Grande Cache
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grande Cache clear sky chart from 2005-11-06 to 2025-02-25.

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	8% 10% 4 10% 7% 6% 6% 5% 7% 7% 31%	111620
	Night	5% 10% 4 10% 6% 6% 6% 5% 7% 8% 33%	115688
Transparency	Day	48% 11% 16% 16% 7% 1	111355
	Night	45% 12% 18% 18% 7% 1	115599
Seeing	Day	21% 21% 29% 19% 9% 2	16599
	Night	15% 20% 30% 24% 10% 1	24968

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	2 10% 5% 11% 9% 9% 6% 5% 7% 9% 28%	2719
	11th to 20th	Day	2 9% 4 14% 8% 7% 6% 4 9% 8% 29%	2975
	21st to end	Day	2 9% 4 13% 7% 5% 6% 5% 7% 9% 35%	3841
February	1st to 10th	Day	3 10% 5% 14% 8% 7% 8% 5% 8% 7% 28%	1600
	11th to 20th	Day	2 11% 7% 14% 7% 6% 7% 5% 6% 6% 28%	1663
	21st to end	Day	1 9% 5% 14% 11% 6% 7% 6% 7% 6% 26%	1539
March	1st to 10th	Day	1 6% 3 13% 10% 10% 7% 5% 7% 10% 28%	3138
	11th to 20th	Day	4 9% 3 7% 5% 5% 5% 5% 8% 10% 38%	3269
	21st to end	Day	3 14% 6% 18% 9% 5% 5% 4 6% 6% 24%	4156
April	1st to 10th	Day	3 14% 5% 12% 9% 7% 7% 7% 7% 7% 20%	2280
	11th to 20th	Day	5% 18% 5% 13% 8% 6% 5% 4 7% 6% 24%	2386
	21st to end	Day	7% 15% 5% 10% 7% 6% 6% 5% 6% 7% 26%	2660
May	1st to 10th	Day	8% 15% 6% 10% 7% 6% 6% 5% 6% 5% 27%	4420
	11th to 20th	Day	10% 15% 4 9% 7% 6% 6% 5% 6% 6% 25%	4586
	21st to end	Day	16% 16% 5% 11% 7% 6% 6% 4 5% 6% 19%	5410
June	1st to 10th	Day	17% 11% 4 8% 6% 5% 6% 4 6% 7% 26%	3040
	11th to 20th	Day	20% 13% 4 11% 7% 6% 6% 5% 6% 6% 17%	3040
	21st to end	Day	14% 9% 3 10% 7% 7% 6% 5% 6% 7% 26%	3040
July	1st to 10th	Day	13% 7% 2 7% 6% 4 4 3 5% 6% 43%	5060
	11th to 20th	Day	8% 4 2 6% 5% 5% 5% 5% 6% 8% 44%	5060
	21st to end	Day	10% 6% 3 7% 6% 7% 7% 6% 8% 9% 31%	5026
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 4 5% 6% 7% 42%	4401
	11th to 20th	Day	14% 7% 2 7% 6% 6% 5% 4 6% 8% 34%	4330
	21st to end	Day	15% 8% 3 8% 6% 5% 6% 4 6% 7% 32%	4511
September	1st to 10th	Day	12% 9% 3 9% 5% 6% 5% 3 5% 7% 37%	2280
	11th to 20th	Day	12% 11% 3 7% 5% 5% 5% 4 6% 8% 36%	2255
	21st to end	Day	11% 9% 4 9% 5% 4 5% 5% 6% 7% 35%	2071
October	1st to 10th	Day	6% 10% 4 10% 7% 6% 7% 5% 6% 8% 31%	3180
	11th to 20th	Day	5% 10% 5% 9% 5% 6% 7% 5% 8% 10% 30%	2943
	21st to end	Day	5% 10% 4 10% 5% 5% 5% 6% 7% 9% 34%	3084
November	1st to 10th	Day	5% 14% 6% 9% 6% 6% 6% 5% 7% 8% 28%	1567
	11th to 20th	Day	3 11% 4 11% 6% 7% 4 5% 8% 8% 34%	1400
	21st to end	Day	3 11% 5% 10% 8% 6% 7% 4 7% 11% 30%	1400
December	1st to 10th	Day	4 16% 6% 13% 7% 6% 7% 5% 6% 6% 24%	2036
	11th to 20th	Day	4 10% 4 9% 6% 6% 8% 5% 8% 8% 32%	2262
	21st to end	Day	1 10% 4 11% 5% 6% 4 4 6% 8% 40%	2992
January	1st to 10th	Night	2 8% 4 14% 8% 6% 6% 5% 7% 9% 30%	7226
	11th to 20th	Night	3 11% 4 12% 6% 6% 8% 6% 9% 8% 28%	6954
	21st to end	Night	2 10% 4 12% 6% 6% 6% 5% 7% 8% 34%	7114
February	1st to 10th	Night	3 12% 5% 11% 8% 8% 6% 5% 8% 8% 26%	2800
	11th to 20th	Night	2 12% 5% 13% 7% 6% 7% 4 8% 8% 27%	2620
	21st to end	Night	2 10% 5% 14% 9% 5% 8% 5% 7% 7% 28%	2009
March	1st to 10th	Night	1 7% 5% 14% 10% 9% 5% 3 6% 7% 32%	3736
	11th to 20th	Night	2 9% 3 7% 5% 5% 6% 5% 9% 10% 40%	3706
	21st to end	Night	2 11% 6% 17% 6% 4 4 3 5% 7% 33%	3497
April	1st to 10th	Night	5% 12% 4 9% 8% 6% 6% 5% 8% 9% 29%	1900
	11th to 20th	Night	6% 14% 5% 9% 7% 4 5% 6% 7% 8% 30%	1743
	21st to end	Night	8% 13% 5% 9% 5% 5% 5% 4 6% 7% 33%	1520
May	1st to 10th	Night	6% 16% 5% 8% 5% 4 5% 4 8% 7% 30%	2540
	11th to 20th	Night	9% 12% 4 9% 5% 6% 6% 4 7% 7% 30%	2305
	21st to end	Night	17% 14% 5% 9% 7% 5% 5% 3 6% 7% 22%	2161
June	1st to 10th	Night	14% 10% 4 7% 6% 4 3 4 6% 6% 35%	1140
	11th to 20th	Night	18% 11% 4 8% 6% 6% 7% 6% 6% 7% 22%	1140
	21st to end	Night	11% 8% 2 7% 7% 6% 5% 5% 6% 7% 36%	1140
July	1st to 10th	Night	12% 5% 1 4 4 4 4 3 6% 7% 50%	1900
	11th to 20th	Night	10% 6% 2 5% 4 4 5% 4 7% 8% 47%	1900
	21st to end	Night	10% 6% 2 5% 5% 5% 6% 5% 8% 10% 37%	2523
August	1st to 10th	Night	13% 7% 2 5% 3 3 3 2 5% 7% 48%	2529
	11th to 20th	Night	15% 8% 3 8% 5% 4 4 4 6% 8% 35%	2497
	21st to end	Night	12% 10% 3 8% 6% 4 6% 4 6% 8% 33%	3154
September	1st to 10th	Night	13% 8% 3 7% 5% 4 5% 4 7% 6% 38%	1900
	11th to 20th	Night	11% 11% 3 6% 4 4 4 3 7% 7% 40%	1924
	21st to end	Night	9% 8% 3 7% 4 5% 5% 4 7% 9% 38%	2106
October	1st to 10th	Night	7% 11% 4 9% 6% 7% 7% 6% 7% 8% 29%	3800
	11th to 20th	Night	4 7% 2 9% 6% 6% 6% 5% 8% 11% 36%	3988
	21st to end	Night	3 11% 5% 10% 7% 5% 6% 5% 8% 8% 32%	4473
November	1st to 10th	Night	6% 13% 6% 10% 5% 5% 4 5% 7% 9% 32%	2730
	11th to 20th	Night	2 11% 5% 11% 6% 5% 5% 5% 7% 9% 33%	2980
	21st to end	Night	3 12% 4 10% 8% 6% 6% 4 8% 9% 29%	3000
December	1st to 10th	Night	4 12% 5% 12% 7% 6% 6% 5% 7% 8% 30%	5139
	11th to 20th	Night	4 12% 5% 11% 6% 8% 7% 5% 7% 8% 28%	5930
	21st to end	Night	1 10% 5% 10% 5% 5% 6% 5% 7% 9% 36%	7964

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	48% 11% 20% 14% 6% 1	2719
	11th to 20th	Day	48% 8% 13% 15% 13% 4	2996
	21st to end	Day	42% 13% 26% 11% 8%	3841
February	1st to 10th	Day	52% 11% 11% 10% 13% 3	1600
	11th to 20th	Day	56% 9% 12% 13% 9% 1	1671
	21st to end	Day	53% 11% 12% 14% 8% 2	1539
March	1st to 10th	Day	44% 16% 17% 12% 10% 2	3138
	11th to 20th	Day	38% 13% 16% 24% 8%	3269
	21st to end	Day	54% 14% 12% 9% 9% 1	4156
April	1st to 10th	Day	58% 15% 15% 9% 2	2280
	11th to 20th	Day	59% 9% 13% 14% 5%	2386
	21st to end	Day	55% 13% 13% 15% 4	2660
May	1st to 10th	Day	55% 11% 14% 15% 4	4420
	11th to 20th	Day	51% 16% 15% 13% 6%	4590
	21st to end	Day	61% 13% 13% 11% 3 1	5411
June	1st to 10th	Day	55% 12% 12% 15% 5% 1	3040
	11th to 20th	Day	65% 11% 12% 10% 2	3040
	21st to end	Day	55% 12% 15% 13% 5% 1	3040
July	1st to 10th	Day	40% 7% 12% 16% 17% 8%	4981
	11th to 20th	Day	36% 9% 13% 28% 13% 1	4973
	21st to end	Day	43% 13% 17% 17% 11%	4867
August	1st to 10th	Day	39% 10% 13% 30% 8%	4384
	11th to 20th	Day	45% 11% 15% 21% 9%	4372
	21st to end	Day	49% 9% 18% 18% 4	4510
September	1st to 10th	Day	46% 11% 17% 17% 6% 2	2280
	11th to 20th	Day	45% 12% 15% 19% 7% 1	2255
	21st to end	Day	46% 12% 17% 15% 8% 2	2071
October	1st to 10th	Day	47% 10% 20% 18% 5% 1	3180
	11th to 20th	Day	44% 10% 20% 22% 4	2943
	21st to end	Day	42% 10% 23% 20% 4	3084
November	1st to 10th	Day	52% 11% 17% 15% 5% 1	1567
	11th to 20th	Day	44% 13% 19% 15% 6% 2	1400
	21st to end	Day	46% 14% 22% 11% 5% 4	1400
December	1st to 10th	Day	58% 12% 15% 10% 5%	2036
	11th to 20th	Day	45% 11% 19% 20% 5% 1	2264
	21st to end	Day	38% 10% 15% 24% 11% 1	2992
January	1st to 10th	Night	47% 10% 15% 16% 10% 1	7226
	11th to 20th	Night	47% 10% 14% 14% 13% 2	6995
	21st to end	Night	43% 13% 25% 12% 7% 1	7114
February	1st to 10th	Night	52% 10% 15% 12% 10% 2	2800
	11th to 20th	Night	51% 12% 13% 14% 7% 2	2622
	21st to end	Night	51% 9% 13% 18% 8% 1	2009
March	1st to 10th	Night	48% 11% 16% 15% 11% 1	3736
	11th to 20th	Night	34% 15% 18% 25% 8%	3707
	21st to end	Night	45% 12% 15% 18% 10%	3497
April	1st to 10th	Night	47% 13% 19% 16% 5%	1900
	11th to 20th	Night	48% 12% 17% 16% 7%	1743
	21st to end	Night	47% 12% 14% 20% 6% 1	1520
May	1st to 10th	Night	47% 12% 16% 20% 6%	2540
	11th to 20th	Night	43% 16% 17% 16% 8%	2305
	21st to end	Night	54% 16% 14% 12% 3 1	2161
June	1st to 10th	Night	47% 11% 15% 18% 9%	1140
	11th to 20th	Night	56% 13% 15% 14% 2	1140
	21st to end	Night	43% 13% 20% 19% 4 1	1140
July	1st to 10th	Night	30% 9% 14% 24% 21% 2	1870
	11th to 20th	Night	32% 11% 19% 27% 10% 1	1864
	21st to end	Night	36% 11% 20% 23% 9%	2443
August	1st to 10th	Night	36% 8% 17% 35% 4	2521
	11th to 20th	Night	46% 9% 17% 24% 5%	2521
	21st to end	Night	47% 10% 26% 13% 3	3154
September	1st to 10th	Night	43% 12% 19% 18% 6% 1	1900
	11th to 20th	Night	41% 10% 18% 22% 7% 1	1924
	21st to end	Night	39% 15% 20% 16% 10%	2106
October	1st to 10th	Night	47% 12% 20% 16% 6%	3800
	11th to 20th	Night	37% 12% 22% 24% 5%	3988
	21st to end	Night	46% 11% 19% 18% 5% 1	4473
November	1st to 10th	Night	47% 11% 18% 16% 6% 1	2730
	11th to 20th	Night	46% 14% 18% 15% 7% 1	2980
	21st to end	Night	48% 15% 19% 11% 6% 2	3000
December	1st to 10th	Night	51% 10% 19% 14% 6% 1	5139
	11th to 20th	Night	52% 11% 16% 16% 5%	5927
	21st to end	Night	40% 11% 18% 23% 7% 1	7964

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	15% 21% 29% 24% 10% 2	200
	11th to 20th	Day	18% 23% 27% 23% 9% 1	200
	21st to end	Day	24% 25% 30% 16% 5%	220
February	1st to 10th	Day	20% 23% 28% 19% 10% 1	200
	11th to 20th	Day	20% 23% 28% 20% 8% 1	303
	21st to end	Day	18% 23% 38% 14% 5% 1	364
March	1st to 10th	Day	18% 24% 31% 18% 8% 1	460
	11th to 20th	Day	18% 26% 32% 14% 7% 2	460
	21st to end	Day	21% 25% 28% 17% 8% 1	506
April	1st to 10th	Day	23% 24% 31% 15% 7% 1	460
	11th to 20th	Day	20% 22% 31% 16% 9% 1	452
	21st to end	Day	19% 24% 30% 17% 7% 3	460
May	1st to 10th	Day	20% 24% 30% 18% 7%	463
	11th to 20th	Day	19% 13% 30% 23% 12% 3	606
	21st to end	Day	23% 13% 23% 23% 15% 3	879
June	1st to 10th	Day	22% 21% 28% 19% 8% 1	870
	11th to 20th	Day	26% 20% 26% 18% 8% 1	870
	21st to end	Day	20% 17% 27% 22% 12% 3	870
July	1st to 10th	Day	21% 13% 30% 21% 13% 2	856
	11th to 20th	Day	16% 17% 32% 22% 11% 2	852
	21st to end	Day	16% 18% 30% 23% 10% 3	676
August	1st to 10th	Day	16% 14% 32% 22% 12% 4	487
	11th to 20th	Day	18% 20% 32% 20% 8% 2	477
	21st to end	Day	18% 24% 29% 17% 10% 1	539
September	1st to 10th	Day	23% 19% 30% 18% 8% 2	490
	11th to 20th	Day	24% 25% 26% 17% 7% 2	490
	21st to end	Day	31% 28% 22% 13% 5% 1	490
October	1st to 10th	Day	29% 21% 25% 14% 9% 2	490
	11th to 20th	Day	24% 25% 25% 19% 6% 2	362
	21st to end	Day	20% 25% 31% 17% 6% 1	327
November	1st to 10th	Day	30% 25% 29% 11% 6% 1	200
	11th to 20th	Day	23% 25% 33% 15% 4 2	200
	21st to end	Day	27% 20% 29% 16% 7% 3	200
December	1st to 10th	Day	16% 29% 30% 17% 8% 2	200
	11th to 20th	Day	19% 23% 31% 22% 6% 1	200
	21st to end	Day	16% 21% 30% 22% 8% 3	220
January	1st to 10th	Night	14% 22% 34% 23% 5% 1	1100
	11th to 20th	Night	17% 23% 33% 20% 6% 1	998
	21st to end	Night	19% 26% 30% 19% 5% 1	915
February	1st to 10th	Night	18% 17% 28% 24% 12% 1	800
	11th to 20th	Night	11% 23% 30% 27% 8% 1	784
	21st to end	Night	11% 18% 37% 21% 12% 1	646
March	1st to 10th	Night	12% 21% 32% 26% 9% 1	760
	11th to 20th	Night	7% 23% 37% 22% 9% 1	760
	21st to end	Night	10% 18% 27% 31% 13% 1	836
April	1st to 10th	Night	12% 20% 30% 25% 12% 1	760
	11th to 20th	Night	15% 15% 29% 28% 13%	615
	21st to end	Night	16% 14% 29% 30% 11% 1	380
May	1st to 10th	Night	12% 13% 29% 33% 13% 1	380
	11th to 20th	Night	12% 6% 26% 40% 14% 1	374
	21st to end	Night	18% 11% 21% 24% 24% 2	411
June	1st to 10th	Night	18% 12% 25% 31% 13% 1	380
	11th to 20th	Night	22% 11% 24% 27% 14% 1	380
	21st to end	Night	14% 9% 27% 28% 19% 2	380
July	1st to 10th	Night	17% 8% 23% 34% 18% 1	370
	11th to 20th	Night	17% 12% 18% 32% 20% 1	368
	21st to end	Night	11% 10% 21% 30% 26% 2	416
August	1st to 10th	Night	15% 9% 21% 26% 27% 2	378
	11th to 20th	Night	13% 10% 26% 31% 20% 1	376
	21st to end	Night	17% 12% 29% 28% 13% 1	622
September	1st to 10th	Night	19% 13% 24% 30% 13% 1	760
	11th to 20th	Night	16% 17% 23% 27% 16% 1	760
	21st to end	Night	20% 24% 25% 21% 9% 1	760
October	1st to 10th	Night	20% 20% 26% 24% 10% 1	760
	11th to 20th	Night	16% 23% 29% 25% 6% 1	760
	21st to end	Night	19% 22% 34% 19% 6% 1	836
November	1st to 10th	Night	20% 26% 30% 16% 7%	780
	11th to 20th	Night	17% 28% 33% 18% 4	980
	21st to end	Night	19% 26% 33% 18% 4 1	1000
December	1st to 10th	Night	15% 26% 33% 18% 8%	1073
	11th to 20th	Night	13% 24% 36% 21% 6%	1100
	21st to end	Night	14% 21% 37% 20% 8% 1	1210

Page generated at 2025-02-22 04:51:33. Version ac07

Copyright © 2025 Attilla Danko.