Lookum Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lookum Observatory clear sky chart from 2005-11-06 to 2024-01-13.

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	15% 11% 5% 8% 6% 3 4 4 5% 7% 31%	74550
	Night	15% 12% 6% 9% 7% 3 5% 4 5% 6% 28%	75174
Transparency	Day	52% 8% 13% 18% 8% 1	74256
	Night	56% 7% 13% 16% 7% 1	75003
Seeing	Day	20% 16% 32% 21% 8% 2	14626
	Night	20% 13% 31% 25% 9% 1	24503

By Month

Percentage Cloud Cover NE (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	11% 17% 11% 15% 11% 5% 4 4 4 5% 13%	1517
	11th to 20th	Day	14% 14% 10% 16% 13% 5% 4 4 4 4 13%	1456
	21st to end	Day	12% 17% 8% 14% 12% 5% 6% 4 4 5% 14%	1584
February	1st to 10th	Day	9% 13% 7% 14% 14% 6% 7% 4 6% 4 16%	1458
	11th to 20th	Day	11% 15% 7% 14% 11% 6% 5% 5% 4 5% 18%	1740
	21st to end	Day	12% 16% 8% 15% 10% 3 6% 3 4 6% 18%	1480
March	1st to 10th	Day	12% 12% 8% 13% 9% 4 5% 4 4 6% 23%	1798
	11th to 20th	Day	12% 12% 6% 12% 6% 3 4 4 4 6% 30%	1875
	21st to end	Day	15% 14% 6% 11% 7% 3 4 4 5% 5% 25%	2330
April	1st to 10th	Day	18% 14% 6% 8% 5% 3 4 4 5% 6% 27%	2160
	11th to 20th	Day	20% 14% 6% 7% 4 2 4 4 4 6% 30%	2199
	21st to end	Day	15% 11% 4 7% 5% 3 5% 5% 4 7% 33%	2340
May	1st to 10th	Day	16% 12% 4 6% 6% 3 4 4 4 7% 36%	2380
	11th to 20th	Day	16% 10% 4 5% 4 3 4 4 5% 6% 39%	2479
	21st to end	Day	12% 6% 2 6% 5% 3 5% 5% 7% 8% 42%	2725
June	1st to 10th	Day	13% 6% 3 6% 5% 3 5% 6% 6% 8% 40%	2618
	11th to 20th	Day	15% 7% 3 5% 4 2 5% 5% 6% 8% 39%	2700
	21st to end	Day	16% 6% 2 5% 5% 2 5% 5% 7% 8% 39%	2620
July	1st to 10th	Day	9% 4 2 4 4 3 5% 5% 7% 8% 49%	2520
	11th to 20th	Day	12% 5% 2 5% 4 3 4 5% 5% 8% 46%	2520
	21st to end	Day	11% 5% 2 4 4 2 4 6% 6% 9% 48%	2743
August	1st to 10th	Day	11% 4 2 5% 5% 4 5% 6% 5% 8% 46%	2507
	11th to 20th	Day	14% 5% 2 5% 5% 3 4 4 6% 7% 46%	2315
	21st to end	Day	17% 6% 2 5% 4 3 3 4 5% 8% 43%	2434
September	1st to 10th	Day	19% 6% 2 5% 4 3 4 4 6% 7% 39%	2160
	11th to 20th	Day	15% 6% 3 5% 5% 4 5% 5% 6% 9% 38%	2012
	21st to end	Day	20% 8% 3 5% 6% 3 5% 5% 7% 8% 30%	1980
October	1st to 10th	Day	21% 9% 3 7% 5% 4 4 5% 5% 6% 31%	1962
	11th to 20th	Day	26% 15% 3 7% 7% 3 3 5% 5% 6% 19%	1666
	21st to end	Day	24% 24% 4 8% 5% 3 4 4 4 4 15%	1782
November	1st to 10th	Day	19% 17% 5% 7% 5% 3 4 4 5% 6% 25%	1672
	11th to 20th	Day	16% 20% 10% 10% 6% 4 5% 4 5% 5% 15%	1710
	21st to end	Day	16% 21% 11% 11% 7% 3 3 3 4 6% 15%	1443
December	1st to 10th	Day	12% 17% 12% 16% 8% 3 4 5% 4 6% 15%	1330
	11th to 20th	Day	14% 19% 9% 16% 7% 4 5% 4 4 5% 14%	1671
	21st to end	Day	19% 25% 14% 13% 6% 2 3 3 4 3 7%	2664
January	1st to 10th	Night	13% 14% 9% 15% 9% 5% 4 4 4 6% 16%	2648
	11th to 20th	Night	13% 16% 8% 16% 14% 4 5% 3 4 5% 13%	2549
	21st to end	Night	12% 13% 8% 15% 12% 5% 6% 5% 5% 5% 14%	2772
February	1st to 10th	Night	9% 13% 7% 13% 12% 6% 6% 5% 4 5% 19%	2500
	11th to 20th	Night	10% 12% 6% 13% 10% 5% 5% 5% 5% 6% 23%	2105
	21st to end	Night	13% 15% 9% 13% 7% 5% 4 4 4 5% 19%	1776
March	1st to 10th	Night	11% 9% 6% 10% 9% 5% 5% 5% 5% 6% 30%	2149
	11th to 20th	Night	12% 12% 7% 10% 7% 4 4 4 4 5% 31%	2083
	21st to end	Night	16% 10% 6% 9% 7% 4 4 5% 5% 6% 29%	2042
April	1st to 10th	Night	17% 14% 5% 8% 6% 4 6% 5% 5% 6% 24%	1800
	11th to 20th	Night	22% 8% 5% 5% 4 3 5% 4 4 5% 34%	1710
	21st to end	Night	15% 11% 5% 7% 5% 3 4 4 5% 7% 34%	1620
May	1st to 10th	Night	16% 10% 3 6% 5% 3 6% 4 5% 8% 34%	1578
	11th to 20th	Night	19% 9% 3 6% 6% 2 5% 6% 5% 5% 33%	1416
	21st to end	Night	12% 7% 3 6% 5% 3 5% 5% 5% 9% 40%	1551
June	1st to 10th	Night	13% 6% 3 5% 6% 3 5% 5% 6% 7% 42%	1337
	11th to 20th	Night	14% 6% 2 4 6% 3 5% 7% 6% 8% 37%	1260
	21st to end	Night	15% 6% 2 5% 6% 2 5% 3 5% 8% 42%	1333
July	1st to 10th	Night	11% 4 3 4 4 2 4 4 6% 9% 49%	1440
	11th to 20th	Night	13% 7% 2 4 5% 3 5% 5% 5% 8% 44%	1440
	21st to end	Night	9% 4 2 5% 3 3 4 5% 6% 8% 51%	1575
August	1st to 10th	Night	8% 5% 2 4 6% 3 5% 5% 6% 9% 47%	1432
	11th to 20th	Night	9% 3 2 5% 5% 3 5% 4 6% 9% 50%	1585
	21st to end	Night	14% 6% 2 5% 4 3 3 4 5% 6% 49%	1919
September	1st to 10th	Night	14% 7% 2 5% 5% 3 3 4 4 7% 46%	1800
	11th to 20th	Night	17% 7% 2 6% 5% 3 4 4 5% 6% 41%	1930
	21st to end	Night	19% 8% 3 6% 5% 3 5% 5% 5% 7% 35%	1980
October	1st to 10th	Night	20% 9% 3 5% 5% 3 5% 4 5% 8% 33%	1996
	11th to 20th	Night	21% 13% 4 6% 5% 4 4 5% 4 5% 28%	2294
	21st to end	Night	25% 18% 4 8% 6% 3 4 4 4 5% 19%	2574
November	1st to 10th	Night	16% 19% 4 6% 6% 3 4 4 5% 7% 26%	2406
	11th to 20th	Night	17% 21% 7% 10% 7% 3 5% 3 3 4 20%	2470
	21st to end	Night	16% 17% 12% 11% 6% 3 4 5% 3 5% 17%	2730
December	1st to 10th	Night	13% 18% 10% 13% 6% 3 5% 5% 5% 5% 17%	2850
	11th to 20th	Night	14% 14% 8% 18% 7% 4 5% 4 5% 5% 16%	3526
	21st to end	Night	21% 24% 13% 14% 6% 3 3 2 3 3 8%	4998

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	72% 8% 9% 9% 2	1518
	11th to 20th	Day	74% 6% 11% 8% 1	1456
	21st to end	Day	70% 6% 12% 9% 3 1	1584
February	1st to 10th	Day	66% 9% 12% 10% 3	1458
	11th to 20th	Day	68% 7% 12% 9% 4	1750
	21st to end	Day	67% 8% 8% 11% 6% 1	1480
March	1st to 10th	Day	62% 8% 10% 12% 7% 1	1800
	11th to 20th	Day	55% 5% 15% 16% 7% 2	1875
	21st to end	Day	60% 5% 13% 15% 6% 1	2330
April	1st to 10th	Day	58% 7% 10% 17% 7%	2160
	11th to 20th	Day	55% 5% 9% 18% 11% 1	2199
	21st to end	Day	50% 8% 9% 18% 12% 3	2340
May	1st to 10th	Day	49% 6% 11% 17% 13% 2	2380
	11th to 20th	Day	45% 7% 11% 25% 12% 1	2483
	21st to end	Day	39% 9% 15% 23% 11% 2	2725
June	1st to 10th	Day	39% 9% 15% 21% 14% 2	2618
	11th to 20th	Day	41% 9% 13% 22% 14% 1	2700
	21st to end	Day	40% 9% 18% 22% 10% 1	2620
July	1st to 10th	Day	29% 8% 17% 30% 15% 1	2451
	11th to 20th	Day	36% 8% 14% 26% 15% 2	2445
	21st to end	Day	32% 9% 18% 27% 14% 1	2591
August	1st to 10th	Day	35% 9% 17% 27% 11% 1	2492
	11th to 20th	Day	38% 8% 15% 27% 12%	2315
	21st to end	Day	40% 7% 16% 24% 12% 1	2434
September	1st to 10th	Day	43% 8% 13% 25% 11%	2160
	11th to 20th	Day	41% 9% 21% 24% 5%	2012
	21st to end	Day	49% 8% 17% 20% 6%	1980
October	1st to 10th	Day	53% 8% 14% 19% 7%	1962
	11th to 20th	Day	64% 7% 12% 13% 3	1666
	21st to end	Day	71% 6% 11% 8% 3	1782
November	1st to 10th	Day	59% 6% 12% 17% 5%	1672
	11th to 20th	Day	69% 8% 12% 7% 4	1710
	21st to end	Day	71% 8% 13% 6% 2	1443
December	1st to 10th	Day	71% 8% 13% 6% 2	1330
	11th to 20th	Day	72% 7% 10% 9% 1	1671
	21st to end	Day	83% 5% 6% 5% 1	2664
January	1st to 10th	Night	69% 7% 13% 9% 2	2648
	11th to 20th	Night	72% 8% 13% 6% 1	2549
	21st to end	Night	69% 7% 13% 9% 1	2772
February	1st to 10th	Night	64% 6% 12% 13% 4	2500
	11th to 20th	Night	61% 6% 14% 15% 3	2106
	21st to end	Night	65% 6% 10% 13% 6%	1776
March	1st to 10th	Night	52% 7% 14% 20% 7% 1	2160
	11th to 20th	Night	53% 6% 12% 17% 11% 1	2084
	21st to end	Night	53% 7% 11% 19% 8% 1	2042
April	1st to 10th	Night	59% 7% 11% 14% 9% 1	1800
	11th to 20th	Night	52% 6% 9% 21% 11% 2	1710
	21st to end	Night	50% 6% 12% 18% 12% 2	1620
May	1st to 10th	Night	48% 6% 11% 19% 12% 3	1578
	11th to 20th	Night	50% 8% 7% 23% 10% 1	1417
	21st to end	Night	40% 7% 15% 21% 14% 3	1552
June	1st to 10th	Night	39% 7% 13% 27% 12% 2	1337
	11th to 20th	Night	41% 10% 12% 21% 15% 2	1260
	21st to end	Night	39% 7% 15% 22% 15% 2	1333
July	1st to 10th	Night	30% 9% 15% 30% 15% 2	1400
	11th to 20th	Night	35% 7% 14% 27% 15% 2	1392
	21st to end	Night	30% 8% 16% 27% 17% 2	1487
August	1st to 10th	Night	32% 9% 17% 28% 12% 2	1423
	11th to 20th	Night	31% 7% 17% 31% 12% 1	1585
	21st to end	Night	36% 6% 14% 28% 14% 1	1919
September	1st to 10th	Night	39% 7% 13% 27% 14% 1	1800
	11th to 20th	Night	43% 7% 18% 24% 7%	1930
	21st to end	Night	46% 9% 17% 21% 6%	1980
October	1st to 10th	Night	46% 9% 17% 20% 7% 1	1996
	11th to 20th	Night	56% 9% 15% 14% 5% 1	2294
	21st to end	Night	65% 8% 10% 13% 3	2574
November	1st to 10th	Night	57% 9% 13% 15% 7%	2406
	11th to 20th	Night	68% 7% 11% 9% 6%	2470
	21st to end	Night	70% 7% 13% 8% 1	2730
December	1st to 10th	Night	68% 9% 13% 8% 2	2850
	11th to 20th	Night	69% 8% 13% 7% 2	3525
	21st to end	Night	82% 5% 8% 4 1	4998

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	19% 20% 41% 18% 1	270
	11th to 20th	Day	24% 16% 45% 13% 2	246
	21st to end	Day	21% 19% 42% 16% 3	264
February	1st to 10th	Day	22% 23% 39% 13% 3	240
	11th to 20th	Day	22% 29% 34% 11% 3	240
	21st to end	Day	27% 30% 35% 8%	198
March	1st to 10th	Day	23% 24% 37% 13% 3	240
	11th to 20th	Day	22% 19% 37% 18% 3 1	315
	21st to end	Day	28% 21% 26% 19% 5% 1	462
April	1st to 10th	Day	26% 18% 31% 20% 4 1	420
	11th to 20th	Day	28% 18% 27% 18% 8% 1	412
	21st to end	Day	23% 21% 29% 20% 5% 2	420
May	1st to 10th	Day	23% 20% 31% 15% 9% 3	463
	11th to 20th	Day	20% 17% 36% 18% 8% 2	625
	21st to end	Day	17% 13% 34% 24% 11% 2	679
June	1st to 10th	Day	15% 14% 35% 26% 9% 1	630
	11th to 20th	Day	18% 13% 34% 25% 8% 2	630
	21st to end	Day	18% 10% 33% 28% 10% 2	630
July	1st to 10th	Day	9% 8% 34% 31% 14% 3	621
	11th to 20th	Day	12% 11% 33% 29% 12% 3	618
	21st to end	Day	11% 7% 37% 27% 13% 5%	684
August	1st to 10th	Day	10% 9% 30% 33% 13% 4	626
	11th to 20th	Day	16% 7% 22% 28% 21% 7%	451
	21st to end	Day	20% 12% 26% 25% 12% 4	495
September	1st to 10th	Day	20% 13% 29% 20% 12% 5%	450
	11th to 20th	Day	15% 10% 35% 26% 10% 3	450
	21st to end	Day	23% 14% 25% 27% 10% 2	450
October	1st to 10th	Day	26% 14% 28% 22% 9% 2	432
	11th to 20th	Day	34% 22% 30% 13% 1	270
	21st to end	Day	38% 22% 30% 8% 2	297
November	1st to 10th	Day	24% 21% 34% 17% 4	270
	11th to 20th	Day	23% 31% 34% 10% 3	270
	21st to end	Day	29% 24% 31% 12% 3	233
December	1st to 10th	Day	23% 24% 31% 16% 6% 1	180
	11th to 20th	Day	27% 23% 35% 9% 5% 1	180
	21st to end	Day	25% 27% 35% 11% 3	265
January	1st to 10th	Night	19% 21% 38% 19% 3	950
	11th to 20th	Night	22% 19% 35% 18% 5% 1	911
	21st to end	Night	19% 15% 40% 20% 5% 1	990
February	1st to 10th	Night	15% 19% 36% 23% 6% 1	900
	11th to 20th	Night	19% 18% 35% 23% 5%	710
	21st to end	Night	21% 16% 40% 20% 4	592
March	1st to 10th	Night	14% 15% 38% 24% 8%	720
	11th to 20th	Night	19% 18% 36% 20% 6% 1	720
	21st to end	Night	19% 14% 31% 27% 8% 1	656
April	1st to 10th	Night	23% 15% 29% 25% 6% 1	540
	11th to 20th	Night	26% 13% 26% 27% 8%	533
	21st to end	Night	21% 11% 31% 27% 9% 1	540
May	1st to 10th	Night	21% 11% 26% 32% 9% 1	540
	11th to 20th	Night	22% 10% 28% 29% 9% 2	531
	21st to end	Night	16% 8% 29% 32% 15%	584
June	1st to 10th	Night	15% 7% 27% 38% 12% 1	437
	11th to 20th	Night	16% 8% 27% 36% 13%	360
	21st to end	Night	16% 6% 26% 36% 13% 2	433
July	1st to 10th	Night	12% 3 25% 40% 18% 2	525
	11th to 20th	Night	13% 6% 26% 37% 16% 2	522
	21st to end	Night	10% 5% 24% 38% 19% 3	591
August	1st to 10th	Night	10% 2 26% 38% 21% 1	537
	11th to 20th	Night	11% 4 20% 38% 25% 3	534
	21st to end	Night	15% 5% 26% 36% 16% 2	594
September	1st to 10th	Night	17% 5% 25% 31% 19% 3	540
	11th to 20th	Night	19% 8% 29% 30% 12% 1	670
	21st to end	Night	21% 9% 27% 29% 13% 1	720
October	1st to 10th	Night	24% 11% 28% 24% 11% 1	720
	11th to 20th	Night	26% 16% 30% 22% 6%	720
	21st to end	Night	36% 14% 28% 17% 4	792
November	1st to 10th	Night	29% 13% 35% 18% 5%	740
	11th to 20th	Night	27% 19% 31% 18% 4	760
	21st to end	Night	26% 20% 32% 18% 4	946
December	1st to 10th	Night	22% 20% 38% 15% 4 1	950
	11th to 20th	Night	20% 20% 36% 17% 5% 1	950
	21st to end	Night	28% 20% 33% 17% 2	1045

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