Observatory Point
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Observatory Point clear sky chart from 2005-11-06 to 2025-10-04.

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	9% 9% 3 8% 6% 5% 5% 4 6% 7% 38%	130690
	Night	9% 8% 3 9% 6% 5% 6% 5% 6% 7% 36%	127413
Transparency	Day	43% 9% 20% 20% 7% 1	130398
	Night	48% 10% 16% 18% 7% 1	127282
Seeing	Day	14% 7% 18% 30% 25% 7%	16632
	Night	17% 8% 18% 29% 21% 6%	24755

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% 20% 6% 13% 9% 7% 7% 5% 6% 5% 10%	3179
	11th to 20th	Day	9% 11% 4 10% 7% 6% 6% 6% 7% 8% 25%	3180
	21st to end	Day	9% 12% 5% 12% 8% 7% 6% 5% 6% 6% 24%	4233
February	1st to 10th	Day	12% 13% 5% 12% 7% 8% 6% 6% 6% 9% 16%	1800
	11th to 20th	Day	13% 15% 4 11% 8% 7% 6% 5% 6% 6% 20%	1755
	21st to end	Day	10% 14% 4 13% 9% 8% 8% 6% 6% 6% 15%	1489
March	1st to 10th	Day	7% 10% 4 11% 8% 8% 7% 6% 9% 9% 21%	4772
	11th to 20th	Day	7% 13% 4 11% 7% 7% 7% 5% 6% 7% 26%	5000
	21st to end	Day	12% 14% 5% 13% 8% 7% 7% 5% 6% 6% 17%	5563
April	1st to 10th	Day	10% 14% 4 11% 8% 6% 8% 7% 7% 8% 18%	2400
	11th to 20th	Day	10% 12% 4 10% 9% 6% 6% 5% 6% 8% 25%	2482
	21st to end	Day	12% 10% 4 10% 8% 6% 6% 4 6% 8% 26%	2600
May	1st to 10th	Day	9% 7% 2 7% 5% 5% 6% 5% 6% 6% 42%	6315
	11th to 20th	Day	8% 7% 2 8% 5% 5% 6% 5% 7% 10% 37%	6277
	21st to end	Day	13% 9% 2 8% 5% 5% 6% 5% 7% 8% 32%	6963
June	1st to 10th	Day	11% 8% 4 9% 6% 5% 5% 4 6% 7% 36%	3000
	11th to 20th	Day	9% 7% 2 9% 7% 7% 7% 5% 6% 9% 32%	3000
	21st to end	Day	8% 7% 2 6% 5% 5% 5% 4 7% 7% 43%	3000
July	1st to 10th	Day	5% 3 1 3 2 2 2 2 4 6% 70%	6780
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 74%	6780
	21st to end	Day	5% 4 1 2 2 2 2 3 4 6% 70%	6931
August	1st to 10th	Day	3 2 1 3 2 2 3 3 4 6% 72%	4365
	11th to 20th	Day	8% 6% 2 5% 5% 5% 5% 4 6% 9% 47%	4156
	21st to end	Day	10% 5% 2 6% 4 4 4 3 5% 7% 50%	4573
September	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 5% 8% 55%	2400
	11th to 20th	Day	7% 7% 4 7% 5% 4 4 4 5% 7% 46%	2358
	21st to end	Day	8% 6% 2 7% 5% 5% 5% 4 5% 9% 43%	2130
October	1st to 10th	Day	10% 7% 3 7% 5% 5% 7% 6% 9% 9% 32%	3211
	11th to 20th	Day	13% 10% 3 8% 6% 7% 6% 4 7% 7% 29%	3161
	21st to end	Day	10% 7% 3 8% 7% 6% 8% 5% 8% 8% 30%	3270
November	1st to 10th	Day	12% 13% 5% 12% 8% 6% 7% 5% 7% 7% 17%	1568
	11th to 20th	Day	11% 12% 4 11% 6% 7% 8% 5% 8% 8% 21%	1600
	21st to end	Day	13% 12% 5% 8% 7% 6% 8% 7% 9% 7% 19%	1600
December	1st to 10th	Day	7% 10% 4 10% 8% 6% 7% 5% 6% 9% 28%	2620
	11th to 20th	Day	11% 14% 5% 12% 7% 8% 6% 5% 7% 7% 16%	2681
	21st to end	Day	12% 15% 5% 13% 8% 7% 8% 5% 7% 6% 15%	3498
January	1st to 10th	Night	12% 15% 6% 15% 8% 7% 7% 5% 6% 6% 13%	6766
	11th to 20th	Night	8% 10% 4 9% 7% 6% 5% 5% 7% 8% 32%	6716
	21st to end	Night	11% 11% 6% 13% 7% 6% 6% 4 5% 6% 26%	6732
February	1st to 10th	Night	13% 13% 5% 11% 8% 6% 6% 4 7% 7% 20%	2600
	11th to 20th	Night	11% 12% 4 11% 7% 6% 6% 5% 5% 6% 26%	2532
	21st to end	Night	12% 13% 3 13% 9% 6% 6% 6% 7% 6% 19%	2140
March	1st to 10th	Night	5% 10% 3 9% 6% 7% 6% 6% 10% 10% 27%	5117
	11th to 20th	Night	5% 7% 3 8% 6% 7% 6% 6% 8% 8% 35%	4999
	21st to end	Night	10% 13% 4 12% 8% 7% 7% 5% 7% 6% 19%	5412
April	1st to 10th	Night	10% 11% 3 10% 7% 7% 7% 5% 7% 9% 25%	2000
	11th to 20th	Night	8% 10% 3 7% 7% 6% 6% 4 6% 9% 33%	1866
	21st to end	Night	10% 9% 3 8% 6% 5% 5% 5% 6% 8% 33%	1800
May	1st to 10th	Night	8% 5% 2 5% 4 5% 6% 4 4 6% 52%	3643
	11th to 20th	Night	7% 6% 2 6% 4 4 5% 4 6% 9% 47%	3616
	21st to end	Night	7% 5% 2 7% 6% 5% 6% 6% 8% 10% 39%	3908
June	1st to 10th	Night	9% 6% 2 9% 6% 4 4 4 4 6% 45%	1400
	11th to 20th	Night	6% 6% 2 5% 6% 4 4 5% 7% 9% 46%	1400
	21st to end	Night	5% 5% 2 5% 3 3 4 5% 7% 8% 53%	1400
July	1st to 10th	Night	3 2 1 4 2 2 2 2 3 5% 75%	3180
	11th to 20th	Night	1 3 1 2 1 1 2 2 3 4 80%	3180
	21st to end	Night	5% 2 1 2 1 2 2 2 4 5% 74%	3896
August	1st to 10th	Night	2 2 1 2 1 1 2 2 4 6% 77%	2788
	11th to 20th	Night	7% 4 1 4 4 3 3 3 6% 8% 57%	2902
	21st to end	Night	10% 4 2 4 3 3 4 3 5% 7% 56%	3338
September	1st to 10th	Night	4 4 1 4 2 3 3 4 5% 8% 62%	2000
	11th to 20th	Night	10% 4 2 6% 4 5% 4 3 6% 7% 50%	2040
	21st to end	Night	9% 5% 2 7% 5% 5% 4 4 5% 7% 48%	2270
October	1st to 10th	Night	8% 5% 3 6% 5% 5% 6% 5% 7% 8% 43%	3836
	11th to 20th	Night	10% 7% 3 9% 7% 7% 6% 4 6% 7% 33%	3777
	21st to end	Night	12% 8% 2 8% 6% 6% 6% 5% 6% 8% 34%	4290
November	1st to 10th	Night	13% 12% 4 9% 7% 7% 7% 5% 7% 7% 22%	2730
	11th to 20th	Night	13% 10% 4 9% 7% 5% 8% 6% 8% 7% 23%	2800
	21st to end	Night	10% 10% 4 12% 8% 7% 7% 5% 7% 8% 21%	2800
December	1st to 10th	Night	7% 9% 3 9% 7% 7% 7% 6% 7% 7% 32%	4560
	11th to 20th	Night	12% 13% 5% 13% 8% 7% 7% 6% 7% 7% 16%	5521
	21st to end	Night	10% 13% 4 12% 8% 8% 7% 6% 8% 7% 18%	7458

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% 10% 8% 9% 2	3179
	11th to 20th	Day	53% 13% 19% 13% 2	3201
	21st to end	Day	64% 10% 9% 13% 3	4233
February	1st to 10th	Day	64% 12% 12% 9% 4 1	1800
	11th to 20th	Day	62% 9% 13% 12% 4	1764
	21st to end	Day	60% 10% 13% 13% 3	1489
March	1st to 10th	Day	55% 12% 14% 16% 4	4772
	11th to 20th	Day	52% 8% 14% 23% 3	5000
	21st to end	Day	62% 10% 15% 11% 2	5563
April	1st to 10th	Day	54% 12% 19% 12% 2	2400
	11th to 20th	Day	47% 10% 19% 16% 7% 1	2482
	21st to end	Day	46% 11% 20% 19% 4 1	2600
May	1st to 10th	Day	36% 7% 21% 26% 9%	6315
	11th to 20th	Day	35% 9% 32% 21% 4	6280
	21st to end	Day	42% 12% 26% 16% 4	6964
June	1st to 10th	Day	41% 11% 25% 16% 6% 1	3000
	11th to 20th	Day	38% 12% 26% 19% 4	3000
	21st to end	Day	33% 11% 27% 19% 8% 1	3000
July	1st to 10th	Day	17% 7% 24% 25% 20% 8%	6706
	11th to 20th	Day	13% 7% 21% 33% 25% 1	6699
	21st to end	Day	16% 9% 33% 30% 11% 1	6736
August	1st to 10th	Day	15% 9% 20% 39% 15% 1	4348
	11th to 20th	Day	34% 8% 30% 20% 7% 1	4195
	21st to end	Day	34% 7% 20% 29% 9%	4572
September	1st to 10th	Day	28% 8% 24% 24% 14% 2	2400
	11th to 20th	Day	38% 9% 18% 21% 13% 1	2358
	21st to end	Day	40% 9% 21% 19% 10% 1	2130
October	1st to 10th	Day	47% 10% 17% 21% 4	3211
	11th to 20th	Day	56% 9% 14% 16% 5%	3161
	21st to end	Day	52% 10% 16% 14% 7% 1	3270
November	1st to 10th	Day	67% 8% 12% 10% 2 1	1568
	11th to 20th	Day	62% 8% 12% 13% 4 1	1600
	21st to end	Day	63% 12% 9% 12% 4	1600
December	1st to 10th	Day	59% 13% 10% 15% 3	2620
	11th to 20th	Day	68% 10% 12% 9% 1	2684
	21st to end	Day	70% 9% 12% 7% 1	3498
January	1st to 10th	Night	72% 13% 7% 6% 2	6766
	11th to 20th	Night	48% 12% 17% 19% 4 1	6757
	21st to end	Night	61% 9% 11% 14% 5%	6732
February	1st to 10th	Night	63% 10% 11% 9% 6%	2600
	11th to 20th	Night	57% 9% 13% 14% 6%	2534
	21st to end	Night	60% 9% 13% 13% 4 1	2140
March	1st to 10th	Night	47% 15% 14% 18% 6%	5117
	11th to 20th	Night	45% 9% 17% 24% 4 1	5000
	21st to end	Night	65% 9% 14% 11% 1	5412
April	1st to 10th	Night	51% 11% 19% 15% 3	2000
	11th to 20th	Night	45% 9% 18% 20% 7% 2	1866
	21st to end	Night	47% 9% 19% 16% 8% 1	1800
May	1st to 10th	Night	35% 5% 17% 30% 12%	3640
	11th to 20th	Night	35% 11% 26% 22% 5%	3616
	21st to end	Night	41% 12% 23% 20% 3	3908
June	1st to 10th	Night	41% 8% 20% 22% 8% 2	1400
	11th to 20th	Night	33% 11% 29% 24% 4	1400
	21st to end	Night	26% 9% 31% 23% 8% 3	1400
July	1st to 10th	Night	17% 7% 22% 28% 16% 10%	3145
	11th to 20th	Night	9% 7% 19% 35% 28% 2	3138
	21st to end	Night	16% 9% 26% 35% 13% 2	3786
August	1st to 10th	Night	13% 9% 19% 36% 22% 2	2780
	11th to 20th	Night	29% 7% 31% 21% 9% 3	2929
	21st to end	Night	31% 8% 18% 32% 11% 1	3338
September	1st to 10th	Night	22% 7% 23% 28% 15% 4	2000
	11th to 20th	Night	37% 7% 17% 19% 17% 3	2040
	21st to end	Night	38% 7% 17% 23% 13% 2	2270
October	1st to 10th	Night	41% 7% 22% 24% 7%	3836
	11th to 20th	Night	51% 6% 18% 20% 4	3777
	21st to end	Night	52% 7% 15% 17% 8% 1	4290
November	1st to 10th	Night	62% 8% 14% 12% 4	2730
	11th to 20th	Night	59% 9% 16% 9% 5% 2	2800
	21st to end	Night	63% 11% 12% 9% 5%	2800
December	1st to 10th	Night	58% 14% 9% 15% 3	4560
	11th to 20th	Night	69% 11% 11% 7% 2	5517
	21st to end	Night	66% 11% 12% 9% 2	7458

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	31% 12% 23% 23% 11%	200
	11th to 20th	Day	25% 10% 22% 28% 16%	200
	21st to end	Day	27% 6% 22% 28% 14% 4	319
February	1st to 10th	Day	25% 13% 25% 25% 12% 1	400
	11th to 20th	Day	23% 14% 18% 30% 15% 1	395
	21st to end	Day	20% 16% 20% 28% 13% 2	334
March	1st to 10th	Day	16% 11% 20% 30% 19% 3	500
	11th to 20th	Day	15% 11% 21% 29% 18% 6%	500
	21st to end	Day	17% 11% 20% 30% 19% 2	550
April	1st to 10th	Day	17% 11% 21% 28% 19% 4	500
	11th to 20th	Day	13% 10% 18% 33% 20% 7%	492
	21st to end	Day	17% 6% 24% 25% 21% 7%	500
May	1st to 10th	Day	11% 8% 17% 31% 28% 5%	503
	11th to 20th	Day	12% 6% 15% 30% 29% 7%	528
	21st to end	Day	10% 5% 21% 32% 25% 7%	594
June	1st to 10th	Day	12% 7% 22% 31% 23% 6%	730
	11th to 20th	Day	7% 7% 28% 36% 17% 5%	730
	21st to end	Day	7% 4 22% 31% 26% 10%	730
July	1st to 10th	Day	7% 2 15% 36% 31% 8%	721
	11th to 20th	Day	4 3 18% 33% 33% 10%	718
	21st to end	Day	3 1 15% 30% 32% 19%	606
August	1st to 10th	Day	5% 2 11% 25% 39% 18%	527
	11th to 20th	Day	4 2 8% 26% 41% 19%	517
	21st to end	Day	7% 4 13% 29% 33% 15%	583
September	1st to 10th	Day	8% 3 8% 31% 38% 12%	530
	11th to 20th	Day	13% 3 11% 26% 35% 13%	530
	21st to end	Day	14% 4 17% 28% 31% 7%	530
October	1st to 10th	Day	17% 6% 14% 28% 27% 9%	503
	11th to 20th	Day	23% 9% 11% 28% 25% 4	490
	21st to end	Day	19% 11% 23% 29% 15% 3	452
November	1st to 10th	Day	29% 9% 23% 24% 16%	200
	11th to 20th	Day	22% 8% 23% 24% 21% 2	200
	21st to end	Day	24% 12% 17% 27% 19% 2	200
December	1st to 10th	Day	23% 11% 15% 34% 17% 2	200
	11th to 20th	Day	26% 13% 23% 28% 11% 1	200
	21st to end	Day	30% 9% 24% 30% 6% 1	220
January	1st to 10th	Night	29% 14% 21% 24% 11% 1	900
	11th to 20th	Night	23% 13% 16% 28% 18% 1	895
	21st to end	Night	23% 9% 17% 29% 18% 4	880
February	1st to 10th	Night	24% 14% 21% 26% 13% 2	800
	11th to 20th	Night	22% 11% 19% 29% 16% 2	784
	21st to end	Night	24% 15% 19% 24% 15% 4	660
March	1st to 10th	Night	16% 11% 18% 31% 19% 5%	800
	11th to 20th	Night	19% 9% 24% 25% 19% 4	800
	21st to end	Night	16% 10% 22% 29% 19% 4	880
April	1st to 10th	Night	20% 8% 21% 29% 20% 4	800
	11th to 20th	Night	14% 9% 22% 32% 18% 5%	678
	21st to end	Night	15% 9% 24% 32% 16% 5%	600
May	1st to 10th	Night	10% 6% 23% 33% 24% 4	403
	11th to 20th	Night	10% 5% 19% 33% 26% 7%	394
	21st to end	Night	9% 6% 20% 36% 26% 2	433
June	1st to 10th	Night	12% 6% 23% 34% 23% 4	400
	11th to 20th	Night	9% 4 26% 41% 17% 4	400
	21st to end	Night	7% 4 24% 31% 28% 7%	400
July	1st to 10th	Night	6% 2 17% 38% 34% 4	390
	11th to 20th	Night	3 2 17% 42% 29% 7%	388
	21st to end	Night	3 1 17% 37% 33% 8%	438
August	1st to 10th	Night	5% 1 10% 32% 41% 10%	499
	11th to 20th	Night	5% 1 7% 33% 38% 16%	594
	21st to end	Night	8% 2 16% 31% 33% 10%	714
September	1st to 10th	Night	7% 2 12% 33% 33% 14%	800
	11th to 20th	Night	11% 4 10% 28% 32% 16%	800
	21st to end	Night	13% 3 16% 30% 28% 11%	800
October	1st to 10th	Night	15% 5% 11% 27% 29% 12%	772
	11th to 20th	Night	19% 6% 13% 29% 23% 11%	760
	21st to end	Night	24% 7% 14% 31% 19% 5%	836
November	1st to 10th	Night	27% 8% 19% 28% 16% 2	780
	11th to 20th	Night	24% 13% 20% 26% 15% 3	800
	21st to end	Night	24% 11% 21% 25% 16% 3	800
December	1st to 10th	Night	24% 9% 18% 28% 19% 2	800
	11th to 20th	Night	26% 13% 21% 27% 12% 1	887
	21st to end	Night	27% 13% 25% 26% 8% 1	990

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