Notes

Outline

Lecture 4: Capricious Clouds

Adiabatic Lapse Rate  Adiabatic lapse rate      = 10°C/km or 1°C/100m      = air temperature in a rising balloon drops 1°C every 100m.

Environmental Lapse Rate The environmental (or ambient) lapse rate is referred to the vertical change in temperature through still air. The environmental lapse rate is not fixed. It changes from day to day and from place to place.

Water Vapor In the Air Evaporation: the process whereby molecules break free of the liquid volume. Condensation: water vapor molecules randomly collide with the water surface and bond with adjacent molecules.

Specific .vs. Relative Humidity Specific Humidity: How many grams of water vapor in one kilogram of air (in unit of gm/kg). Relative Humidity: The percentage of current moisture content to the saturated moisture amount (in unit of %). Clouds form when the relative humidity reaches 100%.

How Much Water Vapor Is Evaporated Into the Atmosphere Each Year?  On average, 1 meter of water is evaporated from oceans to the atmosphere each year.  The global averaged precipitation is also about 1 meter per year.

How Much Heat Is Brought Upward By Water Vapor? Earth’s surface lost heat to the atmosphere when water is evaporated from oceans to the atmosphere. The evaporation of the 1m of water causes Earth’s surface  to lost 83 watts per square meter, almost half of the sunlight that reaches the surface. Without the evaporation process, the global surface temperature would be 67°C instead of the actual 15°C.

“Runway” Greenhouse Effect If a planet has a very high temperature that the air can never reach a saturation point Water vapor can be added into the atmosphere. More water vapor traps more heat (a greenhouse effect) The planet’s temperature increases furthermore  Ever more water evaporated into the atmosphere  More greenhouse effect  More warming  More water vapor  …..

How to Saturate the Air? Two ways: Increase (inject more) water vapor to the air (Aà B). Reduce the temperature of the air (A à C).

Why Clouds Form?    Clouds form when air rises and becomes saturated in response to adiabatic cooling.

Four Ways to Lift Air Upward

Cloud Type Based On Properties

Cloud Types Based On Height

Cloud Classifications

High Clouds High clouds have low cloud temperature and low water content and consist most of ice crystal.

Middle Clouds Middle clouds are usually composite of liquid droplets. They block more sunlight to the surface than the high clouds.

Low Clouds Low, thick, layered clouds with large horizontal extends, which can exceed that of several states.

Clouds With Vertical Development They are clouds with substantial vertical development and occur when the air is absolute or conditionally unstable.

Slide 19

Cloud Role In Climate Change Cloud effects is one of the most uncertain parts of climate prediction. Typically, clouds increase albedo è a cooling effect      clouds reduce longwave radiation è a heating effect The net effect of clouds on climate depends cloud types and their optical properties, the insolation, and the characteristics of the underlying surface. In general, high clouds tend to produce a heating (positive) feedback. Low clouds tend to produce a cooling (negative) feedback.