Weather Knowledge · 7 min
Air masses, fronts and pressure
Why this matters
The forecast tells you what is expected. Understanding why tells you what to do when the sky stops matching the forecast, which in the Adirondacks is most days.
By the end of this lesson you will be able to
- Describe the air masses that reach New York and what each brings
- Distinguish a cold front from a warm front by its weather
- Relate falling and rising pressure to what happens next
Air masses over New York
| Air mass | Source | What it brings here |
|---|---|---|
| Continental polar (cP) | Central Canada | Cold, dry, clear. The blue winter high |
| Maritime polar (mP) | North Atlantic or the Pacific | Cool, damp, cloudy, drizzle |
| Maritime tropical (mT) | Gulf of Mexico, subtropical Atlantic | Warm, very humid. Summer thunderstorms |
| Continental arctic (cA) | The high Arctic | The genuinely dangerous cold, a few times a winter |
New York sits in the westerlies, so systems generally track west to east, often with a northeast component. Weather usually arrives from the direction you are looking when you face west, which is a crude but useful field rule.
Fronts
| Cold front | Warm front | |
|---|---|---|
| Motion | Cold air shoves under warm, steeply | Warm air rides up over cold, gently |
| Speed | Fast | Slow |
| Clouds | Towering cumulus, cumulonimbus | A long sequence: cirrus, then cirrostratus, altostratus, nimbostratus |
| Precipitation | Heavy, brief, sometimes violent | Light to moderate, long, widespread |
| Behind it | Clearing, colder, gusty northwest wind, sharp temperature drop | Warmer, humid, often fog and low cloud |
| Guiding concern | Lightning, wind, a sudden 20°F drop | Hours of soaking rain, poor visibility, hypothermia weather |
The cold front is the one that hurts people quickly. The warm front is the one that hurts people slowly, and in New York's shoulder seasons slowly is the more common way.
Pressure
- Falling pressure means a low approaching: cloud, wind, precipitation. Fast fall means fast deterioration
- Rising pressure means a high building: clearing, calmer, and in winter, colder
- Steady high pressure is your good weather window
An altimeter watch is a barometer. If your altimeter reads higher at camp in the morning than it did last night at the same spot, pressure has fallen: weather is coming. That trick alone justifies the watch.
Getting a forecast
Use the National Weather Service point forecast for the coordinates of your objective, not the town in the valley, and read the forecast discussion where the meteorologist explains their confidence. NWS Burlington covers the eastern Adirondacks, Albany the southern Adirondacks and Catskills, Binghamton and Buffalo the west. Check the day before and again the morning of, and know which observation would make you change the plan.
From the field
Cirrus thickening to a milky halo through the afternoon, and your altimeter reads 80 feet high at the same campsite as yesterday. That is a warm front twelve to twenty-four hours out. You are not cancelling; you are moving the long exposed section to tomorrow morning instead of tomorrow afternoon.
This lesson is part of the NYS Licensed Guide Exam course. Open it in the course to answer the retrieval checks that go with it, track what you have read, and quiz the whole unit. Lessons, quizzes and the glossary are free.