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 2025-01-07.

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	17% 18% 4 8% 5% 4 4 3 4 5% 29%	108971
	Night	17% 19% 5% 8% 5% 4 4 3 4 5% 26%	108632
Transparency	Day	59% 3 10% 17% 10% 2	108728
	Night	62% 3 10% 15% 8% 2	108521
Seeing	Day	27% 20% 29% 17% 6% 1	15188
	Night	28% 14% 28% 22% 7%	23641

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	18% 34% 7% 8% 4 3 2 2 3 3 15%	2261
	11th to 20th	Day	11% 22% 6% 15% 5% 3 3 2 3 4 26%	2400
	21st to end	Day	22% 24% 6% 11% 6% 4 3 2 3 4 14%	2794
February	1st to 10th	Day	20% 25% 6% 11% 5% 4 3 2 4 6% 14%	1651
	11th to 20th	Day	15% 27% 5% 9% 5% 5% 4 3 5% 5% 19%	1665
	21st to end	Day	11% 30% 9% 10% 6% 5% 5% 3 4 3 15%	1494
March	1st to 10th	Day	10% 22% 9% 13% 7% 5% 4 3 4 4 20%	3138
	11th to 20th	Day	10% 28% 5% 7% 4 3 2 2 3 4 31%	3455
	21st to end	Day	15% 28% 6% 10% 6% 5% 4 3 4 3 16%	4028
April	1st to 10th	Day	17% 31% 7% 8% 5% 4 4 3 5% 4 11%	2280
	11th to 20th	Day	15% 30% 4 8% 6% 5% 4 3 4 5% 17%	2262
	21st to end	Day	20% 30% 4 9% 4 4 3 3 4 4 16%	2599
May	1st to 10th	Day	15% 26% 3 8% 4 3 3 2 3 4 29%	4420
	11th to 20th	Day	19% 19% 3 7% 4 3 4 3 5% 7% 26%	4377
	21st to end	Day	29% 27% 4 8% 4 4 3 2 3 3 14%	4813
June	1st to 10th	Day	29% 16% 3 7% 5% 5% 3 3 3 4 22%	2779
	11th to 20th	Day	36% 16% 4 8% 4 4 3 2 4 4 15%	2850
	21st to end	Day	23% 9% 3 6% 5% 5% 4 3 5% 5% 31%	2850
July	1st to 10th	Day	9% 4 2 4 3 3 4 3 4 4 60%	4740
	11th to 20th	Day	9% 4 1 3 3 2 2 2 4 4 66%	4626
	21st to end	Day	12% 5% 2 4 3 3 4 3 5% 6% 54%	4790
August	1st to 10th	Day	8% 3 1 3 3 3 3 3 5% 7% 61%	4401
	11th to 20th	Day	11% 10% 3 7% 4 4 4 3 5% 7% 41%	4145
	21st to end	Day	18% 8% 3 8% 5% 6% 4 3 4 4 37%	4314
September	1st to 10th	Day	16% 6% 2 6% 3 4 3 3 5% 5% 46%	2280
	11th to 20th	Day	20% 9% 3 9% 5% 4 4 2 4 5% 36%	2280
	21st to end	Day	22% 12% 2 7% 4 3 4 3 4 5% 33%	2027
October	1st to 10th	Day	19% 12% 3 8% 5% 6% 5% 4 5% 6% 27%	3180
	11th to 20th	Day	27% 14% 4 9% 5% 4 4 3 4 4 23%	3161
	21st to end	Day	20% 23% 4 10% 6% 5% 4 3 4 4 16%	3119
November	1st to 10th	Day	26% 23% 5% 8% 4 3 4 3 4 4 15%	1608
	11th to 20th	Day	22% 28% 5% 7% 4 4 3 2 3 3 19%	1600
	21st to end	Day	20% 25% 4 9% 5% 4 4 3 4 5% 18%	1600
December	1st to 10th	Day	13% 17% 5% 9% 5% 5% 5% 4 5% 6% 27%	2620
	11th to 20th	Day	11% 27% 7% 13% 6% 6% 5% 3 4 4 13%	2866
	21st to end	Day	13% 29% 8% 13% 7% 5% 4 3 4 4 11%	3498
January	1st to 10th	Night	18% 31% 7% 10% 5% 4 3 2 3 4 13%	4816
	11th to 20th	Night	15% 21% 6% 9% 4 3 4 3 4 6% 25%	4481
	21st to end	Night	21% 23% 6% 11% 5% 4 4 3 5% 5% 13%	4862
February	1st to 10th	Night	22% 24% 6% 10% 5% 4 3 3 4 5% 16%	2527
	11th to 20th	Night	16% 27% 6% 10% 5% 4 4 2 3 4 20%	2402
	21st to end	Night	14% 27% 6% 11% 6% 4 5% 4 4 4 16%	1960
March	1st to 10th	Night	9% 18% 7% 12% 6% 6% 5% 5% 6% 5% 21%	3736
	11th to 20th	Night	11% 24% 6% 7% 4 3 3 2 3 4 33%	3520
	21st to end	Night	16% 25% 5% 9% 5% 5% 4 3 3 4 20%	3624
April	1st to 10th	Night	14% 27% 6% 9% 6% 5% 4 3 3 5% 18%	1900
	11th to 20th	Night	15% 27% 4 8% 6% 5% 4 3 4 4 22%	1865
	21st to end	Night	20% 28% 4 7% 5% 3 4 3 4 4 17%	1579
May	1st to 10th	Night	13% 26% 4 7% 4 3 4 2 3 3 33%	2540
	11th to 20th	Night	17% 20% 3 7% 4 4 4 3 4 5% 29%	2516
	21st to end	Night	28% 27% 4 6% 3 3 3 2 3 4 16%	2762
June	1st to 10th	Night	25% 12% 3 7% 4 5% 4 4 5% 5% 26%	1397
	11th to 20th	Night	26% 15% 3 7% 6% 3 4 4 6% 5% 21%	1330
	21st to end	Night	20% 9% 2 8% 5% 4 2 3 4 7% 37%	1330
July	1st to 10th	Night	10% 5% 1 4 2 3 2 2 3 5% 61%	2220
	11th to 20th	Night	8% 3 1 3 3 2 3 2 4 5% 67%	2333
	21st to end	Night	11% 6% 1 4 3 3 3 3 4 6% 56%	2762
August	1st to 10th	Night	6% 3 1 3 2 3 3 3 6% 8% 62%	2529
	11th to 20th	Night	10% 14% 3 7% 4 3 3 4 6% 6% 40%	2686
	21st to end	Night	18% 9% 2 7% 4 4 3 4 5% 7% 36%	3360
September	1st to 10th	Night	15% 8% 3 6% 4 4 3 2 4 5% 47%	1900
	11th to 20th	Night	23% 9% 2 6% 4 3 3 2 3 6% 38%	1900
	21st to end	Night	22% 12% 3 6% 4 4 4 3 4 6% 32%	2152
October	1st to 10th	Night	17% 12% 3 8% 5% 6% 6% 5% 5% 7% 26%	3800
	11th to 20th	Night	28% 13% 3 6% 4 4 3 3 4 5% 26%	3777
	21st to end	Night	20% 20% 5% 9% 5% 4 4 3 5% 5% 20%	4427
November	1st to 10th	Night	27% 21% 5% 8% 4 4 3 3 4 4 16%	2687
	11th to 20th	Night	22% 25% 5% 7% 5% 5% 3 3 4 4 17%	2800
	21st to end	Night	22% 25% 5% 9% 4 5% 4 2 4 4 17%	2800
December	1st to 10th	Night	13% 16% 5% 9% 5% 4 4 3 5% 6% 29%	4560
	11th to 20th	Night	15% 24% 6% 14% 7% 5% 4 3 4 4 13%	5334
	21st to end	Night	13% 28% 8% 13% 6% 5% 5% 3 5% 5% 10%	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	77% 2 5% 11% 4 1	2261
	11th to 20th	Day	64% 4 10% 10% 7% 5%	2424
	21st to end	Day	77% 2 8% 8% 6%	2794
February	1st to 10th	Day	72% 4 8% 10% 5% 1	1651
	11th to 20th	Day	70% 3 9% 9% 7% 2	1674
	21st to end	Day	74% 4 7% 12% 3 1	1494
March	1st to 10th	Day	71% 3 5% 11% 9% 1	3138
	11th to 20th	Day	59% 1 13% 23% 4	3455
	21st to end	Day	76% 2 9% 7% 7%	4028
April	1st to 10th	Day	77% 4 10% 8% 1	2280
	11th to 20th	Day	69% 5% 8% 10% 7% 1	2262
	21st to end	Day	73% 3 9% 9% 6%	2599
May	1st to 10th	Day	60% 3 8% 23% 7%	4420
	11th to 20th	Day	58% 3 18% 19% 2	4380
	21st to end	Day	78% 2 10% 8% 2	4814
June	1st to 10th	Day	67% 3 10% 13% 4 2	2779
	11th to 20th	Day	73% 4 9% 11% 3	2850
	21st to end	Day	54% 4 14% 20% 8% 1	2850
July	1st to 10th	Day	29% 3 6% 22% 28% 12%	4666
	11th to 20th	Day	23% 2 7% 29% 30% 9%	4545
	21st to end	Day	32% 2 14% 29% 19% 5%	4639
August	1st to 10th	Day	25% 2 9% 30% 30% 3	4384
	11th to 20th	Day	41% 2 23% 22% 10% 2	4187
	21st to end	Day	51% 2 10% 25% 11% 1	4313
September	1st to 10th	Day	42% 2 12% 26% 14% 4	2280
	11th to 20th	Day	53% 3 7% 18% 18% 2	2280
	21st to end	Day	56% 2 13% 17% 10% 2	2027
October	1st to 10th	Day	58% 2 12% 20% 7%	3180
	11th to 20th	Day	65% 3 8% 17% 6% 1	3161
	21st to end	Day	73% 2 8% 10% 6%	3119
November	1st to 10th	Day	74% 4 7% 10% 4	1608
	11th to 20th	Day	70% 4 8% 11% 6% 2	1600
	21st to end	Day	69% 4 7% 11% 7% 2	1600
December	1st to 10th	Day	60% 4 11% 15% 7% 3	2620
	11th to 20th	Day	75% 3 8% 10% 3 1	2868
	21st to end	Day	79% 2 7% 8% 3	3498
January	1st to 10th	Night	79% 2 4 10% 4 1	4816
	11th to 20th	Night	64% 3 9% 12% 8% 4	4519
	21st to end	Night	77% 3 7% 7% 5% 1	4862
February	1st to 10th	Night	75% 3 6% 11% 5% 1	2527
	11th to 20th	Night	71% 2 7% 11% 6% 2	2404
	21st to end	Night	74% 3 7% 12% 4	1960
March	1st to 10th	Night	66% 3 8% 10% 10% 2	3736
	11th to 20th	Night	56% 2 13% 24% 5%	3521
	21st to end	Night	72% 3 9% 11% 5%	3624
April	1st to 10th	Night	70% 3 13% 12% 2	1900
	11th to 20th	Night	64% 5% 11% 10% 9% 1	1865
	21st to end	Night	70% 3 9% 12% 5%	1579
May	1st to 10th	Night	58% 3 8% 23% 8%	2540
	11th to 20th	Night	56% 4 18% 18% 4	2516
	21st to end	Night	71% 3 14% 10% 2	2762
June	1st to 10th	Night	59% 6% 14% 13% 6% 2	1397
	11th to 20th	Night	62% 6% 14% 15% 4	1330
	21st to end	Night	47% 6% 19% 20% 7% 1	1330
July	1st to 10th	Night	26% 3 9% 24% 25% 12%	2185
	11th to 20th	Night	20% 3 10% 25% 30% 12%	2291
	21st to end	Night	30% 3 15% 23% 21% 7%	2674
August	1st to 10th	Night	21% 3 12% 35% 28% 1	2521
	11th to 20th	Night	45% 3 19% 21% 11% 1	2710
	21st to end	Night	46% 3 16% 24% 10% 1	3360
September	1st to 10th	Night	40% 4 14% 23% 16% 3	1900
	11th to 20th	Night	50% 3 10% 18% 17% 3	1900
	21st to end	Night	55% 3 13% 17% 9% 2	2152
October	1st to 10th	Night	55% 3 14% 23% 6%	3800
	11th to 20th	Night	61% 2 10% 17% 9%	3777
	21st to end	Night	68% 2 10% 12% 7% 1	4427
November	1st to 10th	Night	73% 3 8% 9% 5% 1	2687
	11th to 20th	Night	72% 4 8% 9% 6% 1	2800
	21st to end	Night	73% 2 5% 10% 7% 2	2800
December	1st to 10th	Night	59% 4 10% 17% 8% 2	4560
	11th to 20th	Night	77% 2 7% 10% 3	5331
	21st to end	Night	80% 2 6% 9% 4	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	24% 28% 30% 13% 5% 1	192
	11th to 20th	Day	26% 22% 29% 18% 4	180
	21st to end	Day	22% 26% 30% 18% 4	198
February	1st to 10th	Day	38% 27% 25% 9% 1	311
	11th to 20th	Day	32% 22% 30% 13% 2 1	365
	21st to end	Day	31% 24% 30% 12% 3	346
March	1st to 10th	Day	28% 24% 31% 12% 4	460
	11th to 20th	Day	29% 24% 23% 17% 6% 1	460
	21st to end	Day	32% 24% 27% 14% 3	506
April	1st to 10th	Day	37% 22% 24% 13% 3	460
	11th to 20th	Day	29% 26% 26% 12% 7% 1	452
	21st to end	Day	38% 22% 22% 12% 5% 1	460
May	1st to 10th	Day	25% 18% 34% 19% 4 1	463
	11th to 20th	Day	28% 14% 29% 21% 7% 1	488
	21st to end	Day	36% 19% 24% 15% 5% 2	529
June	1st to 10th	Day	31% 22% 26% 16% 4	609
	11th to 20th	Day	39% 22% 24% 13% 3 1	680
	21st to end	Day	27% 18% 29% 19% 6% 1	680
July	1st to 10th	Day	19% 16% 40% 19% 5% 1	671
	11th to 20th	Day	17% 18% 39% 19% 5% 2	606
	21st to end	Day	10% 15% 38% 24% 10% 3	531
August	1st to 10th	Day	14% 12% 34% 26% 10% 4	487
	11th to 20th	Day	11% 14% 35% 28% 10% 2	477
	21st to end	Day	18% 19% 35% 18% 9% 1	539
September	1st to 10th	Day	17% 22% 31% 20% 8% 1	490
	11th to 20th	Day	26% 19% 28% 20% 7%	490
	21st to end	Day	29% 18% 27% 18% 7% 1	490
October	1st to 10th	Day	27% 17% 29% 17% 10% 1	490
	11th to 20th	Day	33% 19% 20% 19% 7% 1	490
	21st to end	Day	36% 18% 27% 16% 3	348
November	1st to 10th	Day	37% 15% 28% 18% 2	220
	11th to 20th	Day	40% 18% 23% 13% 7%	200
	21st to end	Day	32% 25% 25% 10% 9% 1	200
December	1st to 10th	Day	30% 18% 24% 18% 10% 2	200
	11th to 20th	Day	34% 22% 29% 11% 3 2	200
	21st to end	Day	30% 23% 32% 15%	220
January	1st to 10th	Night	34% 24% 28% 12% 2	880
	11th to 20th	Night	32% 17% 32% 17% 2	788
	21st to end	Night	26% 19% 36% 17% 2	836
February	1st to 10th	Night	33% 17% 33% 14% 2	760
	11th to 20th	Night	29% 19% 33% 17% 1	744
	21st to end	Night	29% 18% 31% 18% 4	628
March	1st to 10th	Night	26% 17% 28% 23% 6%	760
	11th to 20th	Night	28% 19% 28% 21% 5%	760
	21st to end	Night	29% 19% 30% 17% 5%	836
April	1st to 10th	Night	27% 16% 28% 24% 5% 1	760
	11th to 20th	Night	29% 15% 28% 23% 5%	737
	21st to end	Night	30% 14% 30% 19% 7%	439
May	1st to 10th	Night	24% 11% 26% 29% 11%	380
	11th to 20th	Night	24% 10% 20% 33% 13% 1	374
	21st to end	Night	30% 9% 22% 27% 11%	411
June	1st to 10th	Night	28% 8% 28% 25% 9%	380
	11th to 20th	Night	32% 8% 24% 26% 9% 1	380
	21st to end	Night	21% 7% 26% 33% 13% 1	380
July	1st to 10th	Night	18% 6% 20% 39% 16% 1	370
	11th to 20th	Night	11% 7% 22% 41% 19%	368
	21st to end	Night	9% 5% 19% 43% 24%	416
August	1st to 10th	Night	12% 2 16% 37% 31% 2	378
	11th to 20th	Night	13% 3 19% 38% 26% 1	487
	21st to end	Night	17% 7% 26% 34% 16%	750
September	1st to 10th	Night	18% 8% 28% 29% 18%	760
	11th to 20th	Night	25% 8% 19% 26% 19% 2	760
	21st to end	Night	29% 12% 22% 28% 9%	760
October	1st to 10th	Night	27% 10% 26% 27% 9%	760
	11th to 20th	Night	34% 12% 25% 23% 6%	760
	21st to end	Night	38% 14% 27% 17% 4	836
November	1st to 10th	Night	38% 16% 33% 12% 1	780
	11th to 20th	Night	39% 17% 29% 13% 2	800
	21st to end	Night	36% 20% 26% 15% 3 1	800
December	1st to 10th	Night	26% 17% 37% 17% 3	800
	11th to 20th	Night	34% 22% 32% 11% 1	833
	21st to end	Night	33% 22% 37% 8%	990

Page generated at 2025-01-04 00:23:57. Version ac07

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