day	59% 9% 17% 11% 4	1667
	11th to 20th	Day	52% 9% 16% 17% 7%	1740
	21st to end	Day	49% 9% 16% 16% 8% 1	1570
March	1st to 10th	Day	40% 7% 13% 26% 13%	3138
	11th to 20th	Day	30% 5% 16% 36% 12%	3321
	21st to end	Day	46% 10% 17% 18% 8% 1	4130
April	1st to 10th	Day	40% 8% 16% 23% 13% 1	2280
	11th to 20th	Day	34% 6% 16% 24% 16% 4	2252
	21st to end	Day	30% 7% 15% 27% 18% 2	2418
May	1st to 10th	Day	28% 4 11% 31% 22% 4	4413
	11th to 20th	Day	19% 4 8% 30% 29% 9%	4380
	21st to end	Day	30% 7% 19% 28% 14% 2	4814
June	1st to 10th	Day	33% 6% 11% 27% 19% 5%	2660
	11th to 20th	Day	26% 5% 12% 28% 24% 6%	2660
	21st to end	Day	16% 4 8% 30% 29% 12%	2660
July	1st to 10th	Day	8% 2 4 13% 27% 46%	4351
	11th to 20th	Day	9% 2 4 8% 25% 52%	4345
	21st to end	Day	8% 2 6% 16% 36% 32%	4639
August	1st to 10th	Day	14% 2 6% 15% 34% 29%	4384
	11th to 20th	Day	11% 3 5% 35% 32% 14%	4059
	21st to end	Day	18% 3 8% 28% 33% 11%	4179
September	1st to 10th	Day	12% 3 8% 29% 32% 16%	2280
	11th to 20th	Day	23% 6% 12% 26% 24% 9%	2239
	21st to end	Day	27% 5% 13% 33% 17% 4	2090
October	1st to 10th	Day	22% 4 11% 36% 23% 4	3215
	11th to 20th	Day	30% 7% 21% 27% 14% 1	3143
	21st to end	Day	42% 8% 13% 27% 10% 1	3101
November	1st to 10th	Day	49% 10% 16% 19% 6% 1	1764
	11th to 20th	Day	60% 9% 13% 15% 3	1587
	21st to end	Day	58% 10% 16% 11% 4	1520
December	1st to 10th	Day	62% 9% 13% 13% 3	1520
	11th to 20th	Day	75% 7% 10% 7% 2	1928
	21st to end	Day	71% 9% 10% 9% 1	2794
January	1st to 10th	Night	67% 10% 13% 10% 2	4406
	11th to 20th	Night	62% 9% 15% 12% 2	4420
	21st to end	Night	68% 12% 11% 8% 1	4862
February	1st to 10th	Night	54% 9% 19% 15% 3	2508
	11th to 20th	Night	49% 9% 15% 21% 6%	2337
	21st to end	Night	49% 8% 15% 19% 7% 1	1884
March	1st to 10th	Night	39% 7% 14% 27% 13% 1	3736
	11th to 20th	Night	30% 6% 13% 37% 14%	3655
	21st to end	Night	45% 13% 17% 20% 4	3522
April	1st to 10th	Night	44% 7% 15% 22% 11%	1900
	11th to 20th	Night	38% 6% 13% 26% 14% 4	1879
	21st to end	Night	33% 8% 15% 23% 16% 5%	1762
May	1st to 10th	Night	31% 6% 10% 27% 23% 3	2545
	11th to 20th	Night	20% 5% 12% 34% 25% 4	2516
	21st to end	Night	27% 9% 21% 31% 10% 2	2762
June	1st to 10th	Night	33% 7% 12% 26% 17% 4	1520
	11th to 20th	Night	30% 6% 10% 30% 20% 4	1520
	21st to end	Night	19% 5% 8% 28% 26% 13%	1520
July	1st to 10th	Night	8% 2 5% 15% 26% 45%	2500
	11th to 20th	Night	6% 3 5% 11% 23% 52%	2492
	21st to end	Night	9% 1 4 15% 34% 36%	2674
August	1st to 10th	Night	16% 3 5% 12% 33% 31%	2521
	11th to 20th	Night	12% 3 6% 37% 27% 15%	2829
	21st to end	Night	14% 4 9% 26% 30% 16%	3498
September	1st to 10th	Night	14% 3 8% 24% 30% 20%	1900
	11th to 20th	Night	23% 5% 10% 24% 23% 15%	1939
	21st to end	Night	23% 6% 11% 28% 26% 7%	2090
October	1st to 10th	Night	21% 3 11% 34% 29% 3	3763
	11th to 20th	Night	29% 7% 17% 25% 20% 2	3793
	21st to end	Night	38% 6% 12% 30% 12% 1	4457
November	1st to 10th	Night	48% 7% 13% 25% 7%	2535
	11th to 20th	Night	53% 9% 15% 19% 4	2598
	21st to end	Night	50% 9% 18% 17% 5%	2660
December	1st to 10th	Night	61% 9% 13% 14% 3	2660
	11th to 20th	Night	71% 10% 11% 7% 1	3276
	21st to end	Night	64% 9% 13% 10% 4	4862

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% 21% 19% 14% 4	180
	11th to 20th	Day	27% 13% 12% 28% 17% 4	180
	21st to end	Day	24% 7% 17% 30% 21% 2	198
February	1st to 10th	Day	21% 14% 20% 26% 14% 5%	327
	11th to 20th	Day	20% 13% 22% 23% 17% 5%	396
	21st to end	Day	14% 15% 26% 24% 16% 4	382
March	1st to 10th	Day	17% 13% 27% 22% 16% 4	460
	11th to 20th	Day	13% 11% 21% 30% 16% 9%	460
	21st to end	Day	12% 13% 28% 29% 15% 3	506
April	1st to 10th	Day	13% 17% 25% 23% 14% 8%	460
	11th to 20th	Day	10% 15% 31% 22% 14% 8%	452
	21st to end	Day	7% 14% 28% 26% 17% 8%	460
May	1st to 10th	Day	10% 9% 25% 25% 24% 8%	463
	11th to 20th	Day	11% 7% 24% 23% 21% 14%	488
	21st to end	Day	10% 6% 22% 24% 24% 14%	529
June	1st to 10th	Day	11% 9% 19% 22% 23% 16%	490
	11th to 20th	Day	9% 11% 26% 27% 18% 8%	490
	21st to end	Day	3 8% 18% 25% 29% 17%	490
July	1st to 10th	Day	2 3 15% 29% 40% 10%	486
	11th to 20th	Day	2 5% 15% 33% 30% 15%	484
	21st to end	Day	1 3 14% 23% 34% 24%	531
August	1st to 10th	Day	4 4 11% 24% 33% 23%	487
	11th to 20th	Day	2 3 10% 26% 27% 32%	477
	21st to end	Day	6% 6% 16% 25% 28% 19%	539
September	1st to 10th	Day	3 4 13% 29% 32% 20%	490
	11th to 20th	Day	5% 6% 17% 26% 29% 17%	490
	21st to end	Day	8% 8% 16% 28% 28% 12%	490
October	1st to 10th	Day	11% 7% 19% 25% 25% 13%	490
	11th to 20th	Day	9% 11% 16% 26% 24% 13%	484
	21st to end	Day	15% 11% 20% 34% 12% 8%	330
November	1st to 10th	Day	18% 11% 22% 29% 15% 4	300
	11th to 20th	Day	20% 12% 25% 22% 16% 6%	212
	21st to end	Day	12% 8% 22% 31% 21% 8%	180
December	1st to 10th	Day	12% 8% 28% 27% 12% 13%	180
	11th to 20th	Day	21% 9% 24% 34% 10% 3	180
	21st to end	Day	23% 7% 22% 30% 15% 4	198
January	1st to 10th	Night	25% 10% 22% 26% 15% 2	760
	11th to 20th	Night	21% 7% 19% 34% 18% 1	760
	21st to end	Night	18% 6% 18% 35% 20% 2	836
February	1st to 10th	Night	19% 9% 21% 33% 15% 2	760
	11th to 20th	Night	15% 10% 21% 33% 19% 3	744
	21st to end	Night	16% 13% 24% 31% 15% 1	628
March	1st to 10th	Night	15% 9% 24% 33% 17% 2	760
	11th to 20th	Night	11% 8% 24% 37% 19% 2	760
	21st to end	Night	13% 10% 31% 32% 13% 1	836
April	1st to 10th	Night	16% 8% 26% 32% 17% 2	760
	11th to 20th	Night	11% 11% 29% 28% 18% 3	751
	21st to end	Night	8% 10% 29% 34% 17% 2	622
May	1st to 10th	Night	8% 6% 29% 34% 21% 3	385
	11th to 20th	Night	10% 6% 20% 34% 25% 4	374
	21st to end	Night	6% 4 24% 42% 20% 3	411
June	1st to 10th	Night	9% 6% 28% 34% 20% 2	380
	11th to 20th	Night	10% 8% 35% 32% 14% 1	380
	21st to end	Night	4 5% 25% 39% 26% 1	380
July	1st to 10th	Night	3 3 20% 46% 26% 1	370
	11th to 20th	Night	4 2 24% 46% 23% 1	368
	21st to end	Night	1 1 20% 46% 28% 2	416
August	1st to 10th	Night	3 2 16% 47% 28% 4	378
	11th to 20th	Night	4 2 12% 37% 36% 9%	554
	21st to end	Night	3 3 18% 39% 31% 6%	836
September	1st to 10th	Night	4 2 18% 38% 34% 5%	760
	11th to 20th	Night	5% 3 15% 35% 35% 7%	760
	21st to end	Night	7% 4 19% 31% 33% 7%	760
October	1st to 10th	Night	11% 4 17% 37% 27% 4	760
	11th to 20th	Night	12% 6% 18% 32% 25% 7%	760
	21st to end	Night	13% 6% 24% 34% 19% 5%	836
November	1st to 10th	Night	18% 6% 22% 35% 17% 1	780
	11th to 20th	Night	20% 9% 23% 31% 15% 3	762
	21st to end	Night	16% 7% 19% 35% 19% 3	760
December	1st to 10th	Night	18% 6% 19% 34% 18% 4	760
	11th to 20th	Night	22% 7% 19% 35% 15% 2	760
	21st to end	Night	25% 8% 22% 31% 13% 1	836

Page generated at 2024-11-08 03:44:46. Version ac07

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