ATMS 120 1st Edition Lecture 10 Outline of Last Lecture I Coriolis effect II Development of high and low pressure systems III The jet stream IV Troughs and ridges V Constant pressure maps sloping pressure surfaces VI Air flow alof VII Curvature effects Outline of Current Lecture I Weather around fronts II Freezing rain how it forms III Freezing rain where it occurs Current Lecture I Weather to the east rain freezing rain sleet snow Clouds overcast layered sheet like clouds stratiform clouds Precipitation north of the front depending on vertical temperature profile snow sleet freezing rain or rain may fall Thunderstorms are a possibility if weather is unstable Temperature warm in the warm sector mT cool in the cool sector mP II Ice will always melt at temperatures above 32 degrees F 0 degrees C but liquid water can remain a liquid at temperatures below 32 degrees F 0 degrees C Supercooled water water in the liquid phase with a temperature 0 degrees C It can remain a liquid to 40 degrees C 40 degrees F Liquid water needs help becoming ice All ice is hexagonal 6 sides in its molecular structure to lock into the lattice structure ice nuclei serve as the site onto which ice crystals begin to grow ice nuclei are rare and only effective at temperatures 15 degrees C without ice nuclei the water will remain liquid even though its temperature is below freezing There must be impurities to serve as nuclei in order to make ice clouds etc If there weren t These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute purities in our atmosphere we wouldn t have clouds To get water to condense you need something for it to condense onto a nuclei Supercooled water has no impurities no nucleation site nothing for it to condense onto III Criteria for freezing rain development air near the surface that is below freezing a warm layer of air in the lower levels of the troposphere above the surface subfreezing layer If the temperature is below freezing higher in the atmosphere precipitation will begin as snow If at some point in the atmosphere the temperature rises to above freezing the snowflake will begin to melt and will melt to a certain point This is called the deep melting layer Then if the atmosphere right above the surface is below freezing the water droplet will begin to freeze again This is called the shallow surface sub freezing layer If the snowflake did not completely melt in the deep melting layer it will become sleet there was an ice nuclei impurity for the water to freeze on If the snowflake DID completely melt in the deep melting layer it will become freezing rain there was no ice nuclei for the drop to freeze back onto it was supercooled and remained a liquid until it hit the ground nucleation site at which point the drop froze Freezing rain occurs most ofen in the northeastern region of the US