Morrison Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Morrison Observatory clear sky chart from 2005-11-06 to 2024-11-02.

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	12% 8% 3 6% 4 4 4 3 5% 7% 43%	101259
	Night	13% 8% 3 6% 4 4 4 3 5% 6% 45%	97802
Transparency	Day	40% 5% 13% 26% 15% 2	101063
	Night	40% 4 12% 25% 16% 3	97669
Seeing	Day	15% 8% 26% 29% 18% 5%	15673
	Night	16% 8% 29% 33% 14% 1	25476

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	14% 12% 4 10% 4 4 4 4 5% 7% 33%	2655
	11th to 20th	Day	12% 11% 4 10% 5% 7% 6% 2 4 3 36%	2850
	21st to end	Day	17% 17% 6% 9% 4 3 3 3 4 4 30%	3135
February	1st to 10th	Day	17% 9% 4 8% 6% 5% 3 4 5% 6% 34%	1710
	11th to 20th	Day	13% 10% 3 7% 5% 4 4 3 4 5% 42%	1685
	21st to end	Day	14% 12% 4 6% 4 4 4 3 4 6% 39%	1570
March	1st to 10th	Day	17% 10% 3 5% 3 3 2 2 3 6% 45%	3431
	11th to 20th	Day	12% 7% 3 5% 4 3 3 3 5% 7% 49%	3480
	21st to end	Day	19% 13% 3 8% 5% 4 4 3 4 6% 31%	3839
April	1st to 10th	Day	15% 8% 3 7% 4 3 3 3 4 6% 43%	2280
	11th to 20th	Day	17% 8% 3 7% 4 3 4 3 5% 6% 41%	2254
	21st to end	Day	22% 6% 3 6% 5% 4 4 4 6% 5% 35%	2280
May	1st to 10th	Day	13% 8% 3 7% 5% 4 5% 4 7% 9% 35%	4065
	11th to 20th	Day	17% 9% 3 7% 4 4 3 3 5% 7% 39%	4230
	21st to end	Day	13% 7% 2 6% 5% 4 4 4 5% 7% 44%	4824
June	1st to 10th	Day	7% 4 2 6% 5% 4 5% 5% 6% 10% 47%	2660
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 8% 48%	2660
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 9% 48%	2660
July	1st to 10th	Day	13% 6% 3 6% 4 4 4 4 6% 9% 41%	4440
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 6% 9% 52%	4230
	21st to end	Day	8% 4 2 6% 5% 5% 5% 5% 7% 10% 44%	4465
August	1st to 10th	Day	4 4 2 4 4 4 5% 5% 7% 11% 51%	4104
	11th to 20th	Day	11% 5% 2 6% 5% 4 4 4 6% 8% 45%	3953
	21st to end	Day	6% 4 2 4 4 4 5% 4 6% 9% 53%	4180
September	1st to 10th	Day	10% 5% 2 4 4 4 4 3 4 6% 53%	2280
	11th to 20th	Day	9% 5% 2 5% 4 3 4 3 4 9% 52%	2275
	21st to end	Day	8% 5% 1 6% 5% 4 5% 3 5% 6% 53%	1982
October	1st to 10th	Day	12% 5% 2 6% 3 2 3 3 5% 4 55%	1900
	11th to 20th	Day	15% 6% 2 5% 3 3 3 2 4 5% 53%	1900
	21st to end	Day	18% 7% 2 6% 3 3 2 3 4 5% 47%	2090
November	1st to 10th	Day	11% 7% 2 5% 2 4 2 2 5% 7% 52%	1802
	11th to 20th	Day	15% 7% 2 6% 3 2 2 3 4 7% 49%	1649
	21st to end	Day	18% 10% 3 6% 4 3 3 3 4 6% 42%	1520
December	1st to 10th	Day	10% 12% 4 7% 4 4 3 3 4 5% 43%	1520
	11th to 20th	Day	13% 11% 4 7% 4 3 4 3 3 6% 42%	1918
	21st to end	Day	20% 15% 7% 10% 4 4 5% 3 5% 5% 24%	2783
January	1st to 10th	Night	10% 11% 7% 12% 5% 4 4 3 4 4 35%	4296
	11th to 20th	Night	13% 11% 5% 10% 4 6% 6% 5% 5% 5% 31%	4120
	21st to end	Night	19% 16% 6% 8% 4 3 3 3 3 5% 30%	4532
February	1st to 10th	Night	15% 11% 4 8% 6% 4 4 3 4 5% 35%	2470
	11th to 20th	Night	14% 8% 4 6% 4 4 4 4 5% 6% 41%	2380
	21st to end	Night	15% 9% 4 7% 4 3 3 3 5% 4 43%	1884
March	1st to 10th	Night	16% 10% 3 7% 3 3 3 3 4 5% 44%	3449
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 4 6% 51%	3480
	21st to end	Night	18% 12% 4 9% 5% 5% 4 3 5% 6% 30%	3791
April	1st to 10th	Night	12% 7% 2 6% 3 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	15% 6% 3 6% 5% 3 4 3 5% 6% 46%	1875
	21st to end	Night	18% 6% 3 7% 4 4 4 3 5% 6% 41%	1900
May	1st to 10th	Night	15% 8% 2 7% 5% 5% 4 4 6% 7% 38%	2837
	11th to 20th	Night	13% 7% 3 7% 4 4 4 4 4 10% 41%	2673
	21st to end	Night	10% 5% 2 5% 3 4 4 3 6% 9% 50%	2743
June	1st to 10th	Night	7% 4 2 4 4 4 4 4 6% 8% 54%	1520
	11th to 20th	Night	5% 5% 2 4 5% 4 5% 4 7% 9% 50%	1520
	21st to end	Night	4 3 1 4 5% 4 4 3 6% 9% 56%	1520
July	1st to 10th	Night	13% 6% 2 6% 3 3 4 3 5% 9% 45%	2530
	11th to 20th	Night	4 2 1 4 4 4 4 3 5% 9% 60%	2736
	21st to end	Night	7% 3 2 5% 5% 4 5% 5% 6% 10% 48%	3098
August	1st to 10th	Night	4 2 1 3 2 3 4 4 6% 10% 62%	2836
	11th to 20th	Night	12% 5% 2 4 3 4 3 3 5% 7% 52%	2881
	21st to end	Night	4 3 1 4 3 4 4 4 6% 9% 57%	3476
September	1st to 10th	Night	11% 4 1 4 3 3 4 3 4 5% 59%	1900
	11th to 20th	Night	9% 5% 1 5% 3 4 4 4 6% 7% 51%	1900
	21st to end	Night	8% 5% 2 6% 3 3 3 3 4 6% 56%	2178
October	1st to 10th	Night	9% 4 1 4 2 3 3 3 5% 7% 58%	2280
	11th to 20th	Night	14% 6% 2 5% 3 3 3 2 4 5% 54%	2280
	21st to end	Night	17% 8% 2 5% 3 3 3 3 4 5% 48%	2508
November	1st to 10th	Night	10% 4 2 4 4 3 3 3 4 6% 57%	2299
	11th to 20th	Night	12% 7% 2 6% 4 3 3 3 5% 6% 50%	2527
	21st to end	Night	18% 8% 2 5% 4 3 3 2 3 4 46%	2660
December	1st to 10th	Night	14% 10% 4 7% 4 4 3 3 5% 7% 41%	2660
	11th to 20th	Night	14% 9% 3 7% 4 3 4 3 4 6% 42%	3279
	21st to end	Night	21% 18% 6% 9% 4 4 3 2 3 4 26%	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	51% 4 10% 24% 9% 1	2656
	11th to 20th	Day	55% 2 12% 21% 10%	2850
	21st to end	Day	58% 4 10% 16% 11% 2	3135
February	1st to 10th	Day	52% 3 12% 21% 12% 1	1710
	11th to 20th	Day	46% 4 12% 20% 16% 3	1694
	21st to end	Day	48% 2 9% 23% 15% 3	1570
March	1st to 10th	Day	43% 2 9% 25% 17% 2	3431
	11th to 20th	Day	34% 5% 12% 28% 17% 4	3480
	21st to end	Day	54% 4 17% 19% 5%	3839
April	1st to 10th	Day	43% 3 11% 27% 15% 1	2280
	11th to 20th	Day	44% 3 11% 26% 13% 3	2254
	21st to end	Day	49% 5% 11% 22% 13% 1	2280
May	1st to 10th	Day	44% 6% 15% 23% 12% 1	4103
	11th to 20th	Day	46% 5% 13% 27% 10%	4234
	21st to end	Day	38% 6% 10% 28% 17% 1	4837
June	1st to 10th	Day	31% 7% 16% 30% 15% 1	2660
	11th to 20th	Day	32% 6% 13% 29% 17% 3	2660
	21st to end	Day	31% 6% 17% 28% 15% 4	2660
July	1st to 10th	Day	40% 6% 12% 25% 15% 2	4371
	11th to 20th	Day	28% 6% 15% 32% 19% 1	4157
	21st to end	Day	33% 9% 18% 29% 12% 1	4325
August	1st to 10th	Day	26% 5% 18% 34% 16% 1	4090
	11th to 20th	Day	36% 5% 13% 28% 16% 1	3992
	21st to end	Day	26% 6% 18% 32% 14% 4	4179
September	1st to 10th	Day	32% 4 11% 25% 22% 6%	2280
	11th to 20th	Day	31% 4 10% 27% 23% 4	2275
	21st to end	Day	31% 5% 10% 28% 23% 4	1982
October	1st to 10th	Day	32% 3 10% 26% 25% 4	1900
	11th to 20th	Day	35% 3 8% 26% 23% 5%	1900
	21st to end	Day	42% 2 9% 24% 19% 3	2090
November	1st to 10th	Day	33% 3 13% 24% 22% 5%	1802
	11th to 20th	Day	37% 2 10% 27% 21% 3	1649
	21st to end	Day	45% 3 8% 28% 15% 1	1520
December	1st to 10th	Day	43% 4 12% 28% 12% 1	1520
	11th to 20th	Day	45% 3 14% 27% 9% 2	1915
	21st to end	Day	63% 4 10% 15% 8% 1	2783
January	1st to 10th	Night	53% 3 10% 23% 10% 1	4296
	11th to 20th	Night	55% 3 12% 19% 10% 1	4120
	21st to end	Night	58% 4 10% 19% 8% 1	4532
February	1st to 10th	Night	51% 4 11% 20% 11% 3	2470
	11th to 20th	Night	43% 4 11% 22% 17% 2	2381
	21st to end	Night	44% 3 10% 22% 17% 4	1884
March	1st to 10th	Night	44% 4 10% 23% 17% 3	3449
	11th to 20th	Night	35% 4 8% 26% 23% 4	3480
	21st to end	Night	55% 5% 12% 23% 5% 1	3791
April	1st to 10th	Night	37% 5% 10% 26% 19% 3	1900
	11th to 20th	Night	40% 3 9% 28% 16% 5%	1875
	21st to end	Night	44% 4 10% 23% 18% 1	1900
May	1st to 10th	Night	44% 6% 15% 19% 15% 1	2864
	11th to 20th	Night	41% 5% 14% 29% 11% 1	2674
	21st to end	Night	32% 5% 13% 24% 24% 2	2751
June	1st to 10th	Night	27% 5% 16% 31% 17% 4	1520
	11th to 20th	Night	29% 7% 11% 27% 22% 4	1520
	21st to end	Night	25% 4 16% 30% 20% 5%	1520
July	1st to 10th	Night	37% 4 12% 27% 16% 3	2490
	11th to 20th	Night	21% 5% 14% 37% 21% 2	2686
	21st to end	Night	29% 7% 21% 30% 13% 1	2999
August	1st to 10th	Night	19% 4 20% 34% 21% 1	2826
	11th to 20th	Night	32% 5% 13% 28% 19% 2	2908
	21st to end	Night	23% 5% 17% 34% 16% 4	3476
September	1st to 10th	Night	27% 4 11% 24% 27% 7%	1900
	11th to 20th	Night	30% 5% 9% 26% 24% 6%	1900
	21st to end	Night	30% 4 7% 29% 27% 4	2178
October	1st to 10th	Night	27% 4 10% 25% 29% 6%	2280
	11th to 20th	Night	34% 3 6% 27% 22% 7%	2280
	21st to end	Night	40% 3 9% 22% 23% 3	2508
November	1st to 10th	Night	30% 3 12% 27% 21% 6%	2299
	11th to 20th	Night	36% 3 8% 27% 22% 4	2527
	21st to end	Night	44% 3 9% 25% 17% 2	2660
December	1st to 10th	Night	45% 3 13% 26% 13% 1	2660
	11th to 20th	Night	44% 4 10% 27% 13% 1	3281
	21st to end	Night	65% 3 8% 15% 8% 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	21% 14% 34% 26% 6%	270
	11th to 20th	Day	32% 13% 37% 15% 2	270
	21st to end	Day	19% 11% 45% 20% 5%	297
February	1st to 10th	Day	23% 14% 42% 17% 4	270
	11th to 20th	Day	21% 19% 36% 18% 6%	270
	21st to end	Day	23% 17% 40% 14% 4	225
March	1st to 10th	Day	19% 17% 41% 21% 3	270
	11th to 20th	Day	28% 10% 34% 23% 5%	270
	21st to end	Day	36% 15% 29% 15% 4	310
April	1st to 10th	Day	23% 13% 33% 24% 6% 1	460
	11th to 20th	Day	22% 13% 31% 23% 9% 2	452
	21st to end	Day	24% 9% 31% 26% 9% 1	460
May	1st to 10th	Day	19% 9% 32% 29% 9% 1	636
	11th to 20th	Day	20% 7% 24% 30% 14% 4	675
	21st to end	Day	16% 5% 29% 25% 19% 6%	734
June	1st to 10th	Day	7% 3 18% 36% 26% 10%	680
	11th to 20th	Day	10% 2 22% 35% 24% 7%	680
	21st to end	Day	7% 5% 26% 38% 19% 5%	680
July	1st to 10th	Day	10% 2 14% 27% 33% 14%	671
	11th to 20th	Day	7% 1 17% 35% 30% 10%	668
	21st to end	Day	8% 2 14% 35% 31% 9%	739
August	1st to 10th	Day	7% 2 15% 37% 32% 7%	676
	11th to 20th	Day	10% 2 14% 37% 29% 8%	616
	21st to end	Day	9% 2 12% 31% 35% 11%	539
September	1st to 10th	Day	10% 2 13% 34% 29% 11%	490
	11th to 20th	Day	8% 3 14% 35% 29% 11%	485
	21st to end	Day	9% 5% 26% 28% 25% 7%	300
October	1st to 10th	Day	15% 6% 28% 26% 17% 7%	300
	11th to 20th	Day	13% 17% 34% 25% 10% 1	300
	21st to end	Day	26% 15% 32% 22% 6%	330
November	1st to 10th	Day	16% 9% 36% 23% 11% 4	273
	11th to 20th	Day	17% 15% 30% 27% 9% 2	270
	21st to end	Day	27% 10% 30% 27% 7%	270
December	1st to 10th	Day	13% 16% 36% 25% 8% 3	270
	11th to 20th	Day	20% 14% 35% 24% 4 1	270
	21st to end	Day	25% 20% 36% 19% 1	297
January	1st to 10th	Night	16% 14% 37% 26% 7%	950
	11th to 20th	Night	18% 15% 37% 24% 5% 1	950
	21st to end	Night	18% 12% 38% 28% 4	1045
February	1st to 10th	Night	20% 12% 38% 27% 4	950
	11th to 20th	Night	20% 13% 36% 27% 4	906
	21st to end	Night	21% 13% 40% 21% 4	628
March	1st to 10th	Night	17% 13% 39% 26% 4	589
	11th to 20th	Night	22% 10% 32% 30% 6% 1	570
	21st to end	Night	26% 11% 34% 24% 4	627
April	1st to 10th	Night	16% 10% 39% 29% 5%	570
	11th to 20th	Night	18% 11% 32% 30% 9%	563
	21st to end	Night	20% 6% 36% 33% 6%	570
May	1st to 10th	Night	15% 6% 31% 38% 10%	570
	11th to 20th	Night	18% 4 22% 39% 16% 1	561
	21st to end	Night	13% 5% 24% 34% 22% 1	617
June	1st to 10th	Night	8% 3 15% 45% 26% 3	570
	11th to 20th	Night	5% 3 21% 43% 25% 2	570
	21st to end	Night	4 2 21% 52% 20% 1	570
July	1st to 10th	Night	12% 2 10% 37% 35% 4	555
	11th to 20th	Night	6% 1 12% 43% 35% 3	552
	21st to end	Night	7% 2 12% 44% 34% 2	624
August	1st to 10th	Night	7% 1 10% 47% 32% 3	567
	11th to 20th	Night	7% 1 11% 44% 34% 3	564
	21st to end	Night	6% 1 10% 39% 40% 4	627
September	1st to 10th	Night	12% 1 13% 41% 30% 4	570
	11th to 20th	Night	11% 2 14% 42% 27% 5%	570
	21st to end	Night	9% 5% 23% 36% 23% 3	658
October	1st to 10th	Night	12% 3 26% 33% 22% 3	760
	11th to 20th	Night	17% 8% 32% 32% 10%	760
	21st to end	Night	21% 10% 29% 30% 9%	836
November	1st to 10th	Night	12% 8% 32% 36% 12%	745
	11th to 20th	Night	15% 10% 38% 29% 8%	817
	21st to end	Night	21% 10% 33% 29% 7%	950
December	1st to 10th	Night	19% 11% 37% 27% 6% 1	950
	11th to 20th	Night	22% 9% 32% 32% 6%	950
	21st to end	Night	24% 13% 33% 27% 3	1045

Page generated at 2024-10-30 00:22:59. Version ac07

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