大气科学基础课件第五章云物理学基础

✓Convergence is an atmospheric condition that exists when there is a horizontal net inflow of air into a region.
✓When air converges along the earth's surface, it is forced to rise since it cannot go downward.
(2) 云的形成
• 凝结 Condensation
✓ Condensation occurs when the relative humidity exceeds 100%； ✓ Water only condenses on a surface；
✓ Dew and frost condense on surfaces such as plants or windshields； ✓ In the atmosphere ， water condenses on condensation nuclei (CN).
第五章 云物理学基础
• §1 云的分百度文库与形成条件 • §2 主要云属的宏观和微观特征 • §3 云滴的凝结和碰并增长 • §4 自然降水过程 • §5 人工影响天气基础
Clear skies——晴 Scattered Cloud——少云 Broken Clouds——多云 Overcast——阴
This can happen if one or both of the following occurs:
1) The air is cooled, reducing the saturation vapor pressure; 2) Water vapor is added to the air.
✓Radius typically from 0.1 to 1 microns (micron = 10-6 meters)
✓Concentrations from 1 to 1000 per cm3 of air
• 冰核 Ice Nuclei ✓Water does not always freeze at 32 F；
(3) 云的分类
Cloud
Cumulus
积云
Stratus
✓Clouds form when water vapor in the atmosphere condenses, condensation only occurs on CN;
✓Water vapor condenses when the relative humidity exceeds 100%;
✓Above this point the cloud droplets grow by condensation in the rising air; If the rising motion is sufficiently intense and enough water vapor is present, precipitation will develop.
• Cloud Formation Imagine an air parcel, rising upward through the atmosphere. The air parcel expands as it rises and this expansion causes the temperature of the air parcel to decrease.
• 凝结核 Condensation Nuclei (CN) ✓CN are tiny particles suspended in the atmosphere；
✓CN stay aloft in the air for many days. They are so small that their weight is less than their air resistance.
✓Water existing at temperatures below freezing is called “super-cooled”；
✓Some particles cause super-cooled water to freeze, these particles are known as ice nuclei；
Why does air rise ?
✓An air parcel will rise naturally if the air within the parcel is warmer than the surrounding air (like a hot air balloon).
✓As the earth is heated by the sun, bubbles of hot air form (called thermals) and rise upward from the warm surface.
✓Without ice nuclei, pure water would need to be – 40F to freeze；
✓Some CN are also good ice nuclei, others are not.
• Clouds in the Atmosphere ✓Clouds are a collection of water drops and/or ice crystals;
Temperature
✓As the parcel rises, it cools, and the humidity increases until it reaches 100%;
✓When this occurs, cloud droplets begin forming as the excess water vapor condenses on CN particles;