Fronts, Station Models, and Storms

A front is the boundary where two air masses meet. The air masses do not mix. The denser cold air pushes under the lighter warm air and forces it to rise. Rising moist air makes clouds and precipitation. That is why fronts are where most weather happens.

At a cold front, cold air pushes forward and shoves warm air steeply upward. You get a narrow band of tall clouds, then brief heavy showers or thunderstorms, then colder and clearer air behind it. At a warm front, warm air slides gently up over cold air along a shallow slope. High thin clouds arrive about a day ahead. They thicken and lower into long, steady precipitation. A stationary front is a boundary that has stalled. An occluded front forms when a fast cold front catches up to a warm front and lifts the warm air completely off the ground.

Fronts rarely travel alone. The main storm system at our latitudes is the mid-latitude cyclone. It is a large low-pressure center with a cold front and a warm front spiraling counterclockwise around it, with cP air on one side and mT air on the other. The whole system rides the prevailing southwest winds across the country from west to east. Crossing takes several days.

That structure makes New York's weather fairly predictable. As a low approaches from the west, the warm front arrives first. High cirrus clouds thicken and lower into a long, steady rain, followed by warmer, humid air. A day or so later the cold front sweeps through. You get a brief violent line of showers or thunderstorms, then wind shifting to the northwest, rising pressure, and cool, dry, clear air behind it. Watch a barometer fall and then rise and you have tracked the whole storm without looking outside.

A station model packs one station's weather into a small symbol. Your Reference Tables show how to decode it. Temperature and dew point, in Fahrenheit, sit at the upper left and lower left. The shading of the circle shows cloud cover. The line sticking out points into the wind, which shows the direction the wind is blowing from. The feathers on that line give the speed. A long barb is 10 knots and a short one is 5.

The tricky number is pressure, at the upper right. It is a three-digit code with the decimal point and the leading 9 or 10 removed. To decode it, put the decimal point before the last digit. Then try a 9 in front and a 10 in front, and keep whichever answer lands between about 950 and 1050 millibars. So 196 means 1019.6 mb, and 988 means 998.8 mb.

The three-digit pressure code is the part of the station model that trips most people up at first. Slide through codes and watch the rule work. Insert a decimal point before the last digit, then put a 9 or a 10 in front, and keep whichever result lands in the realistic range of about 950 to 1050 millibars.

Storms get stronger when three things come together: moisture, unstable air, and lift. Thunderstorms build where warm humid air rises quickly, usually along cold fronts on summer afternoons. Tornadoes are narrow spinning columns that drop out of the strongest thunderstorms. Hurricanes are huge low-pressure systems that form over warm tropical oceans. They get their energy from evaporating seawater, which is why they weaken once they move over land. New York also gets winter nor'easters and lake-effect snow. Lake-effect snow happens when cold cP air crosses the still-warm Great Lakes, picks up moisture, and dumps it in narrow, heavy bands east of the lakes.

A forecast gives you a probability, not a promise. Forecasters feed millions of observations from stations, weather balloons, radar, and satellites into computer models of the atmosphere. The result comes out as a chance. A 70 percent chance of rain means that in conditions like these, it rains 7 times out of 10. Forecasts get less accurate the further ahead they look. They are very good for tomorrow and rough beyond a week, because small errors in today's data grow larger as the model runs forward.

Two words are worth knowing exactly. A watch means conditions are right for severe weather, so stay alert. A warning means severe weather is happening now or is about to, so act right away. Knowing the difference is the most practical fact in this unit.

Fronts are the boundaries where air masses meet, and each type behaves differently. Cold fronts are steep, brief, and violent. Warm fronts are shallow, gray, and slow. Stationary fronts have stalled. Occluded fronts form when a cold front catches a warm one. Fronts usually travel together inside a mid-latitude cyclone that crosses the country from west to east. That gives New York its familiar pattern of a warm front followed by a cold front. Station models pack each station's weather into one symbol, and the three-digit pressure code is the only tricky part. Thunderstorms, tornadoes, hurricanes, nor'easters, and lake-effect snow all run on the same physics, just stronger. Forecasting turns all of it into probabilities, watches, and warnings.

Ten Regents-style questions, one at a time in a focused view, each with an instant explanation. The set reshuffles when you reach the end, so you can keep practicing as long as you like.