Here's a sight with plenty of information click here

Basically there are a number of avenues that give partial information about why there are planes leaving white trails in the stratosphere.  I'll try to break the information I've found into several distinct groups  

1. Commercial,
2. Academic, 
3. Government, and
4. Everyone Else

Commercial Links
​a website for a company that sells their services for solutions to atmospheric necessities:
  www.weathermodification.com 

Also see
Weather Modification, Inc 
Who We Are
Source:  http://www.weathermodification.com/about-us.php
Weather Modification, Inc., is a global atmospheric sciences company committed to continued advances in the field of weather modification. Whether it’s identifying the potential benefits of cloud seeding or monitoring air quality, we are dedicated to providing sound, scientific atmospheric solutions.
Nearly 50 Years of Expertise in Atmospheric Sciences
Weather Modification Inc., was founded in 1961, in the small town of Bowman, ND. Bowman is located in an area of the state where the average rainfall is near desert amounts and hail is common. In order to preserve the livelihood of the agricultural community, two pilots/farmers decided to enter the forefront of weather modification.

Academic Studies  (there are many studies available)  Univ of North Dakota seems to have a large interest in this areaa
Published studies of research resulting from testing results of atmospheric airesole injections can be found at these links:
​Journal Article Abstract
On The Climate response to Cirrus Cloud Seeding
Tuesday, 6 January 2015: 2:30 PM
211B West Building (Phoenix Convention Center - West and North Buildings)
Trude Storelvmo, Yale University, New Haven, CT; and W. R. Boos and N. Herger
Recorded Presentation
Climate engineering, the intentional alteration of Earth's climate, is a multifaceted and controversial topic. Numerous climate engineering mechanisms (CEMs) have been proposed, and the efficacies and potential undesired consequences of some of them have been studied in the safe environments of numerical models. Here, we present a global modeling study of a so far understudied CEM, namely the seeding of cirrus clouds to reduce their lifetimes in the upper troposphere, and hence their greenhouse effect. Different from most CEMs, the intention of cirrus seeding is not to reduce the amount of solar radiation reaching Earth's surface. This particular CEM rather targets the greenhouse effect, by reducing the trapping of infrared radiation by high clouds. This avoids some of the caveats that have been identified for solar radiation management, for example the delayed recovery of stratospheric ozone or drastic changes to Earth's hydrological cycle. We find that seeding of mid- and high latitude cirrus clouds has the potential to cool the planet by about 1.4K, and that this cooling is accompanied by only a modest reduction in rainfall. Intriguingly, seeding of the 15% of the globe with the highest solar noon zenith angles at any given time yields the same global mean cooling as a seeding strategy that involves 45% of the globe. In either case, the cooling is strongest at high latitudes, and could therefore serve to prevent Arctic sea ice loss. With the caveat that there are still significant uncertainties associated with ice nucleation in cirrus clouds and its representation in climate models, cirrus seeding appears to represent a powerful CEM with reduced side ....


Also see:
​Planned Weather Modification through Cloud Seeding
 
Source:   https://www2.ametsoc.org/ams/index.cfm/about-ams/ams-statements/statements-of-the-ams-in-force/planned-weather-modification-through-cloud-seeding/
An Information Statement of the American Meteorological Society 
(Adopted by the AMS Council on 2 November 2010)

Government Links
http://www.globalchange.gov/

Everyone Else
http://www.geoengineeringwatch.org/

Another grouping I've considered is

1. Weather Control,
2. Global Warming, and
3. Chemtrails

Weather Control
Weather Modification Program
Source:  http://www.weathermodification.org/
VISION:
"Free exchange of accurate information regarding the efficacy, safety, methodology and cost-effectiveness of weather modification activities."
MISSION:
"Enabling persons, political entities, and other organizations to make informed decisions about the application of weather modification technologies, to provide for adequate water supplies, and reduced natural weather hazards."

Global Warming
Preliminary Observations on Geoengineering Science, Federal Efforts, and Governance Issues
Source:     http://www.gao.gov/assets/130/124271.pdf
A September 2009 study from the Royal Society4—the United Kingdom’s
national academy of science—categorized most geoengineering proposals
into two approaches: solar radiation management (SRM), which would
offset temperature increases by reflecting a small percentage of the sun’s
light back into space, thus reducing the amount of heat absorbed by the
earth’s atmosphere and surface, and carbon dioxide removal (CDR), which
would address what scientists currently view as the root cause of climate
change by remving carbon dioxide—a greenhouse gas—from the atmosphere.5
 
Footnotes:
3 Geoengineering is also referred to as climate engineering or climate intervention.
4 The Royal Society, Geoengineering and the climate: science, governance and uncertainty (London: September 2009).
5 In addition to these two types of approaches, other large-scale interventions in the earth’s climate system, such as removing other greenhouse gases from the atmosphere, have been considered as part of a potential response to reduce the impacts of climate change.

Chemtrails
​Is there any link between cloud seeding and chemtrails?       
Source:   http://www.weathermodification.org/faq.php
No. The WMA is unaware of any connection between cloud seeding as is practiced by its members and to what some refer to as “chemtrails” (chemical trails).  Atmospheric scientists even dispute the existence of “chemtrails”.  What some chose to call chemtrails are actually “contrails” (condensed engine exhaust trails), which are well-understood atmospheric phenomena. Contrails are defined as “streaks of condensed water vapor created by an airplane or rocket at high altitudes.”  These condensation trails are the result of normal emissions of water vapor, carbon dioxide, and some carbon-containing particulates from piston engines and jet engines at high altitudes in which, given the right atmospheric conditions, the water vapor condenses into a visible cloud. Actually, due to the very cold temperatures at high altitudes, the water droplets that initially form this cloud rapidly freeze, forming an ice cloud similar to naturally occurring cirrus clouds.  Under certain conditions contrails can merge or contribute to the development of a larger area of cirrus clouds. Contrails are normally observed on otherwise clear days, when cloud seeding would not be conducted. The cloud seeding nuclei from ground or airborne sources do not produce such visible clouds.