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 2024-11-05.

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% 5% 6% 7% 37%	105543
	Night	9% 9% 3 9% 6% 5% 5% 5% 6% 7% 35%	102111
Transparency	Day	43% 10% 19% 19% 8% 1	105293
	Night	47% 9% 17% 18% 7% 1	102007
Seeing	Day	14% 7% 18% 30% 24% 7%	15446
	Night	18% 8% 18% 29% 21% 6%	23621

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	12% 18% 6% 13% 9% 6% 6% 5% 7% 6% 12%	2219
	11th to 20th	Day	13% 14% 4 11% 9% 7% 7% 7% 7% 6% 16%	2206
	21st to end	Day	13% 14% 5% 15% 10% 7% 7% 6% 6% 6% 11%	2953
February	1st to 10th	Day	13% 14% 4 12% 7% 7% 7% 6% 6% 9% 16%	1710
	11th to 20th	Day	13% 15% 4 11% 8% 7% 6% 5% 6% 6% 19%	1665
	21st to end	Day	10% 14% 5% 13% 9% 8% 8% 6% 6% 6% 16%	1417
March	1st to 10th	Day	5% 10% 4 12% 9% 8% 8% 7% 9% 9% 20%	3314
	11th to 20th	Day	6% 11% 3 10% 7% 5% 6% 5% 6% 7% 33%	3490
	21st to end	Day	11% 14% 4 13% 8% 6% 6% 5% 6% 6% 21%	3876
April	1st to 10th	Day	10% 14% 4 11% 8% 6% 7% 6% 7% 8% 18%	2280
	11th to 20th	Day	10% 13% 4 10% 9% 6% 6% 5% 6% 8% 24%	2356
	21st to end	Day	13% 11% 4 10% 9% 6% 6% 5% 6% 7% 24%	2470
May	1st to 10th	Day	10% 8% 3 9% 6% 5% 5% 4 5% 5% 39%	4415
	11th to 20th	Day	5% 5% 2 7% 5% 5% 5% 5% 7% 10% 43%	4377
	21st to end	Day	16% 11% 3 9% 6% 5% 6% 4 7% 6% 27%	4859
June	1st to 10th	Day	12% 9% 4 9% 6% 6% 5% 4 6% 7% 33%	2850
	11th to 20th	Day	9% 7% 2 9% 7% 7% 7% 5% 6% 9% 32%	2850
	21st to end	Day	8% 6% 2 6% 5% 5% 5% 4 7% 7% 44%	2850
July	1st to 10th	Day	5% 3 1 4 2 2 2 2 4 4 71%	4740
	11th to 20th	Day	4 3 1 3 3 2 2 2 3 5% 71%	4740
	21st to end	Day	7% 5% 1 3 2 2 2 3 4 6% 65%	4833
August	1st to 10th	Day	3 2 1 3 2 2 3 2 4 6% 72%	4230
	11th to 20th	Day	7% 6% 2 5% 5% 5% 5% 4 6% 9% 47%	4026
	21st to end	Day	10% 5% 2 6% 4 4 4 3 5% 7% 50%	4433
September	1st to 10th	Day	6% 5% 2 5% 3 4 3 4 5% 8% 55%	2280
	11th to 20th	Day	7% 8% 4 7% 5% 4 4 4 5% 7% 45%	2240
	21st to end	Day	8% 6% 3 8% 5% 5% 6% 4 5% 9% 42%	2024
October	1st to 10th	Day	10% 7% 3 7% 5% 5% 7% 6% 9% 9% 32%	3180
	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% 11% 8% 6% 7% 5% 7% 7% 18%	1520
	11th to 20th	Day	11% 12% 4 12% 6% 6% 8% 5% 8% 8% 21%	1520
	21st to end	Day	13% 12% 5% 8% 7% 6% 7% 7% 9% 7% 20%	1520
December	1st to 10th	Day	9% 14% 5% 11% 8% 6% 5% 5% 6% 7% 23%	1520
	11th to 20th	Day	10% 15% 6% 12% 8% 7% 7% 5% 7% 7% 16%	1707
	21st to end	Day	11% 14% 6% 14% 8% 6% 7% 5% 7% 7% 14%	2442
January	1st to 10th	Night	11% 15% 5% 14% 9% 7% 7% 5% 6% 6% 15%	4726
	11th to 20th	Night	11% 13% 5% 11% 9% 7% 6% 5% 7% 7% 19%	4690
	21st to end	Night	14% 13% 6% 15% 8% 7% 6% 5% 6% 6% 12%	4707
February	1st to 10th	Night	13% 13% 5% 11% 8% 6% 6% 4 7% 7% 20%	2470
	11th to 20th	Night	11% 12% 4 11% 7% 7% 6% 5% 5% 6% 25%	2402
	21st to end	Night	12% 13% 4 13% 9% 6% 6% 6% 7% 6% 19%	2036
March	1st to 10th	Night	5% 10% 3 10% 7% 7% 7% 6% 10% 10% 24%	3555
	11th to 20th	Night	5% 6% 2 7% 6% 6% 6% 7% 8% 7% 40%	3489
	21st to end	Night	10% 12% 4 13% 9% 7% 8% 5% 6% 5% 22%	3777
April	1st to 10th	Night	10% 11% 3 10% 7% 7% 7% 5% 7% 9% 24%	1900
	11th to 20th	Night	9% 10% 3 7% 8% 6% 6% 4 6% 8% 32%	1772
	21st to end	Night	10% 10% 3 9% 7% 5% 5% 5% 6% 8% 31%	1710
May	1st to 10th	Night	10% 6% 2 6% 4 4 4 3 5% 6% 50%	2543
	11th to 20th	Night	5% 5% 2 6% 4 3 4 3 6% 8% 55%	2516
	21st to end	Night	9% 5% 3 8% 7% 6% 6% 5% 7% 8% 37%	2712
June	1st to 10th	Night	9% 6% 2 9% 6% 4 4 4 4 6% 43%	1330
	11th to 20th	Night	6% 6% 2 5% 6% 4 5% 5% 7% 9% 45%	1330
	21st to end	Night	5% 4 2 5% 3 3 4 5% 7% 8% 54%	1330
July	1st to 10th	Night	3 2 1 4 2 2 2 2 3 5% 75%	2220
	11th to 20th	Night	2 4 1 3 2 1 2 2 3 5% 75%	2220
	21st to end	Night	8% 3 1 2 2 2 2 2 4 5% 69%	2717
August	1st to 10th	Night	2 2 2 1 1 2 2 4 6% 77%	2703
	11th to 20th	Night	7% 4 1 4 4 3 3 3 6% 8% 57%	2812
	21st to end	Night	10% 4 2 4 3 3 4 3 5% 7% 56%	3236
September	1st to 10th	Night	4 3 1 4 2 3 3 4 5% 8% 63%	1900
	11th to 20th	Night	10% 4 2 6% 4 5% 4 3 6% 7% 48%	1938
	21st to end	Night	9% 5% 2 7% 4 5% 5% 4 5% 7% 48%	2156
October	1st to 10th	Night	8% 4 3 6% 5% 5% 6% 5% 7% 8% 43%	3800
	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% 23%	2658
	11th to 20th	Night	13% 10% 4 9% 7% 5% 8% 5% 7% 7% 23%	2660
	21st to end	Night	10% 10% 4 12% 8% 7% 7% 5% 7% 8% 21%	2660
December	1st to 10th	Night	9% 13% 4 10% 8% 7% 6% 6% 7% 6% 24%	2660
	11th to 20th	Night	9% 13% 5% 13% 8% 7% 7% 6% 8% 7% 17%	3495
	21st to end	Night	10% 14% 4 14% 8% 8% 6% 6% 7% 7% 17%	5214

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% 11% 10% 9% 2	2219
	11th to 20th	Day	61% 11% 17% 9% 2	2227
	21st to end	Day	75% 10% 8% 6% 1	2953
February	1st to 10th	Day	64% 12% 12% 8% 4	1710
	11th to 20th	Day	61% 9% 13% 12% 4	1674
	21st to end	Day	59% 10% 14% 13% 3	1417
March	1st to 10th	Day	49% 15% 16% 15% 4	3314
	11th to 20th	Day	44% 9% 18% 26% 2 1	3490
	21st to end	Day	55% 12% 19% 12% 2	3876
April	1st to 10th	Day	55% 12% 19% 12% 2	2280
	11th to 20th	Day	48% 11% 18% 15% 7% 1	2356
	21st to end	Day	47% 11% 20% 17% 4 1	2470
May	1st to 10th	Day	37% 8% 23% 24% 8%	4415
	11th to 20th	Day	28% 11% 35% 22% 5%	4380
	21st to end	Day	48% 13% 23% 14% 3	4860
June	1st to 10th	Day	42% 11% 25% 15% 6% 1	2850
	11th to 20th	Day	38% 13% 26% 19% 4	2850
	21st to end	Day	32% 11% 27% 20% 9% 1	2850
July	1st to 10th	Day	16% 6% 13% 28% 26% 12%	4666
	11th to 20th	Day	14% 8% 16% 35% 27% 1	4659
	21st to end	Day	20% 8% 27% 30% 15% 2	4680
August	1st to 10th	Day	14% 9% 20% 40% 16% 1	4213
	11th to 20th	Day	33% 8% 30% 20% 7% 1	4065
	21st to end	Day	35% 6% 21% 29% 9%	4432
September	1st to 10th	Day	28% 8% 23% 24% 14% 2	2280
	11th to 20th	Day	38% 9% 18% 20% 13% 1	2240
	21st to end	Day	40% 9% 21% 20% 10% 1	2024
October	1st to 10th	Day	47% 10% 17% 21% 4	3180
	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	66% 8% 13% 10% 2 1	1520
	11th to 20th	Day	62% 8% 12% 12% 4 1	1520
	21st to end	Day	63% 12% 9% 12% 4	1520
December	1st to 10th	Day	64% 6% 12% 14% 4	1520
	11th to 20th	Day	65% 13% 12% 8% 2 1	1710
	21st to end	Day	69% 10% 13% 7% 2	2442
January	1st to 10th	Night	68% 14% 9% 6% 3	4726
	11th to 20th	Night	58% 10% 14% 15% 2 1	4731
	21st to end	Night	73% 8% 8% 8% 2	4707
February	1st to 10th	Night	63% 10% 11% 9% 6%	2470
	11th to 20th	Night	56% 10% 13% 14% 6%	2404
	21st to end	Night	59% 9% 13% 13% 5% 1	2036
March	1st to 10th	Night	47% 17% 15% 17% 4	3555
	11th to 20th	Night	37% 11% 22% 27% 3 1	3490
	21st to end	Night	60% 9% 17% 12% 1	3777
April	1st to 10th	Night	51% 12% 19% 14% 3	1900
	11th to 20th	Night	47% 9% 17% 19% 6% 2	1772
	21st to end	Night	49% 9% 18% 15% 8% 1	1710
May	1st to 10th	Night	35% 7% 20% 28% 10% 1	2543
	11th to 20th	Night	26% 10% 30% 25% 8%	2516
	21st to end	Night	43% 13% 23% 18% 3	2712
June	1st to 10th	Night	43% 8% 19% 22% 6% 2	1330
	11th to 20th	Night	33% 11% 29% 23% 4	1330
	21st to end	Night	26% 9% 30% 24% 9% 3	1330
July	1st to 10th	Night	16% 5% 15% 28% 21% 14%	2185
	11th to 20th	Night	12% 6% 15% 34% 32% 2	2178
	21st to end	Night	19% 6% 20% 34% 18% 2	2631
August	1st to 10th	Night	13% 9% 18% 36% 23% 2	2695
	11th to 20th	Night	29% 7% 31% 21% 8% 3	2839
	21st to end	Night	32% 7% 18% 32% 10%	3236
September	1st to 10th	Night	21% 7% 22% 29% 16% 5%	1900
	11th to 20th	Night	38% 7% 17% 18% 17% 4	1938
	21st to end	Night	37% 8% 16% 23% 14% 2	2156
October	1st to 10th	Night	40% 7% 22% 24% 7%	3800
	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	61% 8% 14% 12% 4	2658
	11th to 20th	Night	59% 9% 16% 9% 5% 2	2660
	21st to end	Night	62% 11% 12% 10% 6%	2660
December	1st to 10th	Night	60% 10% 12% 13% 5%	2660
	11th to 20th	Night	63% 15% 14% 6% 2	3491
	21st to end	Night	64% 13% 12% 9% 3	5214

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	29% 12% 24% 23% 12%	180
	11th to 20th	Day	27% 11% 21% 27% 15%	180
	21st to end	Day	28% 6% 23% 27% 13% 2	292
February	1st to 10th	Day	25% 14% 25% 23% 12% 1	370
	11th to 20th	Day	23% 13% 19% 29% 16% 1	365
	21st to end	Day	19% 17% 20% 29% 14% 2	310
March	1st to 10th	Day	16% 12% 20% 30% 19% 3	460
	11th to 20th	Day	15% 11% 21% 28% 19% 6%	460
	21st to end	Day	17% 11% 21% 30% 19% 2	506
April	1st to 10th	Day	17% 12% 22% 27% 19% 4	460
	11th to 20th	Day	14% 11% 18% 33% 19% 6%	452
	21st to end	Day	18% 7% 25% 26% 20% 4	460
May	1st to 10th	Day	11% 8% 17% 32% 28% 4	463
	11th to 20th	Day	12% 6% 15% 31% 31% 7%	488
	21st to end	Day	11% 5% 21% 31% 25% 7%	549
June	1st to 10th	Day	13% 7% 23% 31% 22% 4	680
	11th to 20th	Day	7% 6% 29% 37% 16% 5%	680
	21st to end	Day	6% 4 23% 33% 24% 10%	680
July	1st to 10th	Day	7% 2 15% 36% 30% 9%	671
	11th to 20th	Day	4 3 19% 32% 32% 10%	668
	21st to end	Day	3 1 16% 30% 32% 18%	561
August	1st to 10th	Day	5% 2 12% 24% 39% 19%	487
	11th to 20th	Day	3 1 8% 26% 42% 20%	477
	21st to end	Day	7% 4 14% 30% 32% 13%	539
September	1st to 10th	Day	9% 3 8% 33% 37% 11%	490
	11th to 20th	Day	13% 3 11% 27% 34% 12%	490
	21st to end	Day	13% 4 17% 28% 30% 7%	490
October	1st to 10th	Day	17% 5% 14% 28% 26% 9%	490
	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% 10% 22% 26% 13%	188
	11th to 20th	Day	23% 8% 23% 23% 21% 2	180
	21st to end	Day	25% 11% 19% 26% 17% 2	180
December	1st to 10th	Day	25% 9% 16% 34% 15% 1	180
	11th to 20th	Day	24% 14% 21% 29% 11% 1	180
	21st to end	Day	31% 7% 23% 32% 7% 1	198
January	1st to 10th	Night	28% 14% 22% 24% 12% 1	850
	11th to 20th	Night	24% 13% 16% 28% 18% 1	846
	21st to end	Night	23% 9% 18% 29% 18% 4	836
February	1st to 10th	Night	24% 14% 21% 26% 13% 2	760
	11th to 20th	Night	21% 11% 19% 29% 17% 3	744
	21st to end	Night	24% 14% 19% 23% 16% 4	628
March	1st to 10th	Night	16% 11% 19% 30% 19% 5%	760
	11th to 20th	Night	19% 9% 23% 25% 19% 4	760
	21st to end	Night	16% 10% 22% 30% 19% 4	836
April	1st to 10th	Night	21% 8% 21% 28% 19% 3	760
	11th to 20th	Night	14% 9% 22% 32% 17% 5%	644
	21st to end	Night	15% 9% 24% 33% 15% 4	570
May	1st to 10th	Night	10% 6% 22% 33% 24% 4	383
	11th to 20th	Night	10% 5% 19% 33% 27% 6%	374
	21st to end	Night	10% 7% 20% 36% 26% 2	411
June	1st to 10th	Night	12% 6% 23% 33% 22% 3	380
	11th to 20th	Night	9% 4 27% 40% 17% 3	380
	21st to end	Night	6% 4 25% 31% 27% 7%	380
July	1st to 10th	Night	6% 1 17% 38% 34% 4	370
	11th to 20th	Night	3 2 18% 41% 30% 7%	368
	21st to end	Night	3 1 18% 38% 32% 8%	416
August	1st to 10th	Night	4 1 11% 32% 41% 10%	474
	11th to 20th	Night	4 1 7% 33% 38% 17%	564
	21st to end	Night	8% 2 17% 32% 32% 9%	678
September	1st to 10th	Night	7% 2 12% 33% 33% 14%	760
	11th to 20th	Night	11% 4 10% 27% 32% 16%	760
	21st to end	Night	13% 3 16% 30% 28% 11%	760
October	1st to 10th	Night	14% 5% 11% 27% 29% 13%	760
	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% 9% 19% 29% 15% 2	760
	11th to 20th	Night	23% 13% 20% 26% 15% 3	760
	21st to end	Night	24% 11% 21% 24% 16% 4	760
December	1st to 10th	Night	25% 10% 18% 28% 18% 2	760
	11th to 20th	Night	26% 13% 21% 27% 12% 1	838
	21st to end	Night	27% 13% 25% 25% 9% 1	935

Page generated at 2024-11-02 23:11:10. Version ac07

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