Understanding Weather: Why Weather Changes (1992)

[Music] Wesley calling Wesley the weather Bureau of Investigation is calling Wesley are you there Wesley this is Wesley sir are you ready for another day of training yes sir you're doing well Wesley soon you'll be a real agent like all of us at the wbi not just an agent in training what is my assignment today sir why weather changes [Music] Wesley come in please Wesley I'm in sir I mean I'm out but here I am well what have you observed about weather changes so far well sir the weather hasn't changed since I've been observing it it's a nice spring day here it's been warm and pleasant like this for several days the sky sort of hazy blue well you're in a warm air mass a warm air what sir Mass Wesley I'll explain look at her weather watch you know that our planet Earth is surrounded by an atmosphere air that is constantly moving but large masses of air can stay over a region for a long period of time when they do they take on the characteristics of that region I don't quite follow that sir well for example air that is over the ocean in a tropical region becomes very warm and moist and we call it a tropical air mass an air that stays over the polar regions for a long time gets very cold and has less moisture than air from a tropical ocean and it's called a polar air mass very good Wesley and what's also important to know about air masses is that they move they move yes look at your weather watch we're both in North America cold dry polar air masses form in the Northern parts of the continent warm moist tropical air masses form over water near the southern parts of the continent polar air masses usually move toward the South and tropical air masses usually move toward the north I'm in a polar air mass now it's cooler and there's less moisture in the air here than in a tropical air mass which is what you're in now but we're going to move you to another place in that air mass why sir so you can be where the action is I don't understand sir you will a wbi van should be pulling up right now yes it is Sir they're honking at me well get in Wesley they'll take you to the airport from there you'll fly in a wbi plane to a wbi station closer to the cold air mass I'm in sir are you there sir yes Wesley what is it well I was just looking at a map I drew I drew the two air masses you showed me kind of where I figure they are if they keep moving like you said they were doing they're going to bump into each other oh they've already met Wesley air masses are bumping into each other all the time well sir what happens when air masses meet Ah that's what you're going to find out Wesley you mean there'll be weather changes That's Right Wesley and big ones too especially now in early spring when there's a big temperature difference between tropical and polar air masses but just be patient I'll try to be sir well are you finally set up Wesley yes sir and waiting patiently very good Wesley anything to report not in the weather sir but I've been noticing the weather maps that were in the newspaper the last few days I saw some things in them that I wanted to ask you about take the map for two days ago for instance here's my map for that day what was your question well the map doesn't show the tropical or polar air masses that you showed me but there's an H an H stands for the center of a high press region that's just another term for one kind of air mass there's also a high pressure over here it's the one I'm in now a polar mass that moves South that's right and the one down there must be the air mass over where I am a tropical air mass that moved northward very good Wesley and this curved line between them shows where they're meeting that's called a front now the prevailing westerlies change the direction of these moving air masses and winds high above the ground steer them the winds move the air masses toward the East and the front is moved toward the East too we moved you so you'd be closer to it it'll be passing over you in a day or two what's the L mean you mean this you'll find out soon now did they issue you the training instruments at the wbi office yes sir they'll help you see the sort of changes that take place as the front passes over you what instruments do you have uh thermometer to measure temperature uh barometer uh yes that's right sir barometer they told me it measures air pressure don't forget to set the pointer to Mark what the air pressure is now so you'll be able to notice any change I'm doing that sir good now go on I have a Weather Vein I know what that's for it tells which direction the wind is blowing from the southwest now and how fast and an umbrella that was good thinking Wesley your idea no theirs but does that mean I can expect rain when the front passes over me you'll see Wesley it won't be long use your eyes and your instruments observe the changes and report them to me then we'll talk about them all right sir there's been a small change in the weather sir what sort of change Wesley you know the Southwest wind I've been watching well it seems to be blowing harder and I see dark puffy clouds in the west those are called cumula Nimbus cumulus rainclouds now look at your barometer what's happening to the air pressure the needle's pointing to a lower number I'll reset the pointer does this mean a lower air pressure yes that often happens as a front approaches we say the barometer is falling what else is happening the clouds are closer now they're very tall clouds and their bottoms look closer to the ground than they were before uh-oh it's starting ter rain umbrella up time I guess so sir wow the wind's getting even stronger and hey the barometer's falling some more air pressure is getting even lower I want to show you something on your weather watch here's yesterday's weather map now can you guess what the L means well H stands for high does the L stand for low pressure right between two air masses there's a low pressure region the wind in the two highs move clockwise the direction the hands of a clock move but in the low between them the winds move in the opposite direction counterclockwise air is being drawn from each High into the low and the warm air and cool air are beginning to mix the low is called a cyclone and the highs anticyclones cyclone and anticyclone got it sir the front is passing over your now Wesley we call this a cold front because the cold air mass is advancing pushing the warm air mass ahead of it it'll take several hours for the front to completely pass I'll be patient sir I have something to report what's that Wesley the wind has changed Direction and that's what should have happened Wesley when part of this weather system was over you winds were blowing from the south West now the cold air mass is beginning to pass over you and the winds where you are are blowing from the north and what's happening in the sky the sky is clearing sir pretty fast and the temperature has that changed it sure has it's a lot cooler now what about the air pressure that seems to be going up again sir I guess that must be because the cold air mass has moved over me and it's a high pressure region like the warm Mar Mass was quite right Wesley and the weather change is over here's the weather map that was in today's newspaper this curved line with the pointed symbols on it represents the cold front the boundary line between the air masses it's raining to the east of you now where the front has moved if the temperature had been below freezing in the polar air mass would there have been snow yes a cold front like this can produce snow in early spring could you show me that cold front on the map again thanks what's that line attached to it this one with the rounded symbols yes is that another cold front well it's a front but the symbols mean it's a warm front here the cold air is retreating allowing the warm air to mix with it there's precipitation in this area too but the weather in places that the warm front passes over will get warmer and instead of the cumulous rain clouds that the cold cold front usually produces a warm front usually produces the flat straightest rain clouds why are the clouds different well look at your weather watch and I'll explain when air masses meet a cold air mass usually moves under a warm air mass and begins to lift it off the ground if the warm moist air is pushed up rapidly cumulus rain clouds form thunderheads that's typical in a strong cold front where cold air is advancing at a warm front where the cold air mass is retreating and the warm air is pushing forward the warm air in this case is slowly lifted as it gradually slides up the cold air mass so the clouds tend to form in sheets straight As clouds in this way warm and cold fronts usually follow each other moving generally Eastward across the continent changing the weather in the place of as they pass over what's the weather like now Wesley Clear Blue Sky cooler than it was before the front past a high pressure region an air mass is over me now an anticyclone but I'll keep watching the sky and the barometer and have my umbrella ready because I know the weather's going to change but from now on I'll know why [Music] n for