NATS 101

Lecture 2

Basic weather symbols and fronts

We describe weather in terms of:

temperature wind patternshumidity clouds visibilitypressure precipitation sunlight/UV

We “feel” weather as the heat we feel, the “weight” of the air we breathe.

If we observe these weather elements at an instance in time, then we obtain a measure of the weather

If we measure these weather elements over many years, then we obtain the “climate” of the region.

Therefore:

- climate represents an average of daily weather over a long period of time

- Weather is the instantaneous condition of the atmosphere.

“Climate is what we expect”

“Weather is what we get”

Atmospheric pressure impacts every aspect of weather although we do not easily recognize differences in pressure.

- Air moves from high pressure to low pressure → wind

- Air tends to rise in regions of low pressure and sink in regions of high pressure

Pressure Units: mb, hPa, inches Hg, mm Hg

Wind Units: m/s, mph, km/h, kts

Pressure and Wind

- tends to change gradually in horizontal or vertical directions

- also changes with time for the same weather system → diurnal cycle

- the only place there are rapid changes in temperature is in the vicinity of fronts

Units:- °C, °F, K

Temperature

- Two common ways to express the amount of water vapor in the air

- Relative Humidity:- is the amount of water vapor in the air relative to the maximum amount that could be present in the air. Units: %

- Dew Point Temperature:-

Td > 15°C is humid.

Td > 20°C is very uncomfortable.

Td < 5°C is dry.

Moisture: water vapor

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H

Fronts:

Cold air

Warm airWarm air

Cold air Cold air

Warm air

Cold front Warm front Stationary front

Pressure Systems:

low pressure

high pressure

Surface Map:

Upper-level Maps:500 mb (hPa) (5 – 6 km or 16000 – 19000 ft)

Upper-level Maps: 300 mb (hPa) (9 – 10 km or 30,000 – 33,000 ft)

Weather Map Symbols

Ref:- pg 525, Appendix C, Aguado and Burt

N

dd

ff

TT CM PPPCH

VV ww ppa

TdTd CLNh WRt

h RR

N – total cloud coverdd – wind directionff – wind speed (kts)ww – present weather

PPP – barometric pressure (hPa) (add a 9 or 10 and place a decimal point to the left of last number)

TT– air temperature in °FTdTd - dewpoint temperature in °F

VV ppa

34 CM 247CH

30 CLNh WRt

h RR

Z (UTC) = MST + 7 i.e., 0000 UTC = 5:00 pm MDT 1200 UTC = 5:00 am MDT

Some basics of Frontal Systems (Chapter 9)

1. Fronts are boundaries that separate air masses with differing temperature and other characteristics.

2. Often represent boundaries between polar and tropical air - marked by sharp temperature changes over a relatively short distance.

3. Cold air is typically more dense than warm air → no mixing. Instead, the denser air forces the warmer air upward.

5. Fronts are marked by wind shifts.

4. This lifting of air upward can cause cloud formation and precipitation.

6. Fronts are marked by pressure and pressure changes.

Fronts:Cold air

Warm airWarm air

Cold air

Cold front Warm front Stationary front

Types of fronts:

Cold air

Warm air

Occluded front

Warm air

Cold air Coolerair

Cold FrontsCold fronts occur when a cold air mass “catches up” with a warm, generally unstable, air mass.

Cold air

Warmair

(relatively)

2 3°C-2°C-25°C

Day BirminghamSt. LouisMinneapolis

1 5°C0°C-5°C

3 -5°C-12°C-28°C

Coldair

Warmair

Cold air advection

Cold Fronts

The cold air catches up with the warm air and it…

- The thunderstorms can produce very intense precipitation. - They only form right along the frontal boundary, - the fast movement of these fronts means that the precipitation is usually of short duration and clearing skies will soon follow.

The cold air mass moves in a different direction (W through N) than the warm air mass ( SW through S).

It is also moving faster.

1. pushes the warm, unstable air up because the cold air is denser 2. the cold air has a steep slope, because friction causes the lowest winds to slow down compared with winds higher up3. unstable rising air mass produces cumulo-type (thunderstorm) clouds

So what would our observer on the ground expect to see and feel with the passage of a “classic” cold front?

West-northwesterlyGusty, shiftingSouth-southwesterlyWinds

Steadily droppingSudden dropWarmTemperature

Rising steadilyMinimum, then

sharp risefalling steadilyPressure

Often cumulusStrong cumulus

cloudsIncreasing cumulus-

type cloudsClouds

Decreasing intensity, then

clearing

Heavy showers of rain or snow, sometimes

with hail, thunder, and lightening

Short period of showersPrecipitation

loweringSharp dropHigh: remains

steadyDew Point

After PassingWhile PassingBefore PassingWeather element

Warm Fronts

Warm fronts occur when a warm, stable air mass “catches up” with a colder air mass.

Cold air

(relatively)

Warmair

Coldair

Warmair

Warm air advection

The warm air is moving faster than the cold air.The warm air is less dense than the cold air

The warm air “runs up” along the cold air boundary, which is not as steep as in the cold front case (over-running).

There are three consequences of this.

3. The precipitation out of this stratiform type cloud tends to be light and continuous, but owing to the large horizontal area of the cloud, and slow movement of these kinds of fronts, the rain can persist for days.

2. The warm air is stable and so it doesn’t form cumulo-type clouds as in the cold front case. Instead, as it is forced to rise, it condenses gradually forming a series of stratiform clouds, in a broad area.

1. The clouds and precipitation form well ahead of the surface front.

So what would our observer on the ground expect to see and feel with the passage of a “classic” warm front?

south-southwesterlyvariableSouth-southeasterlyWinds

Warmer, then steady

Steady riseCool – cold slowly

warmingTemperature

Slight rise, followed by fall

Leveling offUsually fallingPressure

Clearing with scattered Sc

Stratus-type cloudsCi, Cs, As, Ns, St,

fogClouds

Usually none, sometimes light rain

or showersDrizzle or none

Light-to-moderate rain, snow, sleet, or

drizzlePrecipitation

Rise, then steadysteadySteady riseDew Point

After PassingWhile PassingBefore PassingWeather element

Stationary Fronts

Stationary fronts occur when the front stalls.

Coldair

Warmair

No air advection

The structure is the same as in other fronts, with the front sloping over the cold air mass.

There is no air movement across the frontal boundary, thus, there is no real weather.

Occluded Fronts

Are associated with midlatitude cyclones that have both a cold front and a warm front associated with them.

Occlusion refers to “closure”. In this case, a faster moving cold front closes with the warm front.

As the cold front reaches the warm front, (and thus the cooler air in front of the warm front), the warm air mass is separated from the surface.

Because at the surface, the cold air mass is now “catching up” with a cooler air mass rather than a warm air mass the temperature change observed at the surface is not as dramatic.

An additional change is that now the warm, unstable air is no longer being strongly lifted by the cold front.

Thus, where the occlusion has occurred, only stable stratiform cloud develops accompanied by light but persistent rain similar to the warm front.

The cooler air that has replaced the warm air at the surface is not unstable.

Fronts:Cold air

Warm airWarm air

Cold air

Cold front Warm front Stationary front

Summary:- Types of fronts:

Cold air

Warm air

Occluded front

Warm air

Cold air Coolerair

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