Deer Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Deer Park 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	11% 8% 3 7% 4 4 4 3 5% 6% 45%	130055
	Night	13% 10% 3 7% 4 4 4 4 5% 7% 40%	128012
Transparency	Day	39% 7% 12% 22% 15% 5%	129781
	Night	43% 8% 12% 20% 13% 5%	127863
Seeing	Day	15% 10% 24% 27% 18% 6%	15994
	Night	18% 6% 17% 35% 22% 2	25055

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	21% 23% 8% 14% 6% 4 3 3 4 4 11%	3639
	11th to 20th	Day	16% 11% 5% 10% 6% 6% 5% 3 4 6% 30%	3616
	21st to end	Day	22% 16% 4 7% 5% 4 4 2 4 6% 26%	4004
February	1st to 10th	Day	22% 15% 4 10% 5% 5% 4 3 5% 6% 22%	1600
	11th to 20th	Day	15% 15% 5% 9% 5% 4 4 3 5% 6% 27%	1863
	21st to end	Day	17% 15% 5% 8% 6% 4 4 4 5% 5% 29%	1650
March	1st to 10th	Day	10% 11% 3 7% 5% 5% 6% 5% 7% 7% 33%	4518
	11th to 20th	Day	12% 9% 3 7% 5% 4 4 3 4 6% 43%	4682
	21st to end	Day	15% 13% 3 9% 6% 5% 6% 4 6% 8% 24%	5925
April	1st to 10th	Day	14% 10% 4 8% 5% 6% 6% 6% 6% 7% 28%	2400
	11th to 20th	Day	11% 8% 3 7% 6% 5% 4 4 6% 6% 39%	2440
	21st to end	Day	9% 8% 2 8% 6% 6% 5% 4 7% 8% 37%	2600
May	1st to 10th	Day	7% 5% 2 6% 5% 5% 5% 4 6% 8% 46%	6232
	11th to 20th	Day	8% 7% 3 7% 5% 6% 6% 4 7% 10% 37%	6277
	21st to end	Day	7% 6% 2 7% 5% 5% 5% 5% 7% 8% 43%	6963
June	1st to 10th	Day	15% 7% 2 6% 5% 4 4 3 5% 7% 42%	3000
	11th to 20th	Day	12% 6% 3 6% 5% 5% 5% 5% 6% 8% 40%	3000
	21st to end	Day	6% 4 2 5% 3 4 4 4 5% 6% 57%	3000
July	1st to 10th	Day	4 2 1 3 3 2 3 2 3 5% 72%	6396
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	6320
	21st to end	Day	3 2 1 3 2 2 2 2 4 5% 75%	6874
August	1st to 10th	Day	3 2 1 3 2 2 3 2 5% 7% 69%	4541
	11th to 20th	Day	3 3 1 3 3 3 3 3 5% 7% 67%	4294
	21st to end	Day	3 3 1 4 3 4 4 3 4 7% 63%	4391
September	1st to 10th	Day	4 3 1 3 2 3 3 3 4 6% 67%	2400
	11th to 20th	Day	7% 4 2 4 4 4 4 4 5% 7% 54%	2293
	21st to end	Day	8% 4 2 6% 4 4 5% 4 6% 8% 50%	2200
October	1st to 10th	Day	8% 5% 2 4 4 3 4 3 4 8% 56%	3379
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	2973
	21st to end	Day	18% 8% 2 7% 4 4 5% 4 5% 7% 36%	3101
November	1st to 10th	Day	24% 12% 3 7% 5% 3 4 3 4 6% 28%	1764
	11th to 20th	Day	23% 13% 4 9% 5% 4 4 3 4 5% 25%	1685
	21st to end	Day	20% 18% 6% 13% 5% 4 4 4 4 4 18%	1452
December	1st to 10th	Day	16% 10% 6% 14% 7% 5% 4 4 4 7% 22%	2290
	11th to 20th	Day	24% 24% 6% 11% 5% 5% 3 2 3 4 13%	2642
	21st to end	Day	22% 27% 6% 10% 5% 4 4 3 4 4 11%	3651
January	1st to 10th	Night	25% 20% 6% 11% 6% 4 5% 3 4 4 12%	6306
	11th to 20th	Night	15% 11% 3 10% 5% 5% 5% 5% 5% 8% 30%	6282
	21st to end	Night	21% 17% 4 7% 3 3 3 3 5% 5% 29%	6952
February	1st to 10th	Night	17% 17% 5% 8% 5% 4 4 4 5% 6% 26%	2800
	11th to 20th	Night	16% 12% 4 9% 5% 5% 5% 5% 5% 5% 30%	2421
	21st to end	Night	13% 12% 3 8% 6% 5% 4 4 6% 6% 33%	1980
March	1st to 10th	Night	8% 10% 3 8% 5% 4 4 4 7% 8% 40%	5376
	11th to 20th	Night	14% 11% 3 6% 4 3 3 2 4 5% 45%	5317
	21st to end	Night	14% 10% 3 8% 6% 7% 6% 5% 8% 9% 24%	5034
April	1st to 10th	Night	11% 9% 3 7% 6% 5% 5% 5% 6% 8% 35%	2000
	11th to 20th	Night	11% 9% 3 7% 6% 5% 4 4 4 6% 42%	1911
	21st to end	Night	10% 7% 2 7% 5% 6% 5% 5% 5% 7% 40%	1800
May	1st to 10th	Night	8% 5% 2 6% 4 5% 5% 4 6% 8% 48%	3717
	11th to 20th	Night	10% 7% 3 7% 5% 4 5% 4 6% 8% 41%	3616
	21st to end	Night	9% 5% 3 6% 4 4 4 4 7% 8% 47%	3908
June	1st to 10th	Night	12% 5% 2 6% 4 4 4 4 6% 9% 44%	1400
	11th to 20th	Night	9% 5% 3 5% 6% 5% 6% 4 7% 7% 43%	1400
	21st to end	Night	5% 5% 2 4 3 4 3 4 7% 7% 56%	1400
July	1st to 10th	Night	3 2 1 3 2 2 2 2 4 6% 73%	3545
	11th to 20th	Night	2 1 1 2 2 2 2 2 2 4 80%	3640
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 76%	3969
August	1st to 10th	Night	3 3 1 3 2 2 3 2 4 7% 71%	2609
	11th to 20th	Night	3 3 1 4 3 3 4 4 6% 6% 63%	2759
	21st to end	Night	3 3 2 5% 3 4 4 3 5% 7% 62%	3523
September	1st to 10th	Night	4 2 1 3 2 3 4 3 6% 7% 65%	2000
	11th to 20th	Night	7% 3 2 6% 4 4 4 4 5% 8% 53%	2106
	21st to end	Night	9% 5% 2 5% 3 3 4 3 5% 7% 54%	2200
October	1st to 10th	Night	6% 3 1 4 3 3 3 3 6% 8% 60%	3664
	11th to 20th	Night	10% 5% 2 5% 3 4 4 4 6% 8% 49%	3958
	21st to end	Night	14% 6% 2 6% 4 5% 5% 4 5% 7% 41%	4457
November	1st to 10th	Night	21% 12% 3 7% 5% 3 4 4 5% 7% 28%	2535
	11th to 20th	Night	20% 11% 4 8% 5% 5% 5% 4 5% 6% 28%	2714
	21st to end	Night	22% 13% 3 8% 6% 5% 4 4 5% 6% 24%	2925
December	1st to 10th	Night	11% 10% 5% 10% 6% 5% 5% 4 6% 8% 30%	4890
	11th to 20th	Night	22% 20% 6% 11% 6% 4 4 3 4 5% 14%	5550
	21st to end	Night	23% 20% 6% 12% 6% 5% 4 3 3 4 15%	7348

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% 10% 7% 6% 1	3639
	11th to 20th	Day	54% 9% 15% 15% 4 2	3640
	21st to end	Day	60% 7% 12% 16% 4	4004
February	1st to 10th	Day	63% 9% 12% 8% 6% 1	1600
	11th to 20th	Day	57% 11% 13% 13% 7%	1872
	21st to end	Day	57% 9% 12% 15% 7% 1	1650
March	1st to 10th	Day	45% 8% 14% 19% 14%	4518
	11th to 20th	Day	42% 6% 18% 28% 7%	4682
	21st to end	Day	56% 10% 16% 14% 4	5925
April	1st to 10th	Day	52% 8% 17% 17% 5% 1	2400
	11th to 20th	Day	43% 7% 19% 19% 11% 2	2440
	21st to end	Day	43% 8% 17% 21% 10% 2	2600
May	1st to 10th	Day	31% 7% 13% 27% 22% 1	6232
	11th to 20th	Day	37% 8% 15% 28% 11% 1	6280
	21st to end	Day	34% 8% 18% 29% 9% 1	6964
June	1st to 10th	Day	41% 6% 12% 22% 17% 2	3000
	11th to 20th	Day	38% 8% 14% 23% 14% 3	3000
	21st to end	Day	26% 6% 14% 29% 20% 5%	3000
July	1st to 10th	Day	16% 3 7% 21% 30% 22%	6327
	11th to 20th	Day	11% 2 5% 22% 31% 29%	6245
	21st to end	Day	13% 3 6% 28% 32% 18%	6681
August	1st to 10th	Day	16% 3 6% 24% 35% 15%	4524
	11th to 20th	Day	17% 4 9% 39% 25% 5%	4336
	21st to end	Day	21% 5% 11% 33% 26% 4	4390
September	1st to 10th	Day	19% 5% 12% 29% 25% 10%	2400
	11th to 20th	Day	28% 8% 14% 25% 22% 4	2293
	21st to end	Day	31% 9% 18% 27% 12% 4	2200
October	1st to 10th	Day	29% 5% 15% 35% 16% 1	3379
	11th to 20th	Day	37% 8% 18% 24% 13%	2973
	21st to end	Day	47% 8% 16% 19% 9% 1	3101
November	1st to 10th	Day	58% 9% 15% 14% 4 1	1764
	11th to 20th	Day	62% 10% 11% 11% 4 1	1685
	21st to end	Day	68% 10% 11% 9% 3	1452
December	1st to 10th	Day	63% 11% 14% 10% 3	2290
	11th to 20th	Day	77% 6% 9% 6% 2	2644
	21st to end	Day	75% 8% 9% 6% 2	3651
January	1st to 10th	Night	73% 9% 8% 8% 1	6306
	11th to 20th	Night	50% 12% 16% 15% 6% 1	6320
	21st to end	Night	57% 8% 14% 18% 4	6952
February	1st to 10th	Night	58% 9% 14% 12% 7% 1	2800
	11th to 20th	Night	54% 8% 16% 13% 8% 1	2423
	21st to end	Night	50% 8% 15% 17% 9% 1	1980
March	1st to 10th	Night	38% 7% 16% 24% 15% 1	5376
	11th to 20th	Night	42% 5% 14% 26% 13%	5318
	21st to end	Night	51% 10% 17% 18% 4	5034
April	1st to 10th	Night	44% 8% 15% 23% 9% 1	2000
	11th to 20th	Night	41% 6% 12% 22% 16% 2	1911
	21st to end	Night	40% 8% 13% 23% 15% 2	1800
May	1st to 10th	Night	30% 6% 10% 25% 26% 2	3714
	11th to 20th	Night	36% 8% 13% 27% 15% 1	3616
	21st to end	Night	31% 8% 14% 33% 12% 1	3908
June	1st to 10th	Night	36% 7% 10% 21% 20% 5%	1400
	11th to 20th	Night	36% 8% 13% 19% 19% 5%	1400
	21st to end	Night	25% 6% 10% 26% 25% 8%	1400
July	1st to 10th	Night	13% 2 6% 19% 31% 28%	3505
	11th to 20th	Night	10% 2 4 17% 27% 39%	3592
	21st to end	Night	12% 2 3 23% 34% 25%	3857
August	1st to 10th	Night	15% 2 5% 19% 38% 21%	2601
	11th to 20th	Night	20% 5% 6% 31% 29% 9%	2783
	21st to end	Night	21% 5% 11% 25% 29% 10%	3523
September	1st to 10th	Night	16% 5% 8% 27% 28% 15%	2000
	11th to 20th	Night	28% 6% 11% 23% 24% 8%	2106
	21st to end	Night	29% 6% 14% 29% 17% 5%	2200
October	1st to 10th	Night	23% 4 10% 42% 20%	3664
	11th to 20th	Night	33% 7% 16% 24% 20% 1	3958
	21st to end	Night	40% 8% 15% 28% 8% 2	4457
November	1st to 10th	Night	53% 10% 16% 15% 5% 1	2535
	11th to 20th	Night	57% 10% 14% 12% 5% 2	2714
	21st to end	Night	60% 9% 14% 12% 5%	2925
December	1st to 10th	Night	52% 15% 15% 16% 2	4890
	11th to 20th	Night	73% 8% 11% 6% 1	5547
	21st to end	Night	71% 9% 10% 7% 2	7348

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	32% 10% 19% 21% 15% 4	200
	11th to 20th	Day	30% 9% 19% 22% 16% 6%	200
	21st to end	Day	31% 6% 21% 25% 14% 3	220
February	1st to 10th	Day	24% 15% 29% 24% 6% 3	200
	11th to 20th	Day	25% 13% 25% 23% 12% 2	449
	21st to end	Day	21% 14% 25% 26% 13% 1	414
March	1st to 10th	Day	20% 17% 28% 23% 10% 3	500
	11th to 20th	Day	19% 14% 22% 23% 19% 3	500
	21st to end	Day	23% 17% 25% 24% 10% 2	550
April	1st to 10th	Day	19% 15% 27% 22% 13% 5%	500
	11th to 20th	Day	17% 21% 26% 20% 12% 4	492
	21st to end	Day	10% 15% 27% 29% 15% 4	500
May	1st to 10th	Day	11% 10% 31% 28% 16% 4	503
	11th to 20th	Day	17% 10% 27% 25% 16% 5%	528
	21st to end	Day	12% 10% 23% 28% 19% 8%	594
June	1st to 10th	Day	16% 10% 23% 26% 20% 5%	730
	11th to 20th	Day	12% 12% 32% 28% 13% 3	730
	21st to end	Day	5% 7% 25% 29% 28% 6%	730
July	1st to 10th	Day	3 7% 28% 33% 25% 4	563
	11th to 20th	Day	4 8% 26% 35% 19% 7%	524
	21st to end	Day	4 5% 23% 37% 22% 8%	575
August	1st to 10th	Day	6% 6% 20% 35% 25% 9%	527
	11th to 20th	Day	4 4 21% 31% 28% 11%	517
	21st to end	Day	7% 9% 21% 27% 25% 10%	583
September	1st to 10th	Day	6% 5% 23% 28% 25% 13%	530
	11th to 20th	Day	8% 10% 22% 26% 25% 8%	530
	21st to end	Day	12% 12% 25% 25% 20% 7%	530
October	1st to 10th	Day	13% 9% 23% 29% 21% 5%	503
	11th to 20th	Day	16% 11% 17% 26% 21% 9%	379
	21st to end	Day	27% 8% 21% 29% 12% 3	330
November	1st to 10th	Day	31% 7% 17% 30% 14% 1	300
	11th to 20th	Day	34% 9% 17% 24% 12% 4	243
	21st to end	Day	29% 6% 20% 26% 18% 3	200
December	1st to 10th	Day	28% 8% 20% 23% 15% 7%	200
	11th to 20th	Day	33% 10% 22% 25% 10% 2	200
	21st to end	Day	32% 15% 18% 24% 10% 2	220
January	1st to 10th	Night	37% 7% 17% 26% 11% 2	800
	11th to 20th	Night	27% 7% 19% 31% 14% 2	800
	21st to end	Night	25% 5% 19% 33% 17% 1	880
February	1st to 10th	Night	24% 8% 22% 32% 12% 2	800
	11th to 20th	Night	22% 8% 21% 33% 15% 2	784
	21st to end	Night	19% 8% 20% 33% 18% 2	660
March	1st to 10th	Night	20% 8% 18% 33% 19% 2	800
	11th to 20th	Night	20% 8% 17% 34% 20% 1	800
	21st to end	Night	20% 6% 20% 37% 15% 2	880
April	1st to 10th	Night	15% 6% 19% 38% 20% 2	800
	11th to 20th	Night	14% 7% 24% 38% 15% 2	723
	21st to end	Night	13% 6% 23% 36% 21% 2	600
May	1st to 10th	Night	9% 6% 18% 39% 26% 2	438
	11th to 20th	Night	12% 5% 14% 30% 32% 6%	394
	21st to end	Night	12% 4 17% 35% 27% 5%	433
June	1st to 10th	Night	12% 6% 15% 41% 25% 2	400
	11th to 20th	Night	11% 5% 23% 38% 22% 2	400
	21st to end	Night	7% 3 11% 48% 28% 4	400
July	1st to 10th	Night	4 2 9% 43% 39% 3	390
	11th to 20th	Night	2 3 14% 46% 34% 1	388
	21st to end	Night	1 2 8% 42% 45% 3	438
August	1st to 10th	Night	4 2 11% 37% 41% 5%	398
	11th to 20th	Night	3 3 9% 36% 41% 8%	500
	21st to end	Night	4 4 12% 39% 39% 3	821
September	1st to 10th	Night	5% 2 11% 41% 35% 6%	800
	11th to 20th	Night	7% 4 12% 33% 38% 7%	800
	21st to end	Night	10% 4 17% 36% 28% 5%	800
October	1st to 10th	Night	12% 5% 15% 39% 28% 2	772
	11th to 20th	Night	14% 5% 15% 36% 26% 3	760
	21st to end	Night	20% 3 21% 37% 16% 3	836
November	1st to 10th	Night	26% 7% 17% 36% 14%	780
	11th to 20th	Night	26% 7% 21% 30% 15% 2	800
	21st to end	Night	28% 8% 14% 31% 17% 2	925
December	1st to 10th	Night	24% 6% 16% 35% 17% 2	1000
	11th to 20th	Night	33% 6% 17% 31% 12% 1	1000
	21st to end	Night	36% 7% 16% 31% 9%	1055

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