# Presentation: 462036

## Contrails

- David Duda
- Hampton University/NASA LaRC

## Outline

- Introduction – What are contrails?
- Making a cloud  (how contrails form)
- Why do we study contrails?
- S’COOL and contrails
- Contrail cousins

**Notes:**
Why do we study contrails? – Discuss some historical reasons to study contrails, and some reasons why we study them at LaRC today.
S’COOL and contrails – Explain how surface observations from programs like S’COOL can help our research
Contrail cousins – Show a few examples of aircraft phenomena related to contrails

## Introduction

- Contrails are **CLOUDS** that form in the wake of aircraft.
- Contrail is a shortened name for condensation trails.
- Also known as vapor trails, jet trails, “chemtrails”

**Notes:**
Contrails are clouds that are made up of ice crystals, or rarely, water droplets.

## Contrails have been around for a long time!

- They were first described in the scientific literature in 1919.
- During WWII, contrails sometimes littered the skies during aerial combat.

**Notes:**
Although we tend to associate contrails with jet aircraft, propeller-driven aircraft can sometimes make contrails.
During the Battle of Britain in WWII, dogfights between British Spitfires and German Bf109s occasionally produced contrails.

## Making A Cloud

- Clouds form when water vapor in the air
- condenses into visible water droplets or ice
- crystals.

- Condensation can occur in two ways:
- Increase the water content in the air.
- Cool the air to reach the dew point.

**Notes:**
Clouds can form by many different processes.  Most of these processes involve the second method to produce condensation, that is, they cool the air.

## Making A Cloud (Part 2)

- Nearly all contrails form by the **mixing** of
- the colder, drier atmosphere with the
- warmer, wetter jet exhaust *(mixing cloud).*

**Notes:**
Another type of mixing cloud is the cloud that forms on your breath during a cold winter day.

## Mixing clouds

- Mixing clouds form because the ability of air to ‘hold’ water vapor increases rapidly with temperature.
- Like contrails, the cloud that forms on your breath during a cold day is a mixing cloud.

**Notes:**
When air temperature is 20 deg F, your breath can hold over 16 times as much water vapor as an equal volume of the cold air!

The remarkable thing about mixing clouds is that neither the cold air nor the warm air have to be saturated to make a cloud.  Thus contrails can occur in otherwise clear skies.  Some people mistakenly think that the aircraft must be “spraying” something into the atmosphere since the relative humidity isn’t 100 percent.

## Nearly all contrails are mixing clouds

- Most contrails that you see in the sky form
- at high altitudes (above 26,000 ft) and at
- very cold temperatures (less than -40F).

**Notes:**
Jet airplanes cannot always produce contrails.  Sometimes the atmosphere is too warm and/or too dry so that the mixing of the air with the exhaust doesn’t cause condensation.  Note that the heat from the exhaust is working AGAINST contrail formation!

## Why Do We Study Contrails?

- Contrails cause detection and visibility
- problems for military aircraft.

**Notes:**
Contrails are troublesome (astronomers hate them)!
During WWII, formation flying sometimes produced banks of contrails
Reports exist of planes being unable to see targets, and colliding with other aircraft because of contrails.
Fighter pilots sometimes used contrails as a hiding place to ambush other aircraft.

## When the upper

- atmosphere is
- moist enough,
- the contrails
- continue to grow.
- Under these
- conditions, the
- contrails become
- persistent.

## We currently estimate that contrails add an additional 0.5 to 5 percent warming to the greenhouse gas effect.

- Persistent contrails occasionally cover large areas.
-  Like cirrus clouds, contrails likely contribute to global warming.

## Air traffic and persistent contrail coverage will continue to increase.

## By 2050, the warming due to contrails may be 2.5

- to 25 percent of the current greenhouse gas warming.

**Notes:**
Notice the large uncertainty in our current estimates.  Twenty five percent is something to be concerned about.
2.5 percent is probably not important.

## S’COOL and Contrails

## Our estimates of the climatic effects of persistent contrails are still uncertain.

- We still have trouble estimating contrail coverage.

**Notes:**
We also have problems getting global estimates of contrail thickness and particle size.

## Most contrails are still smaller than the resolution of most satellites.  

- Most contrails are still smaller than the resolution of most satellites.

**Notes:**
Contrails usually don’t last more than a few hours.  We must use geo-synchronous satellites to study them more properly, but these satellites lack the necessary resolution for us to identify all contrails.

## Using inventories of commercial aircraft flight data and satellite data, we are learning more about the growth and coverage of contrails under different atmospheric conditions.  Surface observations from S’COOL would improve our studies.

## Contrail Cousins

## Unconventional Contrails(clouds, but not mixing clouds)

**Notes:**
These clouds can also be called contrails, but they are short-lived trails that usually form under mild, humid conditions.  They consist entirely of water droplets that condense as a result of local reductions in pressure due to the movement of of wing or propeller.

## Contrails (not!)

- This B-52 is producing smoke!

- Skywriting planes also
- produce smoke.  Often
- the smoke is from a type
- of cooking oil!

## Inverse contrails (distrails)

- Aircraft sometimes make holes in clouds!