Hope Slide
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hope Slide clear sky chart from 2005-11-06 to 2026-05-02.

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% 18% 4 8% 5% 4 4 3 4 5% 30%	135574
	Night	15% 19% 5% 9% 5% 4 4 3 4 5% 27%	135604
Transparency	Day	58% 3 10% 17% 10% 2	135290
	Night	61% 3 10% 16% 9% 2	135466
Seeing	Day	28% 20% 29% 17% 6% 1	16856
	Night	28% 15% 28% 22% 7%	25335

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	14% 34% 7% 11% 5% 4 3 2 3 4 13%	4125
	11th to 20th	Day	8% 16% 6% 10% 4 3 3 3 4 6% 38%	4516
	21st to end	Day	13% 17% 5% 11% 6% 5% 4 3 4 4 28%	5186
February	1st to 10th	Day	18% 23% 6% 12% 5% 4 3 2 5% 6% 15%	1825
	11th to 20th	Day	14% 26% 6% 9% 5% 5% 4 3 4 5% 20%	1845
	21st to end	Day	11% 30% 9% 11% 6% 5% 5% 3 4 3 15%	1646
March	1st to 10th	Day	8% 22% 8% 12% 7% 6% 5% 3 4 4 19%	4618
	11th to 20th	Day	8% 30% 8% 11% 5% 4 3 2 3 4 21%	5061
	21st to end	Day	14% 27% 6% 12% 6% 5% 4 3 4 4 14%	5907
April	1st to 10th	Day	16% 29% 7% 8% 5% 4 5% 3 5% 5% 13%	2520
	11th to 20th	Day	14% 29% 5% 8% 5% 5% 4 3 4 5% 18%	2504
	21st to end	Day	19% 28% 4 9% 4 4 3 3 4 5% 19%	2873
May	1st to 10th	Day	14% 20% 3 7% 4 4 4 4 4 4 32%	6335
	11th to 20th	Day	21% 25% 4 8% 4 3 4 3 4 5% 18%	6277
	21st to end	Day	23% 21% 3 8% 5% 5% 4 3 4 5% 20%	6903
June	1st to 10th	Day	27% 15% 3 7% 5% 5% 4 3 4 4 24%	2925
	11th to 20th	Day	34% 15% 4 8% 5% 4 3 3 4 4 16%	3000
	21st to end	Day	24% 10% 3 6% 5% 5% 4 3 5% 5% 31%	3000
July	1st to 10th	Day	10% 4 2 4 3 3 4 3 5% 6% 58%	6780
	11th to 20th	Day	7% 3 1 4 2 2 2 2 4 5% 67%	6624
	21st to end	Day	9% 5% 2 4 3 3 3 3 5% 8% 55%	6874
August	1st to 10th	Day	8% 3 1 3 3 3 3 3 5% 7% 60%	4541
	11th to 20th	Day	11% 10% 3 7% 4 4 4 3 5% 7% 41%	4279
	21st to end	Day	18% 8% 3 8% 5% 6% 4 3 5% 4 38%	4450
September	1st to 10th	Day	15% 6% 2 6% 4 4 3 3 5% 6% 46%	2400
	11th to 20th	Day	20% 9% 3 9% 4 4 4 2 4 5% 38%	2400
	21st to end	Day	22% 12% 2 7% 4 3 3 3 4 5% 34%	2133
October	1st to 10th	Day	19% 12% 3 8% 5% 6% 5% 4 5% 6% 27%	3280
	11th to 20th	Day	26% 15% 4 9% 5% 4 4 3 4 4 23%	3261
	21st to end	Day	20% 23% 4 11% 6% 5% 5% 3 4 4 16%	3219
November	1st to 10th	Day	25% 23% 5% 9% 4 3 4 3 4 4 16%	1690
	11th to 20th	Day	23% 27% 5% 7% 4 4 3 2 3 3 19%	1680
	21st to end	Day	20% 24% 4 9% 5% 4 3 3 4 5% 18%	1680
December	1st to 10th	Day	13% 18% 5% 9% 5% 5% 5% 3 5% 6% 26%	2700
	11th to 20th	Day	12% 27% 7% 13% 6% 6% 5% 3 4 4 13%	2942
	21st to end	Day	13% 28% 8% 13% 7% 5% 4 3 4 4 11%	3575
January	1st to 10th	Night	13% 28% 7% 13% 6% 5% 4 3 4 4 13%	8770
	11th to 20th	Night	13% 14% 5% 9% 4 3 3 3 4 6% 36%	8365
	21st to end	Night	13% 15% 5% 11% 6% 5% 4 3 5% 5% 30%	8994
February	1st to 10th	Night	20% 23% 6% 10% 5% 4 3 3 4 5% 17%	2793
	11th to 20th	Night	15% 26% 6% 10% 5% 4 4 2 3 4 21%	2662
	21st to end	Night	13% 27% 6% 12% 6% 4 5% 4 4 4 15%	2160
March	1st to 10th	Night	7% 18% 7% 10% 5% 6% 4 4 7% 7% 24%	5496
	11th to 20th	Night	8% 25% 7% 11% 6% 4 3 3 3 4 24%	5154
	21st to end	Night	16% 24% 6% 11% 5% 5% 4 3 4 5% 17%	5309
April	1st to 10th	Night	14% 26% 6% 9% 6% 5% 4 3 3 5% 19%	2100
	11th to 20th	Night	14% 27% 4 8% 5% 5% 4 3 4 4 22%	2063
	21st to end	Night	19% 26% 4 7% 5% 3 4 3 4 4 20%	1745
May	1st to 10th	Night	11% 21% 3 6% 4 5% 6% 3 3 3 35%	3648
	11th to 20th	Night	22% 25% 4 8% 4 3 3 3 3 4 21%	3616
	21st to end	Night	22% 22% 4 6% 3 3 4 3 5% 6% 22%	3972
June	1st to 10th	Night	24% 11% 3 7% 4 5% 4 4 5% 5% 27%	1470
	11th to 20th	Night	25% 15% 3 8% 6% 4 4 4 6% 5% 22%	1400
	21st to end	Night	20% 10% 3 8% 4 4 2 4 4 6% 36%	1400
July	1st to 10th	Night	11% 5% 1 4 3 3 3 3 4 5% 60%	3180
	11th to 20th	Night	6% 3 1 3 3 2 3 2 4 5% 68%	3335
	21st to end	Night	9% 6% 2 4 3 3 3 3 4 6% 57%	3969
August	1st to 10th	Night	6% 4 1 3 2 3 3 3 6% 8% 61%	2609
	11th to 20th	Night	10% 14% 3 7% 4 3 3 4 6% 6% 40%	2772
	21st to end	Night	18% 9% 2 7% 4 4 3 4 5% 7% 37%	3466
September	1st to 10th	Night	15% 8% 3 6% 4 4 3 2 4 5% 46%	2000
	11th to 20th	Night	22% 9% 2 6% 3 3 3 2 3 6% 39%	2000
	21st to end	Night	22% 12% 3 6% 4 4 4 3 4 6% 32%	2265
October	1st to 10th	Night	17% 12% 3 8% 5% 6% 6% 5% 5% 7% 27%	3920
	11th to 20th	Night	28% 14% 3 7% 4 4 3 3 4 5% 27%	3897
	21st to end	Night	20% 20% 5% 9% 5% 4 4 3 5% 5% 20%	4569
November	1st to 10th	Night	27% 21% 5% 8% 5% 4 3 3 4 5% 16%	2825
	11th to 20th	Night	22% 24% 5% 7% 5% 5% 3 3 4 4 17%	2940
	21st to end	Night	22% 25% 5% 9% 5% 5% 4 2 4 3 17%	2940
December	1st to 10th	Night	14% 17% 5% 9% 4 4 4 3 5% 6% 28%	4700
	11th to 20th	Night	16% 24% 7% 14% 7% 5% 4 3 4 4 13%	5477
	21st to end	Night	13% 28% 8% 13% 6% 5% 5% 3 5% 5% 10%	7623

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	79% 2 4 11% 3 1	4125
	11th to 20th	Day	51% 3 9% 19% 15% 4	4540
	21st to end	Day	63% 2 7% 12% 15%	5187
February	1st to 10th	Day	70% 4 7% 10% 7% 2	1825
	11th to 20th	Day	69% 3 8% 11% 8% 2	1854
	21st to end	Day	75% 4 7% 11% 3	1646
March	1st to 10th	Day	71% 3 5% 9% 11% 1	4618
	11th to 20th	Day	68% 2 9% 17% 4	5061
	21st to end	Day	76% 2 8% 8% 5%	5907
April	1st to 10th	Day	75% 3 10% 10% 1	2520
	11th to 20th	Day	68% 5% 9% 11% 7% 1	2504
	21st to end	Day	69% 3 9% 11% 7%	2873
May	1st to 10th	Day	55% 3 8% 26% 8%	6335
	11th to 20th	Day	70% 2 13% 13% 1	6280
	21st to end	Day	69% 3 11% 14% 2	6904
June	1st to 10th	Day	65% 3 11% 14% 5% 2	2925
	11th to 20th	Day	72% 4 8% 12% 4	3000
	21st to end	Day	55% 4 14% 19% 8% 1	3000
July	1st to 10th	Day	29% 3 13% 23% 23% 9%	6706
	11th to 20th	Day	22% 2 13% 31% 25% 6%	6543
	21st to end	Day	30% 2 17% 29% 18% 3	6681
August	1st to 10th	Day	26% 2 10% 30% 29% 3	4524
	11th to 20th	Day	42% 2 22% 22% 10% 2	4321
	21st to end	Day	50% 2 10% 25% 11% 2	4449
September	1st to 10th	Day	41% 2 12% 26% 15% 4	2400
	11th to 20th	Day	51% 2 8% 19% 17% 2	2400
	21st to end	Day	55% 2 13% 17% 10% 2	2133
October	1st to 10th	Day	58% 2 13% 20% 7%	3280
	11th to 20th	Day	65% 3 8% 17% 7% 1	3261
	21st to end	Day	73% 2 8% 10% 6%	3219
November	1st to 10th	Day	74% 4 7% 11% 4	1690
	11th to 20th	Day	71% 3 7% 11% 5% 2	1680
	21st to end	Day	69% 4 7% 12% 7% 2	1680
December	1st to 10th	Day	61% 4 10% 14% 7% 3	2700
	11th to 20th	Day	76% 2 7% 10% 3 1	2944
	21st to end	Day	79% 3 7% 8% 3	3575
January	1st to 10th	Night	79% 1 5% 10% 4 1	8770
	11th to 20th	Night	52% 3 10% 17% 15% 3	8403
	21st to end	Night	61% 2 8% 14% 14% 2	8994
February	1st to 10th	Night	72% 3 6% 10% 7% 1	2793
	11th to 20th	Night	70% 3 6% 11% 7% 3	2664
	21st to end	Night	75% 3 6% 12% 3	2160
March	1st to 10th	Night	61% 3 9% 13% 12% 2	5496
	11th to 20th	Night	65% 2 10% 18% 4	5155
	21st to end	Night	74% 3 9% 11% 4	5309
April	1st to 10th	Night	70% 3 13% 12% 2	2100
	11th to 20th	Night	64% 4 12% 11% 9% 1	2063
	21st to end	Night	68% 3 11% 12% 6% 1	1745
May	1st to 10th	Night	53% 3 11% 23% 9%	3645
	11th to 20th	Night	68% 3 13% 14% 3	3616
	21st to end	Night	65% 3 17% 14% 2	3972
June	1st to 10th	Night	58% 6% 15% 13% 7% 1	1470
	11th to 20th	Night	61% 6% 13% 15% 4	1400
	21st to end	Night	48% 5% 19% 20% 7% 1	1400
July	1st to 10th	Night	28% 2 17% 22% 22% 9%	3145
	11th to 20th	Night	20% 4 16% 28% 25% 8%	3293
	21st to end	Night	31% 2 17% 28% 16% 5%	3857
August	1st to 10th	Night	22% 3 12% 34% 27% 1	2601
	11th to 20th	Night	46% 3 19% 20% 11% 1	2796
	21st to end	Night	46% 3 15% 24% 10% 2	3466
September	1st to 10th	Night	40% 4 13% 23% 17% 3	2000
	11th to 20th	Night	49% 3 10% 19% 17% 3	2000
	21st to end	Night	55% 3 13% 17% 9% 2	2265
October	1st to 10th	Night	54% 3 15% 22% 6%	3920
	11th to 20th	Night	61% 2 10% 17% 9%	3897
	21st to end	Night	68% 2 10% 12% 7% 1	4569
November	1st to 10th	Night	73% 3 9% 9% 5% 1	2825
	11th to 20th	Night	72% 4 8% 9% 6% 1	2940
	21st to end	Night	74% 2 5% 10% 7% 2	2940
December	1st to 10th	Night	60% 4 10% 16% 8% 2	4700
	11th to 20th	Night	77% 2 7% 10% 3	5474
	21st to end	Night	79% 2 6% 9% 3	7623

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	26% 28% 28% 13% 5%	220
	11th to 20th	Day	25% 20% 30% 19% 5%	220
	21st to end	Day	19% 24% 34% 18% 5%	242
February	1st to 10th	Day	34% 26% 27% 11% 2	365
	11th to 20th	Day	30% 22% 30% 15% 3	425
	21st to end	Day	32% 24% 29% 12% 3	402
March	1st to 10th	Day	29% 24% 31% 12% 4	540
	11th to 20th	Day	33% 23% 22% 16% 5% 1	540
	21st to end	Day	34% 24% 27% 13% 3	594
April	1st to 10th	Day	36% 22% 24% 14% 3	540
	11th to 20th	Day	28% 25% 27% 11% 7% 1	532
	21st to end	Day	36% 21% 23% 13% 5% 2	540
May	1st to 10th	Day	26% 19% 33% 19% 4 1	508
	11th to 20th	Day	30% 14% 28% 20% 7% 1	528
	21st to end	Day	36% 19% 23% 15% 5% 1	573
June	1st to 10th	Day	29% 22% 28% 16% 4	655
	11th to 20th	Day	37% 22% 25% 12% 2 1	730
	21st to end	Day	28% 17% 29% 18% 6% 1	730
July	1st to 10th	Day	18% 17% 41% 18% 5% 1	721
	11th to 20th	Day	16% 18% 41% 18% 5% 2	653
	21st to end	Day	10% 15% 39% 24% 10% 3	575
August	1st to 10th	Day	14% 11% 35% 26% 9% 4	527
	11th to 20th	Day	13% 14% 34% 27% 10% 2	517
	21st to end	Day	17% 18% 34% 19% 10% 1	583
September	1st to 10th	Day	16% 21% 29% 22% 9% 2	530
	11th to 20th	Day	25% 19% 29% 20% 7% 1	530
	21st to end	Day	30% 18% 27% 18% 7% 1	530
October	1st to 10th	Day	27% 18% 29% 17% 9% 1	530
	11th to 20th	Day	35% 19% 20% 18% 6% 1	530
	21st to end	Day	36% 19% 27% 15% 3	382
November	1st to 10th	Day	38% 14% 30% 16% 2	242
	11th to 20th	Day	40% 17% 25% 12% 6%	220
	21st to end	Day	33% 25% 24% 10% 8% 1	220
December	1st to 10th	Day	33% 17% 22% 17% 9% 1	220
	11th to 20th	Day	36% 22% 27% 10% 3 2	220
	21st to end	Day	30% 21% 32% 16%	242
January	1st to 10th	Night	34% 24% 29% 12% 2	950
	11th to 20th	Night	30% 18% 33% 17% 1	874
	21st to end	Night	25% 19% 37% 17% 2	924
February	1st to 10th	Night	32% 18% 34% 14% 2	840
	11th to 20th	Night	28% 19% 34% 17% 2	824
	21st to end	Night	30% 19% 31% 17% 3	692
March	1st to 10th	Night	27% 16% 30% 22% 5%	840
	11th to 20th	Night	28% 19% 29% 20% 4	840
	21st to end	Night	29% 19% 31% 17% 5%	924
April	1st to 10th	Night	28% 15% 28% 24% 5% 1	840
	11th to 20th	Night	28% 15% 29% 24% 5%	815
	21st to end	Night	29% 14% 30% 20% 7%	485
May	1st to 10th	Night	23% 10% 28% 29% 10%	402
	11th to 20th	Night	25% 10% 20% 32% 12% 1	394
	21st to end	Night	30% 9% 23% 27% 11%	433
June	1st to 10th	Night	27% 8% 28% 27% 10%	400
	11th to 20th	Night	31% 9% 25% 27% 9% 1	400
	21st to end	Night	22% 7% 26% 33% 12% 1	400
July	1st to 10th	Night	17% 6% 20% 39% 17% 1	390
	11th to 20th	Night	11% 6% 22% 42% 18%	388
	21st to end	Night	9% 5% 18% 43% 25%	438
August	1st to 10th	Night	13% 3 16% 36% 30% 2	398
	11th to 20th	Night	15% 3 19% 38% 25% 1	513
	21st to end	Night	17% 7% 26% 35% 15%	790
September	1st to 10th	Night	18% 8% 27% 29% 19% 1	800
	11th to 20th	Night	24% 8% 19% 27% 19% 2	800
	21st to end	Night	28% 11% 24% 28% 9%	800
October	1st to 10th	Night	26% 11% 27% 27% 9%	800
	11th to 20th	Night	35% 13% 25% 23% 5%	800
	21st to end	Night	38% 14% 28% 16% 4	880
November	1st to 10th	Night	39% 17% 32% 11% 1	820
	11th to 20th	Night	38% 17% 30% 13% 2	840
	21st to end	Night	36% 20% 26% 15% 3	840
December	1st to 10th	Night	27% 17% 36% 16% 3	840
	11th to 20th	Night	36% 22% 31% 11% 1	876
	21st to end	Night	32% 22% 37% 9%	1045

Page generated at 2026-04-28 23:21:47. Version ac07

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