NATS 101 Lecture 2 Basic weather symbols and

NATS 101 Lecture 2 Basic weather symbols and fronts

We describe weather in terms of: temperature humidity pressure wind clouds precipitation patterns visibility sunlight/UV If. We we “feel” observe weather theseas weather the heat elements we feel, atthe an“weight” instanceofinthe time, air then we breathe. 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”

Pressure and Wind 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, h. Pa, inches Hg, mm Hg Wind Units: m/s, mph, km/h, kts

Temperature - 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

Moisture: water vapor - 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.

Surface Map: Pressure Systems: L low pressure H Fronts: Cold air Warm air Cold front Warm front Stationary front high pressure

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

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

Weather Map Symbols Ref: - pg 525, Appendix C, Aguado and Burt ff CH TT dd CM PPP VV ww N ppa Td. Td CLNh WRt h RR N – total cloud cover dd – wind direction ff – wind speed (kts) ww – present weather Z (UTC) = MST + 7 CH 34 CM 247 VV ppa 30 CLNh WRt h RR PPP – barometric pressure (h. Pa) (add a 9 or 10 and place a decimal point to the left of last number) TT– air temperature in °F Td. Td - dewpoint temperature in °F 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. 4. This lifting of air upward can cause cloud formation and precipitation. 5. Fronts are marked by wind shifts. 6. Fronts are marked by pressure and pressure changes.

Types of fronts: Cold air Cooler air Warm air Occluded front Fronts: Cold air Warm air Cold front Warm front Stationary front

Cold Fronts Cold air Cold fronts occur when a cold air mass “catches up” with a warm, generally unstable, air mass. Cold air advection Cold air Warm air Day Minneapolis St. Louis Birmingham 1 -5°C 0°C 5°C 2 -25°C -2°C 3 -28°C -12°C -5°C Warm air (relatively)

Cold Fronts 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. The cold air catches up with the warm air and it… 3. the 1. 2. pushes unstable coldthe air rising has warm, air a steep unstable massslope, produces air because up because cumulo-type friction the cold causes air is the denser winds toclouds lowest (thunderstorm) slow down compared with winds higher up - 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.

So what would our observer on the ground expect to see and feel with the passage of a “classic” cold front? Weather element Before Passing While Passing After Passing Winds South-southwesterly Gusty, shifting West-northwesterly Temperature Warm Sudden drop Steadily dropping Pressure falling steadily Minimum, then sharp rise Rising steadily Clouds Increasing cumulustype clouds Strong cumulus clouds Often cumulus Precipitation Short period of showers Heavy showers of rain or snow, sometimes with hail, thunder, and lightening Decreasing intensity, then clearing Dew Point High: remains steady Sharp drop lowering

Warm Fronts Warm fronts occur when a warm, stable air mass “catches up” with a colder air mass. Warm air advection Cold air Warm air Cold air (relatively)

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. 1. The clouds 3. 2. precipitation warm air and is stable precipitation out ofand thisso stratiform it doesn’t welltype ahead form cloud cumulo-type of the tends to surface be light clouds front. as and in continuous, the cold frontbut case. owing Instead, to the large as it is horizontal forced to area itofcondenses rise, the cloud, and gradually slow movement forming a series of these of stratiform kinds of fronts, the clouds, in arain broad canarea. persist for days.

So what would our observer on the ground expect to see and feel with the passage of a “classic” warm front? Weather element Before Passing While Passing After Passing Winds South-southeasterly variable south-southwesterly Temperature Cool – cold slowly warming Steady rise Warmer, then steady Pressure Usually falling Leveling off Slight rise, followed by fall Clouds Ci, Cs, As, Ns, St, fog Stratus-type clouds Clearing with scattered Sc Precipitation Light-to-moderate rain, snow, sleet, or drizzle Drizzle or none Usually none, sometimes light rain or showers Dew Point Steady rise steady Rise, then steady

Stationary Fronts Stationary fronts occur when the front stalls. The structure is the same as in other fronts, with the front sloping over the cold air mass. No air advection Cold air Warm air 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. The cooler air that has replaced the warm air at the surface is not unstable. Thus, where the occlusion has occurred, only stable stratiform cloud develops accompanied by light but persistent rain similar to the warm front.

Summary: - Types of fronts: L Cold air Cooler air Warm air Occluded front Fronts: Cold air Warm air Cold front Warm front Stationary front