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 2026-03-03.

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	18% 16% 5% 11% 6% 5% 5% 4 5% 5% 20%	135163
	Night	19% 19% 5% 11% 6% 5% 4 3 4 5% 18%	134647
Transparency	Day	67% 9% 11% 10% 3	134854
	Night	69% 8% 10% 10% 3	134534
Seeing	Day	29% 14% 23% 21% 10% 2	17793
	Night	31% 12% 24% 23% 9%	25424

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	13% 23% 7% 13% 7% 5% 5% 4 5% 4 13%	3860
	11th to 20th	Day	16% 14% 4 11% 7% 5% 5% 3 4 6% 25%	4119
	21st to end	Day	23% 23% 8% 10% 5% 4 4 3 3 3 15%	4691
February	1st to 10th	Day	20% 19% 5% 11% 5% 4 3 3 3 4 23%	1680
	11th to 20th	Day	10% 19% 6% 12% 5% 4 5% 4 4 6% 24%	1759
	21st to end	Day	9% 20% 7% 11% 7% 6% 5% 3 5% 4 23%	1599
March	1st to 10th	Day	11% 17% 5% 13% 7% 7% 5% 4 5% 5% 20%	4517
	11th to 20th	Day	11% 20% 5% 13% 8% 7% 6% 4 6% 5% 14%	4944
	21st to end	Day	11% 15% 5% 12% 6% 5% 6% 4 5% 6% 26%	5590
April	1st to 10th	Day	15% 21% 6% 13% 8% 6% 5% 3 4 4 15%	2510
	11th to 20th	Day	17% 19% 5% 10% 7% 5% 4 3 5% 5% 20%	2568
	21st to end	Day	17% 19% 5% 11% 6% 6% 4 4 4 5% 18%	2610
May	1st to 10th	Day	16% 17% 5% 13% 8% 6% 6% 4 6% 6% 13%	6770
	11th to 20th	Day	20% 16% 5% 10% 7% 5% 5% 4 6% 6% 18%	6736
	21st to end	Day	18% 16% 6% 14% 8% 7% 6% 4 5% 4 12%	7405
June	1st to 10th	Day	21% 14% 4 11% 7% 6% 5% 4 5% 6% 19%	3055
	11th to 20th	Day	23% 14% 5% 10% 7% 5% 5% 4 4 6% 17%	3200
	21st to end	Day	23% 13% 5% 10% 7% 5% 5% 4 5% 5% 19%	3200
July	1st to 10th	Day	13% 9% 3 9% 6% 5% 6% 4 6% 8% 30%	7240
	11th to 20th	Day	16% 9% 3 8% 5% 5% 5% 4 6% 9% 30%	6832
	21st to end	Day	16% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	7375
August	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 5% 7% 49%	4726
	11th to 20th	Day	18% 9% 4 9% 6% 4 5% 4 5% 5% 31%	4331
	21st to end	Day	25% 13% 4 11% 6% 5% 4 4 4 6% 18%	4664
September	1st to 10th	Day	28% 13% 4 9% 5% 4 4 3 4 5% 20%	2561
	11th to 20th	Day	25% 13% 4 9% 7% 6% 5% 3 4 4 21%	2200
	21st to end	Day	35% 19% 4 11% 5% 4 4 3 3 3 10%	2200
October	1st to 10th	Day	24% 15% 4 12% 7% 8% 6% 3 4 4 12%	3391
	11th to 20th	Day	29% 16% 5% 12% 7% 7% 6% 4 4 4 7%	2943
	21st to end	Day	20% 17% 5% 10% 7% 5% 4 3 4 5% 19%	3144
November	1st to 10th	Day	26% 23% 5% 13% 7% 5% 3 3 3 3 9%	1641
	11th to 20th	Day	23% 23% 6% 11% 7% 6% 4 3 3 4 9%	1470
	21st to end	Day	20% 25% 6% 11% 6% 5% 4 2 4 5% 13%	1470
December	1st to 10th	Day	21% 24% 8% 15% 5% 4 3 3 3 3 11%	2360
	11th to 20th	Day	18% 21% 7% 15% 6% 5% 4 3 3 4 13%	2715
	21st to end	Day	18% 26% 8% 16% 8% 6% 4 2 3 3 7%	3087
January	1st to 10th	Night	16% 24% 7% 14% 7% 6% 5% 3 4 4 10%	9029
	11th to 20th	Night	17% 17% 4 9% 5% 4 4 3 5% 7% 25%	8779
	21st to end	Night	25% 24% 6% 9% 5% 3 3 2 3 4 16%	9482
February	1st to 10th	Night	20% 22% 5% 10% 5% 4 3 3 4 3 20%	2940
	11th to 20th	Night	14% 20% 6% 12% 6% 5% 4 3 4 5% 21%	2748
	21st to end	Night	9% 20% 6% 11% 7% 7% 4 3 4 5% 24%	2205
March	1st to 10th	Night	12% 19% 5% 9% 6% 6% 5% 5% 6% 6% 22%	5402
	11th to 20th	Night	11% 22% 6% 13% 7% 6% 6% 4 5% 5% 13%	5053
	21st to end	Night	10% 16% 4 11% 5% 5% 5% 4 5% 6% 28%	5364
April	1st to 10th	Night	14% 17% 5% 10% 7% 7% 6% 5% 6% 5% 18%	1890
	11th to 20th	Night	13% 17% 5% 11% 6% 5% 5% 4 5% 7% 21%	1781
	21st to end	Night	17% 15% 4 11% 7% 5% 5% 3 5% 5% 23%	1788
May	1st to 10th	Night	12% 17% 6% 15% 8% 7% 5% 4 5% 6% 15%	3190
	11th to 20th	Night	20% 15% 5% 12% 6% 6% 5% 4 6% 6% 14%	3159
	21st to end	Night	19% 15% 5% 12% 7% 5% 5% 4 6% 5% 15%	3470
June	1st to 10th	Night	18% 12% 3 10% 6% 5% 5% 4 5% 6% 25%	1340
	11th to 20th	Night	20% 14% 5% 11% 7% 6% 6% 4 5% 8% 16%	1200
	21st to end	Night	20% 12% 4 10% 7% 4 6% 5% 6% 6% 21%	1200
July	1st to 10th	Night	15% 10% 4 9% 7% 5% 5% 3 4 5% 33%	2720
	11th to 20th	Night	16% 11% 4 9% 5% 5% 4 4 6% 6% 31%	3124
	21st to end	Night	12% 10% 3 9% 6% 6% 5% 4 5% 8% 33%	3468
August	1st to 10th	Night	11% 6% 2 4 4 4 4 3 4 5% 55%	2426
	11th to 20th	Night	17% 10% 3 9% 6% 5% 4 4 5% 6% 31%	2726
	21st to end	Night	26% 12% 4 12% 6% 4 3 3 4 5% 20%	3245
September	1st to 10th	Night	24% 12% 4 10% 6% 5% 4 3 4 5% 23%	1835
	11th to 20th	Night	23% 13% 3 9% 5% 4 5% 3 4 6% 25%	2200
	21st to end	Night	36% 16% 4 10% 5% 4 4 3 4 3 11%	2200
October	1st to 10th	Night	26% 17% 4 11% 6% 6% 4 4 4 5% 13%	3809
	11th to 20th	Night	27% 19% 6% 13% 7% 6% 5% 3 4 3 8%	4202
	21st to end	Night	24% 20% 5% 11% 6% 4 5% 3 4 4 16%	4654
November	1st to 10th	Night	30% 23% 5% 10% 7% 5% 3 2 2 3 9%	2875
	11th to 20th	Night	21% 27% 6% 13% 7% 4 3 3 3 3 9%	3150
	21st to end	Night	21% 25% 6% 12% 6% 4 3 3 4 4 10%	3150
December	1st to 10th	Night	24% 26% 8% 13% 5% 4 3 2 3 3 11%	5040
	11th to 20th	Night	19% 26% 7% 12% 6% 5% 4 3 3 3 13%	5710
	21st to end	Night	19% 24% 8% 15% 8% 6% 5% 3 3 3 7%	8093

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	75% 7% 8% 8% 2	3860
	11th to 20th	Day	60% 8% 10% 15% 3 4	4140
	21st to end	Day	73% 7% 8% 8% 3	4692
February	1st to 10th	Day	67% 7% 7% 8% 9% 3	1680
	11th to 20th	Day	63% 9% 10% 12% 4 2	1767
	21st to end	Day	63% 8% 10% 13% 4 2	1599
March	1st to 10th	Day	65% 9% 10% 10% 5%	4517
	11th to 20th	Day	69% 10% 9% 9% 3	4944
	21st to end	Day	59% 8% 11% 18% 3	5590
April	1st to 10th	Day	75% 8% 8% 7% 2	2510
	11th to 20th	Day	71% 7% 8% 10% 4	2568
	21st to end	Day	73% 8% 8% 9% 3	2610
May	1st to 10th	Day	67% 13% 11% 7% 2	6770
	11th to 20th	Day	66% 12% 10% 10% 1	6739
	21st to end	Day	76% 8% 8% 7% 1	7406
June	1st to 10th	Day	70% 8% 11% 7% 4	3055
	11th to 20th	Day	72% 7% 10% 9% 1	3200
	21st to end	Day	71% 8% 11% 8% 1	3200
July	1st to 10th	Day	56% 7% 13% 16% 7%	7161
	11th to 20th	Day	53% 11% 19% 16% 1	6749
	21st to end	Day	52% 10% 22% 14% 2	7168
August	1st to 10th	Day	38% 8% 14% 21% 17% 2	4708
	11th to 20th	Day	57% 8% 10% 18% 6%	4373
	21st to end	Day	69% 9% 14% 6% 2	4663
September	1st to 10th	Day	68% 9% 12% 10% 1	2561
	11th to 20th	Day	67% 8% 12% 10% 3	2200
	21st to end	Day	83% 6% 5% 5% 2	2200
October	1st to 10th	Day	78% 7% 8% 5% 2	3391
	11th to 20th	Day	83% 7% 6% 3 1	2943
	21st to end	Day	69% 10% 11% 8% 3	3144
November	1st to 10th	Day	83% 6% 5% 4 2	1641
	11th to 20th	Day	82% 6% 5% 3 3	1470
	21st to end	Day	77% 5% 5% 8% 4 1	1470
December	1st to 10th	Day	76% 8% 10% 5% 1	2360
	11th to 20th	Day	75% 6% 8% 8% 2	2718
	21st to end	Day	81% 9% 4 3 2	3087
January	1st to 10th	Night	76% 6% 7% 7% 3	9029
	11th to 20th	Night	61% 7% 10% 13% 5% 4	8820
	21st to end	Night	72% 7% 8% 9% 3	9482
February	1st to 10th	Night	69% 6% 4 8% 9% 3	2940
	11th to 20th	Night	67% 5% 10% 11% 5% 1	2751
	21st to end	Night	63% 8% 11% 12% 5% 1	2205
March	1st to 10th	Night	59% 8% 12% 15% 6%	5402
	11th to 20th	Night	70% 11% 8% 7% 3	5054
	21st to end	Night	55% 9% 13% 21% 2	5364
April	1st to 10th	Night	71% 10% 9% 7% 3	1890
	11th to 20th	Night	64% 10% 13% 10% 3 1	1781
	21st to end	Night	66% 8% 12% 12% 2	1788
May	1st to 10th	Night	69% 12% 10% 8% 1	3187
	11th to 20th	Night	68% 12% 10% 7% 1	3159
	21st to end	Night	73% 10% 9% 7% 1	3470
June	1st to 10th	Night	64% 11% 13% 10% 3	1340
	11th to 20th	Night	69% 11% 12% 7% 1	1200
	21st to end	Night	65% 12% 15% 7% 1	1200
July	1st to 10th	Night	57% 9% 12% 18% 4	2690
	11th to 20th	Night	56% 10% 24% 10% 1	3084
	21st to end	Night	52% 10% 23% 13% 2	3370
August	1st to 10th	Night	33% 9% 16% 31% 11%	2419
	11th to 20th	Night	56% 11% 14% 15% 4	2750
	21st to end	Night	70% 9% 12% 8% 1	3245
September	1st to 10th	Night	64% 10% 14% 11% 2	1835
	11th to 20th	Night	62% 9% 13% 14% 3	2200
	21st to end	Night	81% 6% 6% 5% 2	2200
October	1st to 10th	Night	72% 9% 10% 7% 2	3809
	11th to 20th	Night	83% 5% 7% 4 1	4202
	21st to end	Night	70% 9% 10% 8% 2	4654
November	1st to 10th	Night	83% 6% 4 5% 2	2875
	11th to 20th	Night	82% 6% 5% 4 4	3150
	21st to end	Night	78% 6% 6% 6% 3	3150
December	1st to 10th	Night	78% 7% 7% 7% 1	5040
	11th to 20th	Night	75% 7% 8% 7% 3	5706
	21st to end	Night	78% 11% 6% 3 2	8093

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% 16% 26% 20% 10% 1	220
	11th to 20th	Day	33% 20% 23% 14% 8% 3	220
	21st to end	Day	35% 11% 24% 19% 10% 1	300
February	1st to 10th	Day	33% 17% 25% 17% 6% 1	430
	11th to 20th	Day	24% 12% 32% 20% 10% 2	425
	21st to end	Day	28% 22% 26% 17% 6% 1	355
March	1st to 10th	Day	22% 20% 24% 20% 10% 3	407
	11th to 20th	Day	26% 19% 24% 19% 9% 2	486
	21st to end	Day	25% 18% 23% 23% 8% 3	550
April	1st to 10th	Day	28% 19% 27% 15% 8% 2	500
	11th to 20th	Day	26% 14% 23% 23% 11% 3	492
	21st to end	Day	28% 17% 25% 17% 9% 3	510
May	1st to 10th	Day	27% 15% 24% 24% 8% 1	703
	11th to 20th	Day	24% 13% 24% 23% 11% 4	725
	21st to end	Day	25% 15% 27% 23% 8% 2	790
June	1st to 10th	Day	31% 18% 22% 22% 6% 1	730
	11th to 20th	Day	28% 13% 26% 22% 8% 3	730
	21st to end	Day	27% 11% 25% 24% 9% 3	730
July	1st to 10th	Day	23% 11% 26% 26% 12% 2	721
	11th to 20th	Day	26% 12% 26% 23% 11% 3	718
	21st to end	Day	28% 14% 23% 20% 12% 4	794
August	1st to 10th	Day	20% 11% 20% 27% 17% 5%	726
	11th to 20th	Day	27% 9% 25% 25% 12% 1	631
	21st to end	Day	32% 14% 22% 16% 9% 7%	583
September	1st to 10th	Day	37% 11% 15% 19% 13% 5%	530
	11th to 20th	Day	33% 12% 19% 21% 12% 4	530
	21st to end	Day	50% 14% 18% 11% 5% 1	530
October	1st to 10th	Day	35% 10% 20% 19% 13% 2	530
	11th to 20th	Day	41% 13% 20% 16% 9% 1	425
	21st to end	Day	37% 14% 20% 19% 9% 2	430
November	1st to 10th	Day	45% 15% 22% 13% 5% 1	220
	11th to 20th	Day	36% 15% 20% 17% 10% 2	220
	21st to end	Day	40% 10% 19% 18% 11% 1	220
December	1st to 10th	Day	31% 15% 19% 23% 12% 1	220
	11th to 20th	Day	26% 8% 30% 25% 10% 1	220
	21st to end	Day	30% 16% 21% 19% 12% 1	242
January	1st to 10th	Night	35% 15% 27% 19% 4	950
	11th to 20th	Night	39% 14% 25% 17% 3	950
	21st to end	Night	37% 13% 24% 19% 7%	1045
February	1st to 10th	Night	33% 12% 29% 20% 5%	950
	11th to 20th	Night	25% 13% 27% 27% 8%	871
	21st to end	Night	20% 15% 34% 24% 7%	692
March	1st to 10th	Night	26% 13% 28% 22% 10% 1	808
	11th to 20th	Night	26% 16% 27% 23% 7%	800
	21st to end	Night	25% 11% 28% 23% 13% 1	832
April	1st to 10th	Night	25% 15% 25% 25% 10% 1	600
	11th to 20th	Night	24% 13% 23% 30% 10%	593
	21st to end	Night	26% 13% 25% 26% 10%	600
May	1st to 10th	Night	22% 10% 25% 30% 13%	410
	11th to 20th	Night	22% 11% 24% 31% 11% 1	394
	21st to end	Night	21% 9% 28% 31% 11%	433
June	1st to 10th	Night	23% 12% 21% 32% 13%	400
	11th to 20th	Night	24% 9% 24% 31% 13% 1	400
	21st to end	Night	25% 6% 26% 31% 12%	400
July	1st to 10th	Night	21% 7% 21% 38% 13%	390
	11th to 20th	Night	27% 7% 18% 34% 13% 1	388
	21st to end	Night	21% 9% 21% 31% 17% 1	438
August	1st to 10th	Night	19% 6% 15% 28% 29% 1	478
	11th to 20th	Night	24% 7% 18% 32% 19% 1	594
	21st to end	Night	34% 8% 17% 27% 12% 2	660
September	1st to 10th	Night	35% 8% 17% 27% 13%	600
	11th to 20th	Night	33% 7% 16% 24% 18% 2	800
	21st to end	Night	47% 12% 18% 16% 7% 1	800
October	1st to 10th	Night	38% 8% 23% 20% 10%	800
	11th to 20th	Night	43% 11% 20% 18% 8%	800
	21st to end	Night	41% 13% 19% 22% 6%	880
November	1st to 10th	Night	44% 14% 23% 15% 4 1	823
	11th to 20th	Night	35% 15% 25% 20% 4	950
	21st to end	Night	37% 14% 25% 20% 5% 1	950
December	1st to 10th	Night	34% 12% 24% 22% 8% 1	950
	11th to 20th	Night	30% 13% 29% 21% 5%	950
	21st to end	Night	31% 14% 28% 20% 6%	1045

Page generated at 2026-02-27 22:40:21. Version ac07

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