Terrace
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Terrace clear sky chart from 2005-11-06 to 2024-12-02.

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	20% 16% 5% 10% 6% 5% 5% 4 5% 5% 20%	106262
	Night	21% 19% 5% 10% 6% 5% 4 3 4 5% 18%	101928
Transparency	Day	67% 9% 10% 10% 4	105997
	Night	69% 9% 9% 9% 3 1	101839
Seeing	Day	30% 15% 24% 21% 9% 2	16251
	Night	32% 12% 25% 23% 9%	23697

By Month

Percentage Cloud Cover NW (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% 22% 6% 12% 6% 5% 5% 4 5% 3 13%	2064
	11th to 20th	Day	21% 19% 4 9% 6% 5% 5% 3 3 4 22%	2199
	21st to end	Day	34% 25% 6% 8% 4 4 4 3 3 2 7%	2520
February	1st to 10th	Day	21% 19% 5% 11% 4 4 3 3 3 3 22%	1520
	11th to 20th	Day	11% 20% 6% 12% 5% 4 5% 4 4 6% 23%	1587
	21st to end	Day	8% 20% 6% 10% 7% 6% 5% 3 5% 4 25%	1451
March	1st to 10th	Day	12% 19% 5% 11% 6% 7% 5% 4 5% 5% 22%	3126
	11th to 20th	Day	15% 20% 4 9% 7% 7% 6% 4 5% 5% 17%	3448
	21st to end	Day	10% 17% 4 11% 6% 5% 5% 3 4 4 30%	3903
April	1st to 10th	Day	15% 22% 7% 12% 8% 6% 4 3 4 4 15%	2384
	11th to 20th	Day	17% 20% 5% 10% 7% 5% 4 3 5% 4 20%	2438
	21st to end	Day	17% 20% 6% 11% 6% 6% 4 4 4 4 17%	2479
May	1st to 10th	Day	16% 17% 4 11% 7% 6% 5% 4 6% 6% 17%	4730
	11th to 20th	Day	23% 17% 5% 10% 7% 5% 5% 4 5% 5% 15%	4696
	21st to end	Day	20% 16% 6% 12% 7% 6% 5% 3 4 5% 17%	5161
June	1st to 10th	Day	22% 14% 4 11% 7% 6% 5% 4 5% 6% 17%	2902
	11th to 20th	Day	23% 14% 5% 10% 7% 5% 5% 4 4 6% 16%	3040
	21st to end	Day	23% 13% 5% 10% 7% 5% 5% 4 5% 5% 19%	3040
July	1st to 10th	Day	12% 8% 2 7% 5% 5% 5% 4 6% 7% 39%	5060
	11th to 20th	Day	15% 9% 3 8% 6% 5% 6% 5% 7% 9% 28%	4778
	21st to end	Day	22% 12% 4 10% 6% 5% 6% 4 5% 5% 20%	5138
August	1st to 10th	Day	10% 6% 2 5% 4 3 4 3 5% 7% 50%	4580
	11th to 20th	Day	18% 9% 4 9% 6% 4 5% 3 4 6% 32%	4195
	21st to end	Day	26% 13% 5% 12% 6% 5% 4 4 4 5% 17%	4521
September	1st to 10th	Day	29% 14% 4 10% 5% 4 4 3 4 5% 19%	2433
	11th to 20th	Day	25% 13% 4 9% 6% 6% 5% 3 5% 4 21%	2090
	21st to end	Day	36% 19% 4 11% 4 4 4 3 3 3 10%	2090
October	1st to 10th	Day	24% 16% 5% 13% 7% 8% 6% 3 4 4 11%	3287
	11th to 20th	Day	29% 17% 5% 12% 7% 7% 6% 4 4 4 6%	2853
	21st to end	Day	20% 17% 5% 10% 6% 5% 4 3 4 5% 19%	3047
November	1st to 10th	Day	26% 23% 5% 12% 7% 5% 3 3 3 3 10%	1562
	11th to 20th	Day	23% 22% 6% 12% 7% 6% 4 3 4 4 10%	1400
	21st to end	Day	21% 26% 5% 10% 6% 4 4 2 4 5% 12%	1400
December	1st to 10th	Day	18% 21% 6% 12% 5% 5% 4 3 4 4 17%	1337
	11th to 20th	Day	24% 23% 6% 11% 4 5% 4 3 3 3 13%	1685
	21st to end	Day	23% 28% 7% 12% 7% 5% 3 2 2 3 9%	2118
January	1st to 10th	Night	23% 26% 8% 13% 6% 4 3 2 3 3 8%	4875
	11th to 20th	Night	22% 19% 4 7% 4 3 4 2 3 5% 26%	4699
	21st to end	Night	37% 27% 6% 7% 4 3 3 2 2 3 7%	5123
February	1st to 10th	Night	21% 22% 5% 10% 5% 4 4 3 4 3 19%	2660
	11th to 20th	Night	15% 21% 6% 12% 7% 5% 4 3 4 6% 18%	2480
	21st to end	Night	9% 19% 5% 11% 7% 7% 4 3 4 5% 25%	2001
March	1st to 10th	Night	13% 18% 5% 8% 5% 5% 4 4 5% 7% 26%	3738
	11th to 20th	Night	15% 22% 5% 11% 7% 6% 6% 4 5% 6% 13%	3530
	21st to end	Night	10% 16% 4 9% 5% 5% 5% 3 5% 7% 32%	3745
April	1st to 10th	Night	14% 18% 5% 10% 7% 7% 6% 5% 5% 4 18%	1796
	11th to 20th	Night	14% 18% 5% 11% 6% 5% 5% 4 5% 7% 20%	1691
	21st to end	Night	17% 16% 4 11% 7% 5% 5% 3 5% 5% 22%	1699
May	1st to 10th	Night	14% 18% 5% 11% 7% 6% 4 3 5% 7% 19%	2230
	11th to 20th	Night	24% 16% 5% 11% 6% 5% 5% 4 5% 5% 14%	2199
	21st to end	Night	18% 14% 4 10% 7% 5% 5% 4 6% 6% 21%	2414
June	1st to 10th	Night	19% 12% 3 10% 6% 6% 5% 4 5% 6% 24%	1273
	11th to 20th	Night	20% 14% 5% 11% 7% 6% 6% 4 5% 7% 15%	1140
	21st to end	Night	20% 12% 3 10% 7% 5% 6% 5% 6% 6% 21%	1140
July	1st to 10th	Night	13% 8% 2 8% 5% 4 4 3 3 5% 44%	1900
	11th to 20th	Night	14% 11% 4 10% 6% 5% 4 4 6% 6% 29%	2178
	21st to end	Night	17% 13% 4 10% 6% 6% 5% 3 4 6% 26%	2414
August	1st to 10th	Night	10% 6% 2 4 4 3 4 3 4 5% 57%	2352
	11th to 20th	Night	17% 10% 3 9% 5% 5% 4 4 5% 6% 32%	2642
	21st to end	Night	26% 13% 5% 13% 6% 5% 3 3 4 4 19%	3146
September	1st to 10th	Night	25% 12% 4 10% 6% 5% 4 3 4 6% 21%	1743
	11th to 20th	Night	23% 13% 3 8% 5% 4 4 3 4 6% 26%	2090
	21st to end	Night	37% 16% 4 10% 5% 4 3 2 4 3 11%	2090
October	1st to 10th	Night	26% 18% 4 11% 6% 5% 4 4 4 5% 13%	3693
	11th to 20th	Night	28% 19% 6% 13% 7% 6% 5% 3 4 3 7%	4072
	21st to end	Night	24% 20% 5% 10% 6% 4 4 3 4 4 16%	4509
November	1st to 10th	Night	31% 23% 5% 10% 6% 4 3 2 2 3 10%	2735
	11th to 20th	Night	21% 27% 6% 13% 7% 4 3 3 3 3 9%	3000
	21st to end	Night	22% 26% 5% 11% 6% 4 3 3 4 4 10%	3000
December	1st to 10th	Night	17% 23% 6% 11% 5% 5% 4 3 5% 5% 18%	2877
	11th to 20th	Night	24% 26% 6% 10% 5% 4 3 3 3 3 13%	3520
	21st to end	Night	24% 27% 6% 12% 6% 4 4 2 3 3 9%	5534

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	71% 11% 9% 8% 2	2064
	11th to 20th	Day	67% 7% 7% 6% 6% 8%	2220
	21st to end	Day	84% 6% 5% 3 2	2520
February	1st to 10th	Day	69% 7% 7% 7% 8% 2	1520
	11th to 20th	Day	65% 10% 9% 11% 4 1	1595
	21st to end	Day	62% 8% 10% 13% 4 2	1451
March	1st to 10th	Day	63% 9% 9% 11% 7% 1	3126
	11th to 20th	Day	67% 13% 9% 10% 2	3448
	21st to end	Day	58% 10% 9% 19% 4	3903
April	1st to 10th	Day	75% 9% 8% 7% 2	2384
	11th to 20th	Day	72% 6% 7% 10% 4	2438
	21st to end	Day	73% 7% 8% 9% 3	2479
May	1st to 10th	Day	62% 15% 12% 8% 2	4730
	11th to 20th	Day	69% 14% 9% 6% 1	4699
	21st to end	Day	70% 11% 9% 9% 1	5162
June	1st to 10th	Day	72% 8% 10% 7% 3	2902
	11th to 20th	Day	74% 7% 10% 8% 1	3040
	21st to end	Day	71% 9% 12% 8% 1	3040
July	1st to 10th	Day	47% 7% 14% 22% 10%	4981
	11th to 20th	Day	54% 13% 22% 10% 1	4695
	21st to end	Day	65% 9% 12% 12% 3	4976
August	1st to 10th	Day	37% 8% 14% 21% 17% 2	4562
	11th to 20th	Day	57% 9% 10% 18% 6%	4237
	21st to end	Day	71% 9% 14% 5% 1	4520
September	1st to 10th	Day	71% 9% 11% 9% 1	2433
	11th to 20th	Day	67% 8% 12% 10% 3	2090
	21st to end	Day	83% 6% 5% 5% 2	2090
October	1st to 10th	Day	79% 7% 8% 5% 2	3287
	11th to 20th	Day	84% 7% 6% 3	2853
	21st to end	Day	69% 10% 11% 8% 3	3047
November	1st to 10th	Day	83% 5% 5% 4 2	1562
	11th to 20th	Day	82% 6% 5% 3 4	1400
	21st to end	Day	78% 5% 5% 8% 4 1	1400
December	1st to 10th	Day	70% 9% 10% 9% 2 1	1337
	11th to 20th	Day	79% 6% 5% 6% 4 1	1688
	21st to end	Day	80% 10% 3 3 3	2118
January	1st to 10th	Night	81% 7% 5% 5% 2	4875
	11th to 20th	Night	63% 6% 7% 8% 8% 8%	4740
	21st to end	Night	86% 5% 4 4 2	5123
February	1st to 10th	Night	71% 7% 4 8% 9% 1	2660
	11th to 20th	Night	70% 6% 9% 9% 5% 1	2483
	21st to end	Night	62% 8% 11% 12% 5% 2	2001
March	1st to 10th	Night	56% 9% 11% 15% 8% 1	3738
	11th to 20th	Night	69% 14% 9% 7% 1	3531
	21st to end	Night	53% 10% 12% 22% 3	3745
April	1st to 10th	Night	72% 10% 8% 7% 3	1796
	11th to 20th	Night	64% 10% 12% 10% 3 1	1691
	21st to end	Night	67% 8% 11% 12% 2	1699
May	1st to 10th	Night	63% 15% 11% 10% 2	2230
	11th to 20th	Night	70% 15% 8% 6% 2	2199
	21st to end	Night	65% 12% 11% 9% 2	2414
June	1st to 10th	Night	64% 11% 12% 10% 3	1273
	11th to 20th	Night	70% 11% 11% 7% 1	1140
	21st to end	Night	65% 13% 16% 6% 1	1140
July	1st to 10th	Night	45% 8% 16% 25% 6%	1870
	11th to 20th	Night	57% 12% 23% 8% 1	2138
	21st to end	Night	60% 9% 14% 15% 2	2337
August	1st to 10th	Night	31% 9% 16% 32% 11%	2345
	11th to 20th	Night	55% 12% 14% 15% 5%	2666
	21st to end	Night	72% 9% 12% 6% 1	3146
September	1st to 10th	Night	67% 9% 12% 10% 2	1743
	11th to 20th	Night	61% 8% 13% 14% 3	2090
	21st to end	Night	82% 6% 5% 5% 2	2090
October	1st to 10th	Night	73% 9% 10% 7% 2	3693
	11th to 20th	Night	84% 5% 7% 3 1	4072
	21st to end	Night	70% 9% 10% 8% 3	4509
November	1st to 10th	Night	83% 6% 4 5% 2	2735
	11th to 20th	Night	82% 6% 5% 4 4	3000
	21st to end	Night	79% 6% 5% 6% 3	3000
December	1st to 10th	Night	70% 7% 10% 10% 2	2877
	11th to 20th	Night	78% 7% 5% 5% 4 1	3516
	21st to end	Night	78% 11% 4 4 3	5534

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% 19% 29% 17% 7% 1	180
	11th to 20th	Day	33% 24% 26% 12% 4 1	180
	21st to end	Day	37% 12% 27% 17% 6%	250
February	1st to 10th	Day	33% 17% 28% 18% 5%	370
	11th to 20th	Day	25% 12% 36% 19% 7%	365
	21st to end	Day	25% 24% 26% 17% 6% 2	307
March	1st to 10th	Day	22% 22% 25% 19% 10% 2	370
	11th to 20th	Day	28% 20% 25% 19% 7% 1	447
	21st to end	Day	25% 20% 23% 22% 8% 2	506
April	1st to 10th	Day	30% 20% 27% 15% 7% 1	460
	11th to 20th	Day	26% 15% 24% 22% 11% 3	452
	21st to end	Day	29% 17% 25% 17% 9% 2	469
May	1st to 10th	Day	27% 15% 24% 25% 8% 1	653
	11th to 20th	Day	25% 13% 24% 23% 11% 4	675
	21st to end	Day	24% 15% 27% 23% 9% 2	735
June	1st to 10th	Day	32% 18% 21% 23% 6% 1	680
	11th to 20th	Day	29% 13% 25% 23% 8% 3	680
	21st to end	Day	26% 12% 26% 24% 10% 2	680
July	1st to 10th	Day	22% 11% 26% 27% 12% 2	671
	11th to 20th	Day	26% 13% 26% 22% 11% 3	668
	21st to end	Day	29% 14% 22% 20% 12% 3	739
August	1st to 10th	Day	19% 11% 19% 28% 17% 6%	676
	11th to 20th	Day	27% 10% 25% 25% 12% 1	585
	21st to end	Day	34% 15% 24% 17% 8% 2	539
September	1st to 10th	Day	39% 12% 17% 19% 11% 2	490
	11th to 20th	Day	32% 12% 20% 20% 12% 4	490
	21st to end	Day	50% 14% 18% 11% 6% 1	490
October	1st to 10th	Day	37% 11% 20% 18% 12% 2	490
	11th to 20th	Day	42% 13% 21% 14% 9% 2	395
	21st to end	Day	35% 15% 21% 19% 9% 2	399
November	1st to 10th	Day	45% 15% 23% 12% 5% 1	200
	11th to 20th	Day	35% 16% 21% 19% 9% 2	200
	21st to end	Day	43% 12% 19% 16% 11% 2	200
December	1st to 10th	Day	23% 16% 23% 25% 14%	182
	11th to 20th	Day	27% 9% 33% 22% 8% 1	180
	21st to end	Day	32% 15% 22% 19% 13%	198
January	1st to 10th	Night	34% 16% 28% 18% 4	850
	11th to 20th	Night	41% 16% 27% 15% 2	850
	21st to end	Night	38% 14% 26% 18% 5%	935
February	1st to 10th	Night	33% 12% 30% 20% 4	850
	11th to 20th	Night	26% 13% 28% 27% 5%	783
	21st to end	Night	19% 16% 34% 24% 7%	628
March	1st to 10th	Night	25% 14% 29% 22% 9% 1	760
	11th to 20th	Night	26% 17% 29% 23% 6%	760
	21st to end	Night	25% 12% 27% 22% 13% 1	791
April	1st to 10th	Night	26% 15% 26% 23% 10% 1	570
	11th to 20th	Night	24% 14% 23% 29% 10%	563
	21st to end	Night	27% 13% 26% 24% 9%	570
May	1st to 10th	Night	23% 10% 26% 29% 13%	390
	11th to 20th	Night	22% 12% 25% 30% 10% 1	374
	21st to end	Night	19% 9% 28% 32% 11%	411
June	1st to 10th	Night	23% 12% 20% 30% 14%	380
	11th to 20th	Night	24% 9% 24% 31% 12% 1	380
	21st to end	Night	25% 6% 26% 31% 11%	380
July	1st to 10th	Night	20% 8% 21% 39% 12%	370
	11th to 20th	Night	28% 7% 18% 33% 13% 1	368
	21st to end	Night	23% 9% 21% 31% 16% 1	416
August	1st to 10th	Night	18% 7% 16% 29% 30% 1	454
	11th to 20th	Night	24% 7% 19% 31% 19% 1	564
	21st to end	Night	35% 8% 18% 29% 9% 1	627
September	1st to 10th	Night	36% 8% 18% 27% 11%	570
	11th to 20th	Night	32% 7% 16% 23% 19% 2	760
	21st to end	Night	47% 13% 17% 15% 8% 1	760
October	1st to 10th	Night	39% 8% 24% 19% 10%	760
	11th to 20th	Night	44% 12% 19% 16% 8%	760
	21st to end	Night	40% 13% 18% 22% 6%	836
November	1st to 10th	Night	45% 14% 23% 14% 3 1	783
	11th to 20th	Night	36% 15% 26% 19% 4	900
	21st to end	Night	38% 14% 26% 18% 4	900
December	1st to 10th	Night	30% 13% 26% 23% 7%	859
	11th to 20th	Night	30% 14% 30% 20% 5%	850
	21st to end	Night	32% 14% 29% 20% 5%	935

Page generated at 2024-11-29 07:43:09. Version ac07

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