Great Falls Observatory Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Great Falls Observatory Park clear sky chart from 2005-11-06 to 2024-11-04.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	101215
	Night	15% 6% 2 6% 4 4 4 4 5% 7% 43%	97745
Transparency	Day	39% 5% 12% 25% 16% 3	104853
	Night	39% 5% 11% 24% 17% 3	102246
Seeing	Day	18% 10% 26% 25% 15% 6%	14984
	Night	18% 10% 29% 30% 13% 1	24491

By Month

Percentage Cloud Cover SE (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% 5% 2 4 4 3 3 4 5% 7% 46%	2595
	11th to 20th	Day	15% 11% 3 7% 5% 4 4 3 4 5% 40%	2840
	21st to end	Day	18% 9% 3 8% 5% 4 4 3 5% 5% 37%	3124
February	1st to 10th	Day	14% 7% 2 8% 5% 5% 4 4 6% 7% 39%	1710
	11th to 20th	Day	18% 7% 2 5% 4 4 4 4 6% 6% 40%	1708
	21st to end	Day	18% 8% 2 7% 5% 4 4 3 5% 5% 39%	1570
March	1st to 10th	Day	23% 6% 2 5% 4 5% 4 3 6% 7% 35%	3394
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 6% 49%	3480
	21st to end	Day	20% 7% 2 5% 4 3 4 3 5% 6% 40%	3868
April	1st to 10th	Day	13% 7% 2 6% 4 4 4 4 6% 7% 43%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 3 4 4 5% 7% 46%	2254
	21st to end	Day	19% 7% 2 5% 4 4 4 3 5% 7% 41%	2280
May	1st to 10th	Day	23% 7% 2 6% 4 5% 4 4 6% 7% 30%	4036
	11th to 20th	Day	23% 7% 3 5% 4 4 6% 4 6% 9% 30%	4265
	21st to end	Day	11% 4 2 6% 4 4 4 4 6% 9% 45%	4823
June	1st to 10th	Day	10% 5% 2 6% 4 4 4 5% 7% 8% 44%	2660
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 41%	2660
	21st to end	Day	9% 5% 2 5% 4 4 6% 4 7% 10% 44%	2660
July	1st to 10th	Day	6% 4 2 6% 5% 6% 5% 5% 8% 11% 42%	4440
	11th to 20th	Day	9% 6% 2 6% 5% 5% 5% 5% 8% 11% 39%	4286
	21st to end	Day	11% 7% 2 7% 5% 6% 6% 4 7% 8% 36%	4466
August	1st to 10th	Day	15% 6% 2 6% 5% 6% 5% 5% 7% 9% 34%	4104
	11th to 20th	Day	10% 4 2 7% 4 5% 6% 5% 8% 8% 41%	3886
	21st to end	Day	10% 4 2 6% 4 3 4 4 6% 9% 48%	4180
September	1st to 10th	Day	14% 5% 2 5% 5% 4 3 4 5% 7% 45%	2280
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 6% 8% 47%	2280
	21st to end	Day	21% 6% 2 6% 3 4 4 4 4 7% 37%	1995
October	1st to 10th	Day	18% 6% 2 4 3 3 4 3 4 6% 46%	1900
	11th to 20th	Day	18% 4 1 4 3 3 3 3 5% 6% 50%	1900
	21st to end	Day	13% 5% 2 5% 3 4 4 4 4 6% 50%	2090
November	1st to 10th	Day	12% 6% 2 4 3 3 4 4 5% 7% 51%	1797
	11th to 20th	Day	15% 5% 2 4 3 3 3 4 6% 5% 50%	1682
	21st to end	Day	15% 6% 1 4 4 3 4 4 5% 5% 48%	1520
December	1st to 10th	Day	15% 6% 3 6% 5% 4 5% 4 6% 7% 40%	1520
	11th to 20th	Day	16% 6% 2 6% 4 4 4 4 5% 7% 45%	1910
	21st to end	Day	17% 8% 2 7% 4 5% 4 4 6% 7% 35%	2772
January	1st to 10th	Night	15% 7% 2 6% 4 4 3 3 5% 6% 45%	4333
	11th to 20th	Night	18% 7% 2 5% 3 3 3 2 4 5% 45%	4120
	21st to end	Night	20% 9% 2 6% 3 3 3 3 5% 6% 41%	4532
February	1st to 10th	Night	15% 7% 3 6% 5% 4 5% 4 5% 6% 41%	2470
	11th to 20th	Night	18% 7% 2 6% 4 4 3 3 5% 5% 43%	2356
	21st to end	Night	18% 8% 2 5% 4 4 3 3 5% 6% 42%	1884
March	1st to 10th	Night	21% 7% 3 5% 4 4 4 4 4 6% 38%	3458
	11th to 20th	Night	11% 5% 2 5% 3 4 4 4 6% 7% 49%	3480
	21st to end	Night	14% 9% 3 6% 4 3 3 3 4 5% 47%	3762
April	1st to 10th	Night	15% 7% 3 5% 5% 4 5% 4 6% 6% 41%	1900
	11th to 20th	Night	14% 5% 2 6% 4 3 4 4 5% 7% 46%	1875
	21st to end	Night	16% 5% 2 5% 3 5% 5% 4 6% 7% 43%	1900
May	1st to 10th	Night	23% 9% 2 7% 4 4 4 4 5% 8% 31%	2889
	11th to 20th	Night	24% 8% 3 6% 5% 4 3 3 5% 6% 32%	2624
	21st to end	Night	10% 4 2 6% 4 4 5% 4 7% 10% 44%	2732
June	1st to 10th	Night	11% 4 2 4 4 5% 5% 5% 6% 8% 45%	1520
	11th to 20th	Night	12% 5% 2 6% 4 4 4 5% 6% 9% 42%	1520
	21st to end	Night	8% 5% 3 6% 5% 4 5% 4 7% 10% 43%	1520
July	1st to 10th	Night	7% 4 3 7% 6% 6% 6% 5% 6% 9% 41%	2520
	11th to 20th	Night	8% 5% 2 7% 5% 6% 6% 5% 8% 10% 38%	2670
	21st to end	Night	11% 7% 3 8% 4 5% 4 4 8% 9% 37%	3087
August	1st to 10th	Night	16% 6% 2 8% 5% 5% 6% 4 6% 9% 32%	2825
	11th to 20th	Night	10% 6% 2 9% 6% 5% 6% 4 6% 8% 37%	2910
	21st to end	Night	11% 5% 2 4 3 4 4 3 5% 8% 51%	3476
September	1st to 10th	Night	14% 6% 2 5% 4 4 4 4 5% 7% 45%	1900
	11th to 20th	Night	10% 6% 2 5% 5% 3 4 3 4 7% 51%	1900
	21st to end	Night	19% 7% 3 7% 4 3 4 3 5% 6% 39%	2180
October	1st to 10th	Night	20% 6% 2 5% 3 3 3 3 4 5% 46%	2280
	11th to 20th	Night	16% 4 1 5% 4 3 3 2 4 6% 52%	2280
	21st to end	Night	14% 5% 2 5% 3 3 3 3 5% 7% 50%	2508
November	1st to 10th	Night	12% 5% 2 5% 4 3 3 3 4 5% 54%	2352
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 50%	2493
	21st to end	Night	13% 4 1 5% 4 4 4 4 5% 7% 48%	2660
December	1st to 10th	Night	17% 7% 2 7% 4 4 4 4 5% 7% 41%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 5% 7% 42%	3285
	21st to end	Night	17% 6% 2 6% 5% 5% 5% 5% 6% 7% 36%	4884

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	38% 4 7% 32% 15% 5%	2596
	11th to 20th	Day	47% 4 9% 21% 15% 5%	2840
	21st to end	Day	49% 2 11% 23% 12% 2	3124
February	1st to 10th	Day	44% 5% 12% 24% 13% 2	1710
	11th to 20th	Day	42% 4 9% 25% 17% 3	1717
	21st to end	Day	47% 3 11% 20% 16% 4	1570
March	1st to 10th	Day	48% 4 9% 22% 12% 4	3394
	11th to 20th	Day	33% 5% 8% 28% 22% 4	3480
	21st to end	Day	46% 3 11% 21% 16% 3	3868
April	1st to 10th	Day	40% 5% 11% 21% 19% 5%	2280
	11th to 20th	Day	38% 4 10% 22% 20% 6%	2254
	21st to end	Day	44% 4 9% 23% 17% 4	2280
May	1st to 10th	Day	52% 4 11% 21% 10% 1	4073
	11th to 20th	Day	49% 6% 13% 23% 8% 1	4269
	21st to end	Day	34% 6% 16% 25% 17% 2	4837
June	1st to 10th	Day	36% 6% 15% 26% 16% 1	2660
	11th to 20th	Day	37% 7% 12% 26% 15% 2	2660
	21st to end	Day	32% 7% 16% 26% 17% 2	2660
July	1st to 10th	Day	33% 9% 20% 21% 12% 5%	4370
	11th to 20th	Day	36% 7% 17% 29% 10% 1	4211
	21st to end	Day	43% 6% 11% 23% 16% 1	4324
August	1st to 10th	Day	44% 7% 16% 25% 7% 1	4091
	11th to 20th	Day	37% 7% 13% 27% 15% 1	3925
	21st to end	Day	34% 6% 13% 28% 18% 2	4180
September	1st to 10th	Day	38% 5% 14% 19% 20% 5%	2280
	11th to 20th	Day	35% 5% 8% 28% 21% 4	2280
	21st to end	Day	46% 5% 9% 19% 18% 2	1995
October	1st to 10th	Day	37% 4 7% 18% 29% 5%	3160
	11th to 20th	Day	25% 3 7% 30% 26% 9%	3140
	21st to end	Day	23% 3 9% 31% 28% 7%	3428
November	1st to 10th	Day	34% 3 10% 27% 20% 5%	1797
	11th to 20th	Day	35% 3 10% 25% 21% 5%	1682
	21st to end	Day	38% 4 10% 28% 18% 3	1520
December	1st to 10th	Day	42% 4 13% 26% 12% 3	1520
	11th to 20th	Day	39% 4 9% 32% 12% 3	1906
	21st to end	Day	46% 5% 13% 25% 11% 1	2772
January	1st to 10th	Night	39% 3 8% 27% 18% 4	4333
	11th to 20th	Night	41% 3 9% 24% 19% 3	4120
	21st to end	Night	46% 3 9% 27% 16% 1	4532
February	1st to 10th	Night	43% 5% 11% 25% 14% 2	2470
	11th to 20th	Night	44% 3 9% 22% 18% 5%	2358
	21st to end	Night	44% 3 10% 22% 16% 5%	1884
March	1st to 10th	Night	47% 4 9% 26% 12% 2	3458
	11th to 20th	Night	32% 5% 10% 28% 22% 4	3480
	21st to end	Night	41% 3 8% 23% 19% 5%	3762
April	1st to 10th	Night	42% 4 9% 20% 21% 4	1900
	11th to 20th	Night	37% 4 12% 22% 20% 6%	1875
	21st to end	Night	38% 6% 11% 25% 14% 5%	1900
May	1st to 10th	Night	53% 4 11% 19% 10% 3	2917
	11th to 20th	Night	52% 6% 10% 21% 9% 1	2625
	21st to end	Night	35% 5% 19% 24% 13% 3	2739
June	1st to 10th	Night	35% 7% 12% 27% 16% 2	1520
	11th to 20th	Night	38% 6% 14% 26% 14% 2	1520
	21st to end	Night	34% 6% 15% 24% 18% 3	1520
July	1st to 10th	Night	37% 8% 21% 18% 13% 4	2480
	11th to 20th	Night	37% 9% 19% 23% 11% 1	2622
	21st to end	Night	41% 8% 14% 20% 16% 1	2988
August	1st to 10th	Night	47% 7% 15% 23% 8% 1	2814
	11th to 20th	Night	42% 8% 11% 26% 12% 1	2937
	21st to end	Night	32% 5% 13% 31% 18% 2	3476
September	1st to 10th	Night	40% 5% 9% 21% 19% 5%	1900
	11th to 20th	Night	34% 5% 8% 28% 23% 3	1900
	21st to end	Night	46% 5% 9% 19% 17% 3	2180
October	1st to 10th	Night	41% 4 9% 21% 20% 5%	3800
	11th to 20th	Night	24% 4 10% 23% 31% 8%	3779
	21st to end	Night	23% 3 9% 32% 26% 7%	4120
November	1st to 10th	Night	34% 3 9% 27% 22% 6%	2352
	11th to 20th	Night	36% 3 8% 24% 23% 6%	2493
	21st to end	Night	36% 4 11% 26% 20% 3	2660
December	1st to 10th	Night	44% 4 10% 25% 15% 2	2660
	11th to 20th	Night	41% 4 10% 24% 18% 4	3288
	21st to end	Night	46% 4 15% 23% 11% 1	4884

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	25% 22% 33% 17% 3 1	204
	11th to 20th	Day	30% 19% 29% 19% 3	270
	21st to end	Day	22% 22% 36% 13% 6% 1	297
February	1st to 10th	Day	23% 22% 37% 14% 4	270
	11th to 20th	Day	34% 20% 34% 10% 2	315
	21st to end	Day	28% 16% 34% 19% 3	382
March	1st to 10th	Day	24% 18% 32% 21% 4	460
	11th to 20th	Day	24% 19% 33% 17% 6% 1	460
	21st to end	Day	23% 16% 32% 20% 8% 1	506
April	1st to 10th	Day	22% 17% 33% 21% 6% 1	460
	11th to 20th	Day	17% 14% 30% 22% 13% 4	452
	21st to end	Day	25% 12% 29% 22% 10% 2	460
May	1st to 10th	Day	24% 11% 28% 20% 14% 3	463
	11th to 20th	Day	21% 7% 30% 23% 15% 5%	487
	21st to end	Day	18% 5% 22% 29% 20% 6%	529
June	1st to 10th	Day	13% 4 27% 27% 21% 8%	490
	11th to 20th	Day	12% 7% 23% 32% 17% 8%	490
	21st to end	Day	10% 4 27% 33% 19% 7%	490
July	1st to 10th	Day	12% 3 15% 32% 26% 14%	485
	11th to 20th	Day	6% 2 17% 31% 33% 11%	484
	21st to end	Day	13% 2 19% 30% 28% 8%	532
August	1st to 10th	Day	12% 3 16% 28% 29% 12%	486
	11th to 20th	Day	11% 4 15% 32% 27% 11%	479
	21st to end	Day	9% 2 18% 32% 21% 19%	539
September	1st to 10th	Day	17% 5% 18% 23% 23% 14%	490
	11th to 20th	Day	15% 3 17% 29% 22% 14%	490
	21st to end	Day	27% 6% 22% 27% 11% 6%	490
October	1st to 10th	Day	21% 7% 18% 26% 21% 6%	490
	11th to 20th	Day	16% 12% 26% 28% 15% 3	490
	21st to end	Day	17% 8% 32% 27% 12% 3	539
November	1st to 10th	Day	17% 9% 30% 28% 12% 3	425
	11th to 20th	Day	19% 11% 32% 24% 12% 2	342
	21st to end	Day	20% 18% 29% 22% 9% 2	180
December	1st to 10th	Day	19% 12% 41% 21% 5% 2	180
	11th to 20th	Day	27% 13% 31% 20% 9%	180
	21st to end	Day	24% 11% 34% 23% 7% 1	198
January	1st to 10th	Night	21% 16% 40% 19% 4	760
	11th to 20th	Night	24% 13% 33% 26% 4	760
	21st to end	Night	20% 15% 39% 24% 2	836
February	1st to 10th	Night	19% 14% 38% 26% 3	760
	11th to 20th	Night	25% 19% 37% 17% 1	743
	21st to end	Night	25% 16% 35% 21% 3	628
March	1st to 10th	Night	20% 15% 39% 23% 3	760
	11th to 20th	Night	22% 16% 34% 22% 4 1	760
	21st to end	Night	19% 14% 38% 24% 4 1	836
April	1st to 10th	Night	19% 18% 37% 22% 4	760
	11th to 20th	Night	16% 11% 36% 28% 9% 1	751
	21st to end	Night	20% 9% 31% 33% 7%	760
May	1st to 10th	Night	20% 8% 28% 31% 12% 1	621
	11th to 20th	Night	21% 5% 25% 37% 12% 1	451
	21st to end	Night	14% 3 21% 40% 21% 1	411
June	1st to 10th	Night	10% 2 26% 40% 21% 1	380
	11th to 20th	Night	13% 4 22% 39% 20% 1	380
	21st to end	Night	9% 2 22% 49% 17% 1	380
July	1st to 10th	Night	10% 2 17% 43% 24% 4	370
	11th to 20th	Night	6% 2 14% 40% 34% 4	453
	21st to end	Night	9% 2 14% 43% 29% 3	624
August	1st to 10th	Night	12% 1 13% 40% 31% 3	567
	11th to 20th	Night	11% 3 12% 44% 28% 3	637
	21st to end	Night	12% 3 14% 40% 27% 4	836
September	1st to 10th	Night	17% 3 16% 31% 28% 5%	760
	11th to 20th	Night	13% 3 13% 37% 32% 3	760
	21st to end	Night	22% 4 21% 32% 20% 1	760
October	1st to 10th	Night	23% 4 19% 30% 20% 4	760
	11th to 20th	Night	16% 9% 27% 32% 16%	760
	21st to end	Night	17% 9% 32% 29% 13%	836
November	1st to 10th	Night	14% 9% 31% 33% 12% 1	755
	11th to 20th	Night	17% 12% 32% 29% 8% 1	760
	21st to end	Night	16% 14% 32% 30% 8% 1	760
December	1st to 10th	Night	22% 12% 34% 25% 7%	760
	11th to 20th	Night	18% 10% 40% 26% 6%	760
	21st to end	Night	20% 11% 35% 30% 3	836

Page generated at 2024-11-01 22:47:37. Version ac07

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