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 2026-07-01.

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% 8% 31%	141904
	Night	5% 10% 4 10% 6% 6% 6% 5% 7% 8% 33%	139147
Transparency	Day	47% 10% 16% 18% 8% 1	141596
	Night	44% 11% 18% 18% 8% 1	139038
Seeing	Day	21% 21% 29% 19% 9% 2	18541
	Night	16% 19% 30% 24% 10% 1	26633

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 11% 5% 11% 8% 8% 6% 4 7% 9% 29%	3527
	11th to 20th	Day	2 7% 3 11% 7% 6% 5% 4 9% 8% 37%	3879
	21st to end	Day	2 9% 4 13% 7% 6% 6% 5% 7% 9% 33%	4981
February	1st to 10th	Day	3 9% 5% 14% 8% 7% 8% 5% 8% 8% 27%	1680
	11th to 20th	Day	2 12% 7% 15% 8% 6% 7% 5% 6% 6% 27%	1748
	21st to end	Day	1 10% 6% 15% 11% 6% 7% 6% 7% 6% 25%	1655
March	1st to 10th	Day	1 8% 4 12% 9% 9% 7% 5% 7% 10% 28%	4618
	11th to 20th	Day	3 8% 4 8% 6% 6% 6% 5% 9% 11% 33%	4791
	21st to end	Day	5% 15% 6% 16% 8% 5% 5% 4 6% 6% 25%	6092
April	1st to 10th	Day	3 14% 5% 13% 9% 7% 7% 7% 7% 7% 21%	2520
	11th to 20th	Day	4 17% 6% 13% 8% 6% 5% 4 7% 7% 24%	2641
	21st to end	Day	7% 14% 5% 11% 7% 6% 6% 5% 6% 7% 27%	2940
May	1st to 10th	Day	6% 10% 4 8% 6% 6% 6% 5% 7% 8% 33%	8195
	11th to 20th	Day	8% 13% 4 11% 7% 7% 7% 5% 7% 8% 22%	8551
	21st to end	Day	12% 11% 4 10% 6% 6% 6% 5% 7% 9% 24%	10122
June	1st to 10th	Day	16% 11% 4 8% 6% 5% 6% 4 6% 7% 26%	3360
	11th to 20th	Day	19% 13% 4 11% 7% 6% 6% 5% 6% 6% 17%	3360
	21st to end	Day	13% 10% 3 10% 7% 7% 6% 5% 6% 7% 25%	3360
July	1st to 10th	Day	10% 6% 2 7% 6% 6% 5% 5% 7% 8% 37%	7241
	11th to 20th	Day	10% 7% 3 8% 6% 5% 6% 5% 6% 8% 38%	7240
	21st to end	Day	11% 6% 3 7% 6% 6% 6% 5% 8% 9% 32%	7222
August	1st to 10th	Day	11% 6% 2 7% 5% 5% 4 5% 7% 7% 41%	4541
	11th to 20th	Day	14% 7% 2 7% 6% 6% 5% 4 7% 8% 33%	4470
	21st to end	Day	15% 8% 3 7% 6% 5% 5% 4 6% 7% 33%	4654
September	1st to 10th	Day	12% 9% 3 8% 5% 5% 5% 4 5% 7% 38%	2400
	11th to 20th	Day	11% 10% 3 7% 5% 5% 5% 4 6% 8% 37%	2373
	21st to end	Day	11% 9% 4 9% 5% 5% 5% 4 6% 7% 34%	2180
October	1st to 10th	Day	6% 10% 4 10% 7% 6% 7% 5% 6% 8% 32%	3280
	11th to 20th	Day	5% 10% 5% 9% 5% 6% 7% 5% 8% 10% 30%	3036
	21st to end	Day	5% 10% 4 10% 5% 5% 5% 6% 7% 10% 34%	3182
November	1st to 10th	Day	5% 14% 6% 9% 6% 6% 6% 6% 7% 8% 28%	1647
	11th to 20th	Day	3 11% 3 10% 6% 7% 5% 5% 7% 8% 35%	1470
	21st to end	Day	2 11% 4 10% 8% 6% 7% 5% 7% 11% 30%	1470
December	1st to 10th	Day	4 16% 6% 13% 8% 6% 7% 5% 6% 6% 24%	2098
	11th to 20th	Day	4 10% 4 9% 6% 6% 8% 5% 8% 8% 32%	2322
	21st to end	Day	1 10% 4 11% 5% 6% 4 4 6% 8% 40%	3058
January	1st to 10th	Night	2 10% 5% 14% 8% 6% 6% 5% 7% 9% 28%	9366
	11th to 20th	Night	2 10% 4 10% 5% 5% 7% 6% 9% 8% 34%	9064
	21st to end	Night	2 11% 4 11% 6% 6% 6% 5% 7% 8% 34%	9198
February	1st to 10th	Night	3 12% 4 10% 7% 8% 6% 5% 8% 9% 27%	2940
	11th to 20th	Night	2 12% 5% 13% 7% 7% 7% 5% 8% 8% 27%	2754
	21st to end	Night	2 10% 5% 16% 9% 5% 7% 5% 7% 7% 28%	2151
March	1st to 10th	Night	1 9% 4 13% 8% 8% 5% 4 6% 8% 34%	5496
	11th to 20th	Night	2 7% 4 11% 7% 7% 6% 5% 9% 10% 32%	5424
	21st to end	Night	5% 14% 5% 13% 5% 4 4 4 6% 8% 31%	5125
April	1st to 10th	Night	4 12% 4 9% 8% 6% 6% 5% 8% 8% 30%	2100
	11th to 20th	Night	6% 14% 5% 9% 7% 4 5% 6% 7% 9% 30%	1927
	21st to end	Night	7% 12% 5% 9% 5% 5% 5% 4 6% 7% 35%	1680
May	1st to 10th	Night	5% 12% 4 8% 4 4 5% 4 7% 9% 37%	4724
	11th to 20th	Night	7% 12% 5% 11% 6% 6% 7% 5% 7% 8% 26%	4307
	21st to end	Night	10% 10% 4 8% 6% 5% 5% 4 7% 10% 31%	4045
June	1st to 10th	Night	13% 10% 4 7% 6% 5% 4 5% 7% 6% 35%	1260
	11th to 20th	Night	19% 11% 3 8% 5% 6% 7% 5% 6% 7% 22%	1260
	21st to end	Night	11% 8% 2 7% 6% 6% 5% 6% 6% 8% 35%	1260
July	1st to 10th	Night	9% 5% 2 5% 4 4 5% 4 7% 9% 44%	2720
	11th to 20th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 7% 40%	2720
	21st to end	Night	11% 6% 3 6% 5% 5% 6% 4 6% 9% 39%	3618
August	1st to 10th	Night	13% 7% 2 6% 4 3 3 3 5% 7% 48%	2609
	11th to 20th	Night	15% 9% 3 7% 5% 4 4 3 6% 8% 35%	2577
	21st to end	Night	11% 9% 3 8% 6% 4 6% 4 6% 8% 34%	3253
September	1st to 10th	Night	13% 7% 3 7% 5% 4 4 4 7% 7% 40%	2000
	11th to 20th	Night	10% 10% 3 6% 4 4 4 3 7% 7% 41%	2026
	21st to end	Night	9% 8% 3 6% 4 5% 6% 4 7% 9% 38%	2217
October	1st to 10th	Night	7% 11% 4 9% 6% 7% 7% 5% 7% 8% 29%	3920
	11th to 20th	Night	4 7% 3 9% 6% 6% 6% 5% 8% 11% 36%	4115
	21st to end	Night	3 11% 5% 10% 7% 5% 6% 5% 8% 8% 32%	4616
November	1st to 10th	Night	5% 12% 6% 10% 5% 5% 4 5% 7% 9% 33%	2870
	11th to 20th	Night	2 11% 4 11% 6% 5% 5% 5% 7% 10% 33%	3129
	21st to end	Night	3 12% 4 10% 7% 6% 6% 4 8% 9% 30%	3150
December	1st to 10th	Night	4 12% 5% 12% 7% 6% 6% 5% 7% 8% 30%	5296
	11th to 20th	Night	4 12% 5% 11% 6% 8% 7% 5% 7% 8% 27%	6090
	21st to end	Night	1 10% 5% 10% 6% 5% 6% 5% 7% 9% 36%	8140

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% 9% 17% 15% 11% 1	3527
	11th to 20th	Day	40% 7% 16% 21% 13% 3	3900
	21st to end	Day	44% 12% 23% 15% 6%	4982
February	1st to 10th	Day	51% 11% 11% 11% 13% 3	1680
	11th to 20th	Day	57% 8% 12% 13% 9% 1	1756
	21st to end	Day	55% 11% 11% 13% 8% 2	1655
March	1st to 10th	Day	45% 12% 16% 13% 13% 1	4618
	11th to 20th	Day	39% 11% 15% 23% 12% 1	4791
	21st to end	Day	55% 11% 10% 12% 12%	6092
April	1st to 10th	Day	58% 14% 15% 10% 3	2520
	11th to 20th	Day	58% 9% 13% 14% 5%	2641
	21st to end	Day	54% 12% 13% 16% 5%	2940
May	1st to 10th	Day	45% 8% 18% 21% 7% 1	8195
	11th to 20th	Day	51% 12% 15% 16% 7%	8555
	21st to end	Day	52% 10% 19% 16% 3	10123
June	1st to 10th	Day	55% 11% 11% 15% 6% 1	3360
	11th to 20th	Day	64% 11% 12% 11% 2	3360
	21st to end	Day	55% 12% 15% 14% 4 1	3360
July	1st to 10th	Day	41% 8% 15% 16% 15% 6%	7162
	11th to 20th	Day	42% 8% 13% 27% 10% 1	7153
	21st to end	Day	42% 11% 17% 21% 9%	7019
August	1st to 10th	Day	40% 10% 13% 29% 8%	4524
	11th to 20th	Day	45% 10% 15% 21% 8%	4512
	21st to end	Day	48% 9% 18% 19% 5% 1	4653
September	1st to 10th	Day	45% 11% 18% 19% 6% 2	2400
	11th to 20th	Day	44% 12% 15% 20% 8% 2	2373
	21st to end	Day	47% 12% 17% 15% 8% 2	2180
October	1st to 10th	Day	47% 10% 20% 18% 5%	3280
	11th to 20th	Day	44% 10% 19% 22% 5%	3036
	21st to end	Day	42% 10% 24% 20% 4	3182
November	1st to 10th	Day	51% 11% 16% 16% 5% 1	1647
	11th to 20th	Day	44% 13% 19% 15% 7% 2	1470
	21st to end	Day	46% 13% 21% 11% 5% 4	1470
December	1st to 10th	Day	58% 12% 15% 10% 5%	2098
	11th to 20th	Day	45% 11% 19% 20% 5% 1	2324
	21st to end	Day	38% 10% 15% 24% 11% 1	3058
January	1st to 10th	Night	49% 9% 13% 15% 12% 1	9367
	11th to 20th	Night	42% 9% 16% 18% 14% 2	9105
	21st to end	Night	43% 12% 24% 14% 7%	9198
February	1st to 10th	Night	51% 10% 15% 12% 10% 2	2940
	11th to 20th	Night	51% 12% 13% 15% 8% 2	2756
	21st to end	Night	51% 9% 12% 18% 9% 1	2151
March	1st to 10th	Night	45% 9% 18% 15% 13%	5496
	11th to 20th	Night	41% 11% 15% 21% 11%	5425
	21st to end	Night	47% 9% 13% 20% 11%	5125
April	1st to 10th	Night	46% 13% 20% 17% 4	2100
	11th to 20th	Night	47% 12% 18% 16% 7%	1927
	21st to end	Night	46% 11% 14% 22% 6% 1	1680
May	1st to 10th	Night	40% 9% 21% 24% 5% 1	4721
	11th to 20th	Night	48% 12% 17% 18% 6%	4307
	21st to end	Night	42% 12% 25% 15% 5% 1	4049
June	1st to 10th	Night	47% 11% 14% 18% 10%	1260
	11th to 20th	Night	57% 12% 14% 15% 2	1260
	21st to end	Night	44% 13% 19% 20% 3 1	1260
July	1st to 10th	Night	32% 9% 20% 20% 17% 1	2690
	11th to 20th	Night	41% 9% 15% 26% 7% 1	2684
	21st to end	Night	36% 10% 23% 23% 7%	3516
August	1st to 10th	Night	36% 8% 18% 34% 4	2601
	11th to 20th	Night	46% 9% 17% 24% 4	2601
	21st to end	Night	46% 10% 26% 14% 4	3253
September	1st to 10th	Night	42% 12% 20% 19% 6% 1	2000
	11th to 20th	Night	40% 10% 18% 22% 8% 1	2026
	21st to end	Night	39% 15% 20% 16% 9%	2217
October	1st to 10th	Night	47% 12% 20% 16% 6%	3920
	11th to 20th	Night	37% 12% 22% 24% 5%	4115
	21st to end	Night	45% 11% 20% 18% 5% 1	4616
November	1st to 10th	Night	46% 11% 18% 17% 7% 1	2870
	11th to 20th	Night	46% 13% 18% 15% 7% 1	3129
	21st to end	Night	47% 14% 18% 12% 6% 2	3150
December	1st to 10th	Night	51% 10% 19% 14% 6% 1	5296
	11th to 20th	Night	51% 11% 16% 16% 5%	6087
	21st to end	Night	40% 11% 18% 23% 7% 1	8140

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	17% 22% 28% 23% 9% 2	220
	11th to 20th	Day	20% 22% 28% 21% 8% 1	220
	21st to end	Day	23% 26% 31% 16% 4	242
February	1st to 10th	Day	20% 23% 30% 17% 9% 1	220
	11th to 20th	Day	21% 24% 27% 20% 8% 1	328
	21st to end	Day	20% 25% 36% 13% 5% 1	408
March	1st to 10th	Day	21% 23% 31% 17% 7% 1	540
	11th to 20th	Day	19% 26% 32% 14% 6% 2	540
	21st to end	Day	22% 24% 28% 17% 7% 2	594
April	1st to 10th	Day	21% 24% 31% 16% 6% 1	540
	11th to 20th	Day	20% 24% 32% 15% 8% 1	532
	21st to end	Day	19% 24% 31% 18% 7% 3	540
May	1st to 10th	Day	19% 25% 31% 17% 7%	543
	11th to 20th	Day	18% 14% 30% 23% 12% 3	699
	21st to end	Day	23% 15% 24% 22% 14% 3	1005
June	1st to 10th	Day	22% 22% 28% 19% 7% 1	990
	11th to 20th	Day	26% 20% 26% 19% 8% 1	990
	21st to end	Day	20% 16% 25% 23% 13% 3	990
July	1st to 10th	Day	21% 13% 30% 22% 12% 2	917
	11th to 20th	Day	18% 17% 31% 22% 11% 2	912
	21st to end	Day	16% 18% 30% 23% 10% 4	728
August	1st to 10th	Day	17% 13% 30% 23% 12% 4	527
	11th to 20th	Day	18% 20% 32% 20% 7% 2	517
	21st to end	Day	17% 23% 27% 18% 12% 3	583
September	1st to 10th	Day	22% 18% 30% 19% 9% 3	530
	11th to 20th	Day	22% 23% 27% 17% 7% 3	530
	21st to end	Day	32% 27% 22% 14% 5% 1	530
October	1st to 10th	Day	29% 21% 26% 14% 8% 2	530
	11th to 20th	Day	24% 25% 26% 19% 5% 2	395
	21st to end	Day	22% 25% 30% 16% 6% 1	359
November	1st to 10th	Day	31% 23% 30% 10% 5% 1	220
	11th to 20th	Day	22% 24% 32% 16% 4 2	220
	21st to end	Day	25% 19% 27% 19% 7% 2	220
December	1st to 10th	Day	20% 27% 28% 16% 7% 2	220
	11th to 20th	Day	19% 23% 32% 21% 5% 1	220
	21st to end	Day	17% 22% 29% 22% 7% 2	242
January	1st to 10th	Night	15% 22% 34% 23% 5% 1	1160
	11th to 20th	Night	19% 22% 33% 20% 5% 1	1053
	21st to end	Night	19% 25% 31% 19% 5% 1	962
February	1st to 10th	Night	18% 17% 29% 23% 12% 1	840
	11th to 20th	Night	11% 24% 30% 27% 8%	824
	21st to end	Night	12% 19% 36% 20% 11% 1	692
March	1st to 10th	Night	13% 21% 31% 26% 9% 1	840
	11th to 20th	Night	8% 23% 38% 22% 9% 1	840
	21st to end	Night	11% 18% 28% 30% 13% 1	924
April	1st to 10th	Night	11% 19% 30% 28% 11% 1	840
	11th to 20th	Night	14% 15% 31% 28% 12% 1	679
	21st to end	Night	15% 13% 29% 31% 11% 1	420
May	1st to 10th	Night	11% 12% 31% 33% 12%	420
	11th to 20th	Night	13% 6% 28% 38% 14% 1	414
	21st to end	Night	17% 11% 23% 25% 22% 2	455
June	1st to 10th	Night	17% 14% 25% 31% 13% 1	420
	11th to 20th	Night	23% 12% 23% 26% 15% 1	420
	21st to end	Night	13% 9% 28% 28% 20% 2	420
July	1st to 10th	Night	16% 8% 24% 33% 18% 1	390
	11th to 20th	Night	17% 12% 18% 32% 20% 1	388
	21st to end	Night	11% 9% 21% 30% 27% 2	438
August	1st to 10th	Night	16% 9% 20% 26% 27% 2	398
	11th to 20th	Night	13% 10% 26% 31% 19% 1	396
	21st to end	Night	17% 11% 28% 28% 16% 1	655
September	1st to 10th	Night	18% 13% 23% 31% 14% 1	800
	11th to 20th	Night	16% 16% 23% 28% 16% 1	800
	21st to end	Night	20% 24% 25% 21% 9% 1	800
October	1st to 10th	Night	20% 20% 26% 24% 9% 1	800
	11th to 20th	Night	16% 23% 29% 25% 6% 1	800
	21st to end	Night	19% 22% 34% 19% 5% 1	880
November	1st to 10th	Night	21% 26% 30% 16% 6%	820
	11th to 20th	Night	16% 28% 34% 18% 4	1029
	21st to end	Night	18% 26% 34% 18% 4 1	1050
December	1st to 10th	Night	16% 26% 32% 18% 8%	1130
	11th to 20th	Night	14% 24% 36% 21% 6%	1160
	21st to end	Night	14% 21% 37% 19% 7% 1	1276

Page generated at 2026-06-28 00:10:48. Version ac07

Copyright © 2026 Attilla Danko.